Selecting And Sizing Spring-And-Diaphragm Actuators And Rela.pdf

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Engineering EncyclopediaSaudi Aramco DeskTop Standards

Selecting And Sizing Spring-And-Diaphragm Actuators And Related Accessories

Engineering Encyclopedia Instrumentation

Selecting and Sizing Spring-And-Diaphragm Actuators and Related Accessories

Saudi Aramco DeskTop Standards

CONTENTS PAGES

SPRING-AND-DIAPHRAGM ACTUATOR FUNCTIONS ....................................................... 1

Actuator Functions......................................................................................................... 1

Position The Control Valve Closure Member .................................................. 1

Hold The Closure Member In The Desired Position ........................................ 2

Provide Adequate Seat Load For Desired Shutoff............................................ 2

Provide Adequate Valve Stem Travel .............................................................. 2

Provide Sufficiently Rapid Stroking Times...................................................... 2

Provide A Fail Mode ........................................................................................ 2

Spring-And-Diaphragm Actuator Specifications ........................................................... 2

Actuator Action: Direct or Reverse .................................................................. 2

Physical Size .................................................................................................... 4

Operational Specifications................................................................................ 5

Basic Sizing Concepts ................................................................................................... 6

Valve Forces..................................................................................................... 6

Actuator Forces ................................................................................................ 6

Spring Forces ................................................................................................... 7

Actuator Bench Set......................................................................................... 10

Bench Set Span And Performance.................................................................. 13

Effects of Valve Friction On Actuator Performance .................................................... 15

Dead Band...................................................................................................... 15

Effects of Dead Band On The Travel Of An Installed Control Valve ............ 16

Importance Of Instrument Over Ranging ....................................................... 17

Effects Of Friction On Process Control .......................................................... 17

Minimizing Friction Effects ........................................................................... 18

PERFORMING DETAILED ACTUATOR SIZING CALCULATIONS .................................. 20

Introduction ................................................................................................................. 20

Nomenclature For A Detailed Analysis Of Actuator Performance............................... 20

Bench Set, Lower And Upper ........................................................................ 20

Stroking Pressure, Lower And Upper............................................................. 20

Friction Band.................................................................................................. 21

Operating Pressure Range .............................................................................. 22

Supply Pressure.............................................................................................. 22




Overview Of The Detailed Actuator Sizing Method Of The Fisher SizingProgram ....................................................................................................................... 23

Selecting The Detailed Sizing Method ........................................................... 23

Calculation Screen.......................................................................................... 24

Migration Of Valve And Actuator Data From The Quick Sizing Method ..... 24

Direct Entry Of Information ........................................................................... 25

Unique Input Fields........................................................................................ 25

Calculated Results .......................................................................................... 26

Bench Set Selection Guidelines...................................................................... 27

Manual Sizing Option .................................................................................... 28

EVALUATING THE IMPACT OF NEGATIVE FLUID FORCE GRADIENTS ..................... 29

Definition And Examples Of Fluid Force Gradients .................................................... 29

Definition Of Fluid Gradients ........................................................................ 29

Examples Of Fluid Gradients ......................................................................... 29

Preventing Valve Plug Instability That Is Caused By Negative Gradients................... 33

Valve Plug Stability Equation ........................................................................ 33

Determining Total Actuator Stiffness............................................................. 34

Quantifying Negative Gradients ..................................................................... 36

Computer Assisted Selection.......................................................................... 38

Relative Frequency Of Problems That Are Caused By NegativeGradients ........................................................................................................ 39

Troubleshooting ............................................................................................. 39

SELECTING ACTUATOR ACCESSORIES TO PROVIDE THE REQUIREDACTUATOR STROKING TIMES............................................................................................ 40

Clarification Of Terminology And Application Requirements .................................... 40

Clarification Of Terminology......................................................................... 40

Clarification Of Application Requirements .................................................... 40

Parameters That Influence Actuator Stroking Time ..................................................... 41

Supply Pressure And Capacity ....................................................................... 42

Cv Of The Loading Instrument ...................................................................... 42

Tubing And Fitting Size................................................................................. 42

Diaphragm Area ............................................................................................. 42

Actuator Volume............................................................................................ 42

Filling And Exhausting Pressures .................................................................. 43

Exhaust Pressures........................................................................................... 43




Vent Cv .......................................................................................................... 43

Performing Stroking Time Calculations With The Fisher Sizing Program .................. 43

Objective Of Calculations .............................................................................. 43

Overview Of The Calculation Procedure And The Stroking TimeCalculation Screen.......................................................................................... 43

Actuator And Spring Entry Fields .................................................................. 44

Actuator Pressure Entry Fields ....................................................................... 45

Air Supply...................................................................................................... 45

Calculated Times............................................................................................ 45

Interpreting The Results Of The Stroking Time Calculations ........................ 46

Reducing The Stroking Time ......................................................................... 46

Increasing The Stroking Time ........................................................................ 46

Accessories And Options That May Be Selected To Reduce The ActuatorStroking Time .............................................................................................................. 47

Volume Boosters............................................................................................ 47

Oversized Piping And Fittings ....................................................................... 50

High-Capacity Supply Pressure Regulators And Filters ................................. 51

Quick Release Exhaust Valves ....................................................................... 51

Enlarged Actuator Vents ................................................................................ 55

Other Stroking Time Considerations ........................................................................... 56

Potential For Valve Damage .......................................................................... 56

Matching Stroking Times To The Requirements Of The Applications .......... 57

Impact Of Response Time On Stroking Time ................................................ 57

SELECTING ACTUATOR ACCESSORIES TO ACHIEVE THE DESIRED FAIL MODE.... 58

Terminology ................................................................................................................ 58

Definition Of A Failure .............................................................................. 58

Inherent Vs. Engineered Fail Mode................................................................ 58

Solenoid Valves ........................................................................................................... 58

Description And Function .............................................................................. 58

Configurations................................................................................................ 58

Applications ................................................................................................... 59

Trip Valves .................................................................................................................. 61

Description ..................................................................................................... 61

Single-Acting Trip Valve ............................................................................... 61

Double Acting Trip Valve .............................................................................. 62




WORK AID 1. PROCEDURES THAT ARE USED TO PERFORM DETAILEDACTUATOR SIZING CALCULATIONS WITH THE USE OF THE FISHER SIZINGPROGRAM ............................................................................................................................... 67

Work Aid 1A. Procedures That Are Used To Perform Actuator SizingCalculations With The Use Of The Quick Sizing Method Of The Fisher SizingProgram ....................................................................................................................... 67

15. Press the F2 key to display a list of potentially acceptable actuatorconstructions. ................................................................................................. 68

Work Aid 1B: Procedures That Are Used To Perform Detailed Actuator SizingCalculations After Developing A Specification With The Quick Sizing Method ........ 68

Migration Of Data From The Quick Sizing Method ...................................... 68

Performing The Actuator Sizing Calculations ................................................ 68

Selecting A Bench Set.................................................................................... 68

WORK AID 2: PROCEDURES THAT ARE USED TO EVALUATE THE IMPACT OFNEGATIVE FLUID FORCE GRADIENTS ON ACTUATOR SIZING ................................... 69

Determining The Fluid Negative Gradient, Kn............................................................ 69

Balanced Valves............................................................................................. 69

Unbalanced Valves......................................................................................... 69

For Flow Down (PTTC)............................................................................................... 69

Performing The Sizing Calculations ............................................................................ 70

WORK AID 3: PROCEDURES AND SPECIFICATION GUIDELINES THAT AREUSED TO SELECT ACTUATOR ACCESSORIES TO PROVIDE THE REQUIREDSTROKING TIME .................................................................................................................... 71

Work Aid 3A: Procedures That Are Used To Calculate Actuator Stroking TimesWith The Use Of The Fisher Sizing Program .............................................................. 71

1. Complete The Worksheet .......................................................................... 71

2. Perform The Stroking Time Calculations .................................................. 71

Hint ................................................................................................................ 72

Worksheet For Calculating Stroking Times ................................................................. 72

Work Aid 3B: Selection And Specification Guidelines That Are Used To SelectActuator Accessories To Provide The Required Stroking Time .................................. 73

General Equipment Selection Guidelines ....................................................... 73

Equipment That May Be Selected To Achieve A Shorter Stroking Time ...... 73

Equipment That May Be Selected To Achieve A Longer Stroking Time ...... 73

Instructions To The Valve Vendor Or Valve Manufacturer ........................... 73

WORK AID 4: GUIDELINES THAT ARE USED TO SELECT ACTUATORACCESSORIES TO ACHIEVE THE DESIRED FAIL MODE ................................................ 74




General Guidelines....................................................................................................... 74

Specific Equipment Selection Guidelines .................................................................... 74

Guidelines For Solenoid Valve Selection....................................................... 74

Guidelines For Trip Valve Selection .............................................................. 74

GLOSSARY .............................................................................................................................. 75



Saudi Aramco DeskTop Standards 1

SPRING-AND-DIAPHRAGM ACTUATOR FUNCTIONS

Actuator Functions

While it is common to think of the actuator as a device that simply moves the control valve closure member,the actuator performs several critical functions. These functions are shown in Figure 1 and they are discussedbelow.

Use Word 6.0c or later to

view Macintosh picture.

Figure 1Control Valve Actuator Functions

Position The Control Valve Closure Member

To position the control valve closure member, the actuator must overcome the packing friction, the seal friction,and the fluid forces that act on the control valve closure member.




Hold The Closure Member In The Desired Position

To hold the closure member in the proper position, the actuator must overcome the forces of valve plugunbalance and the buffeting forces that result from turbulence in the flow stream. The actuators ability to holdthe closure member in a fixed position is a function of the stiffness of the actuator spring.

Provide Adequate Seat Load For Desired Shutoff

The actuator must provide sufficient seat load to achieve the ANSI Class shutoff rating of the control valve. Theforce that is required is a function of the control valve ANSI Class shutoff rating, the valve style, and the valveport diameter.

Provide Adequate Valve Stem Travel

The actuator must have the ability to move the valve stem a distance that is equal to the rated valve travel.

Provide Sufficiently Rapid Stroking Times

In order to satisfy the requirements of the process system, many control valves must stroke the control valve ina very short time.

Provide A Fail Mode

If the supply pressure to the actuator is interrupted, the actuator provides a fail mode of operation. The commonfail modes are fail open, fail closed, and lock in last position (LILP).

Spring-And-Diaphragm Actuator Specifications

Actuator Action: Direct or Reverse

Spring-and-diaphragm actuators are available in direct-acting constructions and in reverse-acting constructionsas shown in Figure 2. In a direct-acting actuator, an increase in diaphragm pressure will cause the actuator stemto move toward the control valve. In a reverse-acting actuator, an increase in diaphragm pressure will cause theactuator stem to move away from the control valve.






Figure 2Direct And Reverse-Acting Actuator Constructions

Fail Mode - The selection of an action is based on the fail mode that is desired. The fail modes that canbe achieved are shown in Figure 3.



Fail Mode Valve Action ActuatorAction

Fail Close PDTC ReversePDTO Direct

Fail Open PDTC DirectPDTO Reverse

Figure 3Control Valve Action And Fail Mode Selection




Physical Size

Actuator selection and sizing begins with the selection of an appropriate actuator physical size. The physicalsize of a spring-and-diaphragm actuator is typically denoted with a numeric designation such as size 30, size33, size 40, size 100, and so forth. The criteria for selecting a particular actuator size are shown in Figure 4 andthey are discussed below.



Figure 3Criterion For Actuator Physical Size Selection

Diaphragm Area - As the actuator diaphragm area increases, more net actuator force is available tooperate the control valve.

Diameter Of The Yoke-To-Valve Connection - The actuator yoke must have a diameter that is equalto the diameter of the yoke boss that is located on the control valve bonnet.

Actuator Stem Diameter - The diameter of the actuator stem must be compatible with the diameter ofthe actuator stem in terms of strength and the availability of a stem connector that will connect theactuator stem with the control valve stem.

Economics - Because small actuators are less costly than large actuators, the smallest possible actuatoris typically selected.




Operational Specifications

The remaining actuator specifications relate the performance of the actuator. These specifications are shown inFigure 5 and they are discussed below.



Figure 4Operational Specifications

Diaphragm Operating Pressure Range - Actuators are rated for specific diaphragm pressure ranges.The most common nominal diaphragm pressure ranges is 3 to 15 psig. Higher pressure ranges such as6 to 30 psig are sometimes specified in order to increase the thrust of the actuator. The output pressurerange of the instrument that supplies that actuator loading pressure must be compatible with theactuator diaphragm pressure range that is selected.

Maximum Diaphragm Pressure Rating - Each actuator is rated for a maximum diaphragm pressure.The upper pressure limit is established to prevent excessive loads from damaging the diaphragm, thediaphragm casings, and the actuator stem.

Spring Rate - The spring rate of the actuator spring (KS) describes the stiffness of the spring. Springrates will be discussed in the next section of this Module.

Bench Set - The term bench set refers to an actuator specification and to the procedure that is used toadjust the actuator spring prior to mounting the actuator on a specific valve. Bench set will bediscussed in the next section of this Module.




Basic Sizing Concepts

Valve Forces

In order to size an actuator, the specifier first calculates the total force that is required (FTFR) to operate thecontrol valve. (TFR means total force required.) FTFR includes all of the static forces that the actuator mustovercome in order to seat the valve plug with sufficient force to achieve the ANSI Class shutoff rating of thecontrol valve. For ease of calculation, these forces (see Figure 6) are defined as follows:

Force A The static unbalance force that is produced by the DPshutoff. Force B The seat load that is needed to achieve the rated ANSI Class shutoff rating of

the valve. Force C The force that is needed to overcome the packing friction. Force D The force that is needed to overcome any other source of valve friction; e.g.,

the piston rings in balanced valve constructions.



Figure 5Control Valve Forces

Actuator Forces

The amount of force that is available from a particular actuator is a function of:

The diaphragm force The spring force

Diaphragm Force - As shown in Figure 7, the maximum diaphragm force (FD) equals the maximumdiaphragm pressure (Pb) multiplied by the diaphragm area (AD). The maximum diaphragm pressure isa function of the instrument that supplies the actuator loading pressure. Diaphragm areas are publishedin manufacturers sizing information.




FD = Pb x ADUse Word 6.0c or later to


Figure 6Factors That Determine The Maximum Diaphragm Force

Spring Forces

To calculate the force that is produced by the actuator spring, the following factors must be evaluated:

The spring rate (KS) of the actuator spring in pounds/inch. The amount of valve travel (TV), in inches). The amount of initial compression (Fi).

Spring Rate, KS - Spring rate is defined as the amount of force that is needed to compress a givenspring 1 inch; for example, if the spring rate of a particular spring is 100 pounds/inch, 100 poundsforce will be needed to compress the spring 1 inch, 200 pounds force will be needed to compress thespring 2 inches, and so forth, as shown in Figure 8. Similarly, if a spring with a spring rate of 100pounds/inch is compressed 1 inch, it will exert 100 pounds force, and, if it is compressed 2 inches, itwill exert 200 pounds force.






Figure 7Spring Rate

Spring Compression Forces: Direct-Acting Actuators - The components of spring compression for adirect-acting actuator are shown in Figure 9.



Figure 8 Components Of Spring Compression In A Direct-Acting Actuator




Initial Compression (Fi) - The minimum diaphragm force is equal to the minimum actuatorloading pressure (Pa) multiplied by the area of diaphragm (AD). To prevent the minimumdiaphragm force from moving the actuator stem away from the 0 percent travel position, an equaland opposing force (Fi) is wound into the spring. Because the spring is preloaded, lost motion ordead band is eliminated and the actuator produces a stem force as soon as the diaphragm forceexceeds the force of initial compression. The distance that the spring should be compressed (theinitial windup) is calculated as follows:

Initial Windup inches P x AK

a DS

( ) =

Spring Compression Over Travel (Fs) - FS is the force that is required to compress the spring adistance that is equal to the rated valve travel (TV); accordingly, FS is the product of the springrate (KS) and the rated valve travel (TV), as shown below:

FS = KS x TV

Spring Compression: Reverse-Acting Actuators - The components of spring compression for areverse-acting actuator are shown in Figure 10, and they are discussed below. Initial Compression (Fi) - In a reverse acting actuator, the force that is required to close the

control valve (FTFR) is always wound into the spring. In some applications, FTFR is the onlycomponent of Fi. If the minimum output pressure from the instrument that provides the loadingpressure to the actuator is greater than 0 psig, an additional force that is equal to Pa x AD isgenerally wound into the spring to ensure that the valve will shut off. The total amount of initialwindup (in inches) is calculated as follows:

Initial WindupFK

P xAK

TFR

S

a D

S= +

Spring Compression Over Travel (Fs) - As was discussed for direct-acting constructions, FS isthe force that is required to compress the spring a distance that is equal to the rated valve travel(TV).






Figure 9Components Of Spring Compression In A Reverse-Acting Actuator

Actuator Bench Set

Many actuator force relationships can be summarized with an actuator specification that is referred toas the actuator bench set, or spring range. For both direct-acting and reverse-acting actuators, the benchset is the range of diaphragm pressures over which the actuator spring is compressed a distance that isequal to the rated valve travel when the actuator is disconnected from all control valve forces; i.e.,when the actuator and valve stems are disconnected. The term bench set derives from an adjustmentthat is made with the spring adjuster while the actuator is on the assembly bench.

Direct Acting Actuator Bench Set Specification - Figure 11 shows how the bench set specificationhelps to identify all the actuator forces that are produced by a direct-acting actuator on a push down toclose valve. The forces are discussed below.






Figure 10Typical Direct-Acting Actuator Bench Set

For purposes of discussion, assume that the actuator operating pressure range is 0 to 18 psig, thediaphragm area is 70 square inches, and the valve travel is 1 inch. As Figure 11 shows, the lower benchset pressure (BSlower) is the diaphragm pressure at which the diaphragm force overcomes the force ofFi; i.e., the diaphragm pressure at which the actuator stem just begins to move away from the up-travelstop. In this example, the force of initial compression is calculated as follows:

Fi = Pa x ADFi = 3 psig x 70 square inches

Fi = 210 pounds force

As the diaphragm pressure is increased from 3 to 11 psig, the diaphragm generates sufficient force tocompress the actuator spring a distance that is equal to the rated valve travel (TV). Therefore, thepressure range of 3 to 11 psig is the bench set specification of the actuator. The slope of the plotindicates the spring rate of the actuator spring. The spring rate is calculated as follows:




KS = pounds force / TV, inchesKS = [(BSupper - BSlower) x AD]/TVKS = (9 psig x 70 square inches)/1 inch

KS = 630 pounds/inchAs the diaphragm pressure is increased from 11 to 15 psig, there is no additional spring compression orstem travel. Instead, the diaphragm force that is produced over this pressure range overcomes all thevalve forces (FTFR); i.e., the forces that must be provided to overcome static unbalance, to overcomepacking friction, and to seat the valve plug to the ANSI Class shutoff rating. The force that is availableto operate the control valve is calculated as follows:

Net actuator force = (Pb - BSupper) x ADNet actuator force = (18 psig- 11 psig) x 70 square inches

Net actuator force = 7 psig x 70 square inchesNet actuator force = 490 pounds force

Reverse Acting Actuator Bench Set Specification - Figure 12 shows how the bench set specificationhelps to identify the actuator forces that are produced by a reverse-acting actuator that is mounted on apush down to close valve. The forces are discussed below.



Figure 11Typical Reverse-Acting Actuator Bench Set




Assume that the actuator operating pressure range is 0 to 18 psig, the diaphragm area is 70 squareinches, and the valve travel is 1 inch. In a reverse-acting actuator, the force that is required to overcomeall the valve forces and to seat the valve is wound into the actuator spring. Therefore, the force ofinitial compression (Fi) must be equal to or greater than the total force that is required to operate thecontrol valve (FTFR) .

As the diaphragm pressure is increased from 0 psig, the actuator stem will not move until thediaphragm produces a force that is greater than Fi. The net actuator force that is available to operate thecontrol valve is calculated as follows:

Net actuator force = (BSlower - Pa) x ADNet actuator force = (6 psig- 0 psig) x 70 square inches

Net actuator force = 6 psig x 70 square inchesNet actuator force = 420 pounds force

As the diaphragm pressure is increased from 6 to 14 psig, the diaphragm force compresses the actuatorspring a distance that is equal to the rated valve travel. Therefore, the bench set specification is 6 to 14psig.The slope of the plot indicates the spring rate of the actuator spring. The spring rate is calculated asfollows:

KS = pounds force / TV, inchesKS = [(BSupper - BSlower) x AD]/TVKS = (8 psig x 70 square inches)/1 inch

KS = 560 pounds/inchAs the diaphragm pressure is increased from 14 psig to 18 psig, there is no additional movement of thevalve stem because the actuator has engaged its upper travel stop. The additional force that is generatedby the diaphragm ensures that the valve will fully open.

Bench Set Span And Performance

During the actuator selection process, the specifier will typically find that several different spring rates andbench set spans are available that will satisfy the basic force requirements of the control valve. Some bench setspans are fairly narrow (for example, 3 to 7 psig for direct-action or 12 to 15 psig for a reverse action), andother bench set spans are very wide (3 to 15 psig for direct-action or 6 to 15 psig for reverse-action). Specifiersshould be aware of the consequences of selecting extremely narrow or extremely wide bench set ranges.

Narrow Bench Set Ranges - If the specifiers major concern is seat load and shutoff, the specifiershould select a spring with a relatively low spring rate. The low spring rate will increase the force thatis available for seat load because less diaphragm force is necessary to compress the spring over therated travel of the control valve, and more diaphragm force is available for shutoff. The selection of alow spring rate will result in a relatively narrow bench set span as shown in Figure 13.




While a narrow bench set span increases the force that is available for control valve shutoff, a narrowbench set span may result in poor controllability. Figure 13 shows that a narrow bench set span resultsin very high static gain (i.e., a small change in the diaphragm pressure will cause a large change in theactuator stem position).



Figure 12High Gain That Results From A Low Spring Rate And A Narrow Bench Set Span

Wide Bench Set Span - When the objective is to provide the best possible control, the specifier couldselect a very high spring rate. The higher spring rate will result in a wider bench set span because morediaphragm force is needed to compress the spring over the rated valve travel. For purposes ofillustration, assume that a spring is selected that results in a bench set specification of 3 to 15 psig asshown in Figure 14.

The static gain is low which results in good controllability. Because the spring is very stiff, the actuatorwill prevent the plug from changing position as a result of pressure transients and other buffetingforces. However, virtually all of the available diaphragm force is needed to compress the spring overthe rated travel of the valve and there is little force available to seat the valve. This example illustratesthe importance of avoiding a very wide bench set span if the tight shutoff is a requirement.






Figure 13Effects Of Wide Bench Set Span And High Spring Rate

Bench Set Guidelines - When specifiers are faced with a choice of spring rates and bench set spans,they must consider the application and the valve specifications to make an appropriate selection. Thefollowing guidelines summarize the prior discussion of bench set specifications. If shutoff is of prime importance, a lighter spring and a narrower bench set span may be

appropriate. If valve plug stability is a concern, and if tight shutoff force is not a requirement, a heavier

spring and wider bench set span should be specified. If shutoff and plug stability are both critical issues, a larger actuator and/or increased

diaphragm pressure may be required to achieve the performance objectives.

Effects of Valve Friction On Actuator Performance

Dead Band

The friction that is associated with valve packing and other valve components can introduce dead band into acontrol valve assembly. In the context of control valves, dead band is the range of diaphragm pressures overwhich there is no change in valve stem travel. Dead band results from the stick-slip friction effect that isproduced by valve packing and seals. The effects of dead band are shown in Figure 15 and they are listedbelow.




The actuator may not begin to stroke until a pressure that is significantly greater than 3 psig isapplied to the diaphragm.

The actuator may not achieve the full rated valve travel until a pressure that is significantly greaterthan 9 psig is applied to the diaphragm.

As the diaphragm pressure is decreased, the actuator stem may not begin to move toward theclosed position until the diaphragm pressure is reduced to a pressure that is significantly less than9 psig.

The actuator may not fully open the valve unless the diaphragm pressure is reduced to a pressurethat is significantly less than 3 psig.



Figure 14Theoretical Effects Of Friction And Dead Band On Bench Set

Effects of Dead Band On The Travel Of An Installed Control Valve

Figure 16 illustrates the effects of friction and dead band on control valve performance when the control valveis installed in service. Note that the valve should be fully open when the diaphragm pressure is 3 psig, and thevalve should be fully closed when the diaphragm pressure is 15 psig. However, because of friction and deadband, a diaphragm pressure of 3 psig may not fully open the valve and a diaphragm pressure of 15 psig may notfully close the valve.






Figure 15Effects Of Friction Band On Control Valve Travel

Importance Of Instrument Over Ranging

The effect of friction on valve travel underscores the importance of being able to supply a diaphragm pressurerange of 0 to 18 psig even though the actuator may be described as requiring a nominal 3 to 15 psig loadingpressure. When the expanded diaphragm pressure range (0 to 18 psig) is available, an additional force that isequal to 3 psig x AD is available to overcome valve friction. The additional force helps to ensure that the valvewill fully open and that the valve will fully close.

Effects Of Friction On Process Control

Limit Cycle - The effects of stick-slip friction and dead band are often observed in the process as adistinct oscillation of the process variable that is referred to as limit cycle. Refer to Figure 17.Limit cycle is uniquely different from the oscillations that result from excessive loop gain. Limitcycle occurs when the valve closure member "sticks" because of static friction, then suddenlyjumps to a new position when the actuator force exceeds the static friction, as shown in the upperplot in Figure 15. The response of the process variable to the abrupt changes in valve stem positionis shown in the lower plot in Figure 17. Note the following:




The distinct shape of the limit cycle is determined by the time constant of the process and otherfactors.

The magnitude of the limit cycle is determined by the proportional gain of the controller. The frequency of the limit cycle is a function of any integral action in the controller or in the

process.



Figure 16Control Valve Dead Band (Upper Plot) And Limit Cycle (Lower Plot)

Minimizing Friction Effects

Equipment Selection To Minimize Dead Band - Because limit cycle is caused by valve friction, anytactic that minimizes dead band will also help to minimize limit cycle. In terms of equipment selection,the selection of any of the following may help to minimize control valve dead band. A control valve positioner. A larger actuator. An actuator with an elevated operating pressure range.

The selection of a control valve positioner is often the easiest and most practical solution. Theimprovement in stem positioning accuracy that can be achieved with a positioner is illustrated inFigure 18.






Figure 17Minimizing Dead Band With A Control Valve Positioner

Minimizing Limit Cycles With Controller Tuning - The controller can be adjusted (tuned) tominimize the limit cycle in an existing system as follows: Reducing the proportional gain of the controller reduces the magnitude of the limit cycle. Changing the integral action. Increased integral action compensates for the reduction in

proportional gain and increases the frequency of the cycles, while reducing the integral action willdecrease the frequency of the cycles. Even if the limit cycles cannot be totally eliminated, theymay be minimized to the extent that their effects on the process variable become very small oreven imperceptible.




PERFORMING DETAILED ACTUATOR SIZING CALCULATIONS

Introduction

While actuator selection and sizing is typically performed with the use of simple methods, there are instanceswhen the specifier will benefit from performing detailed sizing calculations. A detailed analysis may bebeneficial in the following circumstances: The control valve includes high-friction packing. The instrument that provides the actuator loading pressure provides a nonstandard output pressure range;

i.e., any pressure range that is other than 0 to 18 psig or 0 to 33 psig. It is desirable to evaluate the performance of an installed actuator or to predict the performance of an

actuator that is on-hand or available in inventory. Detailed sizing calculations are typically performed with the use of computer software programs such as theFisher Sizing Program. Much of the discussion that follows will be based on the features and the nomenclaturethat are included in the Detailed Sizing Option of the Fisher Sizing Program.

Nomenclature For A Detailed Analysis Of Actuator Performance

Bench Set, Lower And Upper

As described previously, the bench set is the range of diaphragm pressures over which the actuator stem movesa distance that is equal to the rated valve travel when the actuator is disconnected from the valve. Refer toFigure 19.

Stroking Pressure, Lower And Upper

As shown in Figure 19, the stroking pressure range is the range of diaphragm pressures over which the actuatormoves the valve plug a distance that is equal to the rated valve travel when the valve is installed; i.e., with valvefriction present and with pressure in the valve body. When the valve is open, the pressure unbalance from thestem area may tend to open the valve or to close the valve; therefore, the lower stroking pressure may beslightly higher or slightly lower than the lower bench set pressure. The upper stoking pressure does not includethe diaphragm pressure that is required to provide seat load.






Figure 18Bench Set And Stroking Pressure Range

Friction Band

Friction band is the diaphragm pressure that is required to overcome the packing friction. Friction band iscalculated as follows:

Friction Band psig Friction poundsA inchesD

( ) ( )( )= 2

The term friction band is unique to the Fisher Sizing Program. Friction band is equal to half of the total deadband that results from valve friction.

As shown in Figure 20, the stroking pressure range plus and minus the friction band is the range of pressuresthat will be required to move the valve plug a distance that is equal to the rated valve travel when the unbalance

forces and the packing friction are present.






Figure 19Stroking Pressure Range Plus Friction Band

Operating Pressure Range

The operating pressure range is the range of diaphragm pressures that is required to fully open and fully closethe valve and to provide the required seat load, as shown in Figure 21. The operating pressure range will alwaysbe within the limits of Pa and Pb. In the detailed sizing method, the specifier can set Pa and Pb to any pressurevalue.

Supply Pressure

The supply pressure (PS) that is required to ensure optimum control valve performance is also calculated by thesoftware. As shown in Figure 21, the supply pressure should be somewhat higher than the upper operating

pressure.






Figure 20Operating Pressure Range And Supply Pressure, Ps

Overview Of The Detailed Actuator Sizing Method Of The Fisher Sizing Program

Selecting The Detailed Sizing Method

The specifier selects the Detailed Sizing Method from the main menu by first selecting the Ssact option. Fromthe menu that appears, the specifier selects the Spring & Diaphragm option under heading Detailed Sizing.




Calculation Screen

Figure 22 shows the calculation screen for the Detailed Sizing Method.

Rev 1.42 Detailed Sizing Spring & DiaphragmCalculated Results

Actuator Type 657 -------------------------------- -----------Valve Design ED Actuator SizeFlow DOWN Spring

Spring Rate - lbf/inPort Diameter 3.438 in Spring Windup - inUnbalanced Area 0.400 in2 Max Spring Load - lbfValve Travel 1.500 in Min Req'd Air Sup. - psigValve Stem Size 0.500 inValve Friction 230.00 lbf Actr. Outp. Thrust - lbfP1 Max 300.00 psig Req'd Valve Thrust - lbfdP Max 300.00 psidSeat Load 20.00 lbf/in Lower Bench Set - psigUnbal Frce @ Open 0.00 lbf Lower Stroking - psigPa 0.00 psig Low Frict Band (+/-) - psigPb 18.00 psigFluid Neg. Gradient 0.00 in Upper Bench Set - psig

Upper Stroking - psigBellows (Y/N) ? N Up Frict Band (+/-) - psig

Lower Operating - psigUpper Operating - psig

F1-HELP F2-Calc F3-Option F4-Choice F5-Clear F10-Exit

Figure 21Detailed Sizing Screen For Spring-And-Diaphragm Actuators

Migration Of Valve And Actuator Data From The Quick Sizing Method

As shown in Figure 22, the valve port diameter, the valve travel, the unbalanced area, and other valve andactuator information must be located in the appropriate vendor data and entered into the associated fields.Location of the data is time consuming. To simplify data entry, the specifier may select the Quick-Sizingmethod, calculate the actuator sizing information, and, then, transfer the data to the Detailed Sizing Method. Inthe Quick Sizing Method, the information for each entry field is easily entered by placing the cursor in a field,pressing the F4 key and, then, selecting the appropriate data from pull down menus. After the data is entered inthe Quick Sizing Method, one may return to the main menu and select the Detailed Sizing method. This actiontransfers all of the data from the Quick-Sizing method into the Detailed Sizing Method. After the data has beentransferred to the Detailed Sizing Method, the input fields may be changed as necessary.




Direct Entry Of Information

If you are entering the data directly, the pertinent information must be located in the appropriate valvespecification bulletin or in the manufacturers actuator sizing documentation; e.g., Fisher Catalog 14.

Unique Input Fields

The unique input fields of The Detailed Sizing screen are as follows:

Unbal Frce @ Open - For some special valve constructions that throttle at reduced travel and inapplications where the actuator is operated directly by process pressure, the unbalance force at the openposition can impact actuator sizing. For most installations, a value of zero is entered in this field.Diaphragm Pressures Pa and Pb - These entry fields allow the specifier to input the actual diaphragmloading pressure range.Fluid Negative Gradient - This entry field asks for a coefficient that describes the fluid reaction forcesthat tend to destabilize the valve plug. Negative gradients will be discussed in the next section of thisModule.Bellows (Y/N) - This entry asks if the control valve includes a valve stem bellows. If this entry is set toY (yes), the software calculates the force that results from the spring rate of the bellows.




Calculated Results

After all the inputs are entered, the specifier presses the F2 key to perform the sizing calculations. The fields inthe calculated results section of the screen are shown in Figure 23 and they are explained below.

Rev 1.42 Detailed Sizing: Spring & DiaphragmCalculated Results

Actuator Type 657 ------------------------------ -----------------Valve Design ED Actuator Size 40Flow DOWN Spring 1F1770

Spring Rate 275.00 lbf/inPort Diameter 3.438 in Spring Windup 0.958 inUnbalanced Area 0.400 in2 Max Spring Load 676.0 lbfValve Travel 1.500 in Min Req'd Air Sup. 21.00 psigValve Stem Size 0.500 inValve Friction 230.00 lbf Actr.Outp. Thrust 773.0 lbfP1 Max 300.00 psig Req'd Valve Thrust 566.0 lbfdP Max 300.00 psidSeat Load 20.00 lbf/in Lower Bench Set 3.82 psigUnbal Frce @ Open 0.00 lbf Lower Stroking 3.82 psigPa 0.00 psig Low Frict Band (+/-) 3.33 psigPb 18.00 psigFluid Neg. Gradient 0.00 in Upper Bench Set 9.80 psig

Upper Stroking 11.54 psigBellows (Y/N) ? N Up Frict Band (+/-) 3.33 psig

Lower Operating 0.49 psigUpper Operating 18.00 psig

Figure 22Calculated Results For The Detailed Sizing Method

Actuator Size - After calculating all valve forces, the software automatically selects the smallestactuator that will: Physically mount to the selected valve type (compatible yoke boss size and stem connection size). Provide the required actuator thrust. Provide the widest possible bench set span.

Spring Rate - The spring rate of the selected actuator is displayed.Spring Windup - This field lists the initial compression, in inches, that is wound into the spring withthe spring adjuster.




Max. Spring Load - This field describes the maximum spring load for the selected spring. Themaximum spring load is the sum of Fi (the force of initial compression) and FS (the force of springcompression over travel). The software will not select an actuator construction in which the spring isoverloaded; therefore, this field is for information purposes only.

Min Reqd Air Sup. - This field displays the minimum supply pressure, in psig, that is needed forproper actuator operation. The minimum supply pressure is always set to a pressure that is a fixedamount above the upper operating pressure. The additional pressure compensates for deviations inplant pressure, tolerances in regulator ratings and performance, and other conditions that could reducethe available supply pressure.

Lower And Upper Stroking Pressures - As previously described, the stroking pressure range is therange of pressures over which the valve plug of an installed valve will move from one travel stop to theother. This pressure range does not include the pressure that is required to produce the needed seatload.

Lower and Upper Friction Band (+/-) - This field lists the friction band (one-half the dead band), inpsig, that results from valve friction.

Lower and Upper Operating Pressures - These pressure values define the pressure range thatproduces full valve travel plus the seat load, plus an allowance for friction band.Non-Standard Bench Sets -The Detailed Sizing Method calculates mathematically precise bench sets.For example, bench sets such as 1.6 psig to 8.4 psig for a direct-acting actuator, or 8.75 to 16.2 psig fora reverse-acting actuator may be displayed. Non-standard bench sets are troublesome to somespecifiers who are familiar with the standard bench set ranges such as 3 to 9 psig or 9 to 15 psig;however, the non-standard bench sets are simply the calculated pressure values that will ensure properoperation of the control valve assembly.

Bench Set Selection Guidelines

If the calculated bench set specification is non-standard, it may be possible to select a standard bench set undercertain circumstances. The selection guidelines are as follows:

PTFE Packing - If the control valve includes single PTFE packing, and if the instrument that provides thediaphragm pressure can produce a minimum pressure of approximately 0 psig, the calculated bench set canbe shifted to a standard bench set range (3 psig to x psig) with the same approximate span as the non-standard bench set.

Graphite Packing - If the control valve includes high-friction packing, the calculated bench set should not

be shifted to a standard bench set range. An actuator with the calculated bench set should be selected.




Manual Sizing Option

By pressing the F3 key, the specifier may view the options for the detailed sizing method. The options arediscussed below.

Automatic Sizing - When Automatic Sizing is selected, the software automatically selects theoptimum actuator construction.Manual sizing - The manual sizing option is useful when the specifier wishes to determine if anexisting construction is appropriate for a specific application. For example, when troubleshooting aproblematic control valve, the actuator may be evaluated to ensure that the actuator has been properlysized. The manual method may also be selected to determine if an actuator that is in inventory can beadapted to a new application.In the manual sizing mode, the specifier may evaluate the performance of a particular actuator size andspring combination. First, all the appropriate valve information must be entered. The specifier maythen display a list of actuator sizes and a list of actuator springs by placing the cursor in the appropriatefield and pressing the F4 key. The actuator size and spring that are to be evaluated are selected from thelists. When the specifier presses the F2 key, the software attempts to calculate an actuator specificationthat is based on the valve information, the selected actuator size, and the selected actuator spring. Thesoftware will either display the appropriate actuator specification or it will display a report that explainswhy the selected actuator size and spring are not appropriate for the application.




EVALUATING THE IMPACT OF NEGATIVE FLUID FORCE GRADIENTS

Definition And Examples Of Fluid Force Gradients

Definition Of Fluid Gradients

Dynamic Versus Static Considerations - When one is sizing an actuator for a specific control valve,one must consider both the static performance and the installed dynamic performance of the actuatorand control valve assembly. Most basic actuator sizing techniques are based on static performancecriteria; i.e., ensuring that the actuator produces sufficient force to fully open the valve and to fullyclose the valve.A dynamic analysis is performed to ensure that the actuator will be sufficiently stiff to oppose the fluidreaction forces that may tend to destabilize the valve travels from travel stop to the other.

Fluid Force Gradients - The term fluid force gradient is used to describe the changes, over valvetravel, in the forces on the valve stem that are generated by the interaction of the flowing fluid with thevalve plug.

Force Gradients - To evaluate a force gradient, one must evaluate the change in force relative to thechange in position. The basis for evaluation is expressed with:

DF/Dxwhere:

DF = the change in force, pounds forceDx = the change in travel, inches

Examples Of Fluid Gradients

Positive Gradients - Gradients are most easily identified as being positive or negative if one adheres tothe plotting conventions that are shown in Figure 24 . The valve forces that place the valve stem intension are plotted above the baseline and the valve forces that place the valve stem in compression areplotted below the baseline.A positive valve plug force gradient is shown in Figure 24. The plot is typical for an unbalanced valvethat is installed in the flow-up orientation. Because the fluid force gradient is positive over the ratedtravel of the valve, valve plug instability (from negative gradients) will not be a concern.






Figure 23Positive Valve Plug Force Gradient

Negative Gradient: Open Loop System - Figure 25 shows a hypothetical plot of the valve plugreaction forces that might be generated by a balanced valve that is installed in a flow-down orientation.In the flow-down orientation, the fluid pressure tends to open the valve; i.e., the valve plug forcescreate a compressive force on the valve stem. For the discussion that follows, assume the following:

The actuator is being stroked by manually adjusting the set pressure of a pressure regulator; i.e.,there is no feedback control.

The actuator is a direct-acting type and it is somewhat undersized.As the diaphragm pressure is reduced from the maximum diaphragm pressure, the actuator willsmoothly stroke the valve plug from point A to travel point B. The actuator force that is generated attravel point B also satisfies the valve force requirements for travel point D; therefore, a small reductionin diaphragm pressure will cause the valve plug to jump to travel point D without providing smooththrottling control between points B and D. If, at travel point D, the actuator diaphragm pressure isincreased, the valve plug will smoothly travel to travel point C. Because the actuator force at point Calso satisfies the valve force requirement at point A, an additional small increase in actuator force willcause the valve plug to jump to point A. Because the valve plug tends to jump from one travel point toanother, the plug is described as being bi-stable.






Figure 24Bi-Stable Plug In An Open Loop System

Negative Gradient: Closed Loop System - If a feedback device such as a controller or positioner isincluded in the system, the feedback device will attempt to correct the stem position errors that arecaused by the bi-stable valve plug. As the feedback device tries to correct the error, the plug willrapidly cycle (change position) between points D and A. In addition, depending on the gain of thefeedback device, the valve stem may overshoot the bi-stable points of operation. The result may be asystem that is wildly unstable as shown in Figure 26






Figure 25Unstable Plug In A Closed Loop System

Negative Gradient: Balanced Valve, Flow Down Configuration - While balanced valvesoccasionally generate negative force gradients, unbalanced valves in a flow down orientation alwayspresent a significant negative gradient as shown in Figure 27. However, because larger actuators areselected in order to overcome the considerable static forces at shutoff, the actuators are typicallysufficiently stiff to prevent the valve plug positioning problems that are associated with negativegradients. When the actuator is not sufficiently stiff, the plug, as it approaches the seat, will slam intothe seat.



Figure 26Negative Gradient In A Flow-To-Close Unbalanced Valve




Preventing Valve Plug Instability That Is Caused By Negative Gradients

Valve Plug Stability Equation

To ensure valve plug stability, the total actuator stiffness must be greater than the maximum value of DF/Dx thatis calculated for the valve plug forces. The mathematical expression for valve plug stability is as follows:

Ks + Ka >Kn DPwhere:

Ks the mechanical spring rate, pounds/inchKa the air spring rate, pounds/inchKn coefficient for the valve negative gradient, pounds/inch/psidDP the flowing pressure drop across the valve, psidThe basic concept of the stability equation is shown in Figure 28. The terms that are used in the equation arediscussed below.



Figure 27Conditions For Valve Plug Stability




Determining Total Actuator Stiffness

Mechanical Spring Rate - Increasing the spring rate (Ks) of the actuator spring can help to minimizethe effects of negative gradients. However, the relatively high spring rate that would be required toovercome a large negative gradient may lead to the selection of a very large actuator that is impracticalin terms of its physical size and its cost.

Air Spring Effects - The volume of air in the actuator casing provides an air spring effect that alsohelps to stabilize the valve plug. The air spring derives from the compressibility of the fluid. The rateof the air spring effect is referred to as Ka. In most instances, the spring rate of the air spring is muchgreater than the spring rate of the mechanical spring.Determining The Air Spring Rate - The spring rate of the air spring, Ka, is calculated as follows:

KkPA

Va=

2

where:

Ka the spring rate of the air spring, pounds force per inchk the ratio of specific heats of the actuating media (typically air), dimensionlessP the average pressure that is applied to the actuator diaphragm, psigA2 the area of the diaphragm, inches2V the volume of the actuator casing, inches3

Fisher Controls publishes plots that allow the specifier to quickly determine the air spring rate. Figure29 shows the air spring effect, Ka versus the diaphragm pressure for a size 45 actuator.






Figure 28Air Spring Effect Versus Diaphragm Pressure For A Fisher Type 657 or Type 667 Size 45 Actuator

Diaphragm Pressures That Are Used To Calculate Ka - The value of Ka is computed with the useof the average diaphragm pressure. The average diaphragm is the mean pressure between the lowerstroking pressure and the upper stroking pressure.

Role Of A Positioner - The values of Ka that are calculated with the use of the methods that aredescribed above assume that a positioner is included in the control valve assembly. If a positioner isnot included in the control valve assembly, the value of Ka must be reduced by one half. The reasonfor the reduction is explained as follows:

Any form of feedback control adds significantly to the air spring effect at very low cycling frequencies.If the actuator is to be occasionally operated in the manual mode; i.e., the controller will be set tomanual, the air spring effect will be greatly diminished. If a positioner is included in the control valveassembly, a feedback loop will be maintained even if the controller is set to manual; therefore, the fullbenefit of the air-spring effect will be realized. If a control valve that does not include a positioner isoperated in the manual mode, there is no feed back and the air spring effect is diminished.




Quantifying Negative Gradients

Coefficient For Negative Gradients, Kn - Manufacturers, through the use of appropriate laboratorytests, can determine the magnitude of the negative gradient that will be produced by a specific valve.For example, Fisher Controls tests each valve in the laboratory, identifies the maximum negativegradient, and publishes a value of the coefficient Kn for each valve style that is likely to be affected bynegative gradients. The maximum negative gradient is normalized for a 1 psid flowing pressure drop.Therefore, specifiers may estimate the impact of the negative gradient at any flowing pressure drop.The nomenclature for the coefficient Kn is as follows:

KF

xpsidn

=D

D1

Typical Values Of Kn For Balanced Valves - For balanced valve constructions, the values of Kn arepublished in tabular format in Fisher Catalog 14. A table of typical Kn values is shown in Figure 30

Port Negative Gradient, KnDiameter,

InchesLinear

Class 1500Equal Percentage

Class 15001/4 thru --- 01 --- 0.11-1/2 --- 1.61-7/8 2.0 1.42-7/8 4.0 3.13-5/8 4.0 2.55-3/8 5.4 3.6

Figure 29Kn Values For Design HPD and HPT Valves, Flow Down

Values Of Kn For Unbalanced Valves, PTTO - For unbalanced valves where pressure tends to openthe valve (flow up), the value of Kn is 0.Values Of Kn For Unbalanced Valves, PTTO - For unbalanced valves where pressure tends to closethe valve (flow down), the value of Kn is estimated with the use of the following:

Kunbalance area inchesratedvalve travel inchesn

=2 2( , )

,




Valve Plug Stability Equation - To ensure valve plug stability, the value of Kn times the maximumflowing pressure drop must be greater than the mechanical spring rate plus the air spring rate. Themathematical expression for stability is as follows:

Ks + Ka >Kn DPwhere:

Ks the mechanical spring rate, pounds/inchKa the air spring rate, pounds/inchKn negative gradient, pounds/inch/psidDP the flowing pressure drop across the valve, psid

Determining The Flowing Pressure Drop - The flowing pressure drop that is used in conjunctionwith the value of Kn could be the pressure drop at the minimum flow condition, the normal flowcondition, or the maximum flow condition. Negative gradients tend to cause the most significantproblems under high pressure drop conditions; therefore, the largest flowing pressure drop is thepressure drop that should be closely evaluated




Computer Assisted Selection

The Detailed Sizing Method within the Fisher Sizing Program includes the ability to select actuatorconstructions that are adequately sized to prevent the valve plug instability that can occur as a result of negativegradients. In order to account for negative gradients, the appropriate data must be entered in two input fields.The fields are shown in boldface in Figure 31 and they are discussed below.

Rev 1.42 Detailed Sizing: Spring & DiaphragmCalculated Results

Actuator Type 667 ------------------------------- ------------------Valve Design ED Actuator Size 40Flow DOWN Spring 1E8053With Side MO? (Y/N) N Spring Rate 736.00 lbf/inPort Diameter 2.313 in Spring Windup 0.375 inUnbalanced Area 0.270 in2 Max Spring Load 1104.3 lbfValve Travel 1.125 in Min Req'd Air Sup. 20.00 psigValve Stem Size 0.500 inValve Friction 50.00 lbf Actr. Outp. Thrust 276.3 lbfP1 Max 300.00 psig Req'd Valve Thrust 276.3 lbfdP Max 300.00 psid Air Spring Rate 1408.58 lbf/inSeat Load 20.00 lbf/in Lower Bench Set 4.00 psigUnbal Frce @ Open 0.00 lbf Lower Stroking 2.83 psigPa 0.00 psig Low Frict Band (+/-) 0.72 psigPb 18.00 psigFluid Neg. Gradient 1.80 in Upper Bench Set 16.00 psigdP Flowing 150.00 psid Upper Stroking 16.00 psigBellows (Y/N) ? N Up Frict Band (+/-) 0.72 psig

Lower Operating 0.00 psigUpper Operating 16.73 psig

Figure 30Detailed Sizing Screen Of The Fisher Sizing Program

Fluid Neg. Gradient - Here the specifier enters the value of Kn that has been determined for the valvethat is being considered.Flowing dP - Here the specifier enters the flowing pressure drop.




Relative Frequency Of Problems That Are Caused By Negative Gradients

Experienced specifiers have observed that negative gradients are not a problem in a very high percentage of allcontrol valve installations. However, when negative gradients do cause problems, the problems can besignificant in terms of poor valve performance. In order to prevent problems, many specifiers evaluate thepotential for negative gradients whenever the following valve constructions are being considered:

Flow down unbalanced valves Large unbalanced valves (valve sizes > 6-inches) High pressure applications (DP > 300 psid)

Troubleshooting

As mentioned previously, many specifiers completely ignore the potential impacts of negative gradients; i.e.,they do not solve the stability equation prior to valve selection. As a result, performance problems areoccasionally encountered after the valve is installed. When one is called upon to troubleshoot an instabilityproblem, and when the instability problem cannot be readily attributed to excessive loop gain, to limit cycle, orto other causes, the stability equation may be solved in order to determine if a control valve negative gradient isthe source of the instability problem.




SELECTING ACTUATOR ACCESSORIES TO PROVIDE THE REQUIRED ACTUATORSTROKING TIMES

Clarification Of Terminology And Application Requirements

Clarification Of Terminology

Stroking Speed - Specifiers commonly describe the amount of time that is required for a valve totravel from one travel stop to the other as stroking speed. The term stroking speed implies that thespeed of actuator stem travel will be measured in units such as inches per second. However, actuatorperformance is rarely described in terms of a distance per unit of time. Therefore, the term strokingspeed is a misnomer and it should not be used to describe the speed of control valve operation.

Stroking Time - The correct term for describing the speed of operation of control valves is strokingtime. For example, a control valve specification may require a stop-to-stop stroking time of onesecond or less.

Clarification Of Application Requirements

The requirement for a short stroking time depends upon the specific requirements of each application.

Fast Response to Normal Operating Transients - For many applications, the primary objective is toensure that the control valve quickly responds to any normal operating transient. Short stop-to-stopstroking speeds are not critical for the successful control of most systems.

Short Stop-To-Stop Stroking Time For Critical Applications - For some applications such asemergency shutdown, compressor surge, and pump bypass, the successful and safe operation of thesystem may require short stroking times.

Stroking Time Vs. Control Accuracy (Overshoot) - Stroking times are often shortened through theuse of equipment that increases the loop gain. Increased loop gain can also cause system instability.Therefore, specifiers must balance the requirements for a short stroking time with the requirement forsystem stability.

Extended Stroking Times - In some instances, extended stroking times are desirable. Extendedstroking times help to minimize pipeline surges, water hammer, and other hydraulic events that canoccur as a result of rapid changes in pressure and flow conditions.




Parameters That Influence Actuator Stroking Time

The major influences that impact actuator stroking times are shown in Figure 32 and they are discussed below.



Figure 31Influences On Actuator Stroking Time




Supply Pressure And Capacity

The supply pressure and capacity have a direct bearing on stroking time. Factors that influence the supplypressure are the pressure of the plant air system and the set pressure of the filter regulator. The factors thatinfluence the supply capacity include the capacity of the plant air system and the capacity of the filter regulator.

Cv Of The Loading Instrument

Supply Cv - The supply capacity of the instrument that loads the actuator diaphragm has a directbearing on stroking time. The instrument could be an I/P transducer, a positioner, or a volume booster.Exhaust Cv - Each instrument also has an exhaust capacity. The stroking time is directly influenced bythe ability of the instrument to exhaust pressure from the actuator diaphragm casing.

Tubing And Fitting Size

Reduced stroking times can be achieved by specifying oversized components for the tubing, piping, and fittingsthat are used to connect the instrumentation and the actuator diaphragm casing.

Diaphragm Area

The diaphragm area has a significant bearing on stroking time. In general, a larger diaphragm area will requiremore time to pressurize; therefore, larger actuators will typically provide longer stronger times than smalleractuators.

Actuator Volume

The volume between the diaphragm and the pressurized diaphragm casing influences stroking time. Theactuator volume at each extreme of valve travel must be considered.

Clearance Volume, Vo - The clearance volume is the volume between the diaphragm and thepressurized actuator casing when the control valve is at 0 percent travel.

Total Volume, Vm - The total volume is the volume between the diaphragm and the pressurizedactuator casing when the valve is at 100 percent travel. The total volume includes the clearance volumeplus the displacement volume (the volume that is displaced by the actuator diaphragm as the valvestokes from 0 percent travel to 100 percent travel).




Filling And Exhausting Pressures

Initial Filling Pressure, Pi Fill - The initial filling pressure is the pressure at which the actuator juststarts to move away from the 0 percent travel position; i.e., the diaphragm pressure that is required toovercome the force of initial compression and the control valve friction forces.Final Filling Pressure, Pf Fill - The final filling pressure is the pressure at which the valve stemreaches the 100 percent travel position.

Exhaust Pressures

Initial Exhaust Pressure, Pi Exhaust - Pi Exhaust is the pressure at which the valve stem begins tomove from the 100 percent travel position to the 0 percent travel position.Final Exhaust Pressure, Pf Exhaust - Pf Exhaust is the pressure in the diaphragm casing when thecontrol valve reaches the 0 percent travel position.

Vent Cv

The actuator stroking time is also influenced by the capacity of the vent that is located in the non-pressurizeddiaphragm casing. As stroking times become shorter, a standard vent may create a restriction that will trap air inthe non-pressurized diaphragm casing, thereby slowing the movement of the actuator stem.

Performing Stroking Time Calculations With The Fisher Sizing Program

Objective Of Calculations

The Fisher Sizing Program includes a means for estimating the stroking time of a control valve assembly. Thecalculations can be performed to ensure that a selected valve and actuator will meet the applicationrequirements or the calculations may be performed in order to troubleshoot a control valve assembly that isalready in service.

Overview Of The Calculation Procedure And The Stroking Time Calculation Screen

Prior to calculating the actuator stroking time, one must first size an actuator with the use of the Detailed SizingMethod of the Fisher Sizing Program. Much of the data that is required to perform the stroking timecalculations is available from the calculated results section of the Detailed Sizing Method screen. The Spring-And-Diaphragm Actuator Stroking Time screen is shown in Figure 33. The fields are discussed on thefollowing pages.




Rev 1.42 Spring & Diaphragm Stroking Time

Actuator & Spring Air Supply Cv's-------------------- ------------------------------ ------------------------ ----------------------Area at top 69.00 in2 Air Cv Fill 0.140Area at midpoint 69.00 in2 Air Cv Exhaust 0.240Area at bottom 69.00 in2Vo 57.0 in3 Calculated TimesVm 142.0 in3 -----------------------Spring Rate 736.0 lbf/in Prestroke Fill 0.168 sec

Moving Fill 3.348 secActuator Pressures Total Fill Time 3.516 sec-------------------------- --------------------------Pi Fill 3.55 psig Prestroke Exhaust 0.380 secPf Fill 16.33 psig Moving Exhaust 2.496 secPi Exhaust 14.89 psig Total Exhaust Time 2.876 secPf Exhaust 2.11 psigAir Supply 20.00 psigF1-HELP F2-Calc F3-Option F5-Clear F8-Unit F10-Exit

Figure 32Spring-And-Diaphragm Actuator Stroking Time Screen

Actuator And Spring Entry Fields

Area At Top, Area At Midpoint, Area At Bottom - The area of an actuator diaphragm is not alwaysconstant over the rated travel of the control valve. If the area of the diaphragm does change over therated valve travel, and if the diaphragm area at both travel extremes and at the midpoint of valve travelare known, the values may be entered in the appropriate fields. If the diaphragm areas at various travelpoints are not known, a single entry is all that is required. The diaphragm areas of various actuatorsizes are listed in tabular format in the Help Screens of the Fisher Sizing Program.

Vo (Clearance Volume) - The clearance volumes of various actuator types and sizes are listed intabular format in the Help Screens of the Fisher Sizing Program.Vm (Casing Volume) - The actuator casing volumes of various actuator types and sizes are listed intabular format in the Help Screens of the Fisher Sizing Program. Casing volumes vary as a function ofvalve travel. The value of Vm includes the clearance volume (Vo).Spring Rate - The spring rate is listed as a calculated result in the Detailed Sizing Method of the FisherSizing Program.




Actuator Pressure Entry Fields

Pi Fill , Pf Fill, Pi Exhaust, Pf Exhaust- These values may be calculated from the actuator strokingpressure range and the actuator friction band that are determined with the use of the Detailed SizingMethod of the Fisher Sizing Program. The values of Pi Fill , Pf Fill, Pi Exhaust, Pf Exhaust arecalculated as follows:

Pi Fill = lower stroking pressure + friction bandPf Fill = upper stroking pressure + friction bandPi Exhaust = upper stroking pressure - friction bandPf Exhaust = lower stroking pressure - friction bandPs, Supply Pressure - The supply pressure is the maximum pressure that is available to theinstruments that load the actuator diaphragm; i.e, the set pressure of the filter/regulator. This value isdisplayed in the calculated results section of the Detailed Sizing Method as the Minimum RequiredAir Supply.

Air Supply

Air Cv Fill And Air Cv Exhaust - The filling and exhaust Cvs of the instrument or instruments thatload the actuator diaphragm are entered in these fields. The fill and exhaust Cv ratings of variousinstruments are located in the Help Screens of the Fisher Sizing Program.

Summing Cvs - If pneumatic instruments are piped in series, the Cvs of the appropriate instrumentsmust be summed. For example, if a Fisher Type 67AFR filter/regulator provides the supply pressure toa Fisher Type 3582 positioner, the total Cv for filling or exhausting the actuator casing will besubstantially less than the loading or filling Cv of either of the two instruments. A utility is included inthe Fisher Sizing Program that will sum the Cvs of two or more instruments.

Calculated Times

Prestroke Fill - This value is the time that is required to pressurize the actuator casing prior to anyactual movement of the actuator stem.

Moving Fill - This value is the time that is required for the valve stem to move a distance that is equalto the rated valve travel.

Total Fill Time - The total fill time is the sum of the prestroke fill time and the moving fill time; i.e.,the total time that is required to stroke the valve from the 0 percent travel position to the 100 percenttravel position.




Prestroke Exhaust - This value is the time that is required to exhaust the diaphragm casing pressureprior to any actual movement of the actuator stem.

Moving Exhaust - This value is the time that is required for the valve stem to move a distance that isequal to the rated valve travel.

Total Exhaust Time - The total exhaust time is the sum of the prestroke exhaust time and the movingexhaust time; i.e., the total time that is required to stroke the valve from the 100 percent travel positionto the 0 percent travel position.

Interpreting The Results Of The Stroking Time Calculations

Limits Of Accuracy - The stroking time equations that are included in the Fisher Sizing Program aregenerally accurate to within +/- 25 percent of the actual stroking time. However, when the calculatedstroking is 1 second or less, the error has been observed to be as much as 50 percent of actual strokingtime.Exclusion Of Shutoff Forces - The stroking times that are calculated are the times that are required forthe actuator to overcome packing friction, overcome the forces of valve unbalance, and move the valveplug from one travel stop to the other. The calculations do not account for the time that is required togenerate the maximum seat load.

Reducing The Stroking Time

If the stroking time that is calculated is longer than the desired stroking time, one may select different actuatorand instrument options that are designed to reduce stroking times. Then, the stroking time calculations arerepeated in order to determine if the selected options will provide the desired results. The options that arecommonly selected to shorten stroking times will be discussed in the next section of this Module.

Increasing The Stroking Time

If the stroking time that is calculated is shorter than the desired stroking time, one may select different actuatorand instrument options that will increase the stroking time. For example, one may select:

A larger actuator. An actuator loading instrument with reduced Cv ratings. A filter regulator with a reduced capacity. An optional needle valve that can be installed in the piping that supplies the actuator diaphragm

pressure.

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