basic control valve sizing and selection
DESCRIPTION
In this day and age of automated computer control valve sizing, the logic and theories behind it are invisible. In his presentation, Al Holton of Allagash Valve & Controls will look at the basic principles that apply and how they affect the application and installation of a wide range of control valve types. He will also review the reasoning behind valve type selection.TRANSCRIPT
Basic Control Valve Sizing and Selection
DeZurik Operation
October 2005
Valve Sizing
What is Valve Sizing?
It is a procedure by which the dynamics of a process system are matched to the performance characteristics of a valve. This is to provide a control valve that will best meet the needs of managing flow within that process system.
Flow Coefficient (CV)The valve flow coefficient, CV is the number of
U.S. gallons per minute of water at 60 degrees F which will pass through a given flow restriction with a pressure drop of 1 psi.
For example, a control valve which has a flow coefficient, or CV, of 12 has an effective port area that it passes 12 gallons per minute of water with 1 psi pressure drop.
DeZurik Operation
October 2005
Valve SizingBasic information requirements for effective valve sizing
For the system:Pressure before and after the control valve, ΔPFlow rate, quantity and units, QProcess temperature with units, TProperties of the media, (viscous, fiber suspension, gaseousits vapor pressure, sometimes molecular weight)
For the control valve:Flow capacity (Cv), inherent throttling curve, Kc (Cavitation Index), FL² (Critical Flow factor)
Delta P (P)P Sizing is the pressure drop across the valve
used for control valve sizing at a specific flow rate.
Most applications have multiple flows for sizing with different ΔP for each
In a given system, higher flow rates generally result in lower pressure drop across the valve.
Flow Characteristics
Control valve flow characteristics are determined principally by the design of the valve trim.
The three basic flow characteristics available are: Quick Opening Linear Equal Percentage (=%)
A modified characteristic (sometimes called modified percentage) generally falling between the linear and equal percentage characteristics may also be available.
Why Use Equal PercentageProvides equal
percentage increases in rate of flow for equal increments of plug movement.
Provides the best choice
of flow characteristic for most systems.
DeZurik Operation
October 2005
Valve Sizing - CavitationWhat is cavitation and what does it do to valves?
Cavitation is a 2-stage activity where a portion of the liquid media drops below vapor pressure. This part will boil (vaporize).
In stage two, slightly downstream, pressure recovery takes placeand the vapor bubbles collapse.
The condition known as “critical flow” is an extension of cavitationin that it simply gets worse as the pressure drop increases. This is to the point that changes (reductions) in downstream pressureno longer influence flow rate.
Fluid Recovery Factor (FL)
When a fluid passes through the valve orifice, the velocity increases. This velocity increase is accompanied by a proportional decrease in pressure. Velocity reaches a maximum and pressure a minimum at the smallest cross sectional flow area downstream of the orifice (the vena contracta).
Downstream of the vena contracta the fluid decelerates and consequently the pressure increases or recovers (giving us the term pressure recovery). Different valve types exhibit different recovery factors which becomes an important consideration in valve selection.
Fluid Recovery Factor (FL)The fluid recovery factor (FL) is effectively an index of pressure recovery in a control valve. High FL values indicate low pressure recovery where a low FL value is an indication of high pressure recovery. Higher FL values result in better resistance to cavitation.
FL~(P1-P2)/(P1- PVC)
Valve sizing - Cavitation
Collapse of the bubbles created by cavitation produce noise, like gravel in the line.
Bubbles that collapse in contact with the valve or pipe create damage.
Cavitation can be controlled with special valve trims in some types of valves.
Cavitation should be avoided in all circumstances.
DeZurik Operation
October 2005
Valve SizingWhat is “Flashing”?
It is a condition where the downstream pressure is below the vapor pressure of the incoming fluid and allows some of theliquid to become vapor.
Flashing does not create noise or damage in the valve as with cavitation. It can, however, create damage to the downstream piping due to high velocity.
A control valve will function quit well under these circumstances. Yes!
DeZurik Operation
October 2005
Valve SizingCharacteristics
Systems driven by centrifugal pumps lose pressure two waysAs the flow increases the pump pressure declinesAs the flow increases pipeline friction losses increase
Using a valve with equal percentage or parabolic characteristicswill produce an installed characteristic that is more linear.
Inherent vs. Installed Characteristics
Inherent flow characteristic is defined as the relationship between flow and valve stroke at constant pressure drop. It is unique to valve construction and depends primarily on geometry of the throttling trim and body flow passages and is determined by flow testing.
The inherent flow characteristic changes when installed in a piping system. The result is the installed flow characteristic. As system pressure drop absorbed by the control valve is reduced, the greater the deviation from the inherent state.
Ideally a control valve should be sized to absorb 15%-25% of system pressure.
DeZurik Operation
October 2005
Valve Sizing
0
20
40
60
80
100
0 20 40 60 80 100
% Open
% C
v
Equal Percent
Modified Parabolic
Linear
Quick Opening
What is desired ?
- After installation, generally it should be near linear.
Why?
- To provide a more predictable flow change in response to each incremental valve position change
Control Valve Seat Leakage It is becoming more common to expect a control valve to
provide isolation duty, thus allowable leakage becomes more important.
Control valve seat leakage is designated by the classification of ISA/ANSI-70-2
Leakage classes are class I-VI.Classes I-V are measured using water, class VI is measured
using air.The most common class for metal seated control valves is
class IV. (0.01% of rated Cv)Class VI generally applies to resilient seated valves only.
Fluid VelocityExcessive fluid velocity can create erosion, and
accelerate corrosion damage. It also contributes to noise level and vibration.
Valve users will generally define acceptable velocity in valves and pipes for the above reasons. Liquid velocity of up to 15 ft/sec. is common; for gas or steam 400 ft/sec. or higher, depending upon size, is perfectly acceptable.
Noise and Vibration
Noise results from the conversion of the mechanical energy of the flow into acoustic energy as the fluid passes through the valve restriction.
Vibration is a direct result of noise and can create issues with valve life.
OSHA has designated 85-90 Dba as the generally acceptable limit for valves installed in most areas.
Noise can be reduced through two means, internal at the valve trim, and external such as pipe insulation or downstream silencers.
Cage guided globe valves offer the widest range of noise and vibration attenuation solutions.
RangabilityRangability is defined as the ratio of minimum
to maximum controllable flow rates.Equal percent characteristic offers the best
rangability of common types.
0
20
40
60
80
100
0 20 40 60 80 100
% Open
% C
v
Equal Percent
Modified Parabolic
Linear
Quick Opening
Flow Direction
Many valve types have a preferred flow direction to minimize the effects of velocity, to create the tightest shut-off, and to optimize the response to input signal.
DeZurik Operation
October 2005
Globe Valves
DeZurik Operation
October 2005
Globe ValvesHow do they work?
- By linear (stroke) action
- A contoured plug is lifted out of a seat ring to allow flow to pass
- In most (not all) cases flow comes in “under the seat”
- Conversely, pushing the plug down into the seat causes closure
Flow in
DeZurik Operation
October 2005
Globe ValvesUnbalanced / Balanced
Trim
- Unbalanced has system pressure acting against the full area of the seat Single seated plug styles are unbalanced
- Balanced has provisions for offsetting system pressure Available in double seated plug or balanced cage styles
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October 2005
Globe ValvesBodies & Bonnets
- Configured as required to meet valve style and ANSI Class considerations
- Removable bolted bonnets retain and/or support the trim components
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October 2005
Globe Valves 3 Way configurations for
Mixing and diverting.
-Temperature control- Blending
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October 2005
Globe ValvesTrim Components
- Stem, plug, seat, cage (if that style) & guiding components
- Made of alloy materials
- Optional hardened seating surfaces when required
Top guided style
Cage guided style
Noise and cavitation control trims
By directing flow through a series of staged drops, these trims eliminate cavitation in liquid flow and provides multiple pressure breakdown for noise attenuation
DeZurik Operation
October 2005
Rotary Control Valves
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October 2005
Rotary Control Valves
Design & Construction
ANSI & ISA Face to Face
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October 2005
Rotary Control ValvesV ported valves: Uses a ball segment that is Always in contact with the seat. Up to 100:1 rangability.
Eccentric type: Uses a plug that cams away from the seat during opening
Ideal solution for erosive services Such as slurries and steam.
Available with a range of flow capacities.
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October 2005
High performanceButterfly Valve
Ideal economic choice for larger line sizes
50:1 Rangeability
Modified equal percent flowCharacteristic
Fast accurate response to Control signal
ActuatorsThe actuator provides the muscle for the control
valve and is responsible for moving the valve control element to the required stroke position.
Spring diaphragm actuators are the most widely recognized and used by control valve suppliers.Reverse acting; fail closed and direct acting; fail open.Simplistic design ,few moving parts, and easy to
maintain.Double acting pistons
Smaller, lighter, and less expensive than diaphragm.Fail in last position.
Actuators
Accessories
Positioners – pneumatic input 3-15 psi Electro-pneumatic 4-20 ma
(HART, Fieldbus, Profibus)Limit switchesPosition feedback
Review material
ISA Practical Guide PublicationsControl Valves
Manufacturers data and guides.
Al HoltonNor’East [email protected]
DeZurik Operation
October 2005
Rotary Control Valves