# Valve Sizing for Gas and Steam

Written by Jerry Ratzlaff on . Posted in Valve

## Valve Sizing for Gas and Steam

### 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