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Double Block And Bleed Valve (Dbb)


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#1 Afshin445

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Posted 19 November 2014 - 06:25 AM

Dear Forum Experts,

 

Currently I am working in a project which is including installation of a new dehydration skid in existing platform.

Project will execute in two phases. In 1st phase of project we are going to install new tie-in valves for all of tie-in connection from skid to existing lines which planned for next plant shutdown.

 

In 2nd phase of project new skid tie-ins will be connected to tie-in valves which installed during phase 1 when plant is in operation.

 

According client spec for all of hydrocarbon lines with Ansi rating of 300# and above DBB valves is required.

 

The problem I am facing now is client requested all DBB bleed valves shall be routed to flare and closed drain header (For gas service to flare and for liquid service to closed drain), which I never saw this before.

 

Honestly I dont have any experince in such a specific area and also I didnt find any standard or spec for such a problem.

 

Appriciated if you can share your experince and knowledge about this client recommendation.


Edited by Afshin, 23 November 2014 - 07:31 PM.


#2 paulhorth

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Posted 20 November 2014 - 09:25 AM

Afshin,

I have come across this requirement before, in certain client standards, but only for lines in sour service.

 

Be sure to route the bleeds to the Low Low Pressure flare or Atmospheric Vent. If the bleed is connected to a flare system which can be under pressure, it will defeat the purpose of the DBB, which is to ensure that the bleed cavity is at atmospheric pressure so that no leakage can occur across the downstream valve.

Because of this potential risk, in my opinion it is safer not to route the bleeds to any system but to use a hose to route them to atmosphere at a safe location during the operation of the DBB. In sour service this can still be done so long as the hose outlet is carefully located.

 

Paul



#3 shan

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Posted 20 November 2014 - 10:49 AM

If you route the DBB bleed to flare/drain system, how do you detect the possible leak from the pressure side?



#4 paulhorth

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Posted 22 November 2014 - 07:41 AM

Shan,

 

While it's useful to know if the upstream valve is leaking, the safety of the DBB system does not rely on detecting the leak. So long as the bleed cavity is vented to atmospheric pressure, there can be no leak across the downstream valve, and the downstream system can be opened safely.

 

Paul



#5 Afshin445

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Posted 22 November 2014 - 09:51 PM

Paul/Shan,

 

First of all thank you for your useful reply.

 

In the second place I forgot to mentioned gas is not sour service. In this regards, I am totally agree with Paul in routing bleed valves to atm. in safe location with using flexible hose.

 

But the question came across in my mind is this solution is good for gas service what about liquid service? is it correct if we routing liquid service bleed valves to a portable collecting drum in safe location? 

 

What is your recommendation if by opening bleed valves shown upstream valve is leaking? Need plant shoutdown for replacing damage upstream valve or they can continue to doing tie-in connection between new skid and tie-in valves?


Edited by Afshin, 23 November 2014 - 04:13 AM.


#6 fallah

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Posted 23 November 2014 - 02:18 AM

Afshin,

 

Routing liquid service bleed valve to a portable collecting drum in a safe location could be possible provided that the operation will be accompanied with no liquid flashing...

 

To make a proper decision for upstream valve leaking, the leakage intensity and subsequent consequences should be investigated...



#7 Ikka

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Posted 23 November 2014 - 08:27 AM

I believe the rationale behind DBB request from client is to have shutdown free tie-in execution. The bleed provided will be used only before the tie-in connection in your Phase-II.

 

I suggest for liquid, to connect a hose and chicago fitting to the bleeder and route to temporary drum/oily sewer during the tie-in execution then slowly open the bleeder valve to check valve passing. If there is no leakage install a blind upstream of the second valve and continue with tie-in. A pressure gauge can also be use prior to check whether the valve is leaking or not. If leakage is found check the second valve bleeder if that valve is passing. It is very less probable to have both valves leaking.

 

In case of Gase, I suggest to install pressure gauge with venting facility and check if the valve is leaking or not. 

 

If any sour service route to a drum or to sour sump. 



#8 Afshin445

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Posted 23 November 2014 - 10:42 AM

Fallah/Ikka,

 

Thank you for your prompt replies. I summarize your recommendations as per below:

 

1-Install a pressure gauge in bleed valve location for detecting possible leakage from upstream valve.

2-If pressure not detect means no possible leakage and tie-in execution can be continued

3-In case of detecting any pressure, DBB cavity shall be drain/vent to safe location by using flexible hose.

4-After vent/drain DBB cavity and no more leakage detect tie-in execution can be continued.

 

Please correct me if I am wrong.






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