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Thursday, September 18, 2008

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There are quite a number of posts are circulated around the Shell & Tube Heat Exchanger tube rupture :
Nevertheless there are still quite a lot of issues still needs further studies.

Surge Pressure more than MAWP of LPS even NOT blocked-in...
Whenever a tube rupture occurred, compressible fluid (vapor) in High Pressure Side (HPS) flowing into Low Pressure Side (LPS) with incompressible fluid (water), there is momentary surge. This momentary surge would possibly lead to instantaneous peak pressure which possibly exceeded the Maximum Allowable Working Pressure (MAWP) of the Heat exchanger LPS. One shall noted that the word "possibly" but not always the case. It is very difficult to quantify this surge pressure as it subject to where & how tube rupture occurred, fluid impinging the shell wall, resonance occur, etc. A very special phenomenon that you may not believe, surge pressure is possible exceed the MAWP of LPS even though the LPS side is not blocked in especially those system with rotating equipment, check valve, control valve, etc. Of course, blocked in condition would seriously increase the possibility of surge pressure exceeded MAWP. Thus, a dynamic surge simulation & analysis may provide the answer.

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Qualification for Not Having PRD for Tube Rupture...
Many designers still arguing about 2/3 rules and 10/13 rules. The post "Two-third (2/3) rule or Ten-thirteen (10/13) rule ?" would probably provide some idea. Whenever above rules is applied, one shall used the correct terms in rule application. Read more in "Criteria for Requirement of Pressure Relief Device for Tube Rupture". Regardless of which rules is used for a specific case, the principle still same :

  • Corrected Test pressure of low pressure side MORE than Design pressure of high pressure side
In case above statement for LPS is qualified, NO Pressure Relief Device (PRD) is required. Nevertheless, another question surfaced. Where shall the LPS design pressure stop ?.

Does it stop at the first valve at the inlet and outlet of the heat exchanger ?
Does it extended to entire network after the first valve ?

First point : The LPS design pressure shall be at least upto the first isolation valve at the inlet and outlet of the heat exchanger.

Second point : The LPS design pressure will be extended to equipment or device which possibly block-in. For example manual block downstream which is normally open, filter, solid bed, etc. All these items, possible partial or total blockage.

Third point : In the event, there is no equipment or device possibly block-in and overpressure the piping, the LPS design pressure may not need to be extended to the downstream piping. However, another phenomenon shall be addressed and checked to ensure the LPS design need not be extended. Fluid from HPS side will flow into LPS, fluid in HPS (compressible) may expand. Apart large flow passing through the ruptured tube. Both expansion and large flow discharge into LPS will get into the downstream piping network and release some way downstream via PRD. A high back pressure may occur at the location of the ruptured tube heat exchanger. This back pressure might exceed the MAWP of the piping. Thus, one shall calculate the back pressure along the relief path and ensure pressure at any point in the piping is below the MAWP of piping.

Some engineers may consider short term overpressure excursion rule as allowed in ASME code to quality non-credible tube rupture scenario. The only recommendation that can be advised here is this rule may be used with care.

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posted by Webworm, 1:57 PM


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