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+1 vote
answered Aug 14, 2015 by franchaze (1,150 points)
On offshore facilities, credit should not be taken for the 20/33% overpressure allowance in ASME B31.3 in determining the need for pressure protection.  An exception may be made for causes such as water hammer with careful consideration.

Even if facilities are designed for full shut-in wellhead pressure (e.g., on remote satellite platforms), relief protection for fire and thermal contingencies must be considered and provided if required by the applicable codes and/or standards.

Relief valves on vapor/liquid separators should be sized for at least the combined vapor and liquid rates into the separators.

Restriction orifices should be avoided in the design of any facility for the purpose of reducing relief load.  However, there are cases where it may be impractical to design a facility without them (e.g., gas blow by case for high pressure liquid service dumping into a low operating pressure system).  If a restriction orifice is used to limit the resulting relief load for a specific contingency, then it must be treated as a safety critical device for documentation and inspection purposes.  Venturis are preferred over orifice plates for restrictions.
+1 vote
answered Aug 16, 2015 by redonetti (1,280 points)
The basis for determination of the relief load for a particular system is the worst single case contingency. A system for this purpose is all of the equipment/piping a given safety relief valve service is designed to protect. The worst single case contingency corresponds to the largest expected relief load because of a single event. The event may be a fire or failure/mal-operation of a single component of the system (e.g., fire around a single vessel, inadvertent operation of a single valve, etc.). If a single contingency could trigger further contingencies (e.g., fire around a single vessel spreading to other vessels within the same fire zone, failure, of a control valve causing inadvertent response of other control valves, plant-wide power, or instrument air failure, etc.), then the total effect of the contingency should be taken in to account in arriving at the system relief load. For non-fire cases, credit may be taken for normally open flow paths out of the system. For fire cases, it should be assumed that all the feed to and product from the system has ceased to flow.

Safety relief valve sizing should be generally based on API RP 520 PT I and API RP 521. However, specific requirements for basic categories of safety relief valves (e.g., full-flow, fire, and thermal) are typically given per project specific. Sizing requirements for relief of backflow due to check valve failure are also given. In addition, examples of typical systems that often require each type of protection are included. This can be taken only as a starting point since each relief situation needs to be analyzed carefully to make sure all potential relief scenarios have been screened.
0 votes
answered Aug 19, 2015 by bbenoit (710 points)
When evaluating overpressure resulting from net inflow, connected upstream equipment should be assumed to be fitted for maximum flow and pressure capability.  Your client’s Subject Matter Expert should be consulted when this requirement causes a significant increase in the overall sizing of relief headers or flare systems.

It is often easy to change fittings resulting in a change in flow, which would change and possibly invalidate the relief sizing.  (For example, flow through an upstream control valve should be based on the largest trim that can be installed in the valve; pump flow and head should be based on the maximum number and size of impellers or pistons that can be used in the installed pump case, etc.)  In some cases, this requirement may cause a significant cost impact.  After evaluation of cost/benefit, the client's Engineer may choose administrative measures to prevent the future changes that would invalidate the relief sizing such as notes in the P&ID labels on the valve notations in the facility hazard register, etc.
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