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CONTROL VALVES

Obtaining optimum control valve pa-rametersThe following seven steps will prove benefi-cial to obtaining the optimum control valveparameters:

1) Divide the plant pipework into three sec-tionsa) From pressure source - pump/vessel;

start pressure to the flow meter up-stream pressure.

b) From the flow meter downstreampressure to the control valve upstreampressure.

c) From the control valve downstreampressure to the plant end pressure(place of production)

2) Pressure loss calculation for qmax.qnorm. and qmin. section a)

3) Flow meter optimization with residualpressure losses at qmax. qnorm. and qmin.

4) Pressure loss calculation for qmax.qnorm. and qmin. section b) to startwith flow meter downstream pressure.The yields the first result: the valve up-stream pressure characteristic at qmax.qnorm. and qmin.

By Dipl. Ing. Holger Siemers, SAMSON AG

This publication will continue takingmore the aspect of CONTROL

QUALITY into account, which can alsosuffer under the increasing cost and timepressure.The success of the plant - pro-duction quality and production quantity -can directly depend on reasonable valvecontrol quality, especially if valves are in“key” functions.

1. Plant design under cost and timepressure

Optimizing the pump start pressure withpressure loss calculation of the pipeworkis the key to save power and energy in thelong/term as well as to reduce wear,noise, and maintenance cost.[1] Accuratecalculation sheets for the pipework andthe control valve can be printed out with-in 15 minutes for a plant system shown inFigure1b using the manufacturer-inde-pendent CONVAL software for valves,pipes, and pipe devices, generating dy-namic graphics of plant and control valvecharacteristics. See Figure 1a.

Plant design and controlvalve selection under

increasing cost and timepressure - Part I

Following a career spanning three decades, Mr Siemers is well aware of the pitfalls to be avoidedwhen specifying control valves for a range of demanding applications. In his latest paper for ValveWorld, he looks further into plant design and control valve selection when working under increasedtime and cost pressure. This article is split into two parts: broadly speaking, part one looks at controlvalve operating points and provides a case history involving a mismatch. The author then introducesbetter valve sizing practices and uses this theory to resolve the problems introduced in the case history.Part two (scheduled for the June issue) starts by explaining the trends and definitions of inherent valvecharacteristics before focusing on „quick and dirty“ sizing. The paper then addresses cavitation beforeconcluding with the expert software available to help select the optimum valve characteristic form.

Part I

1. Plant design under cost and time

pressure

2. Control valves today are converting

links between budgets!

3. From traditional to modern

Development and Engineering

Practice (DEP) for plant designers

4. The new DEP for trouble-shooting

the mismatched case study in

section 2

Part II

5. Trends and definitions of inherent

valve characteristics for globe and

rotary valves.

6. Detail engineering-sources for plant

and valve designers have dried out!

7. Noise reduction and getting the

plant power under control.

8. Selecting the optimum valve

characteristic form

9. Using software to increase control

quality, reduce cost and save time

for creativity

Note: given the amount of detail in some of the graphics in this article, readers might like to note that a high resolutionPDF version can be viewed at: www.valve-world.net/magazine/controlvalves.asp.

▲

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CONTROL VALVES

5) Pressure loss calculation for qmax.qnorm. and qmin. section c) to startwith any control valve downstream pres-sure e.g. p2 = p1 - ∆pn (∆pn = 1 bar).Compare the result with the plant endpressure and iteratively correct the valvedownstream pressure with the pressuredrop deviation until the end pressure isreached.This yields the final result: the

valve downstream pressure characteris-tic at qmax. qnorm. and qmin.

6) Control valve sizing and optimizationleads to the selection of the most suit-able control valve.Valve parameters tooptimize: Cv100 value and the valve in-herent characteristic

7) Check the control parameters: controlrange, qmax < 0.9xq100, valve gain0.5 < gain < 2 and SPL dB(A) charac-teristic:The loop gain depends on thecontrol variable Flow q, Level L,Tem-perature T or Pressure p: ∆q /∆s;∆L/∆s; ∆T/∆s or ∆p/∆s. p=p1; p2 or∆p. Check the valve max. power con-sumption and select a valve which with-stands its highest stress situation atmax. power. For a new plant, morethan fifty per cent power savings canbeachieved by sizing the plant and valveparameters more accurately. [1, 4]

2. Control valves today are convertinglinks between budgets!

Increasing cost and time pressure haveconsiderably affected plant designers.Toexplain the change of planning parametersfrom the past to today, a simple pneumaticcontrol loop (Figure 2a) could help to un-derstand the upcoming problem.The con-trol valve is the connecting link betweenthe CONTROL EQUIPMENT and theCONTROLLED SYSTEM. Control equip-

ment can include signal transmitters, actu-ators, single controllers or complex DCSsystems.The controlled system includespumps, pipework, and pipe devices likevalves. If all signal transfer devices 2) to 5)operate in a strictly linear way the flowmeter and the control valve as signal trans-fer devices 1) to 6) also need to work as lin-early as possible to achieve an excellentcontrol quality (Figure 2b).If different departments are responsible forthe control equipment and for the con-trolled system with their specific budgetsthe need of the valve pressure differential isquite often forgotten. If the differentialpressure ratio is too small, the control valvewill lose control authority.The responsibility for control quality de-pends on the valve authority, the valve in-herent characteristic quality and the charac-teristic form but also on the selected cv100value. Mismatching can lead to an uncon-trolled process variable and excessive gainfluctuations up to loop hunting. Under theworst-case conditions the investment tar-gets of production may not be met.

Case study:Good stroking,bad controlIn many cases the traditional engineeringpractice does not fit the needs of today.Additional engineering rules should beadded to, or even used to replace, the tra-ditional engineering practice which only

Tool for sizing, calculation, andoptimization of common plantcomponents:

• Control valves• Steam conditioning valves• Actuator forces• Differential pressure flow

elements• Restriction orifice plates• Safety relief valves• Tank depressurization• Pressure loss• Pressure surge

• Pipes:• Sizing• Pipe compensation • Span calculation• Pipe wall thickness

• Shell and tube heatexchangers

• Condensers• Pump motor output

Supported by vendor-independent device databases(control valves, safety reliefvalves), fluid property calculation,material databases, ...

Fig. 1a: Result of proper control valve optimization using program parts: pressure loss; differential pressure flow elements and control valves.

Fig. 1b: Plant system and proper control valveoptimization with the CONVAL software.

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CONTROL VALVES

takes the relationship “valve stroke versusflow” into account.Quick selecting only looks at the tradition-al ”stroke versus flow” requirements forthe given operating point qmax. But thestroke s < 0.8 is not of interest here.Stroking to s=1 will increase the flow onlyby about 1.7 %. Here it is not the valvemanufacturer's responsibility but ratherthe plant designer's problem to get theproduction under control and to increasevalve authority at qmax., for example withmore pump power.Planning mistakes often occur as a result oftoo small budgets and missing controlcompetence.Figure 3 (bottom right) shows that theoperating point qmax is situated at “good”< 0.8 s/s100 stroke but not controllable at98.3 % flow. See also warning alarm in thetop left chart.The plant target to get a rea-sonable control of qmax. (means produc-tion quality as well as production quantity)cannot be achieved.

Sources of planning mistakesPlanning mistakes can result from a num-ber of situations, including: excessivepressure loss due to pipe and pipe devices,insufficient pump power; not enough ex-penditure on plant design; failure to takethe need for necessary differential pres-sure ratio for control valves into account;

no accurate pressure loss calculation withtoo many assumed parameters.To de-bot-tleneck, if neither changing the pipe DNnor saving pressure loss is possible thenone troubleshooting option includes in-

creasing the pump power with new pumpor pump impeller (see Figure 5).

3. From traditional to modern Devel-opment and Engineering Practice(DEP) for plant designers

The history of traditional “valve strokeversus flow” requirements dates back toearlier times (Figure 4a) when heavy-dutytop-guided and bottom-guided or cage-guided valves were oversized and over-en-gineered in stroke and body weight for thestandard applications of today.At thattime, only linear or equal percentage valvecharacteristics were known. In the time ofplant pioneers, new processes were in-stalled for the first time on a lower scaleproduction volume.Valve trims were re-duced several times to double the produc-tion regarding increasing market de-mands. In the same way pumps and pipedevices were installed with flow reserve.

Today plant parameters are well known.From an economic point of view globevalves often selected with the largest seat.If designed with too small a residual pres-sure differential globe valves need to be ▲

Fig. 2a: Control valve: the converting link betweencontrol equipment and controlled system.

Fig. 2b: Importance of valve gainfluctuations to control quality.

Fig. 3: Typical results of time and cost pressure: no control of production target qmax.

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CONTROL VALVES

replaced with high flow capacity rotaryvalves. CONVAL's graphical support canshow and self-explain advantages and disad-vantages and the risk of over-sizing.In general engineering practice, the controlvalves at system end (short circuit perform-ance) should not be oversized.The standardized plant system with totalvalve authority ∆p100 / ∆p0 = 0.1 showsthe impact of an ideal equal percentage andlinear inherent valve characteristic. Figure4b compares good control parameters withan equal percentage and bad control pa-rameters with a linear characteristic.To calculate the control rangeability from 5to 100 % stroke and gain fluctuation:

The gain fluctuations ∆q / ∆s versus flowas a function of the total valve authority [v =∆p100 / ∆p0] (see Figures 4c and 4d).Following the SHELL development and en-gineering practice (DEP) 32.36.01.17GEN control valve selection, sizing andspecification in principle recommendsplant designers follow the plant designrules indicated below for pump, pipework,and pipe device design. Important is for the“split responsibilities” to work together.One recommendation is to replace the “op-erating point versus stroke” requirementsunder the responsibility of plant designersand valve manufacturers to avoid consider-able loss of control quality.

Fig. 4a Fig. 4b

Fig. 4c: Signal proportional q.Fig. 4d: Signal proportional q2.

Figs. 4a to 4d: Plant system trends and how to keep gain fluctuations (control quality) under control.

Characteristic eq. : 1 : 15; 0.5 < ∆q / ∆s < 2 okCharacteristic lin. : 1 : 5; 0.5 < ∆q / ∆s < 2 not ok

Plant designers' responsibility (Figure 5a)The process design flow qmax. shall stay ≤0.9 x q100. q100 as a function of the se-lected cv100 value can be replaced with avalve manufacturer independent relation-ship: - the max system flow : 0.9 x q100can also defined to 0,85 q* as the distanceto the max. system flow. (q* = q_90 x qs= short circuit performance without con-trol valve).At qmax. = 0.9 x q100 =0,85 x qs thevalve authority shall design ∆p90 / ∆p0 ≥0.27. qs=q*/q_90 can be calculated forgas and liquid as a function of p1 and ∆pand minimum selection between qs_1;qs_2; qs_3:

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The short circuit performance of systemupstream pressure characteristic [2] isgiven by:

Liquid:

Gas; Steam;

The short circuit performance of systempressure differential characteristic [2] isgiven by:

Liquid; gas; steam:

(e.g. if ∆p90 / ∆p0 = 0.27 this re-

sults to the valve independent rule for plant design-

ers q_90 = 1/1,17x q* = 0,85 x q*)

Valve manufacturers' responsibility (Figure 5b)The valve cv100 value shall keep the totalvalve authority ∆p100 / ∆p0 ≥ 0.1. If∆p100 / ∆p0 = 0.1 the valve characteris-tic shall be chosen as equal percentage as

possible. Figure 4b shows the relationshipfor any other “flow versus pressure drop”relationship for the plant parameter totalvalve authority ∆p100 / ∆p0 = 0.1 only ifan ideal equal characteristic is selected.Consequently following the new suggestedregulations the mismatched plant system asshown in Figure 3 can be optimized in theearly stage of planning. ▲

Flow q/q100 Valve authority V = ∆p / ∆p0 Stroke,Travel s/s100 eq. char.1 0.1 10.9 0.27 0.850.8 0.42 0.77

Figs. 5a, b, c: How to avoid loss of control quality using proper plant parameters and valve sizing.

Fig 5a: Plant designers' responsibility. Fig. 5b: Valve manufacturers' responsibility.

Fig. 5c: Valve authority Vdyn =0.27 at q_max = 0.9 q100 for alow noise cage ball valve ™(PiBiViesse, Italy), 6 inch.

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CONTROL VALVES

4. The new DEP for trouble shootingthe mismatched case study fromsection 2

Figures 6a and 6b show a new upstreampressure characteristic increase p1 atqmax from 6 to 7.5 bar abs and at qmin.from 8 to 9.5 bar abs with installing high-er pump power.A DN 150 globe valvewith AC low noise trim could be thechoice for SPL < 80 dB(A). (See Figure6a.)

The installed flow characteristic gain vari-ation stays within the engineering practicerule: 0.5 < ∆q / ∆s < 2 in the entirerange of control. From qmin up to q100

presented this reasonable gain borderswith the bottom green line in the top leftand bottom right graphs in Figures 6a and6b. If replacing the globe valve with a ro-tary plug valve of the same cv100 valuethe installed flow characteristic drifts inon-off direction (Figure 6b).The installedflow characteristic's higher gain fluctua-tions still stay within the engineering prac-tice rules: 0.5 < ∆q / ∆s < 2 between theoperating points qmax and qmin: shownwith the bottom green line in the top leftand bottom right graphs.The sound pres-sure level can exceed > 85 dB(A).Thesoftware further indicates choked flow upto 30% travel and min. flow control at

small opening 5%.This can easily be fur-ther optimized with a smaller cv100 valueby reduced seat technology and adding in-tegrated low-noise devices. In case ofhigher DN, high shut down pressure, ex-otic materials, and within the given soundrequirements a rotary plug valve could bea reasonably priced alternative to globevalves. ■

Fig. 6c: The SAMSON AG AC-Trim system can solve upcomingcavitation problems after de-bottlenecking.[3]

Fig. 6a: Correction of planning mistakes from Figure 3 withmore pump power selecting a globe valve.

Fig. 6b: Correction of planning mistakes from Figure 3 with morepump power selecting a rotary plug valve.

Fig. 6d: The VETEC rotary plug valve in specific areas is areasonably priced alternative to the globe valve.

Don't miss the second andconcluding section of this paper inthe June issue of Valve World.

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