What are the criteria for selecting a control valve?

The following discussion is part of an occasional series, "Ask the Automation Pros," authored by Greg McMillan, industry consultant, author of numerous process control books, and ISA Life Achievement Award recipient. Program administrators will collect submitted questions and solicits responses from automation professionals. Past Q&A videos are available on the ISA YouTube channel. View the playlist here. You can read all posts from this series here.

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Hiten Dalal&#;s Question

Are there basic rule of thumb guidelines for control valve sizing outside of relying on the valve supplier and using the valve manufacturer&#;s sizing program?

Hunter Vegas&#; Answer

Selecting and sizing control valves seems to have become a lost art. Most engineers toss it over the fence to the vendor along with a handful of (mostly wrong) process data values, and a salesperson plugs the values into a vendor program which spits out a result.  Control valves often determine the capability of the control system, and a poorly sized and selected control valve will make tight control impossible regardless of the control strategy or tuning employed. Selecting the right valve matters!

There are several aspects of sizing/selecting a control valve that must be addressed:

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Determine what the valve is supposed to do

• Is this valve used for tight control or is &#;loose&#; control acceptable. (For instance, are you trying to control a flow within a very tight margin across a broad range of process conditions or are you simply throttling a charge flow down as it approaches setpoint to avoid overshoot). The requirements for one situation are quite different from the other.
• Is this valve supposed to provide control or tight shutoff? A valve can almost never do both. If you need both, then add a separate on/off shutoff valve. 

Understand the TRUE process conditions

• What is the minimum flow that the valve must control?
• What is the maximum flow that the valve must pass?
• What are the TRUE upstream/downstream pressures and differential pressure across the valves in those conditions? (Note that the P1 and DP at low flow rates will usually be much higher than at full flow rates. If you see a valve spec showing the same DP valve for high and low flow conditions it will be wrong 95%+ of the time.
• What is the min/max temperature the valve might see? Don&#;t forget about clean out/steam out conditions or abnormal conditions that might subject a valve to high steam temperatures.
• What is the process fluid? Is it always the same or could it be a mix of products?

Note that gathering this data is probably the hardest to do.  It often takes a sketch of the piping, an understanding of the process hydraulics, and examination of the system pump curves to determine the real pressure drops under various conditions. Note too that the DP may change when you select a valve since it might require pipe reducers/expanders to be installed in a pipe that is sized larger.

Understand the installed flow characteristic of the valve

This can be another difficult task. Ideally the control valve response should be linear (from the control system&#;s perspective). If the PID output changes 5%, the process should respond in a similar fashion regardless of where the output is.  (In other words 15% to 20% or 85% to 90% should ideally generate the same process response.) If the valve response is non-linear, control becomes much more difficult. (You can tune for one process condition but if conditions change the dynamics change and now the tuning doesn&#;t work nearly as well.)  The valve response is determined by a number of items including:

• The characteristics of the valve itself. (It might be linear, equal percent, quick opening, or something else.)
• The DP of the process &#; The differential pressure across the valve is typically a function of the flow (the higher the flow, the lower the DP across the valve). This will generate a non-linear function.
• System pressure and pump curves &#; pumps often have non-linear characteristics as well, so the available pressure will vary with the flow.

The user has to understand all of these conditions so he/she can pick the right valve plug. Ideally you pick a valve characteristic that will offset the non-linear effects of the process and make the overall response of the system linear. 

If the pressure drop is high, you may have a cavitation, flashing, or choked flow situation

That complicates matter still further because now you&#;ll need to know a lot more about the process fluid itself. If you are faced with cavitation or flashing you may need to know the vapor pressure and critical pressure of the fluid. This information may be readily available or not if the fluid is a mix of products. Choked flow conditions are usually accompanied with noise problems and will also require additional fluid data to perform the calculations. Realize too that the selection of the valve internals will have a big impact on the flow rates, response, etc.  (You&#;ll be looking at anti-cav trim, diffusers, etc.)

Armed with all of that information (and it is a lot of information) you can finally start sizing/selecting the valve

Usually the vendor&#;s program is a good place to start, but some programs are much better than others because some have more process data &#;built in&#; and have the advanced calculations required to handle cavitation, flashing, choked flow, and noise calculations. Others are very simplistic and may not handle the more advanced conditions. Theoretically you could use any vendor&#;s program to do any valve but obviously the vendor program will typically have only its valve data built in so if you use a different program you&#;ll have to enter that data (if you can find it!)  One caution about this &#; some vendors have different valve constants which can be difficult to convert.  

The procedure for finally choosing the valve (roughly)

Run a down and dirty calc to just see what you have. What is the required Cv at min and max flows? Do I have cavitation/flashing/choking issues?

• Run a down and dirty calc to just see what you have. What is the required Cv at min and max flows?  Do I have cavitation/flashing/choking issues? 
• If there is cavitation/flashing/choking then things get a lot more complicated so I&#;ll save that for another lesson.
• Assuming no cavitation/flashing/choking then you can take the result and start to select a particular valve. The selection process includes:

1. Pick an acceptable valve body type. (Reciprocating control valves with a digital positioner and a good guide design will provide the tightest control. However other body styles might be acceptable depending on the requirements and budget.)
2. Pick the right valve characteristic to provide an overall linear response.
3. Now look at the offering of that valve and trim style and pick a valve with the proper range of CVs. Usually you want some room above the max flow and you want to make sure you are able to control at the minimum flow and not be bumping off the seat. Note that you may have to go to a different valve body (or even manufacturer) to meet your desired characteristic and Cv. 
4. Make sure the valve body/seals are compatible with your process fluid and the temperature.

Hope this helped. It was probably a bit more than you were wanting but control valve selection and sizing is a lot more complicated than most realize.

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Greg McMillan&#;s Answer

Hunter did a great job of providing detailed concise advice. My offering here is to help avoid the common problems from an inappropriate focus on maximizing valve capacity, minimizing valve pressure drop, minimizing valve leakage and minimizing valve cost. All these things have resulted in &#;on-off valves&#; posing as &#;throttling valves&#; creating problems of poor actuator and positioner sensitivity, excessive backlash and stiction, unsuspected nonlinearity, poor rangeability, and smart positioners giving dumb diagnostics.

While certain applications, such as pH control, are particularly sensitive to these valve problems, nearly all loops will suffer from backlash and stiction exceeding 5% (quite common with many &#;on-off valves&#;) causing limit cycles that can spread through the process. These &#;on-off valves&#; are quite attractive because of the high capacity and low pressure drop, leakage and cost. To address leakage requirements, a separate tight shutoff valve should be used in series with a good throttling valve and coordinated to open and close to enable a good throttling valve to smoothly do its job.

Unfortunately there is nothing on a valve specification sheet that requires the valve have a reasonably precise and timely response to signals and not create oscillations from a loop simply being in automatic making us extremely vulnerable to common misconceptions. The most threatening one that comes to mind in selection and sizing is that rangeability is determined by how well a minimum Cv matches the theoretical characteristic. In reality, the minimum Cv cannot be less than the backlash and stiction near the seat. Most valve suppliers will not provide backlash and stiction for positions less than 40% because of the great increase from the sliding stem valve plug riding the seat or the rotary disk or ball rubbing the seal.  Also, tests by the supplier are for loose packing. Many think piston actuators are better than diaphragm actuators.

Maybe the physical size and cost is less and the capability for thrust and torque higher, but the sensitivity is an order of magnitude less and vulnerability to actuator seal problems much greater. Higher pressure diaphragm actuators are now available enabling use on larger valves and pressure drops. One more major misconception is that boosters should be used instead of positioners on fast loops. This is downright dangerous due to positive feedback between flexure of diaphragm slightly changing actuator pressure and extremely high booster outlet port sensitivity. To reduce response time, the booster should be put on the positioner output with a bypass valve opened just enough to stop high frequency oscillations by allowing the positioner to see the much greater actuator and booster volume.

The following excerpt from the Control Talk blog Sizing up valve sizing opportunities provides some more detailed warnings:

We are pretty diligent about making sure the valve can supply the maximum flow. In fact, we can become so diligent we choose a valve size much greater than needed thinking bigger is better in case we ever need more. What we often do not realize is that the process engineer has already built in a factor to make sure there is more than enough flow in the given maximum (e.g., 25% more than needed). Since valve size and valve leakage are prominent requirements on the specification sheet if the materials of construction requirements are clear, we are setup for a bad scenario of buying a larger valve with higher friction.

The valve supplier is happy to sell a larger valve and the piping designer is happier that not much or any of a pipe reducer is needed for valve installation and the pump size may be smaller. The process is not happy. The operators are not happy looking at trend charts unless the trend chart time and process variable scales are so large the limit cycle looks like noise. Eventually everyone will be unhappy.

The limit cycle amplitude is large because of greater friction near the seat and the higher valve gain. The amplitude in flow units is the percent resolution (e.g., % stick-slip) multiplied by the valve gain (e.g., delta pph per delta % signal). You get a double whammy from a larger resolution limit and a larger valve gain. If you further decide to reduce the pressure drop allocated to the valve as a fraction of total system pressure drop to less than 0.25, a linear characteristic becomes quick opening greatly increasing the valve gain near the closed position. For a fraction much less than 0.25 and an equal percentage trim you may be literally and figuratively bottoming out for the given R factor that sets the rangeability for the inherent flow characteristic (e.g., R=50).

What can you do to lead the way and become the &#;go to&#; resource for intelligent valve sizing?

You need to compute the installed flow characteristic for various valve and trim sizes as discussed in the Jan Control Talk post Why and how to establish installed valve flow characteristics. You should take advantage of supplier software and your company&#;s mechanical engineer&#;s knowledge of the piping system design and details.

You must choose the right inherent flow characteristic. If the pressure drop available to the control valve is relatively constant, then linear trim is best because the installed flow characteristic is then the inherent flow characteristic. The valve pressure drop can be relatively constant due to a variety of reasons most notably pressure control loops or changes in pressure in the rest of the piping system being negligible (fictional losses in system piping negligible). For more on this see the 5/06/ Control Talk blog Best Control Valve Flow Characteristic Tips.

On the installed flow characteristic you need to make sure the valve gain in percent (% flow per % signal) from minimum to maximum flow does not change by more than a factor of 4 (e.g., 0.5 to 2.0) with the minimum gain greater than 0.25 and the maximum gain less than 4. For sliding stem valves, this valve gain requirement corresponds to minimum and maximum valve positions of 10% and 90%. For many rotary valves, this requirement corresponds to minimum and maximum disk or ball rotations of 20 degrees and 50 degrees.

Furthermore, the limit cycle amplitude being the resolution in percent multiplied by the valve gain in flow units (e.g., pph per %) and by the process gain in engineering units (e.g., pH per pph) must be less than the allowable process variability (e.g., pH). The amplitude and conditions for a limit cycle from backlash is a bit more complicated but still computable. For sliding stem valves, you have more flexibility in that you may be able to change out trim sizes as the process requirements change. Plus, sliding stem valves generally have a much better resolution if you have a sensitive diaphragm actuator with plenty of thrust or torque and a smart positioner.

The books Tuning and Control Loop Performance Fourth Edition and Essentials of Modern Measurements and Final Elements have simple equations to compute the installed flow characteristic and the minimum possible Cv for controllability based on the theoretical inherent flow characteristic, valve drop to total system drop pressure ratio and the resolution limit.

Here is some guidance from &#;Chapter 4 - Best Control Valves and Variable Frequency Drives&#; of Process/Industrial Instruments and Controls Handbook Sixth Edition that Hunter and I just finished with the contributions of 50 experts in our profession to address nearly all aspects of achieving the best automation project performance.

Use of ISA Standard for Valve Response Testing

The effect of resolution limits from stiction and dead band from backlash are most noticeable for changes in controller output less than 0.4% and the effect of rate limiting is greatest for changes greater than 40%. For PID output changes of 2%, a poor valve or VFD design and setup are not very noticeable. An increase in PID gain resulting in changes in PID output greater than 0.4% can reduce oscillations from poor positioner design and dead band.

The requirements in terms of 86% response time and travel gain (change in valve position divided by change in signal) should be specified for small, medium and large signal changes. In general, the travel gain requirement is relaxed for small signal changes due to effect of backlash and stiction, and the 86% response time requirement is relaxed for large signal changes due to the effect of rate limiting. The measurement of actual valve travel is problematic for on-off valves posing as throttling valves because the shaft movement is not disk or ball movement. The resulting difference between shaft position and actual ball or disk position has been observed in several applications to be as large as 8 percent.

Best Practices

Use sizing software with physical properties for worst case operating conditions. The minimum valve position must be greater than backlash and deadband. Based on a relatively good installed flow characteristic valve gains (valve drop to system pressure drop ratio greater than 0.25), there are minimum and maximum positions during sizing to minimize nonlinearity to less than 4:1. For sliding stem valves, the minimum and maximum valve positions are typically 10% and 90%, respectively. For many rotary valves, the minimum and maximum disk or ball rotations are typically 20 degrees and 50 degrees, respectively. The range between minimum and maximum positions or rotations can be extended by signal characterization to linearize the installed flow characteristic.

1. Include effect of piping reducer factor on effective flow coefficient
2. Select valve location and type to eliminate or reduce damage from flashing
3. Preferably use a sliding stem valve (size permitting) to minimize backlash and stiction unless crevices and trim causes concerns about erosion, plugging, sanitation, or accumulation of solids particularly monomers that could polymerize and for single port valves install &#;flow to open&#; to eliminate bathtub stopper swirling effect
4. If a rotary valve is used, select valve with splined shaft to stem connection, integral cast of stem with ball or disk, and minimal seal friction to minimize backlash and stiction
5. Use Teflon and for higher temperature ranges use Ultra Low Friction (ULF) packing
6. Compute the installed valve flow characteristic for worst case operating conditions
7. Size actuator to deliver more than 150% of the maximum torque or thrust required
8. Select actuator and positioner with threshold sensitivities of 0.1% or better
9. Ensure total valve assembly dead band is less than 0.4% over the entire throttle range
10. Ensure total valve assembly resolution is better than 0.2% over the entire throttle range
11. Choose inherent flow characteristic and valve to system pressure drop ratio that does not cause the product of valve and process gain divided by process time constant to change more than 4:1 over entire process operating point range and flow range
12. Tune positioner aggressively for application without integral action with readback that indicates actual plug, disk or ball travel instead of just actuator shaft movement
13. Use volume boosters on positioner output with booster bypass valve opened enough to assure stability to reduce valve 86% response time for large signal changes
14. Use small (0.2%) as well as large step changes (20%) to test valve 86% response time
15. Use ISA standard and technical report relaxing expectations on travel gain and 86% response time for small and large signal changes, respectively

For much more on valve response see the Control feature article How to specify valves and positioners that do not compromise control.

The best book I have for understanding the many details of valve design is Control Valves for the Chemical Process Industries written by Bill Fitzgerald and published by McGraw-Hill. The book that is specifically focused on this Q&A topic is Control Valve Selection and Sizing written by Les Driskell and published by ISA.  Most of my books in my office are old like me. Sometimes newer versions do not exist or are not as good.

Additional Mentor Program Resources

See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in , the Control magazine Engineer of the Year award for the process industry in , was inducted into the Control magazine Process Automation Hall of Fame in , was honored by InTech magazine in as one of the most influential innovators in automation, and received the ISA Life Achievement Award in . Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since . Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in .

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