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Wednesday, January 30, 2008

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I have noticed there are many engineers still asking a simple question. In determining if a Pressure Relief Device is required for tube rupture, should we apply two-third (2/3) rule or ten-thirteen (10/13) rule ?

For those who has read my earlier post “Criteria for Requirement of Pressure Relief Device for Tube Rupture” may have aware that two-third (2/3) rule or ten-thirteen (10/13) rule are basically derivation of above criteria. Both are correct BUT subject to design code and revision of vessel being designed to.

WHY two-third (2/3) rule ?
Earlier revision of ASME required that equipment and piping be tested at 150% of stated design pressure. If the equipment design pressure is 15 barg, then the test pressure must be 22.5 barg, thus 15/22.5 = 2/3. API RP 521 (1997 edition) stated that if the design pressure of the low-pressure side (LPS) is at least 2/3 of the design pressure of the high pressure side (HPS), tube rupture is not considered a credible relieving scenario.

WHY ten-thirteen (10/13) rule?
In latest ASME, it stated that the test pressure of equipment to be 130% of the design pressure. If the equipment design pressure is 15 barg, then the test pressure shall be 19.5 barg, thus 15/19.5 = 10/13. Similarly follow API RP 521 (1997 edition) statement, if the design pressure of the low-pressure side (LPS) is at least 10/13 of the design pressure of the high pressure side (HPS), tube rupture is not considered a credible relieving scenario.

In latest API STD 521, a more reasonable statement is the corrected test pressure of low pressure side (LPS) is more than the design pressure of high pressure side (HPS), tube rupture may not be considered a credible relieving scenario. This statement is applicable to any test pressure requirement.

WHY both are correct ?
One shall remember, the equipment may be designed to latest ASME code which called for test pressure to be 130% of equipment design pressure. Then ten-thirteen (10/13) rule is applied. But if the vessel is designed to AS (Australia), GB (China), etc which both standards still call for equipment test pressure to be 150% of equipment design pressure with stress correction, then two-third (2/3) rule still applied. Thus, whenever consider a tube rupture scenario, the design code of the vessel shall always be taken into consideration. Nevertheless, latest API STD 521 statement still applicable in conjunction with equipment design to any code (ASME, AS, GB, JIS, etc).

Updated :
June 07, 2008 : "...within" changed to "...at least". Thanks to swakee.
Sept 20, 2008 : "22.5/15" to "15/22.5"...

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posted by Webworm, 2:19 PM

4 Comments:

Blogger swakee said...

As per this article for 2/3rd rule, it says "if design pressure of low pressure side (LPS)is within 2/3 of the design pressure of the high presure side, tube rupture is not consideredHigh pressure is not considered a credible relieving scenario".
But API 521 says, the low pressure side is designed for atleast 2/3 of the design pressure of the high pressure side. Please correct the rule accordingly, otherwise this will mislead the people who refers this article.

June 6, 2008 at 4:05 AM  
Blogger Webworm said...

Thanks swakee. Updated.

June 6, 2008 at 1:51 PM  
Anonymous Anonymous said...

In your "WHY two-third (2/3) rule?" paragraph, i think 22.5/15 is more than 1, so it should be 15/22.5.

September 19, 2008 at 4:26 PM  
Blogger Webworm said...

Sharp ! Thx

September 19, 2008 at 5:26 PM  

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