Valve Sizing And Actuator Selection

There are a couple of engineering design questions that need to be answered in order to help maintain design intent of the system. In this guide, you will be provided with the 18 steps on how to properly size and select valves, actuators and assemblies.

A Pressure Independent Control Valve

1. Determine valve type. Knowing this upfront will enable us to make adjustments in our sizing and selection.

2. Determine medium being controlled.

3. Determine flow rate of equipment to be controlled. (This should be provided, or on the coil schedule.)

4. Determine specified pressure drop. For correct valve authority, the pressure drop across the valve should be equal to the total pressure drop across the controlled branch, including the valve.

Valve Sizing – Equation

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The formula for determining Cv for water valves
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When working with water, this may be simplified
Calculating Cv

S = Specific Gravity of media
CV = Flow Coefficient
Q = Volumetric Flow (gpm) with valve fully open
ΔP = Differential Pressure (psi) with valve fully open

Calculating GPM Flow

GPM requirement may be determined if the BTU/hr requirement and water desired ΔT is known.

GPM may be determined more precisely if % glycol is known.
Calculating GPM

GPM = flow in gallons/minute
q = Heat added or removed in BTU/hr
ΔT = Water temperature rise or drop across the coil
S or SG = Specific Gravity of media
Cp = Specific heat of media

Determining a Rated Flow Rate

Common differential temperatures for chilled water equipment is 12ºF, and 20ºF for hot water systems. This should be double checked with the design engineer on what the intended equipment differentials are for the coils as well as the major equipment such as the boilers and chillers in the system.

If glycol is being used in the system, some modifications to the upper equation can be done to accommodate the difference in specific gravity and specific heat of a mixed fluid compared to standard water.

Specific Gravity of Glycol Solutions

To compensate for a water/glycol mixture, the previous equation for GPM requires two additional pieces of information. The first thing you will need is the specific gravity of the water/glycol mixture at the mix percentages. That can be obtained from Specific Gravity of Glycol Solutions chart. In North American hydronic systems, 50/50% water/glycol is typical. Most manufactures have rated their equipment to similar mixture limits.

Specific Gravity- SG –Ethylene Glycol Solution (% by volume)
Temperature (ºF)253040506065100
  • 1)Below freezing point
  • 2)Above boiling point

Specific Heat of Glycol Solutions

You will also need the specific heat of the water/glycol mixture at the design percentages to get the correct rated flow rate. That information is available on the Specific Heat of Glycol Solutions chart below.

Specific Heat Capacity – cp – (Btu/lb.ºF)Ethylene Glycol Solution (% by volume)
Temperature (ºF)253040506065100
  • 1)Below freezing point
  • 2)Above boiling point
  • 1 Btu/(lbmºF) = 4,186.8 J/(kgºK) = 1 kcal/(kgºC)

5. Calculate Cv using the equation for water valves.

6. Determine number of ports (2-way or 3-way).

7. Determine required ANSI Pressure Class rating(125 or 250).

8. Determine required Flow Characteristic; typically Equal Percentage for water applications and Linear for steam applications.

9. Determine Trim Requirements:

  • Bronze / Brass (usually for low ΔP water applications)
  • Stainless Steel (usually for higher ΔP water applications and steam applications)

10. Determine type of packing, if applicable(Standard or High Temperature)

11. Determine type of mechanical connection to the piping system. (NPT-FxF, NPT – FxUM, Flanged, Sweat, etc.)

12. For the actuator, determine Normal Position and Fail safe requirements

  • NO – Normally Open
  • NC – Normally Closed
  • SR – Spring Return or Fail-Safe
  • NSR – Non-Spring Return or Fail-in-Place

13. Determine type of actuator and control signal(2 position, floating, 0-10 vdc, etc.).

14. Determine if Manual Override is required.

15. Based on all of these inputs, select an orderable valve assembly.

16. Check close off pressure(specified, or at least system differential pressure).

17. Calculate actual pressure drop based on valve selected using CV formula

18. Check for Percent Authority, where: Percent Authority should be between 25% and 50%.