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bijucri (Mechanical) (OP)
25 Nov 06 13:01
Can anyone explian what is anti static and double block and bleed in a ball valve, please explain how it works.
BigInch (Petroleum)
26 Nov 06 2:17
Antistatic construction is provided by making a grounding path between the valve body and the ball which prevents static electric charges and resulting voltage potentials from building up and sparking.

For double block and bleed, see these

thread408-164916
thread408-157390

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

MortenA (Petroleum)
27 Nov 06 3:53
A trunnion mounted valves can in some designs be supplied with a bleed valve inbetween the seats - this make a _test_ of the tightness for both seats possible. This will on occation by sales reps be solds as "double block and bleed" BUT THIS IS NOT TRUE since the seats will only be tight with line pressure both upstream and downstream. Something that you usually dont have if you need a DBB.

Best regards

Morten
cliff6361 (Materials)
28 Nov 06 13:12
Neither answer is complete.
1.  Because the ball and stem in a ball valve are susspended on non matalic parts , i.e. the seats and stem seals, there is a possibility when cycled a static charge can build on the ball-stem.  Ball valves usually have a method to maintain a metal-to-metal contact between the rotaing ball/stem and the valve body which will ground any charge to the valve body.  The valve body is then grounded to the pipe by the pipe connection or the addition of a ground strap around the flanges.  This ground need is also present in plug valves.

2.  A true double block and bleed is three valves w/very ggod seating connected by a Tee fitting (some times done with two valves and a smaller drain valve installed between).  Purpose is to provide a method to change products flowing in a system without having them mix.  The two products valves are closed then the space between them is drained then the second line is opened.  A trunnion ball valve can function as a block and bleed as today most designs include a soft seat element that provides a positive seal and the seat holders are spring loaded to provide a seal at the ball without line pressure.  In a true block and bleed one still needs a shut off valve to change fluid.  But a trunnion can be used to shut off line and bleed valve opened to confirm shut-off.  This is not a true block and bleed application.    
MortenA (Petroleum)
29 Nov 06 3:25
cliff6361

I fail to see how your comment chages anything to what i put in my post?

Except that IMHO the a DBB is for positive isolation of a piece of equipment. This can be usefull if you have parallel trains and want to do maintenance on one train while keeping the other train running. Many operators get kind of nevours about this and in civilised countries we may want to make sure that their survival chances are the best. So therefore you have two vavle and you can close both valves, blow down the train that you want to inspect, bleed the cavity between the valves and close it again and if pressure goes up again in the cavity - tehn you have a leaking valve.

I dont see that the valves have to be "especially good" or that it has anything to do with "mixing" product streams.

If you have a process plat where a mistake might mix "incompatible compounds" e.g. something that might blow up or develop toxic by product or similar - then i sugegst that IF you for some reasen need the possiblity of connect these stream then include a spade or spectable blind in the connection. Two valves are not that more difficult to open than one but a spade/spectable blind will make you think twice.

Best regards

Morten
BigInch (Petroleum)
29 Nov 06 3:58
I don't see any difference.  The essential ideas here are "grounded - no sparks" and "positive (more or less) separation between up and downstream".  

I use them between product tanks and a multiple product header, i.e. taking diesel or gasoline or jet fuel from their respective tanks into the same shipping pipeline.  While you're taking one, you don't want any flows from other tanks, so a DBB comes in handy to assure product quality.  And accidental opening would cause an undesireable mixture, but no explosion.  I would definately not use one DDB for isolation of "incompatible" or dangerous combinations of products.  For incompatibles,  maybe two with a flushing connection and another drain inbetween them or something, but for dangerous combinations, it would be far better not to connect those two products in the piping at all.  I'd try to make any required mixing in some special purpose vessel with two independent feeds for each product.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

MortenA (Petroleum)
29 Nov 06 4:56
hmm - i dont work with refineries (mostly) but in offshore we tend only to use DBB for positive isolation. This is (also) a cost issue - and because for blockage what is it that you gain? Two valves are allmost as easy to open as one and any normal small leakage what harm could that do?

As i said (or at least ment smile (for dongerous combinations only) )IF YOU REALLY NEED to connect these two stream then use a spade or spectable blind (or maybe even s spool piece).

For "blending lines" i fail to see that you get much more from a DBB except more cost for valves and maintenance. If your operator wants to open one valve - then he will also open two - but if he has to get out of the control room and dismantle a flange and remove a spade then he will think twice smile

Best regards

Morten
BigInch (Petroleum)
29 Nov 06 5:21
He's gotta' workaround the software interlock first, or disconnect the status signal.

An operator usually won't open a spec blind.  Most of the time he'll have to call Maintenance.  They'll ask him why.  Then he'll have to explain.  That means work... so I'd call that "intrinsically" safe. smile

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

jte (Mechanical)
4 Dec 06 18:11
In many cases, a double block and bleed is not only a reasonable approach for equipment isolation for entry as stated by Morten, but a legal requirement as well if you are dealing in a USA OSHA environment. Some state laws are more stringent with regard to confined space entry.

As noted in http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&;p_id=9797, the OSHA definition of "double block and bleed" is: "Double block and bleed" means the closure of a line, duct, or pipe by closing and locking or tagging two in-line valves and by opening and locking or tagging a drain or vent valve in the line between the two closed valves.

...while "isolation" is defined as: "Isolation" means the process by which a permit space is removed from service and completely protected against the release of energy and material into the space by such means as: blanking or blinding; misaligning or removing sections of lines, pipes, or ducts; a double block and bleed system; lockout or tagout of all sources of energy; or blocking or disconnecting all mechanical linkages.

There may be (probably is…) some exception, but the basic regulatory definition calls for two distinct valves with a drain valve between them. If I were asked to install a single valve which provides the functions of all three, I'd ask the vendor to provide me documentation indicating that their product was an acceptable alternate to the standard approach. In addition to the ball valve discussed in the original post, I've seen essentially double disk gate valves with the part in between gates ported and advertised as DBB. Could be, but I've never installed any nor seen any.

jt

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