Control Valve

Thumb Rules and Tips for control valves selection and sizing

CONTROL VALVE SELECTION
  1. Control valves in process industries are used to regulate various fluids and to control important process parameters like flow, level, temperature, pressure, and so on.
  2. The tips and rules of thumb that engineers used in selecting and sizing control valves are fuzzy, and applying alternative rules may lead to conflicting answers.
  3. Rules of thumb must be used only for reality check submittals from vendors and for preliminary valve selection.
  4. The control valves are cost expensive and are high-tech products.
  5. The final selection and design of the valve should be left to experts such as the valve manufacturing engineers because they can perform a complete system analysis that also includes some considerations for the valve characteristics such as range, accuracy, dead band, response time, noise, cavitation, and so on.
  6. But some engineers who have basic knowledge can ask intelligent questions to ensure that appropriate calculations can be performed by the experts, and challenge vendors to adhere to reasonable performance requirements.

Thumb Rules for control valve selection and sizing:

  • To specify the pressure drops and flow rates of the fluid in the particular line at higher and lower control ranges. A common mistake made by design engineers is they specify a control valve of a wider range than required. When these valves operate nearly fully open for 80 % of valve opening this results in insufficient control.
  • The required flow coefficients for non-compressible fluids like liquids at high and Low Cv conditions can be calculated using a simplified formula.
  • The valve capacities by the company may be published in metric units.
  • For steam application, if Po < 0.4Pi (absolute pressure) then the critical flow or choked flow occurs and the Cv depends only on the flow rate and inlet pressure.
  • Be aware that the harsh or severe calculation used by valve manufacturers includes additional factors that are specific to the selected valve make and model.  For fluids like liquids, the key factor will account for the pressure recovery factor and the limiting pressure drop due to choked flow and the pressure drop in the geometry of the inlet and outlet side of the valve and valve fittings.
  • For compressible fluids, the factors may also account for the specific heat ratio of the gas and the ratio of critical pressure drop which is inherent in the valve design. This correction changes the flow coefficients that are calculated and might have an impact on selecting the specific valve for the particular application.
  • Rangeability for the control valve is defined as the ratio between the maximum flow rates to the minimum flow rates of the fluid governed by the controller.  The valve is selected depending upon the type of body which is consistent for the particular application.
  • The valve is selected depending upon the type of body type which is consistent with the service requirements.
    • For 6 inches and smaller, select a segmented ball valve if pressure, or differential pressure, temperature, required flow characteristic, cavitation, and noise are all acceptable.
    • For 8” and larger select the globe valve. Or high-performance butterfly valve because it is cheaper and less weight.
  • Valve selection: The control valve sizing and selection are done by the following criteria:

For the normal or low condition, The valve must be chosen with wide-open Cv = 1.4 x Cv low.

  1. If Cv high/Cv low < 1.4, then choose a valve with wide open Cv = 1.5* Cv low.
  2. If Cv high/Cv low > 1.4, then choose a valve with wide open Cv = 1.1 *Cv high.

Some Tips for control valves selection and sizing:

  1. The valve must be selected such that it must allow maximum flow when the valve gets opened for 90 %.
  2.   The valve must be selected such that it must allow minimum flow when the valve gets opened only for 10 %.
  3. The valve must be selected such that it must allow normal flow when the valve gets opened at 60-70 %.
  4. In general, the rule of thumb says that the valve must be designed and selected such that there should be only a 10 -15% of total pressure drop.
  5. The control valves must absorb up to 30 % of the line or system pressure drop at a maximum flow rate.
  6. The control pressure drop must be set to 50 to 60 % of the piping systems.
  7. The size of the control valves must be greater than 50 % of the line size.
  8. The valves envisaged for turn-off service must be in line size.
  9. If the diameter of the line is 1 inch, then the valve size must be equal to the line.
  10. If the diameter of the line is larger than 1 inch, then the valve size should not be less than 1 inch.
  11. The valve size or the valve diameter must not be larger than the line size.
  12. The valves are recommended to have a flanged connection as per the line specification with an exception of the valves with a normal diameter smaller than or equal to 1.5 inches which have a minimum ratio of ANSI 300.
  13. The valve body and trim materials for the control valves are selected to match the impeller and pump casing for the case of corrosive fluids.
  14. For the clean liquid, keep the line velocity of 10 Ft/s. and the velocity must be low as possible for abrasive fluids.
  15. In the case of high-pressure drop situations, the fluid velocity must be maintained below 0.3 mach on the pipe inlet and outlet as well as on the valve body.
  16. It is important to sense the pressure where it is needed to control. Because most of the control valves and pressure regulators may not function accurately and may sense in another point where there is no control required.
  17. The positioner must be specified on the valve, otherwise, the transducer used in the control loop will rob the thrust available and the valve gets leakage and is supposed to turn off.
  18. In the case of Cavitating fluids, allow the straight run of downstream pipe after the valve when the control valve has cavitation trim in it if there is a pipe Tee or elbow.
  19. For the given flow rate in a pumped circuit of about 15 PSI, the pressure drop must be equal to 33% of the dynamic loss in the system.
  20.  In the suction or the discharge line of a centrifugal compressor, the pressure drop in the control valve should be 50% of the dynamic losses or 5% of absolute suction pressure.
  21. The pressure drop allocated to the valve must be 50% of the system’s dynamic losses or 10% of lower terminal vessel pressure. Where the static pressure moves the fluid from one vessel to another vessel.
  22. For the steam lines, the pressure drop in the valves must be 10% of the design absolute pressure or 5 PSI.
  23. The gain on the control valve must be greater than 0.5.
  24. The flow in the control valve is much easier to control at a range of 10% to 80%.
  25. Generally, the control valves used in the lines must be one size less than the line size.
  26. The control valves will turn off the high-pressure fluid flow by rotating the ball to 90 Degrees.
  27. The efficiency is high by using the valve actuators having lower torque thus resulting in low cost for the end-user.
  28. These valves provide higher speed and have a low recovery factor.
  29. These valves are less expensive.
  30. The ball valve is less expensive than a globe valve.
  31. Ball valves can shut off or control the flow of fluid.
  32. Ball valves reduce the required number of components used in the system.
  33. Bi-directional valve and symmetric trim: these valve designs allow control flow in both directions and the ball valve is symmetric so the ball valves have no limitations for plant installation.

Some calculation tips are shown below:

  1. Collect the data of physical property for the gas and calculate the specific heat ratio factor.
  2. Calculate the sizing ratio and adjust the flow factor.
  3. Calculate the flow coefficient and expansion factor.
  4. While selecting a valve for liquid or gas application the Cv is used by the valve manufacturer.
  5. The equations for compressible fluids use different units than the equations for liquids, and the meaning of Cv is the same.

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