What are the nominal pressure, opening pressure, discharge pressure, seating pressure, and operating pressure of a safety valve?

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What are the nominal pressure, opening pressure, discharge pressure, seating pressure, and operating pressure of a safety valve? The nominal pressure of a safety valve indicates the maximum allowable pressure under normal temperature conditions; for safety valves used in high-temperature equipment, the decrease in the allowable stress of the materials at high temperatures should not be taken into account. Safety valves are designed and manufactured according to nominal pressure standards. Opening pressure: also known as set pressure, it is the pressure of the medium at which the valve disc of a safety valve begins to rise under operating conditions. Discharge pressure: The pressure on the inlet side when the valve disc reaches the specified opening height. Seat-back pressure: The inlet pressure at which the valve disc presses back against the seat after the safety valve has discharged, and flow stops. The return seat pressure is an important parameter that indicates the quality of a safety valve’s performance; it is generally required to be at least 80% of the operating pressure, with an upper limit set such that the valve disc does not experience frequent oscillations. Opening and closing pressure difference: the difference between the opening pressure and the returning pressure. The setting pressures for the safety valves of the boiler drum and superheater are specified as follows: (1) When the rated steam pressure is less than 1.27 MPa, the setting pressures are respectively the operating pressure + 0.02 MPa and the operating pressure + 0.04 MPa ； (2) When the rated steam pressure is greater than 1.27 MPa but less than 3.82 MPa, the opening pressures are 1.04 times and 1.06 times the working pressure respectively ； (3) When the rated steam pressure is greater than 3.82 MPa, the opening pressures are 1.05 times and 1.08 times the working pressure respectively. The opening pressure of the economizer safety valve is 1.5 times the operating pressure of the economizer. The opening pressure of the safety valve on a pressure vessel must not exceed the design pressure of that vessel.