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Control Valve Selection Sizing Valves for Compressible Fluids Following is a six-step procedure for the sizing of control valves for com- pressible flow using the ISA standard- ized procedure. Each of these steps is important and must be considered during any valve sizing procedure. Steps 3 and 4 concern the determina- tion of certain sizing factors that may or may not be required in the sizing equation depending on the service conditions of the sizing problem. If it is necessary for one or both of these siz- ing factors to be included in the sizing equation for a particular sizing prob- lem, refer to the appropriate factor de- termination section s , which is refer- enced and located in the following text.

If any of the above terms appear to be new or un- familiar, refer to the Abbreviations and Terminology table for a complete defi- nition.

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Determine the equation constant, N. N is a numerical constant con- tained in each of the flow equations to provide a means for using different systems of units. Values for these var- ious constants and their applicable units are given in the Equation Constants table. Which of the two constants to use depends upon the specified service conditions. N7 can be used only if the specific gravity, Gg, of the following gas has been specified along with the other required service conditions.

N9 can be used only if the molecular weight, M, of the gas has been specified. Which of the two constants to use depends upon the specified ser- vice conditions. Related sponsored items Feedback on our suggestions - Related sponsored items. What Is Solid? What Moves? Hard or Soft? Portuguese by Hutchinson, Amelia P.

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Control Valve Sizing

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How to Size a Control Valve for Liquid Flow

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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.

Plug Vibrational Tendencies of Top Guided Throttling Control Valves

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.

Handbook of Control Valves - Google книги

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.


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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.

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. Use sizing software with physical properties for worst case operating conditions. The minimum valve position must be greater than backlash and deadband.