Page 194 - Pipeline Rules of Thumb Handbook

Control Valves 181

The compressibility factor (Z) Valve outlet velocity

The compressibility factor (Z) can be found from Figure 4 Sonic velocity at the valve’s outlet will cause shock waves
after the reduced pressure (P R ) and reduced temperature (T R ) which produce vibration and noise problems. Therefore, you
have been calculated from these equations: should make sure the outlet velocity does not reach sonic
velocity for gas and steam service. The actual valve outlet
Inlet Pressure (Absolute ) diameter must be larger than the calculated diameter from
P R =
Critical Pressure (Absolute ) the following equations:
Inlet Temperature (Absolute ) QG T for gas
G
T R = d = N 10
Critical Temperature (Absolute ) P 2
(°R = °F + 460, °K = °C + 273) W(1 + N T SH )
12
d = N 11 for steam
P 2
Choked flow Low-noise trim

Gas flow through valves can choke when the gas velocity When the pressure-drop ratio (X) is near or exceeds the
reaches the velocity of sound. The pressure drop ratio at this minimum value for choked flow, check the valve for excessive
point has the value: noise using the procedure under “Aerodynamic Noise Pre-
diction.” If noise is excessive, use anti-noise trim; recalculate
X = F X TP the flow-capacity coefficient (C V ) using factors from “With
K
Trim” in the “Valve Reference Data”—Table 4. For proce-
Substitute X T for X TP with line-size valves. If this valve is to dures on noise level prediction, refer to the manufacturer’s
be used under choked conditions, Y is fixed at 0.667 regard- literature.
less of the pressure drop. Substitute 0.667 for Y and F K X T or
F K X TP for X in basic value flow-capacity coefficient equation. Reprinted with permission: Daniel Valve Company

Figure 4. Compressibility versus Reduced Pressure.