Basic Documentation
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The Most Important Variables to Consider When Sizing a Valve:
1. What medium will the valve control? Water? Air? Steam? What
effects will specic gravity and viscosity have on the valve size?
2. What will the inlet pressure be under maximum load demand?
What is the inlet temperature?
3. What pressure drop (differential) will exist across the valve under
maximum load demand?
4. What maximum capacity should the valve handle?
5. What is the maximum pressure differential the valve top must
close against?
When these are known, a valve can be selected by formula
(Cv method) or water and steam capacities tables which can be
found in the Valves section, pages D-7 through D-10. The valve
size should not exceed the line size.
Recommended Pressure Drops for Valve Sizing — Steam
1. With gravity ow condensate removal and inlet pressure less
than 15 psi, use a pressure drop equal to the inlet gauge
pressure.
2. With vacuum return system up to 7" Hg vacuum and an inlet
pressure less than 2 psi, a pressure drop of 2 psi should be used.
With an inlet pressure of 2 to 15 psi, use a pressure drop equal to
the inlet gauge pressure.
3. With an inlet pressure greater than 15 psi, use a pressure drop
equal to 80% of system differential pressure. Example: Inlet
pressure is 20 psig (35 psia) and a gravity return at atmospheric
pressure 0 psig (14.7 psia), use a pressure drop of 16 psi.
4. When a coil size is selected on the basis that line pressure and
temperature is available in the coil of a heating and ventilating
application, a very minimum pressure drop is desired. In this
case, use the following: pressure drop:
Valve Reference
Initial Pressure Pressure Drop
15 psi 5 psi
50 psi 7.5 psi
100 psi 10 psi
Over 100 psi 10% of line pressure
(typically on/off applications)
Valve Sizing and Selection Example
Select a valve to control a chilled water coil that must have a ow
of 35 GPM with a valve differential pressure (∆P) of 5 psi.
Determine the valve Cv using the formula for liquids.
Cv = Q = 35 GPM = 15.6
Select a valve that is suitable for this application and has a Cv as
close as possible to the calculated value.
One choice is 277-03186: a 1-1/4" NC valve with a Cv of 16. Refer to
Flowrite Valves Reference section.
Valve Selection Criteria
1. Flow characteristic — Modied Equal Percentage which provides
good control for a water coil.
2. Body rating and material — Suitable for water.
3. Valve type and action — A single seat NC valve with an
adjustable spring range which can be sequenced with a NO valve
used for heating.
4. Valve actuator — Actuator close-off rating is higher than the
system differential pressure.
5. Valve line size — Its Cv is close to and slightly larger than the
calculated Cv (15.6).
6. For Ball Valves — use the same selection criteria.
1
5 psi
S
P