Piping upstream of safety valves

It is generally recommended that the frictional pressure drop in the piping between the vessel and the safety valve inlet should be limited to no more than 3% of the gauge set pressure of the safety valve[19J in order to prevent instability. This instability occurs because when the valve is closed, there is no flow and no pressure drop, so that the pressure in the reactor is high enough to open the valve.-However, when flow develops the upstream pressure drop causes the pressure at the valve to 

Only the irreversible frictional pressure drop should be included in the calculation of upstream pressure drop, not the momentum pressure drop required to accelerate the fluid to the velocity at inlet to the valve. The irreversible frictional pressure drop includes both friction in the inlet contraction from the reactor.(K = 0.5 for a sudden contraction1151) and friction in the piping, bends and any fittings.  

The calculation of the inlet frictional pressure drop can be done using the HEM model for two-phase flow. A simplified conservative calculation may be made by assuming incompressible flow, with a two-phase density equivalent to that at the inlet-to the safety valve . .  

The flow rate used for this calculation should be the best estimate flow rate for the safety valve. BS 6759 19] requires that any safety factors used in determining G for relief system sizing, including the 10% de-rating of the safety valye discharge coefficient, should be removed. The actual flow area through the valve should be used.,  

A value for 4f of 0.02 is often used for turbulent two-phase flow (see 9.6.1). 

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