I have a very very urgent question about application of double block & bleed valves,single block & bleed valves ; where and for what working classes these are necessary ?
My question is for a crude oil metering system and the maximum working class is 600# ; please inform me about some site addresses

"where and for what working classes these are necessary ?"

They aren't necessary for any piping class in the sense 'over this rating, you  must provide x, y and z'.  Your question is a little broad so I'll go through a very general description.

That said, it's tyically driven out of operability and safety concerns.  Single Check Valves are for stopping the flow.  They also provide isolation if you need to work on downstream equipment.  Think of the block valves around a control valve.

Single block with a bleed valve is to allow you to drain either the upstream or downstream side, depending which side (or both) the bleed is one.  Also common on control valve manifolds for that reason.

Double block and bleed are used to provide additional safety and/or isolation.  Essentially, you have two closed valves with an open bleed between them.  If both valves are closed and the bleed is opening and functioning, you know you can't get flow into the section that has been isolated.  Blinds are another way of providing positive isolation.  On a metering stations, I've also seen double block and bleeds used to ensure no flow through a bypass where it would affect the accuracy of the meters.

To be more specific i have often seen that on parallel trains where one train may be taken out of service while the other are running (and here metres could be a good example) and where the pressure class is =>900# there you will ud DB&B. But the use will often depend on company standards.

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