Principles for setting the set pressure of a safety valve

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Principles for setting the set pressure of safety valves I. According to ASME standards, the set pressure of safety valves is determined in accordance with the following principles. Natural circulation boilers and controlled circulation boilers: 1. When 3 safety valves are installed on the drum: Design pressure = 1.05 × operating pressure. For the first valve, Pset1 = design pressure; B.D% = 4%. For the second valve, Pset2 = design pressure + 2% of the design pressure; B.D% = 6%. For the third valve, Pset3 = design pressure + 3% of the design pressure; B.D% = 7%. 2. When 4 safety valves are installed on the drum: For the first valve, Pset1 = design pressure; B.D% = 4%. For the second valve, Pset2 = design pressure + 1% of the design pressure; B.D% = 5%. For the third valve, Pset3 = design pressure + 2% of the design pressure; B.D% = 6%. For the fourth valve, Pset4 = design pressure + 3% of the design pressure; B.D% = 7%. The total discharge capacity of the drum’s safety valves should be greater than 75% of the boiler’s evaporation rate. 3. The safety valves at the inlet and outlet of the reheater are all set to the same pressure level. The set pressure is based on the data provided at the design site. Its inlet and outlet pressure difference is the reheater resistance. The exhaust flow at the reheater outlet accounts for over 15% of the total exhaust flow. The flow rate at the reheater inlet accounts for 75%~85% of the total flow rate. 4. The number of safety valves installed at the superheater outlet is determined based on the temperature to be maintained during boiler operation. Superheater resistance PZ=14kg/cm2. II. Installation standards for full-range safety valves by ABB+CE Company
1. Superheater safety valves
(1) The minimum set pressure of the superheater safety valve, Pset, is calculated as follows:
Pset = Design pressure of the steam drum – Resistance of the superheater – 20 (Psi)
(2) When installing two safety valves:
First valve: Pset1 = Design pressure of the steam drum – Resistance of the superheater – 20 (Psi)
Second valve: Pset2 = Design pressure of the steam drum – Resistance of the superheater – 20 + 5 (Psi)
(3) When installing three safety valves:
First valve: Pset1 = Design pressure of the steam drum – Resistance of the superheater – 20 (Psi)
Second valve: Pset2 = Design pressure of the steam drum – Resistance of the superheater – 20 + 5 (Psi)
Third valve: Pset3 = Design pressure of the steam drum – Resistance of the superheater – 20 + 10 (Psi)
The discharge capacity of the superheater safety valves must be greater than 20% of the total discharge capacity. 2. Reheater inlet safety valve (1). Minimum setting pressure of the reheater inlet safety valve: Pset = Design pressure of the reheater. (2) When 2 safety valves are installed, the setting pressure for the first valve is Pset1 = Design pressure of the reheater; the setting pressure for the second valve is Pset2 = Design pressure of the reheater + 3% of the design pressure (i.e., 1.03 × Design pressure of the reheater). (3) When 3 safety valves are installed, the setting pressure for the first valve is Pset1 = Design pressure of the reheater; the setting pressure for the second valve is Pset2 = Design pressure of the reheater + 1% of the design pressure (i.e., 1.01 × Design pressure of the reheater); the setting pressure for the third valve is Pset3 = Design pressure of the reheater + 3% of the design pressure (i.e., 1.03 × Design pressure of the reheater). (4) When 4 safety valves are installed, the setting pressure for the first valve is Pset1 = Design pressure of the reheater; the setting pressure for the second valve is Pset2 = Design pressure of the reheater + 1% of the design pressure (i.e., 1.01 × Design pressure of the reheater); the setting pressure for the third valve is Pset3 = Design pressure of the reheater + 2% of the design pressure (i.e., 1.02 × Design pressure of the reheater); the setting pressure for the fourth valve is Pset4 = Design pressure of the reheater + 3% of the design pressure (i.e., 1.03 × Design pressure of the reheater). (5) When 4 or more safety valves are installed, the setting pressure for the first valve is Pset1 = Design pressure of the reheater; the setting pressure for the second valve is Pset2 = Design pressure of the reheater + 1% of the design pressure (i.e., 1.01 × Design pressure of the reheater); the setting pressure for the third valve is Pset3 = Design pressure of the reheater + 2% of the design pressure (i.e., 1.02 × Design pressure of the reheater); the setting pressure for the fourth and subsequent valves is Pset4 = Design pressure of the reheater + 3% of the design pressure (i.e., 1.03 × Design pressure of the reheater). 2. Reheater outlet safety valve (1). Minimum setting pressure of the reheater outlet safety valve: Pset = Design pressure of the reheater – ΔP – 15 (Psi), but it cannot be lower than 1.05 times the outlet pressure of the reheater. (2) Maximum setting pressure of the reheater outlet safety valve: Pset = Design pressure of the reheater + 15 (Psi) – ΔP. Discharge volume: The discharge volume from the reheater inlet safety valve accounts for 75–85% of the total discharge volume, while the discharge volume from the reheater outlet safety valve accounts for 15% of the total. Note: Since the outlet header is located on top of the furnace, in order to save on discharge pipes, CE Company installs all of the reheater safety valves on the outlet header. 2. In accordance with CE requirements, the superheater and the steam drum are designed to have equal strength, therefore they have the same design pressure.