Relief valve fire case temperature and pipe pressure design

> A relief valve is provided on a pig launcher. The relief valve is a flanged 1>
"> 300# inlet x 2> "> 150# outlet. The RV is provided for the purpose of relieving internal pressure that is likely to build up within the closed system in the event of an external fire. The fire case considered is when the launcher is filled with blanketing gas.
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> The relieving temperature has been arrived at 554 deg C. The basis for this is
from the Heat Absorption equation given in API RP 520 Part 1 section D.5.2, where in the upstream gas temperature equals P1/P2*T2 (Suffix 2 represents the normal operating temperature and pressure and suffix 1 represent the relieving set pressure condition. Temperatures are in degree Rankine Absolute). The normal operating pressure is 18 bar (g), RV set pressure is 46.37 bar (g). The set pressure is based on the pipe class design maximum, which in turn is based on the 300# flange pressure rating. The maximum operating temperature is taken as 60 deg C.
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> Consider the pressure design of the connected 1> "> pipe for this case.
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> When the temperature variation is self limiting (due to the pressure relief)
the design code ASME B31.3 para 302.2.4 (f) (2) allows an excess up-to 20% on the allowable stress for the pressure design.
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> The allowable stress at the 554 deg C is only 2.369 ksi including the 20%
additional allowance allowed. The thickness required becomes more than sch 160 provided in the pipe class for the 1> "> size. The pipe material is ASTM A106 Gr B.
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> Request the group to comment on the pipe pressure design considerations above.
Will the fire case fall into the temperature variation as given in the code. The fire case should be occurring for a lesser duration than given therein, > "> no more than 50 hr at any one time and not more than 500 hr/year> "> .
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Sajit Viswan Received on Tue Oct 07 23:18:00 2003