Relief system considerations - hpvalves

HOW TO CONSIDER RELIEF SYSTEM ????? 

The entire relief system must be considered before selecting the appropriate relief device. The relief headers should be designed to minimize pressure drop, thus allowing for future expansion and additional relief loads.

• Conventional spring-loaded-relief-valve considerations. Conventional valves require the relief header backpressure (superimposed plus built up) to be less than 10% of the set pressure of the lowest-set relief valve tied into the header.

• Balanced-spring-loaded-valve considerations. Balanced spring-loaded valves allow the use of smaller relief headers because of the larger pressure drops allowed, under maximum relief-flow conditions, as a result of higher allowable backpressure (40%). Balanced valves and relief headers are designed as a system to operate at a higher backpressure. The balanced valve is more expensive than conventional valves; however, the total cost of the use of balanced valves plus the smaller header system may be lower. Capacity is reduced at the larger backpressure, so it may not be the solution for all backpressure problems. In the bellows model, the bellows is a flexible pressure vessel that has a maximum backpressure limit that is lower in larger valve sizes. Bellows are available in a limited number of materials and may deteriorate rapidly under certain exposure conditions. Bellows should be checked periodically for leakage. A leaking bellow does not provide backpressure compensation, and it allows the relief header to leak to the atmosphere. The balanced valve commonly is used to tie a new low-pressure-relief load into an existing heavily loaded relief header or to protect the relief-valve top works from corrosive gases in the relief header.

• Pilot-operated-valve considerations. Pilot-operated valves should be considered for all clean services within their temperature limitations. They are well suited for pressures below 15 psig and are available with the pilot-pressure sensing line connected to either the valve inlet or to a different point. Pilot-operated valves provide tight shutoff with very narrow margins between operating pressure and set pressure.

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