What are the setting pressure, discharge pressure, and opening pressure of a safety valve?

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The setting pressure, adjustment pressure, and discharge pressure of a safety valve are its key technical parameters, which play an important role in ensuring the safe operation of the system. Although the three are closely related, they each have their own differences. (1) The following is an introduction to these three parameters: 1) Setting pressure: The setting pressure is the predetermined pressure at which the safety valve begins to open, that is, the gauge pressure measured at the inlet of the valve. At this pressure, the force generated by the medium pressure to open the valve balances the force that keeps the valve disc in place on the valve seat. This is the threshold at which the safety valve opens automatically when adjusted to a specific pressure. 2) Starting pressure: The starting pressure refers to the pressure point at which the safety valve actually begins to release pressure. When the actual pressure of the system reaches this value, the safety valve will begin to operate. The starting pressure reflects the sensitivity and protective performance of the safety valve, and is a parameter used to describe the dynamic response characteristics of the safety valve. 3) Discharge pressure: Discharge pressure refers to the set pressure plus the excess pressure. The excess pressure is usually expressed as an increment in percentage of the set pressure. (II) The following illustrates the relationship among the three parameters: 1) Setting pressure and trip pressure a) Definition difference: The setting pressure is the predetermined pressure at which the safety valve begins to open; it is a set value ； The starting pressure is the pressure point at which the safety valve actually begins to release pressure, and it is a dynamic parameter in actual operation. b) Relevance: The set pressure is usually chosen to be at a level below or equal to the system’s maximum operating pressure. When the system pressure reaches this set pressure, the safety valve begins to open; at this point, the system pressure is actually the trip pressure. Ideally, the set pressure and the start-up pressure should be equal, but in practice differences may arise due to factors such as valve performance and environmental conditions. c) Conversion relationship: Typically, the opening pressure of a safety valve is slightly higher than or equal to the set pressure, as the valve needs to overcome static friction and mechanical resistance before it can open. 2) Setting pressure and discharge pressure a) Definition of the difference: The setting pressure is the pressure at which the safety valve opens for the first time, whereas the discharge pressure is the pressure when the safety valve is fully open and discharging fluid steadily. b) Relevance: The discharge pressure is usually the set pressure plus a certain excess pressure. Excess pressure refers to the increase in pressure above the set pressure, usually expressed as a percentage of the set pressure; it is used to ensure that the safety valve opens fully once the set pressure is reached, thereby allowing excess pressure to be effectively discharged. c) Influencing factors: The discharge pressure depends not only on the set pressure but also on the structural characteristics of the safety valve and the discharge medium. The standards for setting the discharge pressure of safety valves in different media (such as steam, air or other gases, water or other liquids) also vary. d) Conversion relationship: Discharge pressure = Set pressure + Excess pressure. The excess pressure is generally expressed as a percentage of the set pressure; for example, if the excess pressure is set at 10% of the set pressure, then the discharge pressure will be 1.1 times the set pressure. 3) Starting pressure and discharge pressure a) Functional relationship: The starting pressure is the point of initiation of the discharge process, marking the moment when the safety valve begins to release pressure ； The discharge pressure represents the end of the discharge process, indicating that the safety valve has reached its maximum discharge capacity. b) Transition relationship: As the system pressure increases, the safety valve will gradually open starting from its setting pressure, until it reaches the discharge pressure at which point discharge occurs stably. During this process, the opening height and flow rate of the safety valve gradually increase until the maximum discharge requirement is met. c) Conversion relationship: There is no direct numerical conversion relationship between the starting pressure and the discharge pressure, as these two parameters reflect the performance of the safety valve at different operating stages. The starting pressure refers to the sensitivity with which the safety valve begins to operate, whereas the discharge pressure relates to the discharge capacity of the safety valve when it is fully open. In general, the relationship among the opening pressure, setting pressure, and discharge pressure of a safety valve is closely linked, and it is crucial for ensuring that the safety valve responds properly to system overpressure. Understanding these parameters and their relationships helps to select, configure, and maintain safety valves more effectively, ensuring the safe operation of the system.