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Double-isolation and bleed valve and Double-block and bleed
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(Mechanical)
(OP)
10 Sep 14 11:38Hi all,
I am very new to valve and pipeline. While reading through the API 6DSS, i have notice about definition of DIB and DBB which im not so clear of
1. Double-Block and Bleed Valve (DBB) &#; 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. NOTE: This valve does not provide positive double isolation
when only one side is under pressure.
2. Double-Isolation and Bleed Valve (DIB) &#; A single valve with two seating
surfaces, each of which, in the closed position, provides a seal against pressure
from a single source, with a means of venting/bleeding the cavity between the
seating surfaces.
I have several questions which i am seeking for the answers. Hope everyone could help me
1. What are the meaning of "Block" and "Isolation" here? Is this about the function of the valve which is block or isolate?
2. Can someone please explain more about "provides a seal against pressure from both
ends of the valve" and "provides a seal against pressure
from a single source"?
3. "This valve does not provide positive double isolation
when only one side is under pressure". Why is this?
I know my questions sound dumb but since i am new to this, please help. I really appreciate it.
Thank you very much.
(Chemical)
10 Sep 14 16:351.- I think in this case Block and Isolation means the different characteristic of these valves which will be explained in the next question2.- For a Double block and bleed valve I think the most common is a Trunnion ball valve (also we can have some other kind of valves like Slab and Expanding gates which also are DBB). Let&#;s use a Trunnion valve as an example, as you know the seats of trunnion ball valves are dynamic, this means they are energized using some springs between the seat ring and body and by the pressure inside the valve (this means the higher pressure differential you have the seal will be better). In the upstream seat the pressure form the line pushes it against the ball providing a tight seal. You can do this on the downstream side also at the same time and you will have the upstream pressurized, downstream pressurized and the body cavity can be vented. You have double block and bleed. But remember you need to have pressure on both sides.Using this valve as an example again, if the upstream seat has a little leakage, the body cavity will start to fill until the pressure is high enough (something like 10% more than WP) to push the downstream seat away from the ball. This is called self-relieving, this feature prevents the body cavity to be over pressurized. This is a DOUBLE BLOCK AND BLEED VALVEScenario 2: We have another trunnion ball valve, in this case the seats are DOUBLE PISTON EFFECT if the upstream seat has a little leakage, the body cavity will start to fill and pressurize the cavity, but in this case the downstream seat is pushed against the ball sealing also, this valve wont self relieve and needs a safety valve on body cavity . This is a DOUBLE ISOLATION AND BLEED VALVE. But how is this achieved? Why in some valves the downstream seat is pushed away from the ball to self relieve and in the other case is pushed against the ball to achieve a seal? The answer is different design of the seat rings, the dimensions, geometry are different, this allow the different behavior.3.- The answers is because seats are self relieving type.You can check this link it contains an explanation of this and includes some pictures which allow better understanding http://www.valvemagazine.com/index.php/web-only/ca...
If you want to learn more, please visit our website Double Block And Bleed Ball Valve.
(Mechanical)
(OP)
11 Sep 14 02:02Thank you very much,JSOSA555
1. As your explanation, i understand that if DBB valve has pressure only 1 side so only 1 seat is energized so only 1 seal is provided. Therefore, we cant call it Double Block and Bleed valve. Is this right?
2. DBB valve provides a seal against pressure from both ends of the valve while DIB provides a seal against pressure
from a single source. What will happen to DIB if there are pressures from both ends of the valve or the pressure comes to the downstream DPE?
3. As in API 6DSS, there is a note for DIB which is "This feature can be provided in one direction or in both directions."
Could you explain for me on this?
4. As i understand, the term "block" and "isolation" are quite similar in meaning. Any reason behind why one is called "double block" and another one is called "double isolation"?
Thank you very much for your time and your patience.
(Industrial)
11 Sep 14 09:47There is a huge problem with the general understanding of the difference and a mistake on many end users parts to request DBB when what they really need is DIB. Here's a decent video with explanation: http://www.youtube.com/watch?v=JqbaU13cBHk BTW I am not a shill for this company. It's just a good video.
(Chemical)
11 Sep 14 11:10I think the Double Block and Bleed term is more like a feature instead of a kind of valve
2.- This is correct, if you have a DIB valve you can test this feature increasing pressure thru the body vent and check leakage on both seats, no self-relieving in this case.
If you increase pressure on both sides of a DIB valve, nothing will happened, it will act as Double Block and Bleed, because you can isolate the body cavity and then vent the pressure from the inside, if pressure comes to the downstream seat the DPE will allow the downstream seat to seal instead of self-relieve.
3.- You can ask the manufacturer a valve with both seats Self relieving, both seats DPE or a combination of both (one self-relieving and one DPE)
4.- Never thought of this before, I think the term is only to state a difference, but I&#;m not sure
2
LittleInch(Petroleum)
11 Sep 14 12:59minhvu,It is a real issue and tends to arise because "Double Block and Bleed" as ais commonly used to describe a situation where there are two valves in series with an intermediate (short) spool piece with a bleed valve or valves. The purpose of most of these is to be able to isolate a piece of pipe and work safely on the other side of it to remove a valve, pump or connect a new pipe with "double isolation". The valves in this arrangement could easily be two ball valves with SPE seals and just about everyone would accept this as providing a high level of isolation.Unfortunately when this function is transferred to a single valve it actually then becomes a Double Isolation Valve (DIB) whereby there are always two seals and a bleed between you and the pressure source. A DBB valve (as defined by API 6D) does not perform this function but instead allows two pressure sources on either side of the valve to be isolated from each other and when a bleed is opened, proves that no fluid is passing in either direction.As to whether a ball valve with DPE seats (either both seats or only the downstream one) and in an internal bleed actually classifies as a DIB is a matter of some debate and different engineers and companies have their own views on this. My own view is that a ball valve with DPE seats does not classify as a DIB valve (it is of course a DBB valve!), but as said it is open to different interpretation and acceptance. What everyone accepts as a DIB is a valve whereby the sealing action on both seats is as a result of the valve closure, not due to energisation of the seat by pressure- think expanding gate valves as an example.My take on your last set of questions1) You would have thought so, but no, you can call this a DBB valve according to API 6D as long as the other seal could seal against pressure on the other side if there was any.2) DIB valves don't care where the pressure comes from so long as the valves are unidirectional. If the pressure comes to a "downstream" DPE seal then it seals like any other ball valve. The issue arises in the case of SPE/DPE valves which are then directional. If you pressure from the DPE end and the seal passes, you can get fluid past the upstream SPE seal. Moral of the story - much better to get double DPE seals then you don't have to worry about installing the valve in the wrong direction(!)3) I just have in answer to no 2.4) Yes they are similar, but different. DBB valve is defined in API 6D as being able to block pressure from either side. In early ball valve design and if you want to go really cheap, you can get single seal ball valves which only seal in one direction. DBB valve definition was introduced to show valves could seal in both directions at the same time. Double isolation means two (double) forms of isolation in any one direction from a pressure source to a open end.Be warned - many people have never properly understood the difference and will try and say they (DBB/DIB) are the same thing, including some valve vendor salesmen who should know better.Try this link and download the pdf file. Does all what has been said above, but with diagrams.I know its confusing, but once you understand you won't forget and can argue the case properly when it comes up.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
(Mechanical)
(OP)
11 Sep 14 21:28Dear LittleInch,JSOSA555 and Gorman3
I am very grateful to find this forum as well as the people like you guys. It helps and enlightens me a lot.
Thank you so much to you guys. Hope you could help me again when any questions come up.
Warm regards.
(Petroleum)
15 Sep 14 04:13Forgot to post the link. Scroll down to where it says "click here for more info"
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
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