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Any time you&#;re going to work on a valve, separator, or other piece of equipment on a well site, we highly recommend you use the practice &#;Double Block and Bleed.&#;

The Hiss

Maybe you&#;ve had this experience.

You&#;re going to repair a valve that&#;s in line. You know this means you need to block off and depressurize the section you&#;re working on. So you close off the ball valve upstream, open the bleed valve to vent pressure, and get to work.

You square up to the valve in need of repair and start unthreading bolts. Then you hear it: a hiss. You quickly snug the bolts back down.

I could have sworn I just isolated this valve, you think. What did I miss?

One possible cause of the hissing in this scenario is that the valve is getting pressure from downstream.

What is Double Block and Bleed?

Double block and bleed (DBB) is the practice of shutting in a section of pipe on both sides of the valve rather than just one.

It means you close the ball valves to block both the upstream and downstream sides of your working area, and then bleed any pressure that remains in the piping and valve.

The practice of double block and bleed reduces the risks of harm to people and the environment, which is why many oil and gas companies have adopted the practice as mandatory when working on field equipment.

Stay safe out there!

(Note: This article refers to the safety practice; double block and bleed may also refer to a specific kind of valve.)

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DBB Valve Selection

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DBB Valve Selection

DBB Valve Selection

Valving

(Civil/Environmental)

(OP)

19 Apr 11 08:09

There are many experienced members and I would like to receive their comments regarding the DBB valve selection which is not very clear anywhere.
In my work, I located that the types of DBB valves are as follows
1.DBB Ball Vales (consisting of 2 valves)
2.Twin DBB Plug Valves (consisting of  - 2 stems 2 plugs)
3.DBB Plug Valves
4.Through Conduit Slab Gate Valve
5.Through Conduit Expanding Gate Valve
6.Trunnion mounted ball valves with bleeding
7.Any other?

I understand that for real critical and high pressure applications and sealing must be granted, 1 and 2 is the best way. Pricewise, probably 2nd will be cheaper.

I read that 3,4,5 may not be providing good tighness.

I would be glad if you could share your comments
1.When you prefer 1 and 2?
2.I believe 1st option has limited sizes.
3.When could 3-4-5 be preferred?
4.Could anyone compare the prices of 1-2-3-4-5?

  
rgds to all

 

Replies continue below

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RE: DBB Valve Selection

sheiko

(Chemical)

19 Apr 11 20:49

Just for information: http://www.eng-tips.com/faqs.cfm?fid=

"We don't believe things because they are true, things are true because we believe them."

RE: DBB Valve Selection

Valving

(Civil/Environmental)

(OP)

21 Apr 11 04:15

Thanks for comments, I did not readh that clarification but similar ones.
My concern is more for other valves. plug, gate etc.  

RE: DBB Valve Selection

zdas04

(Mechanical)

21 Apr 11 07:09

I'm not sure why you've included gate valves in your list.  They tend to not even be bubble tight, let alone suited for a poaitive energy isolation scenario.  Same with plug valves.

And as to your question 4--yes, many people can compare the prices.

David

RE: DBB Valve Selection

Valving

(Civil/Environmental)

(OP)

25 Apr 11 03:49

David thanks for your input.
Few companies specify Gate Valves as DBB and couple of vendors mention that the valves apply under the API6D design and testing, and insist that they are a zero leakage valve. (API598, API6)

Any further comments will be appreciated.

RE: DBB Valve Selection

HitokiriAoshi

(Mechanical)

27 Apr 11 21:27

I will treat this as just, Double BLOCK, and bleed out the leak.

A trunnion ball valve with single piston effect seats on the upstream and downstream side with a bleed hole/valve in the cavity would be your lowest cost bet depending on size.

for gate valve and plug valves, you would need 2 of them, with a blood valve in between the two for it to act as double BLOCK and bleed.

You need to see how critical is this double block and bleed, for example, if it's just water/waste water, and you are just DBBing for maintenance purpose down the 2 valves, lowest cost solution is best, and it may be single ball valve or double gate valves depending on size.

However, if you need absolute isolation, such as poisonous or reactive chemical, i would use bubble tight valves and do true double block and bleed, maybe even the extra mile by using 2 DBI valves in a DBB configuration.

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