Q & A > Question Details
We have a butterfly pressure control valve (PV) in hydrogen product line from hydrogen unit to hydrocracker unit without any block valves and bypass line on it. Another PV on this line is fitted in split range arrangement with first PV and connected to flare line. Can anybody explain about the design criteria about installation bypass and block valves on a control valve?
25/06/2008 A: Amarjit Bakshi, Refining Hydrocarbon Technologies LLC, abakshi@rhtgulfcoast.com
It is possible that the Hydrogen Pressure drop requirement were low due unit requirements, so butterfly valve was installed. By putting a bypass line with gate and globe valves would have required much higher-pressure drop when using bypass operation, and would not have served the purpose, as the pressure would have been too low to feed the equipment/unit. The split range valve allows connected to flare allows the hydrogen line to be depressurized and maintenance can be done on the butterfly valve.
These types of situations in present operations should be looked into by analysis "what if" safety conditions so as to have fail-safe situation even if nothing has happened till now. Safety issues have to be taken more seriously with cause and effect analysis, point by point.
09/01/2008 A: KengYong Chan, Air Products & Chemicals, Inc, chanky@apci.com
The main purpose for bypass and block valves around control valve is for replacement/repair of the faulty control valve without shutting down the unit. Reasons for not installing bypass/block valves could be cost reason or the need for bigger relief valve downstream.