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[areads]

I am working on a natural gas project. The big question at the moment is relief valves or instrumented protection systems. The governing overpressure scenario is (jet) fire.
One argument pro relief gas is that they are the most simple, reliable and traditional way of protection against overpressure.
The arguments pro instrumented protection systems are that the world is moving on and releasing natural gas out to the atmosphere is not the most environmentally wise option these days, and that relief valves wouldn’t offer so much protection against a fire (I don’t really understand why not).

Apparently the trend is moving towards instrumented systems but I am still fairly new in the area and I need opinions that can support either argument.

Thanks!

[JoeWong]

areads,

Let me tell you a point that you won't like to hear. Pressure relief valve do not protect system against overpressure caused by fire. It only buy you some time so that you can take action on other passive protective measure.

External fire continuously introduce heat into system. Pressure and temperature in the system will continuously rise upto the PSV set pressure, PSV begin to open to relieve internal inventory. PSV openning may continuous increase until internal pressure rise upto relieving pressure, approx. 110% PSV set pressure (although code i.e. ASME VIII allow upto 121%) maximum. At this point of time (if PSV is sufficiently sized), the internal pressure would start-up decrease. PSV is reclosable when internal pressure is decreased to just below it set pressure (to be exact reseat at blowdown, a few percent below set pressure). Once PSV is closed, continuous heat input from fire will increase internal pressure again and PSV will open again. This PSV roughly/approximately maintain system pressure between relieving pressure and blowdown. In this sense, you (and also many others) may understand the system is "protected" from overpressure caused by external fire. In real fact, it is NOT !

As heat input into system, other than pressure increase, temperature will also increase. This will also results pressure containment part i.e. vessel temperature continue to increase. Increased temperature results vessel allowable stress reduce and decrease in maximum allowable working pressure (MAWP). Vessel will start to fail when it MAWP lower than internal pressure i.e. PSV relieving pressure. If you have problem in understanding of pressure definition, consider this post "Definition of Terms Related to PRESSURE"

Instrumented protective system (IPS) is used in replacing ultimate overpressure protection system i.e. PSV. Well...this is commonly known as the code case 2211. There was a long discussion in another forum a couple of months ago on validity of code case 2211 in overpressure protection which the cause is fire . I am still can not convince in accepting code case 2211 in such application. If you are interested in getting more information, read "What to Do if Pressure Relief Devices Impractical in Application ?"

In managing risk and consequence due to fire, you may consider

(i) Depressuring (vapor & Liquid)
(ii) Provide Rupture / bursting disc instead of Pressure Relief Valve (for system that will lift PRV)
(iii) External cooling (deluge & spraying)
(iv) Fire Extinguishing agent (with external cooling)
(v) External Insulation
(vi) Sand Cover Storage
(vii) Diversion wall for isolation and proper drainage
(viii) Reliable & Compatible Fire detection system

Above are in point form. If you are interested, more discussion in "Protective Measures against FIRE other than Pressure Relief Device (PRD)"

By the way, please consider post Relief related in RELIEF DEVICES FORUM.

Edited by JoeWong, 07 September 2009 - 10:59 AM.