How It Works: Double Block-and-Bleed and ...

DBB definitions

There are two entities in the United States that define DBB'API and OSHA. According to API, a DBB valve is 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 venting/bleeding the cavity between the seating surfaces." API also notes in this definition that this valve does not provide positive double isolation when only one side is under pressure.

You can find more information on our web, so please take a look.

In contrast, OSHA describes DBB as "the closure of a line, duct, or pipe by closing and locking or tagging two inline valves and by opening and locking or tagging a drain or vent valve in the line between the two closed valves."

API's DBB definition does not achieve the same level of isolation as OSHA's. API allows DBB valves to be one single valve with two unidirectional seats, while the OSHA standard can only be achieved with two separate valves with a method to bleed pressure in between. There are some valves that utilize a twin-valve design. By combining two valves into one body, a twin-valve design reduces weight and potential leaks paths while meeting the OSHA requirements for double block and bleed.

Valve associations usually choose to follow either API's or OSHA's definition, but some have created their own handbook with their own definitions for industry terms. For example, the British Valve and Actuator Association (BVAA) defines DBB as "a manifold that combines one or more isolation valves, usually ball valves, and one or more bleed/vent, usually a needle-style global valve, into one assembly for interface with other components (e.g., pressure measurement transmitters, pressure gauges, and switches)."

Double Block & Bleed HELP!

Double Block & Bleed HELP!

USAeng

(Mechanical)

(OP)

2 Feb 11 07:58

I have been researching double block and bleed valves for inside a boiler... it is for some natural gas piping that is 1-1/4" dia....

They have one actuator presently and wanted to use that with a double block & bleed setup to replace the existing single ball valve... I was under the impression that a double block & bleed consisted of 2 block valves and 2 bleed valve which would need 3 actuators... BUT I keep reading online here and there about a different kind of block and bleed for smaller size piping like this...

The type I read about has a ball valve that seats on both sides with a vent built into the ball part I guess... but I cannot find any actual place selling it!! Just people talking about them on forums... can anyone help me out here?  

RE: Double Block & Bleed HELP!

reverman

(Mechanical)

2 Feb 11 14:47

I have a similar confusion. I was always under the impression a DBB valve was two independent seating surfaces with a bleed valve in between. like two gate or ball valves attached together with a drain valve between them.

However I see many people advertising to what appear to me as parallel slide gate valves with a bonnet drain as DBB valves.

RE: Double Block & Bleed HELP!

reverman

(Mechanical)

2 Feb 11 14:50

http://www.capitalvalves.com/ar.aspx?page=products-dbb

By the way you may want to give capital valves a shot. They seem to make DBB ball valves.I don't know much about them I just ran across the sight when I was trying to figure out something a DBB gate valves.

RE: Double Block & Bleed HELP!

gerhardl

(Mechanical)

2 Feb 11 17:01
Try to search this forum, and others for double block and bleed, use the search button in topline right above the textboxes, and you will perhaps find some indications or part of your answer.

See for instance earlier discussions

Try to search this forum, and others for double block and bleed, use the search button in topline right above the textboxes, and you will perhaps find some indications or part of your answer.See for instance earlier discussions thread408-: DB&B VALVES thread408-: Types of ball valves and thread408-: the DBB ball valve - some informations

RE: Double Block & Bleed HELP!

zdas04

(Mechanical)

2 Feb 11 17:29

David

You might also take a look at FAQ378-: What is Double Block and Bleed and Why do I care? David

RE: Double Block & Bleed HELP!

USAeng

(Mechanical)

(OP)

3 Feb 11 09:59

Question on the trunnion valves... if the valve was put in a natrual gas line, how do you typically vent the gas from the bleed portion of the valve...  

I imagine it venting to the air outside the building somehow... but how do you do this and how do they know when there is a leak if its venting?

I know these are prob. simple questions I dont want to sound dumb for asking... so appologies in advance...

Thanks for the links guys... the faq link had most of the info in it... I also found another post that was good... I thought I searched a decent amount but I should have looked a little farther... so my appologies.Question on the trunnion valves... if the valve was put in a natrual gas line, how do you typically vent the gas from the bleed portion of the valve...I imagine it venting to the air outside the building somehow... but how do you do this and how do they know when there is a leak if its venting?I know these are prob. simple questions I dont want to sound dumb for asking... so appologies in advance...

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Additional reading:
What is the purpose of a gate check valve?

RE: Double Block & Bleed HELP!

zdas04
Anti-corrosion Check Valve Market Research Report, Its ...
10 Questions You Should Know about Industrial Valve for Oil and Gas Industries

(Mechanical)

3 Feb 11 10:06

The trunnion ball-valves come with a nut that has an interal port that is blocked by an Allen screw.  To vent the body, you use an Allen wrench to let the gas out (vents locally).

The nut has standard threads and can be replaced with any number of devices.  For critical spec breaks, I've put PSV's in the body bleed port.  For a trunnion ball valve in a building there isn't any reason that you couldn't replace the nut with a needle valve piped outside the building.  If I did that I would take positive steps to prevent bugs from nesting in the exhuast pipe (wasps and hornet are the bane of open-ended vent piping where I live) by either putting a vent screen on the end or plugging the end when the vent is going to be shut for a long time.

David

RE: Double Block & Bleed HELP!

bcd

(Mechanical)

3 Feb 11 12:27

Hello,

If your handling natural gas, be careful as there are safety standards that may apply as law.  National Fire Protection Agency (NFPA 54) for one.  FM for USA installations, AS for Australia, CSA B149.1 for Canada, EN161 for EU counties, and others define exactly what you must have for safety shut off of natural gas.  hen complying with these standards, it is the norm to use two separate valves for shut off, and one valve in between for venting.  The reason for two separate valves is redundancy.  If you use a single valve with double block and bleed capability, what happens if this valve jams or doesn't operate for some other reqason during an emergency shut down.  The emergency fire personnel will not be to happy if natural gas continues to feed the fire.  Please be careful and rethink the use of a single valve for performing the entire safety shut down.  Again the norm is to use two separate valves when handling fuel gas with a single vent between so there is much higher reliability of the safety shut down function.

RE: Double Block & Bleed HELP!

USAeng

(Mechanical)

(OP)

3 Feb 11 15:50

I cant seem to find any trunnion type ball valve with 1-1/4" npt anyways...

I do find in nfpa 85 that it says: for each igniter a minimum requirement shall be two safety shutoff valves with and intermediate vent valve or equivalent arrangement.

However, it also states that multiple igniters supplied by one set of safety shutoff valves shall be treated as a single igniter...

So I think this would mean that as long as you have 2 shutoff valves in series with a vent valve in-between UPSTREAM of the igniters that could shutoff 16 igniters all at once (if need be) -then you could just have 1 regular ball valve to operate each individual igniter?

     

RE: Double Block & Bleed HELP!

zdas04

(Mechanical)

3 Feb 11 16:05

USAeng,
1-1/4" is not a size I ever use, the smallest trunnion valve I've ever bought is 2-inch and they are always flanged.

I think your approach is correct.  If you have a "master" isolation valve for all of the igniters, and then an individual valve for each igniter (with a vent between the master and the individual-valve header) then you have satisfied both the intent and letter of the code.  In a situation where you need DB&B, you'll have to lock each of the igniter valves and the master, but with the master shut you're not getting gas to the individual igniters anyway.

David

RE: Double Block & Bleed HELP!

tr1ntx

(Mechanical)

4 Feb 11 10:28

USAeng,

Aceco makes a 2" trunnion ball valve with an 1-1/2" port, so I'm sure they could make end connections which reduce to 1-1/4" pipe ID and thread it 1-1/4" NPT.

The bad news is, yes, they would be custom end connections, so they might not be cheap.  The even worse news is that the valve is a 15,000 psi oilfield compact manifold valve, so there is no telling what the sucker costs in the first place.

Those type valves are little short cylinders where the end connections, whether they be weldneck ends or flanged or threaded, are bolted to the end of the valve body, so you can have whatever end connection made that you want.

You might check with FMC and see if they make one in a lower pressure class, i.e., cheaper.  I've seen little trunnion valves somewhere.  I don't think it was SwageLok, but maybe someone else in that segment of the valve business.

RE: Double Block & Bleed HELP!

danw2

(Industrial)

23 Feb 11 11:19

In the US, most states use the CSD-1 code for boilers between 400,000 and 12.5M BTU.   The 1 1/4" gas supply line is most likely supplying more than 400k BTU to the burner.   

CSD-1 requires double block and bleed, with one of the SSOV's having proof of closure that is recognized by the flame safety controller, which might an additional upgrade you should investigate.

SSOV's are built to code requirements to shut down with 1 second, include a proof of shutoff switch, and carry FM and other agency approvals for gas train service.  Even the normally open solenoid valve used for the vent valve is typically agency approved.

Honeywell has manufactured gas train service SSOV's for decades.   Honeywell VA solenoid style come in 1 1/4" size:
http://customer.honeywell.com/techlit/pdf/PackedLit/65-.pdf

ASCO has gas train service valves also:
http://www.ascovalve.com/Applications/Products/asco-modular-gas-shut-off-valves.aspx

The valves you refer to for a boiler gas train are known as SSOV, Safety Shutoff Valves.In the US, most states use the CSD-1 code for boilers between 400,000 and 12.5M BTU. The 1 1/4" gas supply line is most likely supplying more than 400k BTU to the burner.CSD-1 requires double block and bleed, with one of the SSOV's having proof of closure that is recognized by the flame safety controller, which might an additional upgrade you should investigate.SSOV's are built to code requirements to shut down with 1 second, include a proof of shutoff switch, and carry FM and other agency approvals for gas train service. Even the normally open solenoid valve used for the vent valve is typically agency approved.Honeywell has manufactured gas train service SSOV's for decades. Honeywell VA solenoid style come in 1 1/4" size:ASCO has gas train service valves also:

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