CEMP-E
TI 810-11
30 November 1998
b. The designer will use equations that correct the liquid Kv (Cv) for the compressibility of the steam.
The designer will take into account in his calculations that the limiting factor of steam flow through a control
valve is the flow that occurs at critical pressure drop, and will use an appropriate factor for calculating the
critical pressure drop. The designer will calculate the critical pressure drop across the valve by using
equation 2-5.
pcr = k m x pe
∆
(eq. 2-5)
Where:
∆pcr = critical pressure drop, kPa (psid)
km = valve pressure recovery coefficient, (use 0.45)
pe = Absolute pressure of the steam entering the valve, kPa (psia)
c. For steam control valve application using saturated steam at a pressure drop across the valve less
than the critical pressure drop, the Kv (Cv) will be calculated using equation 2-6.
Qs
(eq. 2-6 SI)
Kv=
0.161 ( pe + po ) ∆ p
Where:
3
Kv = flow coefficient, m /hr @ ∆p = 100 kPa (1 bar)
Qs = steam flow, kg/hr
0.161 = compressibility and conversion factor.
pe = absolute steam pressure entering the valve, kPa
po = absolute pressure entering steam coil, kPa
∆p = pressure drop through valve, kPa
Qs
(eq. 2-6 IP)
Cv =
2.11 ( pe + po ) ∆ p
Where:
Cv = flow coefficient, gpm @ ∆p = 1 psi
2-25
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