Page 277 - Valve Selection Handbook

264 Valve Selection Handbook

Figure 7-3. Back Pressure Correction Factor Kb, Accounting for the Effect of Built-Up
and Superimposed Back Pressure on Valve Lift of Bellows Balanced-Pressure Relief
Valves in Gas or Vapor Service. (Reprinted from API RP 520 Part \, December / 976,
Courtesy of American Petroleum Institute.)

Sizing Equations for Dry Saturated Steam

Historically, pressure relief valves for dry saturated steam are sized in
the U.S. on Napier's steam flow equation. The equation has been formulat-
ed empirically for conditions of choked flow. In the lower pressure range
up to 105 bara (1515 psia), Napier's equation gives similar results to sizing
equations applicable for any gas, using an isentropic exponent of k = 1.3.
At higher pressures, however, Napier's equation can lead to gross oversiz-
ing. This led L. Thompson and O.B. Buxton, Jr. to develop a capacity cor-
rection factor (K N) for use in conjunction with Napier's equation. 50

Customary Mixed Imperial Units

= 100 where P! < 1515 psia
K N
0.1906P.-1000 r, ^r • j ™ir •
L
= where P, < 1515 psia and up to 3215 psia
K N N 1
0.2292P,-1061
K SH = superheat correction factor Table 7-3.
Values of the correction factor for high-pressure saturated steam, K N,
may be obtained from Figure 7-4.