exxon dp - control valves

INSTRUMENTATION DESIGN PRACTICESCONTROL VALVES Section

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1 of 29EXXONENGINEERING PROPRIETARY INFORMATION - For Authorized Company Use Only

DateDecember, 1999

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

CONTENTSSection Page

SCOPE ............................................................................................................................................................ 3

REFERENCES ................................................................................................................................................ 3DESIGN PRACTICES............................................................................................................................. 3INTERNATIONAL PRACTICES.............................................................................................................. 3OTHER LITERATURE ............................................................................................................................ 3

BACKGROUND AND DEFINITIONS .............................................................................................................. 3HYDRAULIC CIRCUIT............................................................................................................................ 3CONTROL VALVE.................................................................................................................................. 3VALVE OPERATOR ............................................................................................................................... 3VALVE CHARACTERISTICS.................................................................................................................. 3RANGEABILITY...................................................................................................................................... 4

CONTROL VALVE TYPES AND APPLICATIONS ......................................................................................... 4CONVENTIONAL DOUBLE-SEATED AND SINGLE-SEATED VALVES................................................ 4CAGE VALVES....................................................................................................................................... 5BUTTERFLY VALVES ............................................................................................................................ 5BALL VALVES ........................................................................................................................................ 5THREE-WAY VALVES ........................................................................................................................... 5OTHER TYPES OF CONTROL VALVES ............................................................................................... 6VALVE POSITIONERS........................................................................................................................... 7

SPECIFIC CONTROL VALVE APPLICATIONS............................................................................................. 7DEFINITIONS ......................................................................................................................................... 7APPLICATION LISTING ......................................................................................................................... 8

DESIGN SPECIFICATION REQUIREMENTS .............................................................................................. 13SELECTION OF CONTROL VALVE DIFFERENTIAL PRESSURE...................................................... 13

Philosophy ......................................................................................................................................... 13Inlet and Outlet Static Head Constant ................................................................................................ 13Static Head Variable .......................................................................................................................... 13Special Cases .................................................................................................................................... 13Three Way Valves for Heat Exchanger Bypass Service .................................................................... 13Furnace Feed..................................................................................................................................... 13

CONTROL VALVE SIZING................................................................................................................... 13Philosophy ......................................................................................................................................... 13General Information ........................................................................................................................... 14Sizing Calculations............................................................................................................................. 14

FAIL-SAFE POSITION - (VALVE ACTION ON AIR OR POWER FAILURE)........................................ 18BLOCK AND BYPASS VALVES VS. CONTROL VALVES WITH HANDWHEELS.............................. 19NOISE-INDUCED VIBRATION PROBLEMS ........................................................................................ 20

Changes shown by ➧

DESIGN PRACTICES INSTRUMENTATIONSection

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2 of 29CONTROL VALVES

DateDecember, 1999 PROPRIETARY INFORMATION - For Authorized Company Use Only

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CONTENTSSection Page

TABLESTable 1 Control Valve Sizing Table for Hydrocarbon Mixtures .....................................................21Table 2 Control Valve Sizing Table for Single Compounds..........................................................21Table 3 Control Valve Sizing Table for Two Phase Immiscible Fluids..........................................21Table 4 Equipment Needed to Obtain Fall-Safe Positions ...........................................................22Table 5 Control Valve Capacity (CV), Pressure Drop Ratio (XT) and Pressure Recovery

Factor (FL) Versus Body Size (Inches)............................................................................23Table 6 Control Valve Rangeability ..............................................................................................24Table 7 Control Valve Seat Leakage Rates ANSI/FCI 70-2 1991 ...............................................24

FIGURESFigure 1 Main Types Of Control Valves.........................................................................................25Figure 2 Typical System Head-Capacity Relationship...................................................................26Figure 3 Characteristics Of Linear And Equal Percentage Valves ................................................26Figure 4 Three-Way Valves In Heat Transfer Control Service.......................................................27Figure 5 Use Of Two 2-Way Valves In Place Of One 3-Way Valve...............................................28Figure 6 Cutaway View Of An Angle Valve ...................................................................................28Figure 7 Cutaway View Of A Saunders Valve ...............................................................................29

Revision Memo

12/99 Added pages 7-12 containing specific application guidelines for troublesome controlvalve applications.


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SCOPEThis section covers those aspects of control valves that are considered and decided during process design. This includes valvetype, when it is important from a process design aspect, assigning of pressure drop to the valve, and the application of controlvalves to some common and troublesome services. Comprehensive information on control valve sizing is also given.Descriptive and explanatory material is included for the benefit of those not completely familiar with control valves.

REFERENCES

DESIGN PRACTICESOther Sections of Section XIISection X, PumpsSection XIV, Fluid Flow

INTERNATIONAL PRACTICESIP 1-1-1 Symbols and Abbreviations for DrawingsIP 3-6-2 Piping at Control Valve StationsIP 10-1-1 Heavy Duty Centrifugal PumpsIP 10-1-2 Medium Duty Centrifugal PumpsIP 15-9-1 Control Valves

OTHER LITERATUREANSI/FCI 70-2 1991, Control Valve Seat LeakageANSI/ISA S75.01 1985, Flow Equations for Sizing Control Valves

BACKGROUND AND DEFINITIONS

HYDRAULIC CIRCUITA hydraulic circuit is an assembly of process equipment, piping and valves that interconnect inlet and outlet reference pressurepoints. In most cases the inlet and outlet points are storage tanks, process vessels or process pumps.

CONTROL VALVEA control valve is an engineered variable flow restriction. The input signal to the control valve is the output signal from acontroller. The control valve is constructed such that the stem lift (plug position) is proportional to the input signal. Therelationship between stem lift (plug position) and area open for flow is called the valve characteristic (discussed below). Thisrelationship is important in determining the suitability of a given valve for a given service, and therefore receives much attentionfrom control engineers and valve manufacturers.Figure 1 (see next page) shows the basic construction of the main types of valves which are used, with a synopsis of their uses.These valves are more completely described and discussed under CONTROL VALVE TYPES AND THEIR APPLICATIONS.Figure 10 shows the names of the various parts of a typical control valve.

VALVE OPERATORThe valve operator is the device used to move the stem. The most common operator is a spring-loaded diaphragm. It isactuated with compressed air, whether the controller itself is pneumatic or electronic (see Figure 10). Other operators arepistons (air or hydraulic), electric motor and air motor.

VALVE CHARACTERISTICSThe relationship between the position of a valve plug (or stem lift) and the area open for flow is the valve characteristic. With aconstant ∆P across the valve this relationship would also hold for flow rate.


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BACKGROUND AND DEFINITIONS (Cont)In process services, however, the valve ∆P is seldom a constant. It varies with flow rate due to the influence of flow rate on suchitems as pipe friction and pump head. It varies with time due to such items as exchanger fouling. Thus, the flow-lift relationshipin actual service (also called the effective valve characteristic) is a function of the entire system of which the valve is a part.Figure 2, below, illustrates how control valve ∆P of a typical system varies with system flow rate. In this example, the static headin the system has been arbitrarily chosen at 30% of the pump head at design flow rate.The valve characteristics of the two most commonly discussed types of valves, known as linear and equal percentage, are shownin Figure 3. This figure also illustrates the “deterioration” of the flow-lift relationship as the assigned ∆P across the valve (as a %of total system friction at design flow rate) is reduced from 100% to 5%.Note that a theoretical equal percentage curve does not pass through zero flow at zero lift. Actual valves, however, are madetight seating. Thus, the lower end of the theoretical curve is adjusted to pass through zero flow at zero lift.Because of this, the International Practices (formally Basic Practices) call for the equal percentage characteristic to be supplied.A major exception, for 3-way valves, is discussed under 3-way valve applications.

RANGEABILITYEach control valve type (and size) has a minimum area open to flow, below which operation is not sufficiently consistent orprecise for satisfactory control. The rangeability of the valve is defined as this minimum divided into the area open to flow at fullstem lift.However, actual control valve rangeability is limited by the following factors:• Control valves are specified with 80% open at maximum flow, to insure it is in a controlling position.• The specified valve capacity will rarely match a commercially available size, since the next larger valve is frequently

supplied.• Erosion and corrosion in service will cause a deterioration of the valve's rangeability.See Table 6, Control Valve Rangeability, for a listing of typical values.If a process rangeability greater than 10 is required, it may be necessary to go into more detail on the actual valve type, size andservice to be sure that the process rangeability can be covered by one valve. Another approach, especially if the processrangeability is well above 10, is to use two valves in parallel; the smaller one for small flow rates. The selection of which valve touse should be by manual operation of block valves as it would be expected that the large valve would leak a large percentage ofthe small valve's capacity.

CONTROL VALVE TYPES AND APPLICATIONSIn this section the more common types of control valves are described, their limitations and capabilities are discussed and theirmore normal applications are covered. Valve positioners and their uses are described.

CONVENTIONAL DOUBLE-SEATED AND SINGLE-SEATED VALVESThe most common control valve is the conventional double-seated valve, the basic structure of which is shown underBackground and Definitions. The purpose of using the somewhat complicated double seat construction is to achieve andapproximately “balanced” design. The term “balanced” comes from the feature that the forces at one seat caused by the ∆P andthe stream velocity are approximately balanced by similar and opposite forces at the other seat.For sizes under one inch, normal machining tolerances are too large to give suitable rangeability, characteristic and leakage forthe double-seat design. The single-seat design is then used. Because of the small size, the forces at the one seat (unbalancedbecause of the absence of the second seat) are sufficiently reduced so that they are no longer of great importance. In thesesmall sizes, the usual construction is to machine flutes or variable depth V-grooves in a solid plug. In the smaller sizes (below anominal 1/4 in.), with flow coefficients (Cv below 1), the area open to flow is so small that normal dirt and scale can causetroublesome plugging. In these sizes, consideration should be given to the use of a strainer ahead of the valve.A double-seated valve, with the seats freshly ground-in, and tested at the factory on water at room temperature, will commonlymeet a leakage specification of 1/2% of full open flow. In refinery service at higher (or lower) temperatures (giving differentialexpansion), connected to lines that can transmit forces to the body (causing distortion), and possibly handling corrosive anderosive fluids, the leakage is not predictable but can be expected to be several times greater than the 1/2%. Thus, when tightshut-off is important, such as on a furnace fuel cut-off valve, the single-seat design is the more logical choice.The unbalanced forces in the single seat design, especially in the larger sizes, may require a piston type actuator. This will beestablished during the procurement phase of a project, and is therefore not a primary responsibility of the process designer.


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BACKGROUND AND DEFINITIONS (Cont)However, he should be aware that the piston actuator causes some complication and added cost in achieving the fail-safe action(see comments under Fail-Safe Position) that is so simple with the standard spring-loaded diaphragm. He should therefore keepthe use of single-seated valves in the larger sizes to a minimum.

CAGE VALVESThe cage valve has large guiding surfaces and close tolerances, which eliminate the need for some of the special materials usedin the conventional double and single-seated designs. The unbalance problem of the cage valve, inherent in its single-seatconstruction, is overcome by bringing the downstream pressure above the piston. Leakage, rangeability and characteristics arecomparable to those of conventional double-seated valves.

BUTTERFLY VALVESThe original automatic control applications of butterfly valves emphasized their similarity to dampers. Therefore, they were usedin large air or gas lines with a low ∆P, where limited rangeability was needed and where tight shutoff was not needed.Subsequent developments in their mechanical design, triggered somewhat by process requirements for higher capacities thancould be obtained with the largest practicable double-seated conventional valves (about 12 in. size), have made them usable fora wider range of service conditions.Their flow-lift (rotation) characteristic is between those of linear and equal percentage valves. Their leakage (not wellestablished) should be considered as somewhat greater than that of a double-seated valve, and their torque requirements limitthe services for which the conventional spring-loaded diaphragm actuator can be used. Thus, they are seldom used where aconventional double-seated valve is practicable. An 8-in. butterfly valve, 60 degrees open, has about the capacity of a 10-in.conventional double-seated valve; and it is at about this size that the butterfly valve begins to be the first choice. The butterflyvalve must now also compete with the ball valve.Services that frequently use butterfly valves include:• Suction lines to centrifugal gas compressors and air blowers.• Outlet lines from water disengaging drums.• As a substitute for a large (or unavailable) 3-way (two butterfly valves are needed) in gas lines through and by-passing heat

exchangers. Special needs of this service are covered under 3-way valves.When used in a service characterized by (1) large lines, (2) low velocities and (3) a need for minimum ∆P for the wide open case(maximum throughput and no control except in the down direction), the manifolding and relative sizes of valve and line areimportant, inasmuch as the pressure loss through two reducers will commonly exceed the ∆P of the valve. Note the discussion ofthis point under Block and Bypass Valves.

BALL VALVESThe ball valve was originally offered as a competitor for the gate valve; i.e., primarily for services where it would be either open orshut, rather than throttling. Subsequently, control valve manufacturers developed it further for throttling service, to make itsuitable for use as a control valve, while retaining the characteristics of good shut-off and high capacity. Several variations of theeccentric rotary valve design are available, the Masoneilan Camflex being a good example. The flow-lift (rotation) characteristicof the shaped ball valve approximates that of the equal percentage valve.The trend in control valve services involving high flow rates and large line sizes is to use ball valves in place of butterfly valves,because ball valves have less leakage in the closed position and less unbalanced internal forces, while still offering similar highcapacity. For high pressure drop services, the ball valve is superior to the butterfly valve. The ball valve is also finding use inplace of conventional valves, especially in high flow rate applications, where its smaller size for a given service makes iteconomical.The ball valve design is widely used in industries where flow of slurries must be controlled. In high ∆P gas services, the ballvalve loses some of its capacity advantage, relative to that of other valves. The comments under Butterfly Valves concerningmanifolding and relative line and valve sizes apply also to ball valves.

THREE-WAY VALVESThere are two types of 3-way valves, “double-seated” and “single-seated.” The basic structures of these valves are shown inFigure 1. The terms “double-seated” and “single-seated,” as applied to the two types of 3-way valves, are somewhat confusing,since each type of valve has two seats. However, the body of the 3-way valve labeled “Double Seat-Diverting” (see Figure 1) isadapted from the body of a conventional double-seated valve (called a 2-way valve when the discussion covers bothconventional and 3-way valves). Similarly, the body of the 3-way valve labeled “Single Seat-Mixing” is adapted from the body ofa conventional single-seated valve (also called a 2-way valve to distinguish it from a 3-way valve in discussions covering both).


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BACKGROUND AND DEFINITIONS (Cont)The main application of 3-way valves in refineries and chemical plants is for control of heat transfer, as illustrated in Figure 4.Note that in Figure 4 each valve is shown in its preferred location, with the flow through each port tending to open that port. Ifthe valve were installed in such a location that the flow would tend to close the port, there would be a danger of the valveslamming onto its seat, with resulting line vibration, especially in the larger sizes so often used in our designs.This action is caused by the following phenomenon: as a valve moves toward its seat, the reduction in flow rate caused by thismovement lowers the downstream pressure, thereby increasing the ∆P across the valve. This increased ∆P results in anincreased force acting to move the valve toward the seat. In addition, with the compressibility of air in the diaphragm chamber inthe valve actuator, the added forces causes an additional valve movement toward the seat. With all of these forces tending toclose the valve, the valve closes too far (or all the way), and the controller is then forced to open the valve again. This action -reaction process continues as the valve slams onto the seat, then opens too far, and repeats the operation. This phenomenon issometimes called the “bath tub stopper effect,” as a hand-held stopper will clearly illustrate.It is possible to prevent such valve action by use of a sufficiently large operator or by eliminating the air diaphragm. However,unless the valve and actuator forces are reviewed in detail for a specific application, the valve type for its location should bespecified per Figure 4.Of the two types of 3-way valves shown in Figure 1, the single seat-mixing type is most widely available world-wide and istherefore usually preferred over the double seated-diverting type valve for reasons of procurement.As an exception to the above rule, the double-seated type in diverting service is usually called for when a large density changetakes place in the exchanger, which would cause problems in ∆P and rangeability in a 3-way valve in mixing service. Themagnitude of this change in density at which the double-seated valve in diverting service should be specified, rather than thesingle-seat mixing type, is not well established. Partial vaporization of a liquid stream is considered sufficient reason forspecifying the double-seated diverting type. On the other hand, partial condensation of a gas stream does not have aspronounced an effect on density, and partial condensation of as much as 1/3 of the process fluid is considered acceptable for thesingle-seated mixing type valve.In this 3-way valve service, a linear flow-stem lift characteristic is required to insure a constant total area open for flow in thecircuit for all valve stem positions. This linear characteristic is required by the International Practices.Where the valve size required is greater than 10 inches, which is commonly the case in Powerformer effluent (hot gas) circuits, itis necessary to use two 2-way valves instead of one 3-way valve. These 2-way valves are usually butterfly valves, with one valveopening as the other valve closes, giving the same control result as one 3-way valve. See Figure 5.From the safety aspect, it should be noted that in a 3-way valve the total area open for flow is equivalent to having one portalways open. The arrangement with two 2-way valves, on the other hand, can conceivably have both valves shut at the sametime. This may result in the need for additional safety valves.Special requirements of butterfly valves, which are the most common valves in this service, are:• The characteristic, not being linear, must be made equivalent to linear. A characterizing cam in the valve positioner is a

common method of accomplishing this. This is called for in the International Practices.• The two valves should be the same size. This gives the same result as a 3-way valve, in which the two ports are always of

equal size. Experience shows that it is a mistake to use a much smaller valve in the bypass line of the basis that a smallpercentage of the total stream is expected to be bypassed; clean tubes and slight shifts in temperatures and heat dutieshave resulted in such designs being inoperable. Despite this, since the bypass valve ∆P is 3 times as great as that of theexchanger valve,* and since large valves are involved, it is acceptable, as an exception to the general rule, to use a bypassvalve one size smaller than the exchanger valve.

*See SELECTION OF CONTROL VALVE DIFFERENTIAL PRESSURE where for 3-way valves an exchanger port ∆P of 50% ofthe exchanger pressure drop is called for, thus giving the bypass port a ∆P of 150% of the exchanger ∆P.

OTHER TYPES OF CONTROL VALVESAngle valves (see Figure 6), sometimes called angle-venturi valves, are used for service conditions where the direct jetting ofthe stream from the plug-seat opening into the exit pipe is advantageous. Such services include:• Streams containing solids which would erode conventional valves, if the jet were to impinge on valve surfaces. Cat plant

slurries and fluids having coking tendencies are examples.


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BACKGROUND AND DEFINITIONS (Cont)Note that the stream flowing past the plug and jetting into the outlet line gives the “bathtub stopper effect” mentioned above underTHREE-WAY VALVES. This situation is called “flow tends to close valve.” To prevent this situation from causing valve chatter,line vibration and unsatisfactory control, one can specify a stronger-than-normal operator or the internal design of the valve canbe changed to provide some balancing force. This would be done during procurement. Where angle valves are discussed in thissubsection, the expression “flow tends to close valve” is included, so that this point should not be overlooked. Angle valves canbe used with flow in the reverse direction, so that flow tends to open the valve; however, this is done only in unusual services.Saunders Valves, sometimes called diaphragm valves, are well known but infrequently used. Their main usefulness is forhandling some slurries and corrosive fluids. Their services conditions are rather limited, because the diaphragm will withstandtemperatures only up to the low 200's (°F), pressures up to about 50 psig and only moderate pressure drops. The allowableconditions should be confirmed from available suppliers, before this type of valve is included in a design.

VALVE POSITIONERSIn the discussion up to his point, it has been implied that a control valve always responds to a control signal and takes a positionproportional to it. Several phenomena prevent the valve from behaving exactly in this manner. Ordinary friction on the valvestem in the stuffing box results in a hysteresis effect (between opposite directions of movement of the valve stem) equivalent toabout 1/8 to 1/2 psi on the diaphragm. This corresponds to about 1 to 4% of the 3 to 15 psig operating pressure range of astandard diaphragm - spring assembly. Internal velocity forces commonly amount to another several percent of the forceavailable for the diaphragm - spring assembly, and these forces change with flow rate. Some valves are particularly bad in thisrespect; butterfly valves and single-seated valves are examples.Because of these inconsistencies and hysteresis, and because such valve action would result in unsatisfactory control in manyapplications, valve positioners were developed. The function of a valve positioner is to force the control valve to take a positionproportional to the control signal. In principle, it is a position controller with the control signal being its set point. The InternationalPractices require valve positioners to be supplied for all control valves except those with a simple on-off operation.Applications of Valve Positioners - Valve positioners are also useful for such requirements as “split-ranging” and “linearizing.”Proper linkage adjustments for this purpose are usually available in normal models. In split-ranging, the valve takes full stroke forhalf the range of the control signal. Thus, one valve operates between 3 and 9 psig of the control signal, and a second valveoperates between 9 and 15 psig. The valves themselves are standard control valves with a diaphragm pressure operating rangeof 3 to 15 psig.An example of a split range valve application is suction pressure control for a compressor, where the 3 to 9 psig portion of thecontrol signal range causes the compressor to be progressively loaded (suction valve or speed governor setting), while the 9 to15 psig portion of the control signal range causes a flare release valve to open progressively.An example of linearizing is the use of valve positioners with two butterfly valves through and around a heat exchanger, asdescribed under 3-way valves. In this case, the motion balance is made non-linear, usually with a cam in order to compensatefor the inherently non-linear characteristic of the valve, to result in overall linear/relationship between control signal and valveopening.

➧ SPECIFIC CONTROL VALVE APPLICATIONSThis section is intended to provide suggestions on appropriate choices of the style, type and specific items to be included forprocess services that have proven over time to be particularly troublesome in service. The cases outlined below should be usedas guidelines since no new application reflects exactly the same characteristics as the ones identified in this listing. Engineeringjudgment should be applied as appropriate when making final selections for this equipment. The best guideline is that provenand well documented experience should be used.

DEFINITIONSLow Recovery Valve is one that does not recover very much pressure at the outlet compared to the differential pressure acrossthe valve itself. Examples are cage valves and globe valves with flow upwards against the seat.High Recovery Valve is one that does recover a significant amount of pressure at the outlet compared to the differentialpressure across the valve itself. An example is a streamlined angle valve with flow in the side and out the bottom.Erosion/Corrosion damage is the damage to valve bodies and internal trim due to combined effects of straight erosive materialremoval and corrosion of internal surfaces. This effect can increase loss of material in the range of orders of magnitude whencompared to corrosion or erosion alone.


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SPECIFIC CONTROL VALVE APPLICATIONS (Cont)

APPLICATION LISTING1. Boiler Feed-water or Waste Heat Boiler Supply

Fluid: WaterTypical Operating ConditionsValve Inlet Pressure: 300 - 2500 psig (2068 - 17237 kPa gauge) based on feedwater supply pump discharge pressureValve Outlet Pressure: atmospheric up to header or drum pressure plus ten percent moreTemperature: 100 - 200°F (37 - 93 °C)Most Likely Problem with ApplicationValves tend to be destroyed on startup if they are selected to handle relatively low differential pressure at drum operatingpressures but used to handle full pump pressure when initially filling the steam drum and encountering high differentialpressure.Problem Solving Approaches in Control Valve SelectionUtilize two separate valves, one for initial filling and one for running. Alternatively use a single high pressure globe styleangle valves with flow direction under the seat to out the side outlet. An example of this type of valve is the Fisher DBAQ.SizingLiquid sizing methods should be used for normal operation (Method 1). Single compound (water) Flashing methods shouldbe used for steam drum filling (Method 7).

2. Sulfur Vapor to Eductor (Sulfur Pit Application)Fluid: Sweep Air from Sulfur PitTypical Operating ConditionsValve Inlet Pressure: 3 in. (76 mm) of water vacuumValve Outlet Pressure: 14 in. (355 mm) of water vacuumTemperature: 300°F (149°C)Most Likely Problem with ApplicationPlugging of the valve with condensed sulfur vapor forming solid sulfur deposits is the main problem.Problem Solving Approaches in Control Valve SelectionLine size rotary valves should be used. Either body jacketing or well installed (heavy) steam tracing should be used.Butterfly, eccentric or in some cases segmented ball valves should be used.SizingMethod 2 should be used

3. Liquid Sulfur to StorageFluid: Liquid SulfurTypical Operating ConditionsValve Inlet Pressure: 30 psig (206 kPa gauge)Valve Outlet Pressure: 20 psig (138 kPa gauge)Temperature: 300°F (149°C)Most Likely Problem with ApplicationPlugging of the valve with solid sulfur is the main problem.Problem Solving Approaches in Control Valve SelectionLine size rotary valves should be used. Full body jacketing must be used. Butterfly, eccentric or in some cases segmentedball valves should be used.SizingLiquid sizing methods should be used (Method 1).


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SPECIFIC CONTROL VALVE APPLICATIONS (Cont)4. Hydrofluoric Acid Service

Fluid: Various dilutions of HFTypical Operating ConditionsWide ranging and not clearly definableMost Likely Problem with ApplicationLoss of containment in valve body due to incorrect body material, selection or process condition change and foreign materialpresent due to body testing at delivery.Problem Solving Approaches in Control Valve SelectionCarbon steel body should be used for service to about 300°F (149°C). Initial corrosion of the surface creates a protectivebarrier to limit further corrosion. Abrasion or water can remove this barrier. Monel body should be used services above300°F (149°C). Since the consequences of process fluid leakage are highly significant detailed attention must be given toadequate body inspection including materials verification. Valve acceptance testing should not employ water as a test fluiddue to the negative effects of having water inadvertently introduced into the valve as a leftover from tests at the factory.Voids of all types must be avoided in either body or internal trim parts.SizingMethod 1 is used in most cases.

5. Slurry Service - FCCU Bottoms, FCCU Pump AroundFluid: Heavy hydrocarbon mixtures with solids present (1% to 15% wt range variable particle size 10 to 200 micron)Typical Operating ConditionsValve Inlet Pressure: 80 psig (550 kPa gauge)Valve Outlet Pressure: 20 psig (138 kPa gauge)Temperature: 500 - 600°F (260 - 315°C)Most Likely Problem with ApplicationErosion of valve body, internals and downstream piping are the main to be dealt with in this application.Problem Solving Approaches in Control Valve SelectionStream lined angle valves with flow entering the side and exit the bottom are the best choice for this service. Hard materialsfor trim application are also a plus. Downstream piping must straight. Free of elbows and obstructions for a length of 20inside pipe diameters.Where hydrostatic pressure is not sufficient to employ streamlined angle valves, full ball rotary valves can be used.Materials used for the ball must be hard. The same suggestions for downstream piping as outlined above should be used.SizingMethod 1 is used in most cases.

6. Reactor Hot Separator Level Control Letdown ValveFluid: High temperature hydrocarbon mixture from Reactor effluent usually the result of hydrogen and hydrocarbonmaterials reacted in the presence of catalyst to form lighter materials.Typical Operating ConditionsInlet Pressure: typical 1000 - 2500 psig (6895 - 17237 kPa gauge)Outlet Pressure: typical 200 psig (1379 kPa gauge)Temperature: 850°F (455°C)Most Likely Problem with ApplicationErosion damage to the valve, flashing/cavitation conditions leading to incorrect valve sizing are the primary problems.Problem Solving Approaches in Control Valve SelectionThere are two approaches in handling this problem. The first is to use a streamlined angle valve where particulate is thoughtto be significant (above 1% by wt). Additional purging of the control valve bonnet with gas oil (high boiling range) should beconsidered to maintain plug to guide bushing freedom from coke build-up.An alternate, where particulate levels are low, is to use a low pressure recovery cage valve. Straight bore cage types withholes that are not counter bored should be used. Downstream piping should be straight from the outlet of the valve for adistance of 20 diameters.


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SPECIFIC CONTROL VALVE APPLICATIONS (Cont)SizingSizing these valve applications is particularly difficult since two phase flow is involved. Method 4 should be used in mostcases.

7. Reactor Cold Separator Level Control Letdown Valve (for cases where hot and cold vessels are linked with heatexchangers to reduce temperature)Fluid: Hot separator overhead vapor cooled by a heat exchanger.Process ConditionsInlet Pressure: typical 1000 - 2500 psig (6895 - 17237 kPa gauge)Outlet Pressure: typical 200 psig (1379 kPa gauge)Temperature: 200 - 300°F (93 - 149°C)Most Likely Problem with ApplicationErosion damage to the valve, flashing/cavitation conditions leading to incorrect valve sizing are the primary problems.Problem Solving Approaches in Control Valve SelectionThere are two approaches in handling this problem. The first is to use a streamlined angle valve where particulate is thoughtto be significant (above 1% by wt). An alternate, where particulate levels are low, is to use a low pressure recovery cagevalve. Straight bore cage types with holes that are not counter bored should be used. Downstream piping should be straightfrom the outlet of the valve for a distance of 20 diameters.SizingSizing these valve applications is particularly difficult since two phase flow is involved. Method 4 should be used in mostcases.

8. Hot and Cold Separator Water Draw Boot Level Control ValvesFluid: Relatively pure waterTypical Operating ConditionsInlet Pressure: typical 1000 - 2500 psig (6895 - 17237 kPa gauge)Outlet Pressure: typical 50 psig (344 kPa gauge)Temperature: 200 - 300°F (93 - 149°C)Most Likely Problem with ApplicationLow flow rates require cyclic opening and closing of the control valve corresponding to high and low liquid levels. Flashingconditions complicate valve sizing.Problem Solving Approaches in Control Valve SelectionThe most successful approach in solution of this application problem is to use a single seated globe valve with flow enteringunder the seat. Body style can be either straight or angle pattern. Specialty multi stage valves an be considered for theservice. Masoneilan model 78200 is an acceptable design.SizingMethod 3 should be used in most cases.

9. Depressurization between Intermediate Reactor effluent vesselsFluid: Hydrocarbon mixture with particulate presentTypical Operating ConditionsInlet Pressure typical: 1000 - 2500 psig (6895 - 17237 kPa gauge)Outlet Pressure typical: 400 - 700 psig (2758 - 4826 kPa gauge)Temperature: 850°F (455°C)Description and most Likely Problem with ApplicationThis application takes two forms.The first is a sequential depressurization of the reactor outlet. The hot separator recommendation listed above should beused for this case.The second case is depressurization between the overhead vapor in the hot and cold separator vessels. This application isnormally a hydrocarbon mixture vapor depressurization but can exhibit droplet carry over depending upon vapor rates andvessel size.


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SPECIFIC CONTROL VALVE APPLICATIONS (Cont)Problem Solving Approaches in Control Valve SelectionSolution of case two is accomplished by means of a cage valve using straight bore holes.SizingMethod 2 is normally used.

10. Rich and Lean MEA Letdown Valve ApplicationsFluid: Rich and Lean MEA solution containing H2S dissolved in the solution.Typical Operating ConditionsRich Valve Inlet Pressure: 150 - 300 psig (1034 - 2068 kPa gauge)Rich Valve Outlet Pressure: 20 - 70 psig (138 - 483 kPa gauge)Lean Valve Inlet Pressure: 100 - 200 psig (689 - 1379 kPa gauge)Lean Valve Outlet Pressure: 5 - 20 psig (35 - 138 kPa gauge)Temperature: 100 - 250°F (38 - 121°C)Most Likely Problem with ApplicationErosion of valve body, internals and downstream piping are the main problems to be dealt with in this application. The causeis entrained particulate that occurs due to contamination of the MEA solution. This occurs due to startup debris or corrosionof internal vessel and piping which accumulates due to the constant recycle of solution.Problem Solving Approaches in Control Valve SelectionUse of cage valves is suggested for both applications. Cage holes should be straight bore holes with hard materialsemployed where possible. Counterbored holes should not be used since reduced internal hole orifices tend to be erodedaway resulting in a control valve capacity increase caused by the extra hole area. If downstream safety valves are in servicethese can be undersized if the controlling blow through rate is based upon valve capacity.SizingMethod 4 is used in most cases but expert advise is often needed.

11. Rich Catacarb Letdown ValveFluid: Rich Catacarb solution containing CO2 dissolved in the solution.Typical Operating ConditionsRich Valve Inlet Pressure: 150 - 300 psig (1034 - 2068 kPa gauge)Rich Valve Outlet Pressure: 20 - 70 psig (138 - 483 kPa gauge)Temperature: 100 - 250°F (38 - 121°C)Most Likely Problem with ApplicationErosion of valve body, internals and downstream piping are the main problems to be dealt with in this application. The causeis entrained particulate and rapid vaporization that occurs as the fluid with dissolved CO2 is depressurized into the absorbertower. This occurs due to startup debris or corrosion of internal vessel and piping which can accumulate due to the constantrecycle of solution.Problem Solving Approaches in Control Valve SelectionUse of cage valves is suggested for the application. Cage holes should be straight bore holes with hard materials employedwhere possible. Counterbored holes should not be used since reduced internal hole orifices tend to be eroded awayresulting in a control valve capacity increase caused by the extra hole area. The use of an angle pattern valve is suggestedwith flow in the side and out the bottom. The bottom connection should be mounted on the vessel flange close coupled tothe tower.SizingMethod 4 is used in most cases but expert advise is often needed.

12. High Solids (10-25% wt.) in Multiphase Hydrocarbon Mixtures (Low Pressure and High Pressure)Fluid: Hydrocarbon mixture carrying particulate with a wide range of size and weight distribution.Typical Operating ConditionsInlet Pressure: 100 - 2500 psig (689 - 17237 kPa gauge)Outlet Pressure: 20 - 200 psig (138 - 1379 kPa gauge)Temperature: ambient - 850°F (455°C)


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SPECIFIC CONTROL VALVE APPLICATIONS (Cont)Most Likely Problem with ApplicationErosion damage to the valve, flashing/cavitation conditions leading to incorrect valve sizing are the primary problems.Problem Solving Approaches in Control Valve SelectionA streamlined angle valve should be used with fluid entering the side and exit the bottom. Additional purging of the controlvalve bonnet with gas oil (high boiling range) should be considered to maintain plug to guide bushing freedom.Where pressures are low and not sufficient to allow use of a streamlined angle valve, then rotary valves can be used. Theseand their installation must be carefully designed.SizingEither method 1 or 4 should be used depending upon the process conditions.

13. Compressor or Blower (Centrifugal) Anti-surge Control ValvesFluid: Gas or VaporTypical Operating ConditionsInlet Pressure: Compressor dischargeOutlet Pressure: Compressor Suction or Intermediate StageTemperature: variable all levelsLikely Problem with ApplicationCompressor (centrifugal) recycle control valves are used to provide a return path for compressor discharge flow. Theprimary purpose of the valve is to prevent low flow conditions in the compressor that result in damage to the machine. Themain problem with the application is valve speed of response. The valve or its operator must operate quickly to avoidmachine damage after surge conditions are detected.Problem Solving Approaches in Control Valve SelectionIn most cases relatively high control valve capacity is required to meet flow requirements imposed by the machine. Hencerotary valves tend to be used. High speed actuation is achieved by means of dedicated high pressure air cylinders, local tothe valve or through hydraulic operators.SizingMethod 2 is normally used.

14. Pump Re-circulation (low flow bypass) ValveFluid: Hydrocarbon mixture or single compoundTypical Operating ConditionsInlet pressure (pump discharge): 100 - 900 psig (689 - 6200 kPa gauge)Outlet Pressure: pump suctionTemperature: 100 - 500°F (38 - 260°C)Most Likely Problem with ApplicationIn most cases this application is a typical liquid application. In a small number of cases the re-circulated fluid changes statewhen passing though the valve. Typically a state change to vapor causes cavitation rather than flashing due to pump inletconditions typically being all liquid.Problem Solving Approaches in Control Valve SelectionIf there is no state change in the fluid then no special requirements should be imposed.State changes should normally be handled through the use of specialty multi-stage valves. One acceptable type is theMasoneilan model 78200.Caveat - Single body combined discharge and recycle valves should not be used where fluid change of state is predicted. Inaddition the use of block valves in series with fixed restriction devices should be carefully reviewed prior to their use.SizingMethod 1 should be used for all liquid with no state change. Other methods selected based on the fluid should be employedfor sizing these applications.


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DESIGN SPECIFICATION REQUIREMENTS

SELECTION OF CONTROL VALVE DIFFERENTIAL PRESSURE

Philosophy

Assignment of valve pressure drop is sometimes as simple as checking the operating pressures of up and downstreamequipment and using those values as the valve pressure drop by definition when no other parameters influence the selection. Inother cases the hydraulic considerations are more complex. In these cases, frictional pressure loss in piping, process equipmentand external pressures defined as static head at the inlet and outlet of a hydraulic circuit play important roles. Available valvedifferential pressure must be great enough to allow reduction of circuit flow to the minimum design rate yet not so great as tosignificantly increase the cost of providing the differential pressure to operate the hydraulic loop.The key question is, how much differential pressure is enough to achieve process objectives in all operating cases. Guidelinesfor the most common cases are outlined below:

Inlet and Outlet Static Head Constant

Clearly, when process flow rates are at the maximum, hydraulic circuit installed process equipment uses maximum and thecontrol valve is using minimum differential pressure. Conversely when flow rates are at the minimum process equipment usesminimum and the control valve absorbs maximum differential pressure. The pressure range on the control valve over themaximum to minimum flow rates is not linear with the flow. It generally varies with the square root of the flow. Ideally the controlvalve should be wide open when the flow is at maximum and closed to a point considered its minimum useful operating capacity.Hence there are really two control valve associated selections to be made. These are the assigned differential pressure and theinherent rangeability of the control valve itself which has been discussed earlier in this document.The differential pressure is selected by making frictional pressure loss calculations of all the equipment installed in the hydraulicloop including the piping at the maximum and minimum flow rates anticipated not including the control valve. The differencebetween these two numbers is the minimum differential pressure that must be available to the control valve to achieve flowcontrol in the hydraulic circuit. The guideline that should be used is 20% of the overall circuit differential pressure at maximumflow rate.

Static Head Variable

If the process inlet and outlet static head for the hydraulic circuit is variable, the impact of the minimum and maximum hydraulicloop pressure swings for all operating cases must be considered in assignment of control valve differential pressure. Ideally thehigh and low range should be used in the calculations described above to obtain minimum control valve differential pressure. Forcases where single phase liquid is present throughout the hydraulic loop, the guideline on control valve differential pressure is toincrease the allowable pressure between 5 and 10% of the maximum frictional hydraulic circuit loss beyond those considerationslisted in previous paragraphs. This results in a typical overall differential pressure of 25 to 30% of the overall hydraulic circuit atmaximum process flow rate.

Special Cases

Three Way Valves for Heat Exchanger Bypass Service

Three way valves or valve combinations, should be sized utilizing a differential pressure of 50% of the maximum fouleddifferential pressure for the valve in series with the exchanger itself and 150% of the maximum fouled differential pressure for thevalve in parallel with the series valve and exchanger itself. Maximum flow rates should be used.

Furnace Feed

Furnace pass flow control valves should generally be sized using a minimum pressure differential of 10% of the maximumfurnace pass differential pressure unless other hydraulic equipment in the process loop requires more differential pressure basedon the methods outlined above.

CONTROL VALVE SIZING

Philosophy

Selection of Control Valve size, normally characterized by the term Cv, should be calculated using the methods described in thissection. This section provides recommended methods, a summary of control valve parameters and general information on thesizing process. Data on Control Valves provided in Table 5 should be used when details for specific valves are not available.


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DESIGN SPECIFICATION REQUIREMENTS (Cont)

General Information

The results of specific control valve sizing calculations should be used to select a control valve that provides the calculated Cv atan opening that represents 80% of the full open capacity of the valve. For butterfly valves the calculated Cv should occur at anopening of 60° from full closed.Variations in effective control valve Cv's due to valve body size reduction in comparison to line size have not been included in thisdocument. This is due to the wide range of swaging effects on control valves. Selection of a control valve with a body size lessthan the line size always reduces the effective Cv. The amount of reduction depends upon the type of valve and the degree ofreduction. The effects of control valve body size reduction for a given line size should be discussed with a Control ValveSpecialist.

Sizing Calculations

Sizing calculations should start with knowledge of the process fluid. Tables 1 - 3 identify hydrocarbons, pure compounds andimmiscible gas/liquid combinations with specific guidelines on how control valves should be sized in each category. The methodsidentified should be used.Method 1 - Liquid to Liquid

)pp(gNwC

)pp(G

NqC

2116v

21

f

1v

−γ=

−=

where: q = volumetric flow rate (gpm, m3/h)w = mass flow rate (lb/h, Kg/h)p1 = absolute inlet pressure (psia, kPa)p2 = absolute outlet pressure (psia, kPa)Gf = liquid specific gravity at upstream conditions referenced to water at 60°Fγ1 = specific weight at upstream conditions (lb/ft3, kg/m3)N1 = 1.0 for Customary units (gpm, psia); = 0.0865 for Metric units (m3/h, kPa)N6 = 63.3 for Customary units (lb/h, psia, lb/ft3); = 2.73 for Metric units (kg/h, kPa, kg/m3)

Method 1 - Example CalculationProcess Conditions to be used for the example calculationTemperature = 500°RMinimum Control Valve Differential Pressure = 20 psi @ a volumetric flow rate of 200 GPM.Maximum Control Valve Differential Pressure = 40 psi @ a volumetric flow rate of 50 GPMFluid is all liquid because the vapor pressure at temperature is below all calculated hydraulic circuit pressuresGf = 0.95 @ process conditions

71.7)40(

95.00.10.5C59.43

)20(95.0

0.1200C flowratemin@vflowratemax@v ====

Selection of Control ValveTable 5 lists a number of valve types in the 2 and 3 in. body size range that have enough capacity to handle this application.First calculate the maximum Cv required including the 80% factor:

49.5480.059.43C maxv ==

Selecting a 3 in. Cage Valve with a Cv of 65 satisfies the capacity requirement of the calculations. Calculating Rangeability usingthe Table 5 value and the smaller Cv from the calculation above yields the rangeability.

43.871.7

65C

CtyRangeabiliValveControlflowratemin@v

maxv ===

Checking with Table 6, the figure calculated is below the maximum number listed for a Cage Valve, hence the selection iscorrect.


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DESIGN SPECIFICATION REQUIREMENTS (Cont)Method 2 - Gas / Vapor to Gas / Vapor

xMT

YpNqC

xTG

YpNqC 1

19v

1g

17v ==

MxT

YpNwC

xpgYNwC 1

18v

116v =

γ=

T1

21xk142.2

x1Yp

ppx −=−=

where: xT = the pressure drop ratio factor empirically determined for each valveq = volumetric flow rate (cubic feet per hour at 14.73 psia and 60°F or cubic meters per hour

at 101.3 kPa and 15.6°C)w = mass flow rate (lb/h, Kg/h)p1 = absolute inlet pressure (psia, kPa)p2 = absolute outlet pressure (psia, kPa)k = the ratio of specific heats (Cp/Cv)Gg = gas/vapor specific gravity at upstream conditions referenced to air both at standard

conditionsγ1 = specific weight at upstream conditions (lb/ft3, kg/m3)T1 = absolute upstream temperature (°R,°K)M = molecular weightN6 = 63.3 for Customary units (lb/h, psia, lb/ft3); = 2.73 for Metric units (kg/h, kPa, kg/m3)N7 = 1360 for Customary units (scfh, psia, °R); = 4.17 for Metric units (kg/h, kPa, °K)N8 = 19.3 for Customary units (lb/h, psia, °R); = 0.948 for Metric units (kg/h, kPa, °K)N9 = 7320 for Customary units (scfh, psia, °R); = 22.5 for Metric units (kg/h, kPa, °K)

Limitations on equationsThe range of differential pressures represented as x that is allowable for these equations is limited to:

40.1xkx T≤

for k between 1.25 –> 1.40. If x, when calculated through pressure ratios, is above the limit then the limit value should be usedfor x. If k falls outside the range indicated above, the assistance of a control valve sizing specialist should be obtained for atechnical recommendation on sizing procedures.Method 2 - Example CalculationProcess Conditions to be used for the example calculationTemperature = 500°RMinimum Control Valve Differential Pressure = 20 psi @ a mass flow rate of 50,000 lb/h and a line pressure of 250 psia.Maximum Control Valve Differential Pressure = 40 psi @ a mass flow rate of 20,000 lb/h and a line pressure of 300 psia.Fluid is a light hydrocarbon gas with a MW = 16, k = 1.26Line Size is 4 in.


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DESIGN SPECIFICATION REQUIREMENTS (Cont)A 4 in. Butterfly valve has been arbitrarily selected for this case by simple inspection noting that the available pressure for thevalve for the maximum flow rate is relatively low. Extracting valve parameters from Table 5 and proceeding with the calculations:

37.040.1

)41.0)(26.1(x =≤

23.220)08.0)(16(

500)93.0)(250)(3.19(

000,50C flowratemax@v ==

93.0)37.0)(4.1(142.2

08.01Y08.025020x flowratemax@ =−===

17.60)13.0)(16(

500)89.0)(300)(3.19(

000,20C flowratemin@v ==

88.0)37.0)(4.1(142.2

13.01Y13.030040x flowratemin@ =−===

Calculating the maximum Cv required including the 80% factor:

29.27580.0

23.220C maxv ==

Selecting a 4 in. Butterfly valve with a Cv of 220 does not meet the capacity requirement of the calculations. A new valve typemust be selected and the calculations repeated. Consulting Table 5, a selection of a 4 in. Segmented Ball valve seemsappropriate:

23.040.1

)25.0)(26.1(x =≤

12.230)08.0)(16(

500)89.0)(250)(3.19(

000,50C flowratemax@v ==

88.0)23.0)(4.1(142.2

08.01Y08.025020x flowratemax@ =−===

52.64)13.0)(16(

500)83.0)(300)(3.19(

000,20C flowratemin@v ==

81.0)23.0)(4.1(142.2

13.01Y13.030040x flowratemin@ =−===

Calculating the maximum Cv required including the 80% factor:

65.28780.0

6.222C maxv ==

Selecting a 4 in. Segmented Ball Valve with a Cv of 550 satisfies the capacity requirement of the calculations. CalculatingRangeability using the Table 5 value and the smaller Cv from the calculation above yields the rangeability.

52.852.64

550CVALVECtyRangeabiliValveControl

flowratemin@v

maxv ===

Checking with Table 6, the figure calculated is below the maximum number listed for a Segmented Ball Valve, hence theselection is correct.


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DESIGN SPECIFICATION REQUIREMENTS (Cont)Method 3 - Single Compound Liquid to Liquid/Vapor

c

vF

vF1

f

L1v p

p28.096.0FpFp

GFN

qC −=−

=

where: q = volumetric flow rate (gpm, m3/h)p1 = absolute inlet pressure (psia, kPa)p2 = absolute outlet pressure (psia, kPa)pv = absolute vapor pressure of the liquid at inlet temperature (psia, kPa)pc = absolute critical pressure of the fluid (psia, kPa)FL = pressure recovery factor empirically determined for each valve typeGf = liquid specific gravity at upstream conditions referenced to water at 60°FN1 = 1.0 for Customary units (gpm, psia); = 0.0865 for Metric units (m3/h, kPa)

Method 4 - Liquid Hydrocarbon Mixture to Combined Liquid/Vapor Mixture HydrocarbonStep 1 - Compute or estimate the following process parameters for the control valve application:Inlet Liquid Flow RateOutlet Liquid Flow RateOutlet Vapor Flow RateInlet PressureOutlet PressureOutlet Density or Specific Weight for the liquid partOutlet Molecular Weight or Specific Weight for the vapor partStep 2Using Method 1 compute the control valve capacity attributable to the liquid portion of the outlet flow based upon:Outlet Liquid Flow RateInlet PressureOutlet PressureOutlet liquid Density or Specific Weight at conditionsStep 3Using Method 2 compute the control valve capacity attributable to the vapor portion of the outlet flow based upon:Outlet Vapor Flow RateInlet PressureOutlet PressureOutlet Vapor Molecular WeightInlet TemperatureStep 4Add the results of the calculations from Steps 2 and 3 above to obtain the control valve capacity.Method 5 - Liquid/Vapor Hydrocarbon Mixture to Liquid/Vapor Hydrocarbon MixtureUse Method 4 if the amount of vapor at the inlet is less than 3% by volume.If Greater than 3% by volume proceed as detailed below:Step 1 - Compute or estimate the following process parameters for the control valve application:Inlet Liquid Flow RateInlet Vapor Flow RateOutlet Liquid Flow RateOutlet Vapor Flow RateInlet PressureOutlet PressureInlet Vapor Molecular WeightOutlet Liquid Density or Specific WeightOutlet Vapor Molecular Weight


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DESIGN SPECIFICATION REQUIREMENTS (Cont)Step 2Using Method 1 compute the control valve capacity attributable to the liquid portion of the outlet flow based upon:Outlet Liquid Flow RateInlet PressureOutlet PressureOutlet liquid Density or Specific Weight at conditionsStep 3Using Method 2 compute the control valve capacity attributable to the vapor portion of the outlet flow based upon:Outlet Vapor Flow RateInlet PressureOutlet PressureInlet Temperaturek of the outlet vapor mixtureM as calculated below:

−−=

rOutletVaporOutletVapo

InletVaporrOutletVapoInletVaporrOutletVapo MW

)MM(WMM

where: MOutlet Vapor = the molecular weight of the outlet vaporMInlet Vapor = the molecular weight of the inlet vaporWInlet Vapor = the mass flow rate of the inlet vaporWOutlet Vapor = the mass flow rate of the outlet vapor

Step 4Add the results of the calculations from Steps 2 and 3 above to obtain the control valve capacity.Method 6 - Single Compound Liquid/Vapor to Liquid/VaporStep 1 - Use Method 3 to compute valve Cv attributable to inlet liquid conversion to outlet liquid and vapor.Step 2 - Use Method 2 to compute valve Cv attributable to inlet vapor conversion to outlet vapor. The basis must be that inlet

rates be used in the equations.Step 3 - Add the results of the calculations from Steps 1 and 2 above to obtain the control valve Cv.Method 7 - Liquid/Gas Immiscible FluidsStep 1 - Use Method 1 to compute valve Cv attributable to inlet/outlet liquid.Step 2 - Use Method 2 to compute valve Cv attributable to inlet/outlet gas.Step 3 - Add the results of the calculations from Steps 1 and 2 above to obtain the control valve Cv.

FAIL-SAFE POSITION - (VALVE ACTION ON AIR OR POWER FAILURE)Spring-Loaded, Diaphragm-Actuated Valves - With the conventional spring-loaded diaphragm actuator, a control valve will bepushed by the spring to an extreme position on loss of air pressure. This extreme position can be specified to be either the openor the closed position.When the controller is electronic the valve construction does not change. It is operated by a spring-loaded diaphragm. Anelectric-to-air (called I/P) transducer is located between the electric output from the controller and the air input to the valve. Thereare then two external energy sources and the failure of either or both must be considered. However, specifying the fail-safeposition can be simplified as follows: It is not necessary to consider each energy source separately but simply to call for aposition on failure of the operating media. Thus, the International Practices call for FO (fail open) or FC (fail close) to cover thefailure position on either an all-pneumatic or a combined electric-pneumatic system.Where it is preferable to have a valve stay in the failed position until put back in service manually (e.g., a furnace fuel valve wouldusually be in this category), the letter R (for manual “Reset:) is added to FO(R) or FC(R) designation.Sometimes a satisfactory fail-safe position is neither fully open nor fully closed, but for the valve to hold the position it had whenthe failure occurred. Because the stationary position is obtained by closing a valve to lock the air in the diaphragm head, airleakage causes a drift towards the zero air pressure position of the valve stem. Table 4 outlines the equipment needed for theseactions.Piston-Actuated Valves - Air is the actuating medium for almost all piston operated valves. Hydraulic operation is rare, becauseof its cost, and will need special attention for fail-safe action. The following paragraph is based on use of air.


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DESIGN SPECIFICATION REQUIREMENTS (Cont)With piston actuation there is sometimes a spring to push the valve to a fail-safe position. Without a spring on loss of actuatingenergy the valve would tend to drift, as governed by unbalanced internal forces and leakage past the piston. This is usually quiteslow, compared to the speed of process responses. With a springless piston-actuated valve, an open or shut fail-safe position isobtained by means of an air storage reservoir plus checks and pilots. This is a relatively complex system and should be avoidedunless it is imperative that the valve be forced to the extreme position. Table 4 outlines the equipment needed for these actions.Choice of Failure Position - When selecting the positions to which the valves go on air or power failure the goal is to requireminimum operator attention to put the unit in the safest possible standby condition; to minimize upsets to associated units; and toease the problem of returning to service when the failure is corrected. So-called general rules are seldom “generally” applicable;there usually will be some valves that are contrary to the rule. However, the following rules can be useful as a point of departure:1. Close valves feeding heat and material to the unit.2. Close valves on streams leaving the unit.

These two action “bottle-up” the unit and may cause actuation of safety valves.3. Open valves in heat absorbing circuits such as furnace coils, heat exchangers, the exchanger port of 3-way valves, reflux

streams, pumparounds, etc. Note that on process units such as crude pipe stills and steam crackers this rule will supersederule No. 1.

There are many special situations in which the equipment characteristics or the process will govern the choice. Examplesinclude:Centrifugal compressors, on which surge prevention and overload prevention are both important.Refrigeration systems with associated machinery; these systems are small “process units” in themselves.Ultra-low temperature systems.The above discussion applies to failure of air and electricity supply to a process unit. Consideration must also be given to failureof the supply to a single valve. This is probably a more frequent hazard than failure of the supply for the entire process unit. Inworking out the consequences of a control valve failing in the open position it will often be necessary to know the size andcapacity of the valve. In these cases a valve with a capacity well beyond design rates may cause serious problems and justifyputting an upper limit on the valve capacity.Obviously, choosing the best failure position can be difficult. General rules have limited applicability and each valve situationmust be analyzed for the effect of its opening and its closing. The best answer may often appear to be to have the valve remainwhere it was at the moment of failure; however, the debits for this action as outlined under the two preceding headings must thenbe considered.

BLOCK AND BYPASS VALVES VS. CONTROL VALVES WITH HANDWHEELSGuidelines for choosing between the use of two block valves plus a bypass valve versus a handwheel on the control valve aregiven below:Blocks and Bypass - Use these for:1. Control valves 2 inches and smaller.2. Control valve 3 inches and larger with expected life due to service conditions less than unit run length and where valve

removal without blocks and bypass would require unit shutdown.3. Control valves of all sizes when used in Protective Systems.Handwheels - Use these for control valves 3 inches and larger where the above criteria do not apply.The incentive to use a handwheel on the control valve in place of two blocks and a bypass valve is cost reduction. It should beobvious that the handwheel is not the equivalent of blocks and bypass; however, it has proven generally satisfactory. The costdecreases with decreasing line size and at about 2 inches (steel lines and valves) the saving is small or disappears. Also, as thesize decreases the probability of plugging with dirt and trash increases. Thus, for 2-in. valves and below, blocks and bypassesare used.Note that judgement is needed to select the 3-in. and larger valves for which blocks and bypass are specified. Some examplesof service conditions that properly require blocks and bypass are: streams carrying coke particles (e.g., vacuum tower bottoms),slurries (0.1 lb/gal and up): corrosive streams where short life would be expected; and high pressure drop service where erosionwould cause short life.Bypass Valve Type - The International Practices call for bypass valves (for control valves above 1/2 in. size) to be gate or glovetype. In practice this means gate, because of cost and availability. Gate valves are generally satisfactory. However, in highpressure drop service a gate valve, being designed essentially for open or shut service, erodes rapidly and will then leak whenshut. Therefore, for such services globe valves should be specified. The ∆P level above which to specify globe valves is not welldefined. A figure of 100 psi is suggested. Globe valves are generally available up to 12-in. size.


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DESIGN SPECIFICATION REQUIREMENTS (Cont)Bypass Valve Size - From the Cv values of Table 5 it is seen that a globe valve one size larger than a conventional doubleseated equal percentage valve would be needed to obtain a Cv between 100 and 200% of that of the control valve, as called forin the International Practices. Table 5 also indicates that that a gate valve two sizes smaller than that of the control valve wouldbe needed to meet these criteria. Gate and globe valve Cv values are not well known and it is doubtful that a contractor wouldselect sizes in accordance with the International Practices unless the criteria were specifically brought to his attention.For situations in which a flow rate many times normal is needed for occasions such as filling a unit, a larger than normal orperhaps a second bypass valve may need to be specified. Conversely, as pointed out under “Choice of Failure Position,” forcontrol valves, there may be situations that justify putting an upper limit on the bypass valve capacity.Block Valve Size - The International Practices now call for block valves to be lines size gate valves or ball valves with equivalentflow coefficients. This requirement originated with the desirability of being able to change the control valve size up to line sizewithout the need to change the block and bypass piping. There can be cases where it is logical to call for block valves smallerthan line size, for example in expensive alloy piping. In these cases it would be normal to limit the block valve size reduction toone size below the line size.The International Practice requirement will mean that most control valves will be smaller than the block valves, and many will betwo or more sizes smaller. The Cv values of table 5 and the Cv values published by manufacturers are arrived at by tests inwhich control valves sizes and line sizes are equal. When the valve is smaller than the adjacent line the Cv is reduces, primarilydue to contraction and expansion losses on the valve inlet and outlet. The reduction for globe type valves (single and doubleseated and 3-way) is not serious, tests showing a maximum reduction of 5%. However, V-shaped ball valves are reported ashaving reduction of 20% for a 1.5/1 ratio of line to valve diameters and 25% for a 2/1 ration. The effect on 90° open butterflyvalves is given as 16 to 24%, depending on shaft to body diameter ratio and line to valve diameter ration.Thus, it may sometimes by necessary when using ball and butterfly valves in services where minimum overall pressure drop isdesired at design conditions to have some consideration given to the effect of the line to valve diameter ratio. Larger valves thanfirst indicated may be required, or it may be desirable to consider several combinations of line and valve sizes.

ACOUSTICALLY-INDUCED VIBRATION PROBLEMSControl valves handling large flow rates at high ∆P dissipate large amounts of energy that can create high noise levels, acousticturbulence and vibration. Proper measures to prevent problems have not yet been detailed but the following outline defines theconditions beyond which noise and vibration experts should be consulted for an evaluation of possible problems and theirsolution. This topic is covered in more detail in Section XV-D under DESIGN PROCEDURES.


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TABLE 1CONTROL VALVE SIZING TABLE FOR HYDROCARBON MIXTURES

SERVICEDESCRIPTION INLET PHASE PHASE IN VALVE OUTLET PHASE

SIZING METHODTO BE

EMPLOYED

Liquid LiquidHydrocarbonMixture

LiquidHydrocarbonMixture


Method 1

Cavitation LiquidHydrocarbonMixture

Greater than2% volVaporization


ContactControl ValveSizingSpecialist

Flashing LiquidHydrocarbonMixture

Conversion ofLiquid to VaporGreater than2% volVaporization

LiquidHydrocarbonMixture plusHydrocarbonVapor Mixture

Method 4

Condensing LiquidHydrocarbonMixture plusHydrocarbonVapor Mixture

Conversion ofVapor to Liquid


ContactControl ValveSizingSpecialist

Mixture of Phases LiquidHydrocarbonMixture plusHydrocarbonVapor Mixture



Method 5

All Gas/Vapor HydrocarbonVapor Mixture

HydrocarbonVapor Mixture

HydrocarbonVapor Mixture

Method 2

TABLE 2CONTROL VALVE SIZING TABLE FOR SINGLE COMPOUNDS

SERVICEDESCRIPTION INLET PHASE PHASE IN VALVE OUTLET PHASE

SIZING METHODTO BE

EMPLOYED

Liquid Liquid Liquid Liquid Method 1Flashing Liquid Conversion of

Liquid to VaporLiquid/VaporMixture

Method 3

Mixture ofPhases

Liquid/Vapor Liquid/Vapor Liquid/Vapor Method 6

All Gas/Vapor Vapor Vapor Vapor Method 2

TABLE 3CONTROL VALVE SIZING TABLE FOR TWO PHASE IMMISCIBLE FLUIDS

INLET PHASE PHASE IN VALVE OUTLET PHASESIZING METHOD

TO BEEMPLOYED

Liquid/Gas Liquid/Gas Liquid/Gas Method 7


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TABLE 4EQUIPMENT NEEDED TO OBTAIN FALL-SAFE POSITIONS*

Spring loaded Diaphragm Actuated Valves

Control Signal from Controller, I/P Transducer,or Positioner

Fail Open FO Fail Closed FC

Control Signal

Manual Reset Device

Fail Open,Manual ResetFOR

Fail Closed,Manual ResetFOR

From Air Supply

Fail Closed-Remain StationaryFO (S)

Fail Open-Remain StationaryFO (S)

SolenoidFrom Elec. Supply

Control SignalFail Open-Remain StationaryFO (S)

Fail Closed-Remain StationaryFO (S)

Piston Operated Valves

Control Signal

PositionerPositionFeed-Back

PistonCylinder

VentVentTrip Valve

Air Supply

Check

TankBracketsValve Stem

Control SignalAir Supply

PositionerVent

PositionFeed-Back

Trip Valve

BracketsPiston

Cyl.

To Valve StemLock-in-Valves

Control Signal

This most common arrangement accomplishes the fail open orfail closed requirement without extra equipment. On restorationof air pressure the valve goes to the position called for by thecontrol signal. To obtain the same action for either a failure ofair or a failure of electricity (with electronic control equipment)requires that the I/P transducer or the positioner be the directlyproportional type: this limits the choice of direct or reverseaction to the controller.

This arrangement accomplishes the fail closed requirementplus manual reset to return the valve to operation onrestoration of air pressure.

This arrangement accomplishes the requirement that the valveremain stationary on the air failure. On restoration of airpressure the valve goes to the position called for by the controlsignal. For such an arrangement the FO or FC part of thedesignation is inoperative except for failures other than in airsupply (for example, a broken diaphragm connection).

* This table convers typical equipment arrangements to accomplish the more common actions. Other actions and combinations of actionsare somewhat obvious. For some actions, especially with pistons, there are several alternative arrangments available from suppliers toaccomplish the desired result; the above gives examples of some simple methods. All arrangements using extra equipment will requireextra maintenance and checks and thus their use should be minimized.

This arrangement accomplishes the requirement that the valveremain stationary on electrical failure. On restoration ofelectrical supply the valve goes to the position called for by thecontrol signal. For such an arrangement the FO or FC part ofthe designation is inoperative except for failures other than inelectrical supply.

Fail Open, FO, or Fail Closed, FC. This is a frequently usedmethod for accomplishing the fai l open or fai l closedrequirement with a piston operator. The arrangement showndrives the valve stem up on failure of air supply. The trip valvecan be the type requiring manual reset (FOR or FCR) or thetype allowing the control valve to go to the position called for bythe control signal on restoration of the air supply.

This is a common method of accomplishing the "RemainStationary" requirement for a piston operated valve. The tripvalve can be of the manual reset type or the type that allowsthe control valve to go to the position called for by the controlsignal on restoration of the air supply.

Remain Stationary.

DP12Ft04


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DateDecember, 1999


TABLE 5CONTROL VALVE CAPACITY (CV), PRESSURE DROP RATIO (XT) ANDPRESSURE RECOVERY FACTOR (FL) VERSUS BODY SIZE (INCHES)

NOMINALBODYSIZE

INCHES

CAGE VALVEFULL TRIM

REDUCED TRIM

ECCENTRICV 500

CAMFLEXTHREE WAY BUTTERFLY

@ 90°°°°

CV XT FL CV XT FL CV XT FL CV XT FL

1 12.214

0.600.39

0.850.67

2 3515

0.700.70

0.900.90

4650

0.440.44

0.740.74

60 0.87 0.61 40 0.41 0.74

3 6525

0.700.70

0.900.90

142130

0.460.44

0.770.74

125 0.67 0.90 110 0.41 0.74

4 14090

0.700.70

0.900.90

250230

0.440.44

0.760.74

220 0.69 0.90 220 0.41 0.74

6 250150

0.700.70

0.900.90

255500

0.420.44

0.710.74

450 0.73 0.88 750 0.37 0.74

8 550250

0.700.70

0.900.90

1050850

0.360.44

0.670.74

480 0.82 0.90 1250 0.37 0.74

10 900400

0.700.70

0.900.90 1300 0.44 0.74

750 0.82 0.90 2000 0.37 0.74

12 1200500

0.700.70

0.900.90 2350 0.44 0.74 — — —

3000 0.37 0.74

16 2200900

0.700.70

0.900.90 3650 0.44 0.74 — — —

5000 0.37 0.70

SEGMENTEDBALL @ 90°°°°

FULL BALL@ 90°°°°

GLOBESINGLE PORTDOUBLE PORT

NOMINALBODYSIZE

INCHES CV XT FL CV XT FL CV XT FL

1 50 0.19 0.46 100 0.13 0.44 15 0.70 0.90

2 100 0.1 0.38 450 0.10 0.38 5048

0.700.70

0.900.90

3 260 0.08 0.35 1200 0.08 0.35 120110

0.700.70

0.900.90

4 550 0.25 0.32 2120 0.07 0.32 200195

0.700.70

0.900.90

6 1100 0.28 0.32 5100 0.07 0.32 325450

0.870.70

0.890.90

8 2000 0.31 0.31 9300 0.07 0.31 430750

0.770.70

0.890.90

10 3200 0.31 0.29 15000 0.06 0.29 9501160

0.720.75

0.900.90

12 4500 0.32 0.28 22400 0.05 0.28 11801620

0.750.70

0.890.90

16 8500 0.30 0.31 37000 0.05 0.31 — — —


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EXXONENGINEERING


TABLE 6CONTROL VALVE RANGEABILITY

CONTROL VALVE TYPE MAXIMUM RANGEABILITY

Cage Valve 10Eccentric Rotary Valve 10Three Way Valve 10Butterfly Valve 6Segmented Ball Valve 12Full Ball Valve 15Globe Valve 8

TABLE 7CONTROL VALVE SEAT LEAKAGE RATES

ANSI/FCI 70-2 1991

CLASS RATE TEST REQUIRED

I 0.5% of Rated Cv No

II 0.5% of Rated Cv Yes See Note 3

III 0.1% of Rated Cv Yes

IV 0.01% of Rated Cv Yes

V See Note 1 Yes

VI See Note 2 Yes

Notes:(1) A leakage rate of 5x10-4 milliliters per minute of water per inch or orifice diameter per psi differential pressure. In Metric

Units, 5 x 10-12 m3 per second of water per millimeter of orifice diameter per bar differential pressure.(2) Please refer to the Standard ANSI/FCI 70-2 1991 for procedures and leak rates for Class VI.(3) Leakage rates that occur in service may be higher than test leakage rates due to the low test pressures used (45 - 60

psig) under FC/70 - 2 1991. If control of leakage rates are required for higher pressures, then a note should be added toprocess flow plans, and a special test must be performed to reflect process service conditions.


XII-FPage


DateDecember, 1999


FIGURE 1MAIN TYPES OF CONTROL VALVES

The most common valve.Called "balnced" becausemost forces on plug at topseat are "balanced" bysimilar and oppositeforces at bottom seat.

Double-Seated ValveUsed for tight shutoff.Unbalanced forces are aproblem; they limit sizeunless special actuatorsare used. Flutted or"splined" valve are aspecial case of singleseat, for small flow rates.

Single-Seated ValveUsed for low ∆P, high flowservice where double-seateddesign unavailable. Minimumsize used is about 8 in. Oftenneeds piston actuator. Needsspecial sizing rules for gaswhen ∆P>10% of upstreampressure. See Table 2.

Butterfly ValveUseful for same services asbutterfly valves, but has bettershutoff. Not common above 16 in.size. Beginning to complete withdouble-seated valves. Needsspecial sizing rules for gas when∆P>10% of upstream pressure.See Table 2.

Ball Valve

A single-seat type valve balancedby bringing downstream pressureabove the piston. Becoming firstchoice, displacing theconventional double-seat valve.

Cage ValveSingle-Seated Mixing ValveMost common application of 3-way valves is through and aroundexchangers to control heat transferred. Two designs are used, to allow achoice so that flow tends to open port as valve closes that port, to combatchatter. Single seat design is more available, but most models have smallerCv. See Table 2.

Note: For names of important control valve parts, see Figure 10.

Double-Seated Diverting Valve3-Way Valves

DP12FF01

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EXXONENGINEERING


FIGURE 2TYPICAL SYSTEM HEAD-CAPACITY RELATIONSHIP

Static Head

Friction Head

Pump Head

Control Valve ∆P

Control Valve ∆P atDesign Flow Rate

Flow Rate,% of Design

1005000

50

100

Hea

d,%

of S

yste

mR

equi

rem

ent a

t Des

gin

Flow

Rat

e

DP12Ff02

FIGURE 3CHARACTERISTICS OF LINEAR AND EQUAL PERCENTAGE VALVES

0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 100

10

20

30

40

50

60

70

80

90

100

10

20

30

40

50

60

70

80

90

100

% Valve Lift % Valve Lift

VALVE ∆P, % ofsystem frictionat design flow.

VALVE ∆P, % ofsystem frictionat design flow.

LINEAR VALVE EQUAL-PERCENT VALVE

Perc

ent o

f Max

imum

Flo

w

510

20

40

60 80

100

510

2040

80

100

DP12Ff03


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DateDecember, 1999


FIGURE 4THREE-WAY VALVES IN HEAT TRANSFER CONTROL SERVICE

FromTRC

FromTRC

Single Seated Design, for Mixing Service

Double Seated Design, for Diverting ServiceDP12Ff04


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EXXONENGINEERING


FIGURE 5USE OF TWO 2-WAY VALVES IN PLACE OF ONE 3-WAY VALVE

ExchangeValve

BypassValve

DP12Ff05

FIGURE 6CUTAWAY VIEW OF AN ANGLE VALVE

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DP12Ff06


XII-FPage


DateDecember, 1999


FIGURE 7CUTAWAY VIEW OF A SAUNDERS VALVE

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Diaphragm

DP12Ff07

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exxon dp - control valves

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