13 replies to this topic

[Sandeep01]

Hello Adam

To start with, DBB is not positive isolation. Spade/blind or removal of a spool piece are examples of positive isolation.

Now, on positive isolation, which of the above two methods is used depends on the operating philosophy. Some feel better to see a piece of spool "missing" before, say vessel entry. They may be sceptical about an operator not properly assessing the spade position.

On DBB, you would require DBB between a live system and a flange you may wish to break (above a certain pressure rating/operating pressure), for example to install a blind and achieve positive isolation. DBB should not be used as a "long term" means of isolation and where vessel entry, or similar scenarios, are involved. As to guidelines, I would refrain from guiding you to any specific company (majors) guidelines, as all have been developed with years of experience going in to it.

Regards
Sandeep

[Art Montemayor]

Adam:

You are asking a legitimate and “hard” engineering question. It isn’t “hard” to answer; it begs a hard, factual, personal answer. And I like it like that because there is no “easy” answer.

After 47 years in engineering, I’ve arrived at a pointwhere I have developed firm and stern engineering positions – especially where it concerns Human Safety. I would address your basic query with a question: Would you enter a confined space – such as the 15^th tray on a 30 tray distillation tower - knowing that only a double-block-and –bleed (D-B+B ) arrangement on all connected process nozzles stands between you successfully getting out alive? Or would you demand that an “air-break” – a completely, total disassembly of all inlet and outlet nozzles be done complete with their being 100% blinded and physically separated from the column? I think you would rest much better if you had the latter. And that, I believe, is the essence of your query. A D-B+B arrangement is not bad if it is used to segregate benign streams from physically coming into contact and mixing. However, in my opinion, it is a potential hazard when used to protect human beings from a potential fatal hazard.

Having said that, I believe all professional engineers realize that we are constantly pressured by the economics of an operation when operating a processing unit. We have to rely on our organizations to cooperate in ensuring that our personnel’s safety is always at the forefront. A D-B+B may be much less expensive to impose on a turn-around and may be the system of choice for some companies. If so, I would work diligently to change that policy if I were employed by them. I would also, at the same time, demand a 100% control on all maintenance blind flanges used to secure a complete process isolation. This means that a qualified and experienced engineer is used to design and make sure that the correct blinds are always used on the corresponding processes. I have found that this is also an area of potential errors or stupid mistakes.

To answer your direct questions:

1. The only advantage of a D-B+B arrangement over an air break is probably economic.
2. The advantage is not because higher pressure piping systems become too bulky for blind installations.

I hope this response addresses your concerns and strengthens your resolve.

[Art Montemayor]

Adam:

It's not about the valve(s) spontaneously opening on their own. It's all about YOU and other human beings coming by and accidently or stupidly opening or moving the valve handle. This is a HUMAN factor, not a mechanical factor.

These are important Hazop issues and if you haven't participated in one, then you should start to groom your thoughts to taking part in one. Operating a plant or process does not only involve the design and control of machinery. It also involves the selection, training, and control of human operators. --- And that is where the big problem raises its head! Controlling the mechanical and physical components is the easiest portion! Sometimes, depending on the process, simple vibration and harmonic effects in the machinery can crack a ball valve open.

D+B &B valves are important to ensure that a critical fluid doesn't escape or migrate out of one of the valves in the configuration. The "bleed" valve is the one that maintains a monitoring over that effect. For a critical separation between fluids, three valves are indeed better than one.

I hope the above addresses your concerns. Your questions are valid; I think your exposure to critical and toxic fluids is limited, however, and that will change with time and exposure.

Keep up the curiousity and always challenge the engineering behind an application. If the engineering is sound, there will be sound answers to your concerns.

[vicini]

The double block and bleed allows the operator or maintenance people to monitor and positively assure that the valve does not leak not even one gram of the fluid into the area they are working, it is not for a spontaneous failure. ALL valves leak!! The DBB arrange it used wher the designer knows the system will have to be shutdown and oped up for repairs , like pumps, compressor, catalyst beds, exchangers that foul, ect. Distillation towers may never be looked at, so DBB valves are not specified. As stated, most PHA's will identify where they should be included. There is no law, rule, standard that madates under severe penalties, the designer and operations/maintenance people can give guidance.

[djack77494]

The "search" function brought me to this old thread, so I apologize if the interest has been lost.
I take issue with the first "bullet point". DBB is NOT positive isolation. It is the principle alternative to positive isolation which is valved isolation. The other bullet points are basically opinions and local practices, so I have no issues with them except for the linkages between positive isolation and DBB. I would like to point out, however, that positive isolation, in the form of a spectacle blind, spade and spacer, drop-out spool, swing elbow, etc., generally is located within valved isolation. The valving isolates the process and allows containment to be broken at a set(s) of flanges so that positive isolation may be implemented.

[SafetyUser]

Hi Adam

can find some guides in this fundamental

"The safe isolation of plant and equipment" HSE UK, 2006

ps_ why don't suggest your Company HSE to charge of this policy ...

[PKS]

The "search" function brought me to this old thread, so I apologize if the interest has been lost.
I take issue with the first "bullet point". DBB is NOT positive isolation. It is the principle alternative to positive isolation which is valved isolation. The other bullet points are basically opinions and local practices, so I have no issues with them except for the linkages between positive isolation and DBB. I would like to point out, however, that positive isolation, in the form of a spectacle blind, spade and spacer, drop-out spool, swing elbow, etc., generally is located within valved isolation. The valving isolates the process and allows containment to be broken at a set(s) of flanges so that positive isolation may be implemented.

I agree.
Double Block and bleed is helpful and ensure us to know if the first isolation valve is passing. the bleed is opened to check for the passing. The second isolation valve helps to hold the fluid. That's it. When it comes at positive isolation, a spectacle blind is provided between the two.

PKS

[Root]

The "search" function brought me to this old thread, so I apologize if the interest has been lost.
I take issue with the first "bullet point". DBB is NOT positive isolation. It is the principle alternative to positive isolation which is valved isolation. The other bullet points are basically opinions and local practices, so I have no issues with them except for the linkages between positive isolation and DBB. I would like to point out, however, that positive isolation, in the form of a spectacle blind, spade and spacer, drop-out spool, swing elbow, etc., generally is located within valved isolation. The valving isolates the process and allows containment to be broken at a set(s) of flanges so that positive isolation may be implemented.

Doug,
DBB is mostly being used in compressor suctions and for double confirmation of positive isolation, if 1st isolation valve is passing and the bleed valve will not allow pressurize the spool of pipe.
Indeed DBB is Personel and equipment safety and MR. art explained will in his reply.

Toor

[djack77494]

Toor,
We are in agreement. I did not intend to "shortchange" the utility of a DBB isolation. I did intend to state that, by definition, DBB is not considered positive isolation. Nonetheless, you can think of DBB as "a good quality isolation" that is often adequate.

[Zubair Exclaim]

i would just like to add that some companies even require ADBB isolation on pumps as well.

There have been incidents with pump casing under repair and valve inadvertantly opened with liquid at its auto ignition temperatures , catching fire instantly. I guess fluid flash / fire point conditions if achieved in operation should also be considered with online maintenance of rotary eqps

Edited by Exclamation, 28 February 2012 - 02:24 AM.

[Robert Montoya]

The double block and bleed allows the operator or maintenance people to monitor and positively assure that the valve does not leak not even one gram of the fluid into the area they are working, it is not for a spontaneous failure. ALL valves leak!! The DBB arrange it used wher the designer knows the system will have to be shutdown and oped up for repairs , like pumps, compressor, catalyst beds, exchangers that foul, ect. Distillation towers may never be looked at, so DBB valves are not specified. As stated, most PHA's will identify where they should be included. There is no law, rule, standard that madates under severe penalties, the designer and operations/maintenance people can give guidance.

I totally agree with vicini, In pumps, sampling on pipes, etc. maintenance is very common, often there is no impact upstream or downstream in production, as we all valves passing along the time and additional valve which are generally ball-type to provide sealing, provide safety during the maintenance and further during the normal operating due that the first valve is normally open.
Actually in pumps maintenance is frecuently used a ball valve followed by a bleed valve then the figure 8 blind (closed) and finally the second valve closed.