My library Help Advanced Book Search. Instrument Society of America, c Public Private login e. Open to the public ; These 3 locations in All: You also may like to try some of these bookshopswhich may or may not sell this item. Physical Description viii, p. From inside the book.
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Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants. This question comes from Hiten Dalal. Hiten has extensive experience in pipeline pressure and flow control. 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! 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. What is the maximum flow that the valve must pass? Note that the P1 and DP at low flow rates will usually be much higher than at full flow rates.
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. Understand the installed flow characteristic of the valve This can be another difficult task. If the valve response is non-linear, control becomes much more difficult.
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.
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 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.
Others are very simplistic and may not handle the more advanced conditions. 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? Run a down and dirty calc to just see what you have. The selection process includes: 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. Pick the right valve characteristic to provide an overall linear response. 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. 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. 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.
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. 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. 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 amplitude in flow units is the percent resolution e.
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. 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 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 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. 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. 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.
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. 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. 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. Most of my books in my office are old like me. Sometimes newer versions do not exist or are not as good.
Instrumentation Consulting Services, Ltd. About the Author Gregory K. Greg was also an affiliate professor for Washington University in Saint Louis. 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.
Control-valve selection and sizing
If you have an automation-related question for this column, write to liptakbela aol. Q: I am an engineer working in operation and maintenance of test facilities in the Indian Space Research Organization. I have a requirement for a new control valve to be installed in our facilities. I am selecting equal percentage characteristics for the control valve, but I need to specify the rangeability for the control valve. For this, I have calculated the ratio of required max Cv to min Cv 10 for the particular case , and I have selected the valve rangeability , which is higher than this ratio. Kindly correct me if my method is wrong. I would be highly obliged if you can guide me on this point.