Relief inlet piping

Each relief valve should be equipped with inlet piping no smaller than the valve inlet flange size, and inlet piping should be as short as practi-... 

Inlet piping is held to a minimum, with the safety device preferably mounted directly on the equipment and with the total system pressure drop loss to pres.sure relief valve inlet not exceeding 3% of the set pressure in psig, of maximum relief flowing conditions [10]. To conform to code (see ASME code. Sect. Vlll, Div. l-UG-127 [1]) avoid high inlet pressure drop and possible valve chatter ... 

Inlet piping sized so that pressure drop from vessel to pressure relief valve inlet flange does not exceed 3 percent of valve set pressure... 

Typical pressure relief valve mounted on long inlet pipe... 

Typical installation avoiding process laterals connected to pressure relief valve inlet piping... 

When designing the inlet piping to a relief valve, what pressure losses are recommended See API 520, Sizing, Selection, and Installation (1994). 

For both conventional and balanced SRVs, the inlet pressure loss, including the mounting nozzle entrance loss, rupture disk flow resistance, and inlet pipe friction, is recommended to stay below 3 percent of the differential set pressure, or else valve instability may occur, resulting in degraded relief capacity. 

Check that the de-rated relief capacity is equal to or greater than the required capacity. If this is not the case, have a close look at either the inlet piping configuration or at increasing the size of the valve. 

While this is the general recommendation, compliance with this is not always possible. As seen earlier in this book, higher oudet pressure drops may result in reduced valve capacity and instability. In that case, other types of SRVs as discussed in Chapter 9 need to be taken into consideration. API RP 521 also states the outlet pressure drop for individual relief valves should be based on the actual rated valve capacity consistent with the inlet piping pressure drop as discussed in Section 6.1. 

In addition, noise suppression, preferred location of relief valves, and pressure drop requirements for stable operation involving inlet pipe to the valve and tail pipe are reviewed. 

Safety devices should be piped with a minimum of inlet pipe runs because excess pressure drop before a safety valve will affect its operation. For a minimum of discharge piping, relief valves are located high on a tower in open relief systems. In closed systems relief valves are located just above the relief header. 

The size of outlet piping required for a safety disc is not necessarily the same as the disc receptacle size. Discs frequently are sized on the basis of pressure requirements rather than capacity requirements. In such instances it is possible for the outlet piping to be smaller than the pipe size of the disc receptacle. If an arrangement of this type is desirable, the pipe diameter must be calculated on the basis of the relief capacity requirements and the maximum allowable upstream pressure. The inlet piping, however, must have an area which is at least equal to that of the receptacle in order to comply with the ASME code. 

The permitted distinct values for the maximum allowable pressure drop depend, among other things, on the exact design of the valve. It is common practice to limit the maximum permitted pressure drop for the inlet pipe to 10% of the set pressure. For the outlet vent line from a balanced bellows safety relief valve a 30% pressure drop with respect to the set pressure is regarded as permissible. The manufacturers of relief valves provide the exact limit values. Therefore the data given here should be looked upon as recommendations without any mandatory character. 

The design of relief valve systems includes the criteria of local stresses at the header-to-relief valve inlet piping junction and the stresses in the relief valve inlet piping and header. 

The inlet pressure of relief fluid to PRV inlet piping is the system pressiue at relief condition. This pressure for non-fire case is 110% Ps, and for fire case, it is 121% Ps. 

Rapid eycling can occur when the pressure at the valve inlet decreases at the start of relief valve flow beeause of excessive pressure loss in the piping to the valve. Under these conditions, the valve will cycle at a rapid rate which is referred to as chattering. The valve responds to the pressure at its inlet. If the pressure decreases during flow... 

The 1997 edition of the API RP 521 extends the two-thirds rule to include the upstream and downstream system. At a minimum, the inlet and outlet piping up to and including isolation valves must be designed for the two-thirds rule to be able to block in the exchanger. If the upstream and downstream equipment is not designed for the two-thirds rule, relief devices may be required on both the inlet and outlet piping to protect the piping and adjaeent equipment. 

This section describes the requirements for the design and installation of pressure relief valve inlet and outlet piping manifolds and valving, including safety valve and flare headers. 

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