Valve Type Selected Used in Emergency Depressurization (EDP) System
We use ball type valves for EDP system in our existing hydro-treating and hydro-cracking units. And they work well.
Now, we want to select Triple Eccentric Butterfly valve for EDP system in our new project. Please advise.
Answers
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26/10/2011
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A:
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G KAMALAKANNAN, Foster Wheeler India Ltd, siddhukamal@yahoo.co.in
Butterfly valves are not considered to be FULL BORE valves. For any isolation valve related to flare system like PSV inlet/outlet valves or Emergency Depressurisation valves shall be of full bore type to avoid blockage of flow.
Secondly as explained in previous answer, please check whether the RO sizing (at upstream of emergency depressuring line) is adequate and can reduce the system pressure to 50% of design pressure in 15 minutes. Accordingly, please check the outlet line sizing as per MACH number criteria. Main flare header and Flare tip capacity should also be checked for the peak flow rate through this line. Larger RO size will cause higher flow during the start of the depressuring and flare header may not be adequate. Smaller orifice will delay the depressuring process and system pressure can not be reduced to 50% of design pressure within 15 minutes. Hence, an optimum size of RO should be selected.
Alternatively, as suggested in previous answer, modulating type control valve may be selected. However, as per my knowledge, control valve can not be connected to ESD (PLC) system. Emergency Depressuring valves should be hard wired to PLC systems to increase the reliability of instrumented system.
Please consider the above factors while designing the EDP valves and ROs.
Question does not indicate the type of system for which EDP is required. Based on the type of system (Liquid filled vessel or Vapor Liquid Equilibrium vessel), the calculation procedure will vary to arrive at the depressuring flow rate.
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24/10/2011
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keith bowers, B and B Consulting, kebowers47@gmail.com
Why change from what works well? Sometimes it is prudent to reduce reactor pressure to less than 60% of rated pressure as quickly as possible to reduce possibility of rupture if fire should impinge directly on reactor shell. Often the maximum venting rates of a hydro-cracking/hydro-processing complex will be very large and require a separate high pressure flare header and flare to avoid high built-up back-pressure difficulties at large low-pressure distillation columns.
Dynamic simulation of these complex systems often shows distillation columns will not overpressure on loss of reflux for 20 minutes or more. IF the large blow-down quantities from the high pressure hydro-cracking units can be completed before distillation towers would potentially over-pressure, then the separate (and very costly) high pressure flare system can be avoided.
However, large reactors may also have maximum depressuring rates allowable to prevent damage to catalyst bed support support structure. It is then desirable to maintain depressuring rate constant at the maximum allowable rate as the reactor pressure decays.
Achieving the sustained maximum allowable rate depressuring of the hydro-cracking complex will require modulating the depressuring valve(s) to limit flow to safe rates when pressure is high, and then increasing the depressuring valve opening as pressures decay. 'Fixed CV' depressuring systems, such as restriction orifices or reduced valve CV to meet maximum allowable depressuring rates, will not achieve the rapid depressuring needed to avoid the separate flare system.
Some valves are not suitable for throttling service such as this. Valve operating forces are often high and flow modulation difficult to achieve reliably. 'Drag Valves' are extremely well suited for this type of service wit low operating force and easy to modulate flow rate capability.
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