Valve Sizing for Gas and Steam

Written by Jerry Ratzlaff on . Posted in Valve

Gas Flow Rate Formula

\(Q_g = 59.64 C_{vl} p_i \sqrt {\frac {\Delta p} {p_i} }   \sqrt {\frac {520} {SG T_a} }   \)

Where:

\(Q_g\) = gas flow rate, SCFH (Use only at very low pressure drop \(\left( \frac {\Delta p} {p_i} \right)\) ratios of 0.02 or less)

\(C_{vl}\) = liquid sizing flow coefficient

\(p_i\) = valve inlet pressure, psia

\(\Delta p\) = pressure differential, pressure drop across valve, psi

\(SG\) = gas specific gravity (air = 1.0)

\(T_a\) = absolute temperature absolute temperature of gas at inlet, degrees Rankine

Critical Flow Rate Formula

\(Q_{cr} = C_{vg} p_i \sqrt { \frac {520} {SG T_a} }   \)

Where:

\(Q_{cr}\) = critical flow rate, SCFH (Use only to determine critical flow capacity at a given inlet pressure)

\(C_{vg}\) = gas sizing flow coefficient

\(p_i\) = body inlet pressure, psia

\(SG\) = specific gravity of fluid (water at 60°F = 1.0000)

\(T_a\) = absolute temperature of gas at inlet, °R

Universal Gas Sizing Formula

\(Q_g = \sqrt { \frac {520} {SG T_a} }     C_{vg} p_i sin \left[ \left( { \frac {59.64} { c_i} } \right)     \left( \sqrt { \frac {\Delta p} { p_i} } \right) \rightarrow \right]           rad \)

\(Q_g = \sqrt { \frac {520} {SG T_a} }     C_{vg} p_i sin \left[ \left( { \frac {3417} { c_i} } \right)     \left( \sqrt { \frac {\Delta p} { p_i} } \right) \rightarrow \right]           deg \)

Where:

\(Q_g\) = gas flow rate, SCFH

\(SG\) = specific gravity of fluid (water at 60°F = 1.0000)

\(T_a\) = absolute temperature of gas at inlet, °R

\(C_{vg}\) = gas sizing flow coefficient

\(p_i\) = body inlet pressure, psia

\(C_i\) = \(\frac {C_{vg}} {C_{vl}}\)

\(\Delta p\) = pressure differential, psi

Steam or Vapor Flow Rate Formula

\(Q_{sv} = 1.06 \sqrt { \rho p_i }     C_{vg}   sin \left[ \left( { \frac {3417} { c_i} } \right)     \left( \sqrt { \frac {\Delta p} { p_i} } \right) \rightarrow \right]   deg \)

Where:

\(Q_{sv}\) = steam or vapor flow rate, lb/hr (use to predict flow for perfect or non-perfect gas sizing, for any vapor including steam, at any service condition when fluid density is known)

\(\rho\) = density of steam or vapor at inlet, lb/cu ft

\(p_i\) = body inlet pressure, psia

\(C_{vg}\) = gas sizing flow coefficient

\(C_i\) = \(\frac {C_{vg}} {C_{vl}}\)

\(\Delta p\) = pressure differential, psi

Steam or Vapor Flow Rate 1000 psig or Less Formula Formula

\(Q_{sv} = \left[ \left( \frac { C_{vs} p_i } {1+0.00065 T_s } \right) \right]   sin \left[ \left( { \frac {3417} { c_i} } \right)     \left( \sqrt { \frac {\Delta p} { p_i} } \right) \rightarrow \right]   deg \)

Where:

\(Q_{sv}\) = steam or vapor flow rate, lb/hr (only to determine steam flow when inlet pressure is 1000 psig or less)

\(C_{vs}\) = steam sizing flow coefficient, \(\frac {C_{vg}} {20}\)

\(p_i\) = body inlet pressure, psia

\(T_s\) = degrees of superheat, °F

\(C_i\) = \(\frac {C_{vg}} {C_{vl}}\)

\(\Delta p\) = pressure differential, psi