An in-depth study of the sizing process for a range of applications, including sizing equations for AD Merkblatt, DIN , TRD, ASME, API, BS6759 and others. Covers more complex issues such as two-phase flow and superheat.
Safety Valve Sizing
Balanced bellows valves
Liquid constant backpressure correction factor for ASME/API RP 520
Balanced bellows valves
Viscosity correction factor for ASME/API RP 520 and EN ISO 4126
This is used to make allowances for high viscosity fluids. In order to account for this, the valve size must first be established, assuming the fluid is non-viscous. Once the size has been selected, the Reynolds number for the valve is calculated and used to establish the correction factor from Figure 9.4.9.
The valve size should then be checked to ensure that the original size chosen would accommodate the flow after the viscous correction factor has been applied. If not this process should be repeated with the next largest valve size.
The Reynolds number can be calculated using Equations 9.4.15 and 9.4.16:
Standard - EN ISO 4126: 2004
Use Equation 9.4.17 to calculate the minimum required orifice area for a safety valve used on dry saturated steam, superheated steam, and air and gas applications at critical flow:
Use Equation 9.4.18 to calculate the minimum required orifice area for a safety valve used on wet steam applications at critical flow; Note: wet steam must have a dryness fraction greater than 0.9:
Use Equation 9.4.19 to calculate the minimum required orifice area for a safety valve used on air and gas applications at sub-critical flow:
Use Equation 9.4.20 to calculate the minimum required orifice area for a safety valve used on liquid applications:
Value of C as a function of ‘k’ for steam, air and gas applications to the EN ISO 4126 standard.
‘k’ values are incorporated into the ISO 4126 standard: (Part 7).
Alternatively, ‘k’ values can be obtained from the Spirax Sarco website steam tables.
Capacity correction factors for backpressure to the EN ISO 4126 standard for steam, air
and gas applications
Size the minimum flow area required for a safety valve designed to EN ISO 4126 to relieve a
superheated steam system of overpressure.
Appendix A - Properties of industrial liquids
Table 9.4.5 Properties of some common industrial liquids
For specific gravity (G) used in ASME liquid sizing calculations, divide density by 998 (density of water).
Properties of industrial gases
Table 9.4.6 Properties of some common industrial gases
For specific gravity (G) used in ASME gas sizing calculations, divide molar mass by 28.96 (molar mass of air).