Block and Bleed Valves

[mariscor]

Is there any pressure request for installation of double block and bleed valves? It is neccessary to install those valves on the low pressure lines or you can get away with just a block valve and a drain?

 

[DRWeig]

What is the pressure level?

 

[DLite30]

In a general sense, block and bleed valves need to be of at the least the same class as your isolation valve, or of the pressure rating of the isolated system.

 

[mariscor]

The pressure in the system is 150psi

 

[BigInch]

Whether you use DBB or not usually does not depend on the pressure of the system, but rather the degree of need to prevent leakage across the valve. For example, you would not want an expensive product leaking across a valve that was supposedly closing off the sales meter, nor would you want a valve from a gasoline line to a common header leaking and contaminating your diesel that was currently passing through the header, or a valve on a concentrated sulfuric acid line leaking across a common drain and backing into the drinking or cooling water supply, high or low pressure, although the leaked volume might be higher with higher pressure, the damage was probably already done no matter what the pressure or valve size.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[KernOily]

What Big said. The need for DB&B is determined by the process hazard and the owner's HES policy on equipment isolation and line isolation. Even then sometimes there are no easy answers. This is not a cut-and-dried issue at all.

 

[zdas04]

DB&B is a class of isolation called "positive energy isolation" Positive energy isolation is required to protect workers during certain evolutions. That is the only time that I know of that it is required by law. There are many ways to accomplish positive energy isolation including: (1) insert blinds; (2) removing or misaligning piping; or (3) double block and bleed. The first two are appropriate for any work evolution. The third has some restrictions where it can be used.

Now, a designer can choose to provide built-in positive energy isolation for the kinds of things that BigInch mentioned. That is prudent design, not usually mandated.

So for the OP question, install permanent DB&B or not depending on your expected risk profile. It has nearly nothing to do with the design pressure in your line.

David

 

[MortenA]

David - even so; Some companies (e.g. Statoil) has defined that DBB must be used for parallel instllations where one line can be opened while the other is still in service.

Best regards

Morten

 

[zdas04]

I know another company that requires DB&B for all maintenance work. I always use it for meter station bypasses. Engineering judgement can be institutionalized or reside in one person.

David

 

[unclesyd]

If you are in the US don't let OSHA find any problems with the isolation and control of hazardous substances or energy.

http://www.osha.gov/Publications/osha3120.pdf

http://erd.dli.mt.gov/safetyhealth/brochures/lockouttagoutstandard.pdf

The OSHA rules have brought out a lot of new style block and bleed valves. We started using the Parker valves after a near hit on two welders, only a small fire no one was hurt.

http://www.spiraxsarco.com/pdfs/sb/p184_10.pdf

http://www.belloil.com/data/pdf/parker/Double Block & Bleed Manifolds & Flanged Products.pdf