Double Block and Bleed Valves Arrangment

It means two block valves with a bleed valve between them. The bleed line downstream of the bleed valve is usually blanked or spaded during normal service.

What kind of block valve you use is up to you or the code you are working to.

A double block and bleed valve, as I think you understand is a block valve that has a cavity between the inlet and outlet seals. This cavity can be drained thus proving the valve is effective and there is no bypass. In my experience, this is done at regular intervals to test the valve or as necessary when searching for an in-system leak.

You can do the same with a pair of any type of valve and a drain valve inbetween the two valves.

Drew

Hi shanii,

The 'double block and vent' application usually refers to the arrangement of valves responsible for the isolation of the gas supply to large commercial/industrial gas burners. The 'Double block' being a pair of closely coupled spring loaded disc valves which have the intervening space connected to atmosphere via a small bore solenoid controlled valve (N.O ). There is usually also a 4th valve another small bore solenoid valve (N.C),across the first of the large isolation valves.

Now for the MO:

1.At 'rest' both the big valves are de-energised and therefore closed whist the small vent valve (v3) is open to atmosphere; the other small valve (v4)is also de-energised and closed. Therefore in this condition if there is any leakage thru' the first of the main valves it will be vented to atmosphere without the risk of gas pressure building up in the space between the valves and possibly infiltrating the combustion chamber ( if the other main valve was also faulty)

2). (a)At start up the vent valve is energised and closed and a pressure switch is used to monitor the space between the two main valves;any increase in pressure above a set value says that the first valve is faulty and the burner goes to lock-out.

(b) Subject to stage (a) being successful the next stage sees the shunt valve across the first main being opened briefly to admit a 'slug' of gas into the void which is now pressurised to line pressure. A second pressure switch now monitors this void for any fall in pressure. This would indicate a leaky second block valve.

(c) With both 'tests' successfully timed out the burner is deemed fit for action and the remainder of the pre-firing tests are undertaken. After establishing and proving the pilot flame a signal is applied to both the spring loaded isolation valves which open simultaneously and put gas onto the burner. If anything goes wrong the signal is removed and the valves run shut under spring power instaneously.

The 'Double block and Vent' arrangement has been around for many years and is a respected safety isolation set-up. It does not do anything other than isolate the burner from the fuel supply and therefore all volumetric control or modulation to vary firing rate is achieved by another valve down stream of the it.

I hope you find this helpful,

Massey.

It is also used similarly with pipeline fuel systems. If you need to prove that a component is isolated you can use a block and bleed valve. The vent line described above serves the same purpose.

Drew

Thanks a lot for very comprehensive answers especialy Massey

Also for sanitary operations to confirm a seal to prevent contamination during a wash cycle, when product is on one side of the block and wash is on the other side with the bleed vented.

p911

A double block and bleed valve is very common in oil refineries, this valve can be shut off to block the flow, bleed off the pressure left in the valve chamber and allow the mechanic the ability to unbult the bottom of the valve and drop out the main seal component and rebuild the valve.

During this process the product in the piping lines is not effected on both sides of the valve thus the term double block and bleed when the valve is rebuilt you simply bolt it back together and open the valve.

Please note that the valve must be installed with sufficient space below the valve to drop the internals out. I had a big job to design a piping system for GATX up in Carteret N.J. the job involved 55 of these double block and bleed valves.

Flaresideone

Mike

Hi Mike,

Good point well made. Taking the guts out of one of these valves can prove quite a challenge if insufficient space has been allowed for unbolting(after securing the spring!!) and removing the bottom plate+spring+ valve disc assembly.

Massey.

actualy no one is going deep in the question. question is whether the arrangment (two separate ball valves with a bleed connection between them and after them+ Level control valve+bleed connection +two ball separate ball valves with bleed connection between them) make any sense??????

i got a P&ID which has this arrangment and i am not satisfied with this. i think two ball vaves upstream anr two ball valves downstream does not make any sense. some of my collegue say that this is double block and bleed valve arrangment whicle we are not using Double block and bleed valves.

please help me in this regard

Shani:

If the schematic is for something other than chemicals and oils I would question the safety of this arrangement, question what happens to the product in the line when you open the bleed valve?

it does make sense as to what your doing to isolate the product between two valves, but you are using four based on the term double block and bleed.

mike

I am asked to use in AMINE SWEETENING PROCESS. Rich Amine is brought back to reboiler through Level control valve assembly but before level control valve two ball valves with the bleed in between them and after level control valve two ball valves with the bleed between them are used. In addtion bleed before and after control valve are also given. so total four bleed connection are there. i.e 1st in between the two ball valves upstream ..2nd after the upstream ball valves and before conrol valve..3rd after control valve and before down stream ball valves..and 4th in between the downstream ball valves...... i think now its very clear.. i dont understand the purpose of two ball valves upstream and two ball valves downstream while one ball valve is sufficient for upstream blockage and on for downstream blockage, thats why i am asking whether its sensible??

The reason given to me for double block and bleed is to assure a positive shut-off of the stream to the rest of the process, for safety reasons. If you have a single valve, when it begins to fail, it will allow bypass leakage of the stream into the process when shut off.

To shut off a double block and bleed system, both block valves are shut while the bleed valve is open to atmosphere. If the first valve leaks, it is vented to atmosphere through the bleed valve.

The second valve can be leaky as well. It isolates the process, but it has no pressure on it from the incoming stream because one side is open to atmosphere via the bleed valve.

Thus, both block valves can be leaky, and the process is still isolated from the stream.

The Amine material you are dealing with must be really toxic? We use the double block and bleed arrangment on equipment that is likely to be taken out of service while the rest of the system is still operating. An example is filters and guard beds I haven't seen that arrangment on a control valve but it depends on the expected failure rate and how reliable your system needs to be.

From the sounds of your description the a double valve and vent is set up on both sides or your control valve so the control valve itself can be pulled safely while still operating the system. The extra drains before and after the control valve are there to insure it's drained well before maintenance is performed on the valve. The double block and bleeds are there to insure there is no leakage while work is performed on this valve. Is is practical that this control valve will need to be pulled while in operation and pulled on a regular basis? If not then you may consider eliminating the double block and bleed. If work is required then you shut down the system completly and empty and wash it of the amine.

After re-reading OP and update, I think Don has a good understanding of the stated situation (better than I did).

The double block and bleed is called thus because it blocks the line twice leaving a cavity in the middle. The OP describes two double block and bleed arrangements with a control valve in the middle. I hope this answers the original question about what a double block and bleed is.

As Don describes, it proves that the control valve is isolated from the system for maintenance. You could use 4 ball valves and 2 vents for this or 2 double block and bleed valves instead. Whichever you prefer for price or complexity.

Drew

Thanks Don ... your comments make sense. I appreciate.

As I did post above, you can simulate the effect of a block and bleed valve in the method you state. It will serve the same purpose many have described here: proof of isolation of the system.

I would need to know more about your system to determine the reason for wanting a double block and bleed. If the diameter of your pipes is small, I see no reason not to use two ball valves and a vent line. If you are talking about 8inch or higher, it might be cheaper to just procure the double block and bleed (unless you have surplus ball valves on hand).

My point is, if your purpose is to verify a section of pipe has no flow going to it your set-up will work. An example of why you might need to verify a system has no pressure leaking past a valve is if it is the line leading into a tank and workers will be present in the tank, or you have to drain a section of pipe and you don't want to worry that pressure / product is flowing into that section past a leaky valve.

Drew

http://www.spiraxsarco.com/about/news-article.asp?news_id=15

Shows a normal double block and bleed. I agree with you, your situation sounds like quadruple block and bleed. Probably not worth changing.

i am familiar with the conventional DBB arrangement in P&ID like what i see in above spirax sarco link. from my understanding, the upstream ball valve is to isolate the line, then the bleed valve is for vent/drain, then the downstream valve is to isolate the control valve/equipment.is it true?