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Valve Sizing Information for Steam
176
© Copyright 2006 TAC. All Rights Reserved. F-27414-1
Cv (FLOW COEFFICIENT) DETERMINATION
The Steam Capacity Tables or Slide Rule (refer to this and
the following two pages) is based on the following formula:
Where: Cv = Coefficient of flow
Q = Lbs per hour of steam
= Differential pressure in psi (pressure drop)
P
2
= 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.
Note: K normally is 1.
Note: K normally is 1 (K = 1 for saturated steam).
These formulas can be used to calculate one of the quantities
if the others are know.
Flow coefficients (Cv’s) for specific valve bodies are given on
specification pages of this catalog.
Cv
QK
3 PP
2
×Δ
----------------------------=
PΔ
Q
3Cv P P
2
×Δ
K
------------------------------------=
PΔ
QK
3Cv
-----------
⎝⎠
⎛⎞
2
1
P
2
Δ
----------×=
P
2
QK
3Cv
-----------
⎝⎠
⎛⎞
2
1
PΔ
-------
×=
RECOMMENDED PRESSURE DROPS FOR STEAM
Refer to specific valve data in this catalog for maximum
allowable drops and close-off ratings.
A. Two Position Zone Valves and Direct Radiator Valves
Use a minimum of 10% of inlet pressure (psig).
B. Proportional Control Valves
Low pressure (15 psig or less): 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.
Valve
Sizing
Infor
matio
n for
Steam
PΔ