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The meaning of UOP words is “the target depressuring rate is the rate that will decrease the system pressure to 20% of normal operating pressure in 15 minutes” is 41*0.20=8.16 Kg/cm2 in 15 minutes. This does not mean 41*0.8=32.8 Kg/cm2
This is because;
1.Actual rate of EDS (Emergency Depsressurization) as we observed at our DHT is 15 minutes and the system pressure reaches ~ 7~8 Kg/cm2 while the operating pressure is 36.5 Kg/cm2. As it is tested many times in our emergency situations of power failure.
2. As per D.S.J. Jones and S.A. Treese Handbook of Petroleum Processing, page 1336, in the hydroprocessing units, the unit should be at flare pressure within about 15 min. But if it is 32.8 it will no fulfill the requirements of depressurization.
3. As the purpose of EDS is to save the reactor from temperature run away or fire, then if the system pressure is still at 32.8 kg/cm2 and all the hydrocarbons are still in the reactor then, temperature run away will still continue.
4. As per API 521, A vapor-depressuring system should have adequate capacity to permit reduction of the vessel stress to a level at which stress rupture is not of immediate concern. For pool-fire exposure and with heat input calculated from, this generally involves reducing the equipment pressure from initial conditions to a level equivalent to 50 % of the vessel’s design pressure within approximately 15 min. The rate of depressurization for the hydrotreater is equivalent to 50 % of the vessel’s design pressure within approximately 15 min, i.e. ~20 Kg/cm2 if the design pressure is 40 Kg/cm2.As per the above references and personal experience, the depressurization rate to 32.8 Kg/cm2 from 41 Kg/cm2 in 15 minutes can not be justified.
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