Chemical & Process Technology: Should we consider JET FIRE for Pressure Relief Valve (PSV) load determination ?

Friday, July 27, 2007

Should we consider JET FIRE for Pressure Relief Valve (PSV) load determination ?

PSV is a final safeguarding device to prevent equipment or system from catastrophic failure. However, this device may not protecting equipment and/or system from FIRE attack. We may only rely on other active and passive protecting system to safeguard the system.

A wet gas KO drum operate at LOW pressure & LOW temperature. External FIRE (regardless of POOL or JET fire) heating up it gas via metal wall. As gas is low in heat transfer (~ 7 - 20 kW/m2), the heat absorbed by vessel wall do not dissipate into gas fast enough and tends to stay in the metal wall. Vessel wall (flanges, gasket, etc) probably failed before the internal pressure reached it PSV set pressure.

JET FIRE

Installing PSV does not protecting the vessel from Fire attacks regardless of pool nor jet fire. Practically, we shall shift the focus to :

emergency depressurization system
high SIL instrumented protective system
fire proofing system
high firewater spray density

Jet fireis initiated from flanges leakage, cracked pipe, damaged of some fitting attached to large pipe, etc. It subject to

internal pressure at release point
flame direction
relative distance between release point and impinged vessel

Jet flame contain very high momentum which impacting to the impinged vessel (but reduce according as pressure depleted), etc. There are many other transient parameters evolve with time such as wind speed, wind direction, etc.

Fire impinged area may expose to very high localised heat flux (>300 kW/m2), as the flame travel around the vessel, the heat flux reduced accordingly. Many documents e.g. "Guidelines for Protection of Pressurised Systems Exposed to Fire", by Scandpower Risk Management AS, quoted jet fire heat flux of 300 kW/m2 is just an average heat flux.

As indicated in latest API Std 521, section 5.16 "...a relief device might not prevent vessel failure from jet fire impingement.", those sizing PSV protecting a vessel may not needs to consider jet fire heat flux.

POOL FIRE

In conclusion,

PSV will only be sized for pool fire using the API 521 equation
Focus on emergency depressurization. Use jet fire heat flux rather than pool fire heat flux. CAUTION : if you use HYSYS, the model only use API 521 equation. Some configuration is required to estimate depressurization rate based on jet fire heat flux.
While conduct depressurizations study, ensure maximum allowable working pressure due to reduce wall stress (reduce according to time) is always above internal pressure exert on the vessel wall.
Increase wall thickness or apply external fire proofing if necessary.
Conduct Scenario and Quantitative Risk Analysis, Jet fire flame pattern analysis etc if above measures are too excessive