General Information Required
1. Fluid controlled:
   • Chilled water, hot water, or steam.
2. Temperature limitations:
   • Fluid, maximum, and minimum.
   • Ambient for actuator.
3. Pressure:
   • Static.
   • Close-off - Fully closed.
   • Differential - Pressure drop across the valve in the fully open position.
4. End fitting:
   • Union end.
   • Globe screwed.
   • Flared.
   • Flanged.
   • Flangeless.
5. For return to a known position (i.e., normally open or closed): Specify spring return actuator.
6. Dimensional data.
7. Cv (flow coefficient) requirement is calculated from flow rate and differential pressure. Refer t0 formulas and tables.

Pressure Drops for Water
Refer to specific valve data in this catalog for max. allowable pressure drops and close-off ratings.

A. Two-Position Valves
Two-position valves are normally selected “line size” to keep pressure drop at a minimum. The differential pressure in psi should be no more than 10% of available inlet pressure in psia.

B. Proportional Two-Way Valves
These are usually selected to take a pressure drop equal to at least 50% of the “available pressure” (i.e., the pump pressure differential available between supply and return mains with design flow at the valve location). As “available pressure” is difficult to calculate, 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 5 psi (34 kPa).

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

Conventional Heating Systems

Secondary Circuits with Small Booster Pumps
50% of Available Pressure Difference (Equal to drop through load, or 50% of booster pump head)

C. Proportional Three-Way Valves
Recommended Pressure Drop - Bypass Application: 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.

Cavitation Limitations on Valve Pressure Drop
A valve selected with too high a pressure drop can cause erosion of discs and wire drawing of the seat. In addition, cavitation can cause noise, damage to the valve trim (and possibly the body) and choke the flow through the valve.

Do not exceed the maximum differential pressure (pressure drop) for the valve selected.

The following formula can be used on higher temperature water systems, where cavitation could be a problem, to estimate the maximum allowable pressure drop across the valve: Pm = 0.5 (P1 - Pv)

Pm = Maximum allowable pressure drop
P1 = Absolute inlet pressure (psia)
Pv = Absolute vapor pressure (refer to Vapor
Pressure of Water Table or Steam Table)

Note: Add 14.7 psi to gauge supply pressure to obtain absolute pressure value.

For example, if a valve is controlling 200ºF water at an inlet pressure of 18 psig, the maximum pressure drop allowable would be:

Pm = 0.5 [(18 + 14.7) - 11.53] = 10.6 psi
(Vapor pressure of 200ºF water is 11.53 psi)

If the pressure drop for this valve is less than 10.6 psi, cavitation should not be a problem.

Systems where cavitation is shown to be a problem can sometimes be redesigned to provide lower inlet velocities. Valves having harder seat materials should be furnished if inlet velocities cannot be lowered.

C_v (Flow Coefficient) Determination
The Water Valve Sizing Table or Slide Rule (refer to the following page) is based on the following formula:

Where: C_v = Coefficient of flow
C_v is defined as the flow in GPM with ΔP = 1 psi
GPM = US gallons per minute (60ºF, 15.6ºC)
ΔP = Differential pressure in psi (pressure drop)

Other variations of this formula are:
     

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

Flow coefficients (Cv’s) for valve bodies are given on valve specification pages of this catalog.

Metric (SI) Units
Kvs is defined as the flow in m3/h with ΔP = 100 kPa (1.0 Bar) with the valve completely open.

Flow is calculated using the following formula:
     

ΔP = Differential pressure (pressure drop) in Bar (1 Bar = 100 kPa)
m3/h = Cubic meters/hour (15.6 °C)

Pressure drop is calculated using the following form of the above formula:
     

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