Sizing Valves for Water & Steam

 

General Information Required

  • Fluid Controlled:
    • Chilled water, hot water or steam
  • Temperature limitations:
    • Fluid, maximum and minimum
    • Ambient for actuator
  •  Pressure:
    •  Static
    • Close-off – fully closed position
    • Differential – Pressure drop across the valve in the fully open position
  • End fitting:
    • Union end
    • Globe screwed
    •  Flared
    •  Flanged
    • Flangeless
  •  
  • For return to a known position (e.g., normally open or normally closed): Specify a spring return actuator.
  • Cv (flow coefficient) requirement is calculated from flow rate and differential pressure. See formulas and tables.

Recommended Pressure Drops For Water

Refer to specific valve data in manufacturer’s catalog for maximum allowable pressure drops and close-off ratings.

Two-Position Valves

Normally selected “line size” to keep pressure drop at a minimum. If desirable to reduce valve below line size, then 10% of “available pressure” normally used to select valve.

Proportional Two-Way valves

Usually selected to take a pressure drop equal to at least 50% of the “available pressure” (that is, the pump pressure differential available between supply and return mains with design flow at the valve location). As “available pressure” is often difficult to calculate, normal procedure will be to select the valve using a pressure drop at least equal to the drop in the coil or other load being controlled (except where small booster pumps are used) but never less than 4 psi (34 kPa).

When design temperature drop is less than 60°F (33oC) for conventional heating systems, higher pressure drops across the valve are needed for good control results.

  • Conventional Heating Systems
    • Design Temp Recommended Multiplier on Drop °F (°C) Pressure Drop Load Drop (% of Available Pressure)*
    • 60 (33) or more 50% 1 x load drop 40 (22) 66% 2 x load drop 20 (11) 75% 3 x load drop
    • * Recommended minimum pressure drop – 5 psi (34 kPa) Secondary Circuits with Small Booster Pumps 50% of available pressure difference (equal to drop through load or 50% of booster pump head)

Proportional Three-Way Valves

Recommended Pressure Drop – Bypass applications: 50% of “available pressure,” or equal to pressure drop through the load at full flow.

Three-way valves in the return used to control heat output by throttling water flow to the load (bypass applications) are controlling output in the same manner as throttling two-way valves, and must be selected using the same high pressure drops if good control results are to be obtained.

Recommended Pressure Drop – Constant flow applications: 20% of “available pressure” or equal to 1/4 of the pressure drop through the load at full flow.

Three-way valves used with individual pumps to control heat output by varying water temperature to the load (constant flow applications) are controlling output by mixing two water sources at different temperatures, and do not require high pressure drops for good control results.

In most cases the required Cv falls between two valve sizes. If the pressure drop of the smaller is acceptable for the application, select the smaller valve.

 

Cv (Flow Coefficient) Determination

The Water Valve Sizing Table is based on the following formula:

Where: Cv = Coefficient of flow gpm = U.S. gallons per minute (60°F/15.6°C) P = Differential pressure in psi (pressure drop)

 

Other forms of this formula are:

 

 

and

 

 

These formulas can be used to calculate one of the three values if the other two are known.

Flow coefficients ( Cv) for valve bodies are generally found on the manufacturer’s valve body data sheets.

 

Recommended Pressure Drops For Steam

Refer to specific valve data in this catalog for maximum allowable drops and close-off ratings.

Two-position Zone Valves and Direct Radiator Valves

Use a minimum of 10% of inlet pressure (psig).

Proportional Control Valves

Low pressure (15 psig or less) P of 80% of gauge inlet pressure.

When Cv required is between two valve sizes and closer to the smaller valve size, re-size for Cv using 42% of the absolute inlet pressure as pressure drop. Use the valve that is larger than the calculated Cv.
For steam pressures greater than 15 psig: 42% of the absolute inlet pressure.

When Cv required is between two valve sizes, select the larger size.

NOTE: Do not size steam valves on higher system pressures using a pressure drop greater than 42% of the absolute inlet pressure.

Cv (Flow Coefficient) Determination

The Steam Capacity Tables are based on the following formula.

 

 

Where: Cv = Coefficient of flow Q = lbs per hour of steam P = Differential pressure in psi (pressure drop) P2 = Outlet pressure in psia (absolute) psig + 14.7 = psia (absolute) K = 1 + (0.0007 x °F super-heat)

NOTE: K normally is 1 (K = 1 for saturated steam)

Other forms of the formula are:

 

 

 

 

 

 

NOTE: K normally is 1 (K = 1 for saturated steam)

These formulas can be used to calculate one of the values if the others are known.