Chemical & Process Technology_ Two-Third (2_3) Rule or Ten-Thirteen (10_13) Rule

5/2/13 Chemical & Process Technology: Two-third (2/3) rule or Ten-thirteen (10/13) rule ?

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Two-third (2/3) rule or Ten-thirteen (10/13) rule ?

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

ASME Vessel Pressure API Flange ASME Flange

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Useful Documents Related to Pressure Relief Valve

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Pressure Relief Valve (PRV) has been widely use in

overpressure protection of pressure containment

equipment. Regardless of vessel/column/drum/tank

designed in accordance to any pressure vessel code

e.g. ASME, JIS, BS, GB, etc,...



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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 Webw orm, 2:19 PM

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4 Comments:

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

Webworm said...

Thanks swakee. Updated.

June 6, 2008 at 1:51 PM

Anonymous said...

Relief Valve

API Flange

ASME Flanges



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

Webworm said...

Sharp ! Thx

September 19, 2008 at 5:26 PM

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Chemical & Process Technology_ Two-Third (2_3) Rule or Ten-Thirteen (10_13) Rule

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