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Sunday, January 27, 2008

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There are many shell & tube heat exchangers used in Oil & Gas production platform and Chemical & Process plants for heat exchange and heat recovery. In many cases the design pressure on shell side and tube side are having large differences (e.g. Water-cooled compressor discharge cooler with 100 barg on gas side (Shell) and 15 barg on water side (tube)). Potential tube leakage or rupture is scenario that shall be engineered during design phase.

A heat exchanger with large design pressure difference between shell and tube side (Shell : 100 barg, Tube 15 barg), a pressure relief device (i.e. PSV & rupture disk) is provided to protect the Low Pressure Side (LPS). However, should a pressure relief device required for a heat exchanger with design pressure of High Pressure Side (HPS) of 18 barg) and LPS of 15 barg ? I noticed that there are still doubt among engineers and heavily discuss.

Criteria for Requirement of Pressure Relief Device for Tube Rupture
We may made reference to API Std 521 - ISO 23251, Fifth edition, Jan 2007 “Pressure-relieving and Depressuring Systems” section 5.9.1
“Complete tube rupture, in which a large quantity of high-pressure fluid flows to the lower-pressure exchanger side, is a remote but possible contingency. Minor leakage can seldom overpressure an exchanger during operation, however such leakage occurring where the low-pressure side is closed-in can result in overpressure. Loss of containment of the low-pressure side to atmosphere is unlikely to result from a tube rupture where the pressure in the low-pressure side (including upstream and downstream systems) during the tube rupture does not exceed the corrected hydrotest pressure (see 3.21 and 4.3.2). The user may choose a pressure other than the corrected hydrotest pressure, given that a proper detailed mechanical analysis is performed showing that a loss of containment is unlikely. The use of maximum possible system pressure instead of design pressure may be considered as the pressure of the high-pressure side on a case-by-case basis where there is a substantial difference in the design and operating pressures for the high-pressure side of the exchanger.”
In addition to 5.9.1, engineer shall not miss the second part in section 5.9.2 :
“This type of analysis* is recommended, in addition to the steady-state approach, where there is a wide difference in design pressure between the two exchanger sides [e.g. 7 000 kPa (approx. 1 000 psi) or more], especially where the low-pressure side is liquid-full and the high-pressure side contains a gas or a fluid that flashes across the rupture. Modelling has shown that, under these circumstances, transient conditions can produce overpressure above the test pressure, even when protected by a pressure-relief device. In these cases, additional protection measures should be considered.”
* Dynamic analysis
Thus, a Pressure Relief Device for tube rupture may NOT required if the following criteria are MET :

  1. Corrected Test pressure of low pressure side MORE than Design pressure of high pressure side
  2. Transient peak pressure of low pressure side LOWER than (100%-X%) * Corrected Test pressure of low pressure side
Where X% being a margin between transient peak pressure and corrected test pressure of low pressure side.

Coming related post...Don't miss
  • How to apply Pressure Relief Valve and Rupture Disk for tube rupture service from action perspective ?
  • Confusion on Two-third (2/3) rule or Ten-thirteen (10/13) rule
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posted by Webworm, 1:19 PM


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