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ProGas
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Join Date: Mar 2010
Location: Middle East
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Double Block and Bleed Valves

03/28/2010 6:27 AM

It's planning to install a double block and bleed valves system configuration( two isolating(block) valves is series and a bleed valve between them) on a 2inch hydrocarbon liquid line. Its obvious that the ball valve is easier to operate since these valves are on/ off but, in view of other points like price, leakage, … I'm doubtful which type of valve( ball, gate, …) can be the best choice. Any comments is appreciated.

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PennPiper
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Engineering Fields - Piping Design Engineering - Retired Piper

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#1

Re: Double block and bleed valves

03/28/2010 8:28 AM

For every process plant piping system (or group of systems) there is a document called "Piping Material Line Class Specification" (or close to that).

This document defines what material (pipe, fittings, flanges, gaskets, bolts, valves, etc) that is to be used for that commodity for a specific pressure/temperature range. One of the primary reasons for this document and this practice is the safety of the plant operators. If you use some other valve, because it is cheaper is easier to operate it could fail and kill somebody.

I STRONGLY urge you to find the Piping Material Line Class Specification for the service you are working on and use only what is called for in that Spec.

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lyn
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#2

Re: Double Block and Bleed Valves

03/28/2010 11:37 AM

Please tell us where this installation will be constructed. I don't want to get within 10 miles of the place.
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Drew K
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#3

Re: Double Block and Bleed Valves

03/28/2010 11:53 AM

With a 2 inch line you should be fine with using 2 ball valves and a bleed in the middle. Usually I have seen the bleed open so it can be drained into a bucket to prove the line is isolated downstream.

For larger systems it is better to use a double block and bleed valve.

The red one is a double block and bleed Plug Valve, the Blue is a ball valve double block and bleed.

With the valve closed you can open the bleed line and prove that the pipe is isolated down stream so work can begin without worry of seepage bypassing the valve. The system could be done with two ball valves and a bleed valve between. The system you use depends on cost. If you are dealing with a pipe six inches or larger, it would probably be best to go with this type due to cost.

Drew

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ProGas
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#6
In reply to #3

Re: Double Block and Bleed Valves

03/29/2010 7:09 AM

Thank you

but, What do you mean "With a 2 inch line you should be fine with using 2 ball valves and a bleed in the middle." And, why do you claim the double block and bleed is better for larger system.

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ducon
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#4

Re: Double Block and Bleed Valves

03/28/2010 1:46 PM

The double-block valves are NC operation and can be Solenoid/Motorized.

The bleed valve is in-between these valves and are always Solenoid NO operation.

When both the double-block valves are on the bleed valve is off and when off(shut down of system) the bleed valve comes on to vent any residue or leakage from the first block valve. The bleed valve is installed vertical and the outlet of the bleed valve is taken out of the roof/ building to vent safely.

Mostly you will find this system in over 90% of gas-train of gas Burners for Stenters, Driers, Furnaces & boilers.

Refer ASCO Solenoid Valves for your best selection as per fluid and application.

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ProGas
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#7
In reply to #4

Re: Double Block and Bleed Valves

03/29/2010 7:14 AM

I mean manual operated valves( not solenoid/ motorized), i.e. two flanged valve with a short spool between them which has a 3/4" bleeder valve connented to the spool. These three valves are normally closed.

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sabriahmad
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#5

Re: Double Block and Bleed Valves

03/28/2010 11:03 PM

Hi

In most plant that I have worked with, the common type of valve for double block and bleeds are gate valves. The gate valves meet the basic purpose of positive isolation.
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ProGas
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#8
In reply to #5

Re: Double Block and Bleed Valves

03/29/2010 7:49 AM

Some people (like you) believe that (WKM) gate valve accommodates positive isolation although its price is about 3 times more than a ball valve.

It should be noted that most valves leak after installation(based on my experience) due to the incompatible design with process contition(fluid nature, … ).

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Drew K
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#9
In reply to #8

Re: Double Block and Bleed Valves

03/29/2010 10:20 AM

If positive seal is what you are after, price does matter. The more you pay for or to maintain a valve the better the seal is going to be. We had some 24 inch lubricated plug valves on our system that had been there for 15 years and did not leak (often). As long as regular maintenance is accomplished they will last a very long time.

A lubricated plug valve uses a thick grease as a seating sealing material. Once a week we would pump a small amount of this grease into a fitting on top of the valve body. It is pushed through cavities to the bottom where it lifts the internals and provides a thick viscus barrier to seepage. Here is a link to the page that describes the valve below.

Drew

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ProGas
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#10
In reply to #9

Re: Double Block and Bleed Valves

03/29/2010 11:45 PM

Thank you Dear Drew K

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