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 three safety valves are installed on the steam drum:
Design pressure = 1.05 × operating pressure
First valve: Pset1 = design pressure; B.D% = 4%
Second valve: Pset2 = design pressure + 2% of the design pressure; B.D% = 6%
Third valve: Pset3 = design pressure + 3% of the design pressure; B.D% = 7%
2. When four safety valves are installed on the steam drum:
First valve: Pset1 = design pressure; B.D% = 4%
Second valve: Pset2 = design pressure + 1% of the design pressure; B.D% = 5%
Third valve: Pset3 = design pressure + 2% of the design pressure; B.D% = 6%
Fourth valve: Pset4 = design pressure + 3% of the design pressure; B.D% = 7%
The total discharge capacity of all safety valves on the steam drum must be greater than 75% of the boiler’s evaporation rate.
3. All safety valves at the inlet and outlet of the reheater are set to the same pressure. 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-type safety valves of ABB+CE company 1. Superheater safety valves (1.) Minimum setting pressure of superheater safety valves: Pset = Design pressure of the drum – Superheater resistance – 20 (Psi) (2.) When 2 safety valves are installed: Pset1 for the first valve = Design pressure of the drum – Superheater resistance – 20 (Psi); Pset2 for the second valve = Design pressure of the drum – Superheater resistance – 20 + 5 (Psi) (3.) When 3 safety valves are installed: Pset1 for the first valve = Design pressure of the drum – Superheater resistance – 20 (Psi); Pset2 for the second valve = Design pressure of the drum – Superheater resistance – 20 + 5 (Psi); Pset3 for the third valve = Design pressure of the drum – Superheater resistance – 20 + 10 (Psi) The discharge capacity of the superheater safety valves should be more 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: Pset1 = Design pressure of the reheater; 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: Pset1 = Design pressure of the reheater; Pset2 = Design pressure of the reheater + 1% of the design pressure (i.e., 1.01 × Design pressure of the reheater); 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: Pset1 = Design pressure of the reheater; Pset2 = Design pressure of the reheater + 1% of the design pressure (i.e., 1.01 × Design pressure of the reheater); Pset3 = Design pressure of the reheater + 2% of the design pressure (i.e., 1.02 × Design pressure of the reheater); Pset4 = Design pressure of the reheater + 3% of the design pressure (i.e., 1.03 × Design pressure of the reheater).
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(5) When 4 or more safety valves are installed: Pset1 = Design pressure of the reheater; Pset2 = Design pressure of the reheater + 1% of the design pressure (i.e., 1.01 × Design pressure of the reheater); Pset3 = Design pressure of the reheater + 2% of the design pressure (i.e., 1.02 × Design pressure of the reheater); Pset4 and subsequent valves = 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 – Pressure loss (△P) – 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) – Pressure loss (△P).
Discharge volume: The discharge volume from the reheater inlet safety valve accounts for 75–85% of the total discharge volume, while that 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 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.