Depressuring within 15 minutes no longer applicable ?

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="background-color: white; font-family: Arial, Helvetica, sans-serif; font-size: 12px; text-align: justify;"> 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.</div> <br style="background-color: white; font-family: Arial, Helvetica, sans-serif; font-size: 12px;" /> <div style="background-color: white; font-family: Arial, Helvetica, sans-serif; font-size: 12px; text-align: justify;"> 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.<br /><br /><span style="font-weight: bold;">API RP 521 Edition Fourth,</span><span style="font-weight: bold;"> "Pressure-relieving and Depressuring Systems</span>" section 3.19 has a statement<br /><blockquote style="background: url(http://webwormcpt.blogspot.com/2008/01/quote.jpg) 10px 0% no-repeat; border: medium none; font-style: italic; margin: 20px 0px; padding: 0px 20px 0px 50px;"> "<span style="color: #6600cc;">...A vapor depressuring system should have </span><span style="color: #6600cc; font-weight: bold;">adequate capacity to permit reduction of the vessel stress to a level at which stress rupture is not of immediate concern</span><span style="color: #6600cc;">. For sizing, this generally involves </span><span style="color: #6600cc; font-weight: bold;">reducing the equipment pressure from initial conditions to a level equivalent to 50% of the vessels design pressure within approximately 15 minutes</span><span style="color: #6600cc;">...</span>"</blockquote> Many engineers has only considered the second part of the statement.<br /></div> <div style="background-color: white; font-family: Arial, Helvetica, sans-serif; font-size: 12px; text-align: justify;"> With the issuance of <span style="font-weight: bold;">API Std 521 - ISO 23251, Edition Five, Jan 2007, "Pressure-relieving and Depressuring Systems</span>". In <span style="font-weight: bold;">section 5.20</span>, it clearly stated that</div> <blockquote style="background: url(http://webwormcpt.blogspot.com/2008/01/quote.jpg) 10px 0% no-repeat rgb(255, 255, 255); border: medium none; color: #6600cc; font-family: Arial, Helvetica, sans-serif; font-size: 12px; font-weight: bold; margin: 20px 0px; padding: 0px 20px 0px 50px;"> <span style="font-style: italic;">"...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. "</span></blockquote> <div style="background-color: white; font-family: Arial, Helvetica, sans-serif; font-size: 12px; text-align: justify;"> Similar to earlier revision (<span style="font-weight: bold;">1997</span>), 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.</div> <br style="background-color: white; font-family: Arial, Helvetica, sans-serif; font-size: 12px;" /><span style="background-color: white; color: #cc0000; font-family: Arial, Helvetica, sans-serif; font-size: 22px; font-weight: bold;"></span> <div style="-webkit-text-stroke-width: 0px; background-color: white; color: black; font-family: Arial, Helvetica, sans-serif; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: justify; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px;"> A depressuring system shall be designed such that the <span style="font-weight: bold;">stress resulted by internal pressure shall always below the rupture stress</span> (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.<br /><br /><span style="color: #cc0000; font-weight: bold;">Updated : Dec 2008</span></div> </div>

Depressuring within 15 minutes no longer applicable ?

Recently i have discussed some issue related to system depressuring with some engineers. One of the topic where we were arguing about was ...

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