CEMP-E
TI 810-11
30 November 1998
f. For superheated steam applications with the pressure drop across the valve at or greater than the
valve's critical pressure, the Kv (Cv) will be calculated using equation 2-9.
Qs (1 + 0.00131 ( T sh ) )
Kv=
(eq. 2-9 SI)
0.133 pe
Where:
3
Kv = flow coefficient, m /hr @ ∆p = 100 kPa (1 bar)
Qs = steam flow, kg/hr
(1 + 0.00131 (Tsh)) = superheat factor
Tsh = superheat temperature, degrees C
0.133 = compressibility and conversion factor.
pe = absolute pressure of the steam entering the valve, kPa
Q s (1 + 0.0007 ( T sh ) )
(eq. 2-9 IP)
Cv =
1.74 pe
Where:
Cv = flow coefficient, gpm @ ∆p = 1 psid
Qs = steam flow, lb/hr
(1 + 0.0007 (Tsh)) = superheat factor
Tsh = superheat temperature, degrees F
1.74 = compressibility and conversion factor.
pe = absolute pressure of the steam entering the valve, psia
g. An example of the calculation required for the Kv (Cv) of a valve designed to handle saturated steam
when the pressure drop will be less than critical is as follows: An air coil requires 113.4 kg/hr (250 lb/hr) of
steam entering the coil at 13.8 kPa (2 psig). The steam is generated by a local boiler with the boiler's
operating pressure switches set at 34.5 kPa (5 psig) "on" and 55.2 kPa (8 psig) "off". Assuming 34.5 kPa (5
psig) at peak load at the valve inlet and 13.8 kPa (2 psig) at the valve outlet, the Kv = 9.8 (Cv = 11.34) as a
result of using equation 2-6. In this example, critical pressure drop is not a limiting factor, because the coil
pressure at critical pressure drop would be (101.4 + 34.5) x .45 = 61.2 kPa ((14.7 + 5) x 0.45 = 8.87 psia),
which is not possible unless the system is designed to operate at a high vacuum. A check of
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