Double Block and Bleed -> Requirements 1

[LSKF]

Hello,

I would like to know what are typical Double block and bleed requirements. On our project we have set the following requirements: Any pipe containing a substance at 40 degress celcius or higher and/or at 50 psi or higher shall have double block and bleed for any equipment or tank. Does this seem excessively stringent?

Can you share some requirements from your projects?

 

[Kreot]

Hi,
It does not seem very strict to me. We evaluate the neccesity for singel or double barriere according to the pipe pressure and medium. I do not see any reason for having a limit of 40 degrees C as we do have hazardious medium below this temp. By doing the correct valve choice you can easily obtain a double barriere with a single valve. Most of our requirements for double barriere is obtained with a single valve.

 

[LittleInch]

There is no such thing as typical as this is set by company and individuals and there are about as many permutations and requirements as there are those two things. It appears excessive to me, but may depend on the substance. Excessive for air or water, but maybe not for chlorine, H2S or hydrogen.

The use of DBB has been creeping slowly further along the line from high pressure to low pressure and the quote above may be someone's attempt to eliminate the thinking part about whether it does or does not apply to different substances or particular projects. Usually written by someone without responsibility for the budget...

For natural gas / liquid petroleum products, if there is a normal it is at #600 you see DBB, normally no DBB at #150, but variable at #300. It is though very much substance dependant and risk averseness (if that is a word).

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Roberto1956]

If you are working with 50 psi I will think you are working in a tank site. Gate valves or globe valves are not necessary a DBB and they are used very well in a tank site using light fluids.
If you want to be sure about seat tightness I would suggest you to specify all your valves with supplementary requirements of API 6D that ask the low pressure test with 5 bar air test and the low low air seat test with 0.5 bar. You will be slightly stric but correctly stric.