Emergency Depressurization (EDP) System of Hydrocracker Reactors:
Our Hydrocraker unit has two emergency depressurization systems : 7 bar/min (100 psi/min) and 21 bar/min (300psi/min). Assuming a pressure of 157 barg when 21bar/min EDP is activated, how long should pressure decrease at a rate 21bar/min? how does depressurization rate vary ?
keith bowers, B and B Consulting, email@example.com
The question cannot be answered with the information available. The EDP valve flow coefficient is fixed, so mass flow rate will decline with pressure drop across the valve. A significant confounding factor is the 'light ends' being produced in the reactors during the de-pressuring. Depending on the cracking severity, time to reach a 'safe pressure' can increase substantially. Dissolution of hydrogen, inerts, and light ends also contributes significantly to total mass to be vented.
The approach to estimating an answer also depends on the level of accurracy desired. Exact answer requires development (and solution)of differential equation precisely describing the behaviors mentioned above and the obvious factors of course. Solution of this equation is possible to reasonable precision with successive approximation.
A physical solution to minimizing depressuring time is a modulating 'drag valve(s), programmed (in the DCS) to maintain the desired pressure decay rate (delta P/T), opening the valve further as the pressure decays.