DB&B VALVES 2

[SIMA85]

Hi
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

thanks a lot

 

[TD2K]

"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 block 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.

 

[MortenA]

I agree with TD2K

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.

Best regards

Morten

 

[Ashereng]

TD2K hit it straight on.

The use of DB&B is independent of pressure classes.

Their use is to ensure a better isolation.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[JLSeagull]

If you look at the GPSA for metering skids you will find a double block and bleed valve with individual takeoffs above and below to divert the flow to a prover.

 

[DiegoMartinez]

Gents,

we are using DB&B for level bridles (LB) on the equipment on site which are mainly class 1500#. When they want to perform CFT on the (LB), they close both blocks while the equipment is in operation. The issue is reopening the first block valve adjacent to equipment is extremely hard due to high delta P across the valve. Can anyone advise what is the best practice to reopen this valve?

The valves we are using are Hi-lock DB&B.

 

[BigInch]

Double block and bleed valves are used where positive isolation and assurance from flow leaking from upstream to downstream of a valve is required, nothing else. It has nothing to do with pressure class and little to do with safety. These valves are usually located in a piping path between two liquids that should not contaminate each other, as in a common suction header to a pump feeding a pipeline from either a diesel tank or a gasoline tank, where you will either transport gasoline or diesel in sequential batches. While transporting gasoline, you would want the diesel valve to have been closed, and bleed down, thus assuring that gasoline is not leaking into the diesel and making a mixture that is being feed into the pipeline. Likewise, while transporting diesel, you would want the gasoline valve to be closed and bleed down.
They are also used between or in front of meters at a custody transfer point where it would be unacceptable to have one meter working while the other meter was leaking product across it too. Likewise, it is also common to find these at any point or pipeline branch where a loss of product would be dangerous or could mean lost revenues. This would typically be at custody transfer points from a gas/oil producer to the meters as they enter a pipeline owned by others, between refineries, between process units where revenues are accounted for separately, etc.

 

[BigInch]

London2002. Put a smaller DDB valve in as a bypass. Or maybe change the valve to a globe, plug or a needle valve if its small thing, something that will give you high control of flow at low %open.

 

[DiegoMartinez]

Big Inch,

The connection is 2". Can we use gate valve?

 

[BigInch]

If its already a 2", I'd say that's not a good sign and agree that you do have problem. 1500# class implies that there can be a very high pressure differential that is probably flashing liquids when you reopen. If I had that differential on a pipeline, where normal operating conditions required flow, I would be forced to use two pressure control valves, because it is almost impossible to get one valve to function for any length of time under such a high pressure diff. Your application is better in that respect, since it is a maintenance condition in which you experience this problem, not normal ops. But still, I would not use a gate. With gates at that pressure diff, they are basically either full open or full closed. I would tend to try a reduced port plug valve, maybe even a special port plug, although a globe may be your best bet. Not knowing the scale of your equipment, I hate to suggest that you put a bypass around a 2", but ... somehow you will need to get some fine pressure control when reopening.

 

[DiegoMartinez]

Thanks Big,

I am just asking with using a globe, do we eliminate this problem? I know this problem exists with balls forever. As this valve upstream of Level Bridle is normally fully open or closed, I do not see any issue with using Gate valves.

Cheers

 

[BigInch]

You could try using a gate. I've seen some special port gates that may open with more control, but I would still prefer a globe or plug. This high dP is really a problem. Its almost impossible to control with a one valve step down in a continuous flow situation, but you will probably be able to get away with it using one globe or one reduced port plug valve, if it does not have to operate all the time. Really, globes or plugs will give you a much better chance trying to control this high dP. You should have this talk with your valve supplier.

 

[DiegoMartinez]

Like your recommendation. It is better to use 2 globes and a bleed in between.

By the way, can I ask where you are working? Would you mind me asking for your email address? I am in Australia.

Rgds

 

[BigInch]

London2002

Sure thing. Please see,

http://virtualpipeline.spaces.msn.com/

 

[morgan1952]

Another justification for double block and bleed is custody transfer. If the well you are producing is owned by a partnership that shares percentages of profit, a demonstratable seal should be required of any block valves involved in the process of measurement and accounting.

 

[MARKFLOW]

Go to

http://www.olivervalves.com,

they have a kick ass D.B & B, using two balls in one ANSI B16.10 Body length. It works real well.

MARKFLOW

 

[dcasto]

I disagree about the safety aspect, a DBB is very desirable where a piece of equipment will be taken out of serfice, like pumps, compressors, site gauges, ect. All our ANSI 1500 pumps had DBB 8" valves on suction and discharge.

 

[MortenA]

dcasto could you elaborate where the disagreement lies?

So far i have only seen arguments that DBB should be used for positive isolation of equipment. One example is HP systems e.g. for parallel trains.

Best regard

Morten

 

[dcasto]

from one post above:
"Double block and bleed valves are used where positive isolation and assurance from flow leaking from upstream to downstream of a valve is required, nothing else. It has nothing to do with pressure class and little to do with safety."

DBB are very much a part of a safety program especially if you have a LT&T system in a facility.

 

[MortenA]

I think that you take biginchs answer out of context. Off course its for the safety of those that you do the positive isolation in favor of - but its not safety the way e.g a PSV or at TAHH alarm is.

Best regards

Morten