If the model of the safety valve is specified as 300lb, then are the inlet and outlet flanges of the safety valve also 300lb?

The grades of the inlet and outlet flanges are generally different. The poster did not specify the exact model of the safety valve, which makes it difficult to analyze the situation. Generally speaking, the main parameters of a safety valve include the design pressure, the setting pressure, the specifications of the inlet flange, and the specifications of the outlet flange. As it is connected to a flare for export, the pressure rating is low

The owner didn’t specify the model clearly; the 300LB ANSI safety valve likely refers to the pound rating of the inlet flange

1. Nominal pressure of the safety valve: The highest operating pressure that the inlet of the safety valve can withstand. Denoted by PN, the nominal pressure of a safety valve shall be selected in accordance with relevant standards, based on the valve material, operating temperature, and set pressure. 2. The setting pressure of a safety valve, also known as the opening pressure, is the predetermined pressure at which the valve begins to open under operating conditions; it is the gauge pressure measured at the inlet of the valve. At this pressure, under the specified operating conditions, the force generated by the medium pressure that opens the valve balances the force that keeps the valve disc in place on the valve seat. 2.1 Pressure vessels: According to GB150, the set pressure should be 1.05–1.1 times the working pressure. 2.2 Steam boilers: When p is less than or equal to 0.8 MPa, the set value (when one safety valve is used) is the working pressure plus 0.03 MPa; when p is between 0.9 and 5.9 MPa, the set value is 1.04 times the working pressure; when P > 5.9 MPa, the set value is 1.05 times the working pressure, with P representing the set pressure of the safety valve. 2.3 Pressure pipelines: The set value is 1.05–1.10 times the normal maximum working pressure. 3. Safety valve return pressure: This refers to the static inlet pressure at which the valve disc comes into contact with the valve seat again after the safety valve has discharged, that is, when the opening height becomes zero. 4. Sealing test pressure: The inlet pressure during the sealing test, at which pressure the leakage rate through the seal surface between the valve disc and the valve seat is measured. 5. The operating pressure of a safety valve is the operating pressure of the system in which the safety valve is installed. The setting pressure of a safety valve is related to the operating pressure, and there is also a direct relationship between the nominal pressure of the safety valve and the operating pressure. The nominal pressure represents the value at room temperature; when the design temperature of the system is higher than room temperature, the allowable stress of the materials at that temperature decreases, and accordingly, the nominal pressure of the safety valve must be increased. Therefore, when manufacturing a safety valve, the orderer must provide the nominal pressure and setting pressure of the valve, and the manufacturer will select the appropriate valve body material as well as the diameter of the spring wire.

300lb is the pressure at the inlet flange, so is it also 300lb at the outlet?

Since the pressure at the outlet of the safety valve is relatively low, it is not necessary for the pressure rating of the outlet flange to be 300LB; 150LB is usually sufficient. The nominal diameter of the outlet flange does not have to be the same as that of the connecting flange.

I have a question: For an acrylic spherical tank equipped with two safety valves, manual valves are installed at both the inlet and outlet of each safety valve to facilitate annual inspections. Under normal conditions, when a safety valve is in operation, both the inlet and outlet manual valves are unlocked. Another safety valve is on standby, and the inlet and outlet valves are locked shut. The problem is: the pressure rating at the inlet of the safety valve is 300 lb, while the pressure rating of the outlet valve is 150 lb. When the system is locked, if there is a slight internal leakage in the inlet and safety valve, overpressure occurs before the outlet valve. Should the pressure at the safety valve outlet flange, the outlet valve, and a short section of pipe be increased to 300 lb?

From a process perspective, the pressure rating generally applies to the inlet of the safety valve, as the outlet of the safety valve is the discharge point; therefore, the pressure rating of the pipeline downstream of the outlet can be lower than that at the inlet； From the perspective of the equipment, the pressure ratings of the inlet and outlet flanges are generally the same. There should be no shut-off valve installed at the outlet of the safety valve; once the safety valve activates, it must discharge and relieve pressure automatically, allowing the fluid to flow into other containers or drainage tanks, thereby maintaining a safe pressure level in the pipes and equipment upstream of the safety valve.

As far as I know, there is no such thing as a backup safety valve designed for the equipment; during normal operation, all safety valves on the equipment must be in use. The number of safety valves installed depends only on the volume of fluid that needs to be discharged under pressure conditions, and not on the pressure level itself. It is normal for safety valves to have inlet and outlet valves, and during annual inspections, a report can be prepared to temporarily deactivate one of the safety valves for a short period of time so that it can be taken apart for testing. I’m not quite sure what exactly \"locking off\" means, but you’re right about the second point: in general, the safety valve setting for propylene tanks is set at a pressure greater than 2.0 Mpa (150 lb), so the pipes, fittings, valves, and gaskets used for the safety valves of such tanks should be of PN5.0 (300 lb) grade.

To be more specific: two safety valves are installed on one tank. As designed, the discharge volume of a safety valve is sufficient to meet the requirements. On the design drawings, for one unit that is in operation, its upstream and downstream valves are open (unlocked), while for the other unit that is not in operation, its upstream and downstream valves are closed (locked). When a safety valve that is in use needs to be tested, the upstream and downstream manual valves of another valve should be locked in the open position (the valves should be open), so as to ensure that there is always a functional safety valve on the tank during its testing. This is done to guarantee that safety valves remain effective at all times while the pressure equipment is in operation. Locking on and locking off are measures in process organization management. In other words, a lock is installed on the valve to ensure that it remains in a safe position; when the valve lock is engaged, it cannot be closed without a key, and vice versa, when the lock is disengaged, the valve cannot be opened without a key.