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During a plant fire, a relief valve may not open in time. Designing for this contingency can save a vessel from destruction
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Pressure relief valves (PRVs) are a favored protection against explosions of pressure vessels due to external fires. Because two Recommended Practices (RPs) issued by the American Petroleum Institute (API; Washington, D.C.) endorse PRVs, many engineers do not realize that PRVs are only one component of a total defense strategy. This article details alternative approaches to protect vessels in addition to using a PRV.
The first step towards understanding starts with a look at the process fluid inside a closed vessel. It can be gas, liquid plus gas, or liquid. Protection techniques depend on the fluid volume and its thermal properties at the· first instant of fire. This article will discuss gas-filled and quasi-gasfilled vessels. We will outline the problem and then solutions.
PRVs for gas-filled vessels
A PRV on a gas-filled vessel, exposed to an external fire very likely will not protect the vessel - it will just provide a false sense of security. A flame has hot spots and cool spots. The heattransfer coefficient between the wall and the gas in the vessel is very small. The steel wall cannot cool down. Tests and actual fire incidents have confirmed that when a gas-filled vessel is exposed to a fire, the vessel wall develops overheat spots where the flame is hottest. The vessel will fail due to heat stress on the wall long before its internal gas pressure reaches the set pressure of the PRV.
Section 3.15.1.2 of API RP 521 [1] warns design engineers about the potential consequences of such a thermal failure. API RP 521 does indicate that a PRV alone may not be adequate to protect gas-filled vessels. At the same time, API RP 520 [2] provides detailed fire-load calculation procedures for gas-filled vessels. Most petroleum refiners use these equations to size PRVs. However, a gas-filled vessel cannot be protected by a PRV alone.
Equations 1 and 2 are used to size PRVs. They are based on the following assumptions:
* The vessel is not insulated
* The vessel wall temperature (recommended maximum: 1,10O0F) will not weaken the wall below its rated rupture stress.