the DBB ball valve - some informations

[lele03]

Can somebody explain why some companies require that the double block be two independent valves separated by some distance and others accept the trunion ball valves with a adequate functioning body bleed?

Does it depend from working conditions? If yes, what?
Moreover, where are used, normally? On which installation?
About the safety both constructions has the same? If no, why?

Thank you
Lele (petroleum)

 

[OCANDOA]

Perhaps at that time, the use of three valves (Two independent elements and a bleed) was the only possible option. And since the design has proven to meet their demand it has been standardized and continues to be applied.

Here are some comments on using a single DB&B valve:
- The use of three valves increases the number of connections, therefore possibility of leaks.
- More weight and space is required which is not good in offshore applications.
- The safety integrity level of the installation is higher, therefore the insurance costs.
- Reduction of installation costs and downtime. (Most DB&B valves have a standard face to face that allows a simple installation)

The applications for DB&B on Oil and Gas are:
Manifolds, Headers, Bypass valves, Water injection valves, Gas injection valves, Isolation valves to control valves, Isolation of compressors, Emergency shutdown, Emergency Blow down, Vents and Drains. ANYWHERE A RELIABLE BLOCK IS REQUIRED FOR DOWNSTREAM MAINTENANCE, ISOLATION, AND/OR A BLOWDOWN OF A PRESSURIZED LINE.

The downside of using a single valve is that their initial cost is higher (but you might save on installation costs and downtime).
Maintenance or failure of one component could have a consequence on the rest, if they are all in one body.

 

[NGiLuzzu]

OCANDOA,
good post, but... why should the insurance costs be higher for higher safety integrity levels?
I would say the opposite...


lele03,
please take also a look at thread408-150132 within this Forum, posts dated "24 Mar 06" and following.


Hope this helps, 'NGL

 

[lele03]

Thank you very much for your replies.

 

[OCANDOA]

I am sorry, you are right. Insurance costs should be lower!

 

[Ashereng]

The use of three valves increases the number of connections, therefore possibility of leaks.

I do not mean to offend.

I would rather say that with more valves, more possible routes for leaks, but with two valves in series, better chance for shutoff.

I think that the 3 valve set-up still has it's place. 2 valves in series gives a better probability of shutting off flow than a single valves (even a single DBB valve).

We often use 2 valves in series in critical shutoff service (also SIL 2 and SIL 3 applications) for the fact that there is a better chance of shutting off the flow with 2 valves, than 1.

The bleed is to relieve thermal expansion between the valves.

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