Double block and bleed valves

[utlandmah]

Hi People,

Please could you let me know your opinions or experience on the following task?

I want to install a 12" class 600 Double Block and Bleed system on a natural gas pipeline at the plant inlet. This line is going to be commissioned in 10 months. I want to know what valves type to use as main isolating valves?

Rgds,
Marcell

 

[jte]

Typically two flanged gate valves with a short spool between them which has a 3/4" bleeder (gate valve) in between them. Less often seen (in my mostly downstream experience) would be to put the line valves back to back and the bleeder in one of the line valves in the space between gates. See B16.34 Fig. 1 (page 13 in the 2004 ed). Use location C or D for the bleeder - just be sure its installed correctly in the field...

There are several existing threads with more info on double block and bleed setups. Use the search feature to find them if you want more backgound.

jt

 

[utlandmah]

Jt,

Why gate valves and not ball valves?

 

[dcasto]

WKM pow r seal or saf t seal is a single reverse gate valve that will always seal. TK ball valves and WKM make a ball valve that has a body blow down that makes it a pretty good DBB valve.

We typically just install an orbit valve at the inlet, or if there is some special sfety issue a Pow r seal or equal true slab gate valve.

 

[utlandmah]

I agree Dcasto regarding perfectness of WKM Gates; but cost is an issue. That's why I want to use a Ball valve!

 

[BigInch]

I'm with dcasto. TK or WKM BALL valves.

http://virtualpipeline.spaces.msn.com

 

[jte]

utlandmah-

Gate or ball, whatever works for you. I'm used to seeing gate valves in most services, especially on lines over 8" or so.

jt

 

[BigInch]

Isolation valves are usually on/off and need a faster close than you can get with a typical gate.

http://virtualpipeline.spaces.msn.com

 

[jte]

Big-

Depends on your perspective... Emergency block valves maybe (typically motor operated and controlled from the control house), but most of the double block and bleeds I've dealt with are not critical to an emergency shutdown of a refinery unit, just for ordinary isolation purposes. Then again, in the refining world you can hardly spit without hitting a valve. I imagine its a bit different on the pipeline side...

jt

 

[BigInch]

I don't know about you, but I sure can't spit 25 miles.

http://virtualpipeline.spaces.msn.com

 

[dcasto]

for a description of a DDB see:

http://www.c-a-m.com/cam/search/showdocw.cfm?DOCUMENT_ID=7345

 

[utlandmah]

Thanks Guys,
Dcasto,

Please note the company is not prepared to pay for a WKM gate as the price is 3 times more than a ball. I have seen ball valves as DBB in many gas plants and refineries and am going to convince them using ball valves. Any suggestions, please let me know!

Rgds Marcell

 

[dcasto]

I've used TK and WKM ball valves as a DBB, but be prepared for the body not blowing down and have you high pressure grease gun ready to use at least semi-annually or quarterly. I've used the ball valves where, if they leaked a little, no problem.

Remember, if you want economy, you will pay for it.

 

[JLSeagull]

In my experience, most valves leak soon after installation. The double block and bleed arrangement can accommodate a small leak. Most will swear by the WKM Power Seal. I mostly deal with automated valves. The quarter turn aspects take me toward ball valves where practical. Process conditions such as high temperature and errosive particulates are factors that move one from ball valves toward gate valves. Some people even consider high-performance butterfly valves. These are not as high on my selection sequence.

 

[utlandmah]

Guys,

Thanks for your supports.

Please note this DB&B is going to isolate a flowline from the plant inlet and must be piggable. So, I think the option of using gate valve drops out (except WKM Gate valve).
Another option is to use a ball valve with proper trunion mounted ball and trim material. I mean if I use peek as seat insert (whicj is a hard material) and heat treat the 316 SS ball with ENP Overlay in order to increase the hardness of the ball, I think I should be fine.

Any comments is appreciated.

If you are agree with what I suggested, could you help me with some supporting documents, articles?

Regards,
Marcell

 

[dcasto]

First off, I'd never put an isolation line in the pigable line. Put it between the plant and the pig trap. Everytime you operate the valve, it will wear, so you do not want your ESD valve used all the time unless you can live with some leakage after the valve is closed.