Boiler Safety Valve Relieving Capacity

The safety valves must pass everything the boiler can generate at full fire with the outlet shut, without the pressure climbing more than the permitted accumulation above the maximum allowable working pressure. The accumulation test is what proves it. Where plants get into trouble is that the burner has been uprated, or the boiler re-rated, or a valve replaced with one of a different stamped capacity, and nobody re-ran the sum. A boiler rated 20,700 lb/hr with valves stamped 11,500 and 10,200 has 21,700 lb/hr installed and passes; uprate the burner to 24,000 lb/hr and the same valves are 2,300 lb/hr short, nothing was done to them, and the boiler is now outside its code case. Two details carry weight. Stamped capacity is at a specific set pressure -- the same valve passes more at a higher one -- so the sum has to be taken at the pressures actually installed, not at a catalogue figure, and a supplementary valve set above the maximum allowable working pressure carries its stamp at THAT pressure. And where two valves are fitted the code governs both the set pressures and the spread between them, so a plant cannot simply install two of whatever adds up. The required capacity itself is not always the nameplate: for a fired boiler it is at least the maximum output the fuel input supports, which is why the fuel input and efficiency are entered here and the larger of the two figures governs. Safety valves are also a maintenance item with a testing interval, and a valve that has not lifted in years may not lift at its set pressure -- its stamped capacity is a statement about a valve that works. This is a capacity comparison from stamped and rated values the user supplies. It is not a relief system design and it does not substitute for the ASME code calculation or for the accumulation test, which is the actual demonstration of compliance. It does not select valves, determine set pressures and their permitted spread, size discharge piping and drip pans -- a discharge line that imposes back pressure or that puts thrust on the valve body defeats it -- evaluate economizer or superheater relief requirements, or address reheaters and other special cases. It does not evaluate valve condition or testing interval. Safety valves are the last line of protection on a pressure vessel that can fail catastrophically: ASME Boiler and Pressure Vessel Code Sections I and IV as applicable, the National Board inspection code, the valve manufacturer, and the jurisdiction's boiler inspector govern.

required relieving capacity = the greater of the boiler's maximum designed steaming capacity and fuel input x efficiency / steam enthalpy rise; installed capacity = the sum of the stamped capacities of the fitted valves at their installed set pressures; accumulation pressure = MAWP x (1 + accumulation limit / 100).

The ASME Section I and IV relieving-capacity requirement, cited not mirrored: the valves must pass everything the boiler can generate at full fire with the outlet shut, no valve is set above the maximum allowable working pressure except as the code permits for supplementary valves, and pressure must not rise more than the permitted accumulation. ASME BPVC Sections I and IV, the National Board inspection code, the valve manufacturer, and the jurisdiction's boiler inspector govern.

A comparison of stamped and rated values the user supplies; no code table or valve catalogue is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: rated_steaming_capacity_lb_hr, fuel_input_btuh, boiler_efficiency, steam_enthalpy_rise_btu_lb, mawp_psig, accumulation_limit_pct, valve1_set_psig, valve1_capacity_lb_hr, valve2_set_psig, valve2_capacity_lb_hr, valve3_set_psig, valve3_capacity_lb_hr, uprated_capacity_lb_hr, capacity_from_fuel_lb_hr, required_capacity_lb_hr, installed_capacity_lb_hr, margin_lb_hr, accumulation_pressure_psig, uprated_shortfall_lb_hr

• Stamped capacity is at a set pressure not a catalogue figureASME BPVC Sections I and IV
• The accumulation test is the demonstration this is a screen, not a code calculationthe National Board inspection code
• Valve condition is not evaluated a valve that has not lifted in years may not liftboiler inspection practice