Safety Valves in Low Pressure Steam Systems

Sizing safety valves after boiler output power in low pressure systems (kW and Btu/hr)

The main purpose of a safety valve is to prevent the pressure in a system to exceed the certification pressure. Above certification pressure, no one can guaranty the systems safety - and especially for a steam system with very hot gas with huge amount of latent heat, the consequences can be dramatically.

The size of the safety valve depends primarily on the maximum boiler output and the operation pressure of the system. The safety valve shall as minimum have the evacuation capacity of all the vapor the boiler can produce running at full power at working (or certification) pressure.

The table below can be used to select a typical safety valve based on boiler output. Before final design, always consult the manufactures documentation.

Pipe Size
DN
CapacityMaximum Boiler Output
(in)(mm)(kg/h)(lb/h)(kW)(Btu/h)
1/2 15 38 86 23 81 270
3/4 20 82 185 50 174 825
1 25 145 324 89 306 180
1 1/4 32 249 557 152 526 365
1 1/2 40 360 807 220 762 615
2 50 536 1 199 328 1 133 055
2 1/2 65 901 2 012 551 1 901 340
3 80 1 244 2 779 761 2 626 155
4 100 2 397 5 355 1 465 5 060 475

Note! The table above is based on low pressure steam of 100 kN/m2 (1 bar) or 15 psi in imperial units. Latent heat of saturated steam is 2201 kJ/kg (945 Btu/lb).

  • 1 N/m2 = 1 Pa = 1.4504 x 10-4 lb/in2 (psi) = 10-5 bar
  • For higher pressure, steam is compressed and require less volume - required size of the valve reduced
  • For lower pressure, steam is expanded and require more volume - required size of the valve increased

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