Vent Header Isolation and Diversion

[bif]

I currently have a relief vent header (potentially containing a flammable gas stream) that is routed to a scrubbing system and subsequent downstream processing.
Under certain circumstances (ie loss of scrubber water flow) there is a requirment to 'bypass' the scrubber and downstream processing line and relieve the vent header locally by an automatic switching of isolation valves in the header.

Obviously by having automated isolation valves in the header there is a potential that the switch over does not work properly and the header line is isolated with both outlet valves closed thus redering the relief devices that relive into the header useless.

Are there any standards that state this cannot be done as it could potentially isolate a relief device?
If it is acceptable are there any standards that cover how reliable the switch over valves need to be?

 

[insult2injury]

I have seen three-way valves (where flow is open to both headers when the valve is in an intermediate position between open and closed) used in similar installations.

I2I

 

[JoeWong88]

I think you need to achieve SIL4 with probability of failure on demand lower than 10-4. I guess it will difficult to achieve it . What you can do is to provide a bypass with rupture pin to cater for failure on demand. But, the new challenge is the setting of this rupture pin.

Whenever the system equipped with "automated" diverter valve, the failure of the diverter valve will lead you to have additional ultimate safeguarding. May consider manual operation with interlock like interlocking system for 2 x 100% PSV inlet & outlet isolation valves...