What is a Double Block and Bleed Valve?

Author: Molly

Jun. 10, 2024

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What is a Double Block and Bleed Valve?

To me, double block and bleed always sounded like a move out of the Cobra Kai karate dojo. Of course a double block and bleed valve has nothing to do with karate, it has more to do with controlling process flow to allow for maintenance, sampling, and flow diversion. This post describes the attributes of a double block and bleed valve, its function, and application.

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What is a double block and bleed valve?

A double block and bleed valve is like having three valves in one.

API 6D defines a double block and bleed valve as a "single valve with two seating surfaces that, in the closed position, provides a seal against pressure from both ends of the valve with a means of vending/bleeding the cavity between the seating surfaces."

There are a couple different styles of double block and bleed valves. One style has two independently seating valves inside, with a bleed or drain valve. This configuration is referred to as a double isolation and bleed (DIB). The DIB valve can isolate either side of the valve (bi-directionally) to vent or bleed the cavity and isolate piping.

This trunnion style ball valve from Val-Matic provides double block and bleed in a single valve. This style can isolate piping on both sides of the valve (uni-directionally) to vent/bleed the valve cavity between the seats.

Using a single double block and bleed valve system versus 3 separate valves saves installation time, weight on the piping system, and space. It also cuts down the number of leak paths to the atmosphere, reducing the risk of hazards when transferring liquids dangerous to humans or the environment.

Which applications require a double bock and bleed valve?

Double block and bleed valves are most commonly used in the oil and gas industry, but can also be helpful in many other industries.

It's typically used where bleeding the valve cavity is required, where piping needs isolation for maintenance, or for any of these other scenarios:

Prevent product contamination
Remove equipment from service for cleaning or repair
Meter calibration
Liquid service near waterways or municipalities
Transmission and storage
Chemical injection and sampling
Isolate instrumentation such as pressure indicators and lever gauges
Primary process steam

What to look for when selecting a double block and bleed valve?

As with any valve, during selection, one should consider the fluid properties, pressure, temperature. Perhaps most importantly, however, is how the valve will be used. With a number of configurations offered, consider which of these will fit your process and needs best.

If you're unsure which double block and bleed valve is right for you process, talk to an engineer. They can help determine which one will achieve your objectives best.

Need help selecting a double block and bleed valve? Ask us about it! We gladly provide technical assistance to businesses in Wisconsin, Minnesota, Iowa, and upper Michigan.

Double Block and Bleed versus Trunnion - Valve engineering

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Double Block and Bleed versus Trunnion

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Double Block and Bleed versus Trunnion

Zeroman

(Petroleum)

(OP)

29 Nov 04 18:39

Hi all,

I'm sorry for posting, but unfortunately the search isn't working.

Could someone please explain the difference between a floating ball valve and a trunnion mounted ball valve? And what is double block and bleed on a trunnion mounted ball valve?

Thanks in advance.

RE: Double Block and Bleed versus Trunnion

zdas04

(Mechanical)

29 Nov 04 23:38

A trunnion ball valve uses springs to push both of the seats against the ball (which is held rigidly against lateral movement by the trunion bearings). A trunnion ball valve has two seating surfaces against the process fluids and has a body bleed to evacuate the cavity in the ball when the valve is shut. This is similar to having two floating ball valves with a vent valve in between.

In cases where safety (or the government) dictates that you have "double block and bleed" (e.g., energy isolation for hot work in a situation where no one is working in a confined space), a trunnion ball valve meets the definitions (it is up to each company to satisfy themselves if it is truly a safe practice, but it does meet the definitions).

On the other hand, some manufacturers sell "double-block-and-bleed" valves which are not trunnion ball valves. These valves typically have two complete valves and a vent in one casting. They use various technologies and probably work as advertized. I'm not terribly familiar with them, but one that I looked at was not piggable and I went back to Trunnion Ball Valves.

A floating ball valve relies on a differential pressure to shift the ball a few microns into the downstream seat. This takes the ball off of the upstream seat and allows upstream pressure to fill the ball cavity.A trunnion ball valve uses springs to push both of the seats against the ball (which is held rigidly against lateral movement by the trunion bearings). A trunnion ball valve has two seating surfaces against the process fluids and has a body bleed to evacuate the cavity in the ball when the valve is shut. This is similar to having two floating ball valves with a vent valve in between.In cases where safety (or the government) dictates that you have "double block and bleed" (e.g., energy isolation for hot work in a situation where no one is working in a confined space), a trunnion ball valve meets the definitions (it is up to each company to satisfy themselves if it is truly a safe practice, but it does meet the definitions).On the other hand, some manufacturers sell "double-block-and-bleed" valves which are not trunnion ball valves. These valves typically have two complete valves and a vent in one casting. They use various technologies and probably work as advertized. I'm not terribly familiar with them, but one that I looked at was not piggable and I went back to Trunnion Ball Valves.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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RE: Double Block and Bleed versus Trunnion

Zeroman

(Petroleum)

(OP)

30 Nov 04 11:12

Thanks Dave. It finally makes sense.

RE: Double Block and Bleed versus Trunnion

MaxPressure

(Industrial)

10 Dec 04 07:09

Dave has just about covered the "how they work issue" although there are some variants. Piggable double block and bleed valves, or DBB's, are available so are floating ball versions, I've acquired them for projects before. Try a google/yahoo search for "alco dbb valves". or try

Zeroman & David,Dave has just about covered the "how they work issue" although there are some variants. Piggable double block and bleed valves, or DBB's, are available so are floating ball versions, I've acquired them for projects before. Try a google/yahoo search for "alco dbb valves". or try www.mono-flange.com this has more details on the actual types and variations you can also refer to the EEMUA 182 standard for a deeper understanding.

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