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Wednesday, February 20, 2008

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This is a rather doubtful query and have debated "n" times in many forums. Reason being tube is not part of pressure vessel code (i.e. ASME Section VIII) and thus NO Pressure relief device (i,e. PSV) is required. I do agree with the statement that PSV is NOT required IF design and fabrication code for tube side do not demand present of PSV. This solely from CODE application perspective.

Nevertheless, we shall also review from other relevant codes compliance and non-code perspectives.

i) API STD 521 compliance - Nowadays in general most plant design to API STD 521 compliance. API STD 521 2007, section 4.3.1 stated "The process-systems designer shall define the minimum pressure-relief capacity required to prevent the pressure in any piece of equipment from exceeding the maximum allowable accumulated pressure. The principle causes of overpressure of overpressure listed in 4.3.2 through 4.3.15 are guides to generally accepted practices." and 4.3.14 stated "plant fire" is one the potential cause of overpressure. As S&T HX tube side front end and rear end is potential expose to direct external fire heat input and there is potential of heat input from external fire to shell side fluid which further transfer to tube side, thus there is potential of overpressure cause by external fire and it demand pressure relieving.

ii) API RP 14C, 2001 compliance - Oil & facilities are commonly designed to API RP 14C, 2001 compliance. Following appendix A.10. Heat Exchanger (Shell-Tube), A.10.C.2 "Each input source is protected by a PSV set no higher than the maximum allowable working plessure of the heat exchanger section and a PSV is installed on the heat exchanger section for fire exposure and thermal relief.", a PSV may be required on tube side.

iii) Pinhole leak - There are many heat exchanger has pinhole leak due to thermal shock, thermal stress, flow induced vibration, erosion, corrosion, etc. These kind of pinhole leak may not be detected during normal operation as the leakage is rather small as compare to process fluid flow. In the event of fire, entire S&T HEX may be totally isolated. External heat input to Shell side and result pressure increase. Pinhole leak would lead to continuous flow from Shell side to tube and subsequent overpressure the tube side.

iv) Inventory associated Risk - Requirement of PSV shall NOT be 100% judged from the design code itself. The RISK and CONSEQUENCE e.g. INVENTORY associates risk & consequences shall come into consideration to define if a overpressure protection i.e PSV is required. Although the code does not call for it but from risk point of view and insurance premium perspective, provision of PSV may be a more practical way of handling it. Read more in "Requirement of overpressure protection devices on system design to PIPING code".

Thus, it is advisable to consider providing a PSV protecting tube side overpressure cause by external fire.

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posted by Webworm, 1:30 PM


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