Chemical & Process Technology: Depressuring within 15 minutes no longer applicable ?

Thursday, January 24, 2008

Depressuring within 15 minutes no longer applicable ?

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Recently i have discussed some issue related to system depressuring with some engineers. One of the topic where we were arguing about was the time limit on the depressuring. These engineers keep on stating that the emergency depressuring system shall be designed to depressure the facilities to 50% of its design pressure or 6.9 barg within 15 minutes.

There are many oil and gas company are practicing this approach. Above statement has been clearly stated in the company design manual. Many engineers has simply used above as criteria without confirming stress induced by internal pressure during the depressuring process upon allowable stress by the weakest component within the system.

API RP 521 Edition Fourth, "Pressure-relieving and Depressuring Systems" section 3.19 has a statement

"...A vapor depressuring system should have adequate capacity to permit reduction of the vessel stress to a level at which stress rupture is not of immediate concern. For sizing, this generally involves reducing the equipment pressure from initial conditions to a level equivalent to 50% of the vessels design pressure within approximately 15 minutes..."

Many engineers has only considered the second part of the statement.

With the issuance of API Std 521 - ISO 23251, Edition Five, Jan 2007, "Pressure-relieving and Depressuring Systems". In section 5.20, it clearly stated that

"...In order to be effective, the depressuring system shall depressure the vessel such that the reduced internal pressure keeps the stresses below the rupture stress. "

Similar to earlier revision (1997), the time limit of 15 minutes is not a criteria. However, an example cited 15 minutes for a particular material and vessel was exposed to pool fire condition.

A depressuring system shall be designed such that the stress resulted by internal pressure shall always below the rupture stress (included fire scenario). This approach has been adopted for many cases and it appear that those thick wall vessel (more than 90 mm wall thickness) would allow longer than 15 minutes depressuring time. Medium wall thickness vessel (30-90mm) would see depressuring in the range of 3-15 minutes. While those vessel with wall thickness (less than 30 mm) would experience less than 3 minutes of depressuring. This leads to an excessive depressuring load and flare capacity.

Updated : Dec 2008