Relief valves liquid-filled vessels

The ASME Boiler Code may require a liquid-filled vessel to have a pressure relief valve if it can be isolated from another vessel that has relief valve protection. Then the designer s obligation is to determine if the flare system can handle the volume of liquid depressuring through the relief valve on the liquid-filled vessel. 

Pilot-operated valves have the advantage of allowing operations n the set point v/ith no leakage, and the set position is not affected by ba pressure. However, they will not function if the pilot fails. If the sens line fills with hydrates or solids, the valve will open at 25% over pressure trapped above the disc (usually the normal operating pre the vessel). For this reason they should be used with care in dirty vice and liquid service. They are used extensively offshore where all platform relief valves are tied into a single header because up to 5 back-pressure will not affect the valve capacity. 

Vessels operating completely filled with liquid must be equipped with liquid relief valves, unless otherwise protected (Par. UG-125-3(g). 

When a pipe or vessel is totally filled with a liquid which can be blocked in, for instance, by closing two isolation valves, the liquid in the pipe or pressure vessel can expand very slowly due to heat gain by the sun or an uncontrolled heating system. This will result in tremendous internal hydraulic forces inside the pipe or pressure vessel, as the liquid is non-compressible and needs to be evacuated. This section of pipe then needs thermal relief (Figure 2.9). 

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