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Positive Isolation - Blinds

Positive Isolation - Blinds

Positive Isolation - Blinds


We have a tank farm where aviation turbine fuel ( ATF) or jet fuel is stored in the refinery.

"Hammer blinds'are being used for positive isolation which will isolate the tanks from the system so that  contamination of the product is prevented.

Operators of the blinds complain that they find it very difficult to operate ( Physically removing the blinds has resulted back injury)  and change the position of the blind each time it is required for On off operations.

In order to bring the positive isolation , motorised valves were  thought of but it was not approved due to various reasons.

Can any one suggest , what is the alternative to implement positive isolation , where positive isolation is possible without HSE prolems



RE: Positive Isolation - Blinds

I am not sure what HSE means (Health and Safety Executive???)
Positive isolation can be obtained with the correct selection of valves.  Use a DBB (double block and bleed) valve and you can have the isolation that is required.  Think about installing DBB trunnion ball valve or DBB plug valve.  Both are often used in tank farms for positive isolation and can be operated by hand.  Note: These valves can only be considered as positive isolation when the cavity is monitored or opened to atmosphere.  In the tank farms that I have seen, these DBB valves are only periodically tested by opening the cavity port to verify isolation of product.

RE: Positive Isolation - Blinds

    Don't wanna to go off-topic, but... may I ask a stupid question?
    What does the adjective "positive" mean in expressions like "positive isolation", "positive shut-off", etc.?
    Is there any "negative shut-off" or similar?
    May be I can't understand just because I am not native English speaking... but I feel there might be a different explanation  

Thanks,       'NGL

RE: Positive Isolation - Blinds

Blinds are a plate inserted between flanges in the piping system.  Another positive isolation is a removable spool with blind flanges on the piping when the spool is removed.  I am not familiar with hammer blinds but the objective is to prevent any possiblity of vapors, etc. to enter a system such as when people enter a confined space for maintenance.

Although I know experience process engineers who would argue, NO VALVE alone provides positive isolation suitable for me to enter the confined space.

HSE and the assorted character orders pertain to health, safety and enviroment.


RE: Positive Isolation - Blinds

Check ASTM F 1020 regarding Line-Blind Valves.  The following definitions may help.  
A blank or what I called a blind is a solid one-piece circular unit inserted into a pipeline to prevent flow.

A line-blind valve—an assembly consisting of a spectacle
plate, bolting, and body, the purpose of which is to provide a convenient means to align a piping system to an open or positively closed configuration. The assembly is designed to provide a simplified method of changing over the flow control spectacle plate without the necessity of plate removal from the valve body.

A spectacle plate or spectacle blind is a figure-eight shaped unit with one end open for flow and the other solid to prevent flow.

The term "positive" means absolutely no flow such as might occur with slight valve leakage.


RE: Positive Isolation - Blinds

Many thanks to all of you !

 Shell DEP defines POSITIVE ISOLATION , as - " A procedure whereby phyisical separation between systems is achieved" With a note on that -  Closing of valves does not achieve the same .

CRG ,Thanks a lot .  Double block and bleed arrangement is being scrutinised .

But CRG , can you PLEASE answer the following

1. Do we get double block and bleed arrangement (  DBB) as a single pre assembled unit or should we have to procure and Install  two isolations valves, and a draining arrangement in between two isolation valves .

2. In the above arrangement of DBB , when you say "cavity"  do u mean to suggest a closed system drain for containing the loss of Hydrocarbon in case of  Upstream valve failure?


Many thaks ! We have been using spectacle blinds but for a 10 inch line ( In the present case) operating spectacle blind also needs a bit of physical and manual intervension  and this is exactly we want to avoid.

As CGS has suggested DBB will be most suitable but if DBB is proven method of positive isolation.

Waiting for CGS / your valuable advice!

RE: Positive Isolation - Blinds

If you purchase the correct valve, only one valve is need for double block and bleed isolation.  These are quality valves and their prices reflect the quality.   See the two brochures (trunnion ball valve & plug valve) below:



RE: Positive Isolation - Blinds

CRG , Thanks a lot !!

RE: Positive Isolation - Blinds

Nordstrom also makes a double block and bleed valve: two lubricated plug valves in one body, with gear operators for manual operation.  

RE: Positive Isolation - Blinds

As I’ve stated several times, in my opinion the only "Positive" isolation of a tank or vessel for personnel entry  is achieved by is a properly designed blank or blind.

I've seen several, too many, failures of block and bleed systems and physical separation to perform the duty of positive isolation.  This is with purchased or site built block and bleed systems and a 30" spool. .  

If one does use a block and bleed system just make sure it’s the proper one.

In 42+ years climbing in and out of chemical process vessels and boilers I've have never had a blank fail to perform in the isolation of a process system.  

RE: Positive Isolation - Blinds

To reinforce Unclesyd, OSHA doesn't recognize double-block-and-bleed as adequate isolation for work in a confined space.  There are many times where DBB is perfectly reasonable (i.e., "perfect" isolation is not necessary), but climbing into vessels with a spark ain't one of them.

David Simpson, PE
MuleShoe Engineering
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RE: Positive Isolation - Blinds

I have to agree with the issue of entering confined space.  DBB are not adequate for safety as stated above by others.  

RE: Positive Isolation - Blinds

Well, I'll have to disagree with the idea that a DBB is not adequate. I don't like statements like "OSHA doesn't recognize double-block-and-bleed as adequate isolation for work in a confined space." unless they are supported by "and here's the quote from the law which says so."

So... from 29 CFR 1910.146 available at http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&;p_id=9797

"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

"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.

So Federal OHSA does recognize DBB as one means of isolating a permit space. It does not state a preference for isolation schemes; as far as isolation is concerned, a DBB is equal to a blind. Having said that, I would prefer that a vessel which I enter is isolated by line misalignment or blinding, but I will not refuse to enter a vessel which is isolated via a DBB scheme. Of course, company policies and states may have stricter guidelines. For example, California recognizes DBB as adequate, but places a preference on disaligning and blinding over DBB and has additional requirements for using a DBB approach.

From the California Petroleum Safety Orders: http://www.dir.ca.gov/Title8/6816.html
Isolation of equipment shall be accomplished by blinding or disconnecting and disaligning. Where equipment, operations or conditions will not allow blinding or disconnecting and disaligning, double blocking with a bleeder (located between the block valves) may be utilized to isolate pipelines or equipment under the following conditions:...


RE: Positive Isolation - Blinds

Can't say I like your tone, but I really appreciate your contribution.  

My statement was a direct quote from a company safety manual that I never bothered to double check.  Maybe the authors of the safety manual got a bit zealous in their statemts or maybe the law has changed.  The reason they were wrong is immaterial, from your reference it is clear they were wrong.

Thanks for setting the record straight.


RE: Positive Isolation - Blinds


All the things you mention are regulations that are supposed to be used in everyday operations, but are only checked or audited when an event happens.   Today most written safety procedures are written not to particularly prevent the event but to be the CMA of management in case of an event.  I’ve been to too many meetings where the primary concern is “ARE We Covered On This”
I’ve made the statement many times on this Forum concerning the same thing.
OSHA is a regulatory authority that has been neutered by the politicians until it's useless in the prevention of incidents, excursions, accidents, or what ever you call them. I call them near hits.
You have to remember that near hits are not reported nor documented by the responsible party. Until you have been part of or witness to a potential fatal event one should stay out of tanks, vessels, etc. until he or she realizes the potentials of ingress to same.  
I can imagine what it's like in the contract maintenance situation that exists today coupled with inexperienced operators along with agressive management.  Most workers wouldn’t recognize potential problems and if they did stumble on one they are scared to report anything in fear of reprisals, being fired, or accused of not being a team player.  

I’m fortunate enough to have survived my career without injury but not without incidents or several near hits.  

What I consider the major accomplishments of my career were the act of directly saving the lives of 5 people.  The last was my co-worker who was in properly prepared and cleared for vessel entry a large double cone dryer locating leaking tubes for repair.  In the approved isolation procedure, part of the VE,  was a item to remove an approximate  30" section of the vent line for the N2 purging system.  The only problem was that the operations people had changed the N2 purge  to pressure control from flow control so that when any dryer was on line the pressure was maintained in the dryer.  By removing the section of vent as per procedure it dropped the back pressure on the system and the remaining dryer N2 pressure control valves opened to maintain pressure in the online dryers with increased flow in the vent header.  I realized what was happening by seeing particulate matter form the vent system  the vent line to the out of service dryer and exiting the entrance flange of the dryer in beam of sun light.  I was on my way to lunch.    
I have several more incidents involving almost everything except a blank.

I still have my wrenches and will be glad to assist you rolling any  blank in and out of a line to isolate equipment that you are to enter.  I might have to have some help when we get to using 1" impacts.  

RE: Positive Isolation - Blinds


Didn't mean to be curt. If you'll be at the PVP conference in July let me know and I'll buy you a beer.


I agree that regulations can be "neutered." I guess I'm fortunate to be working for a major firm which places safety as a high priority. In my past life I worked in several refineries and did see operations which were questionable, so I recognize that they exist. We all have different experiences. I'm familiar with one case where a line blank leaked and wound up spraying some contractors inside a vessel with hydrotest water. That's a long story that I really can't get into here.

Nitrogen is scary stuff. Glad you caught that.

I think the main thing to take away from this discussion is that there are certain legal limits and there are good practices. Just like there are code minimums and customer specifications which go above and beyond the code. Legal limits save lives; good practices save more.


RE: Positive Isolation - Blinds

           how big are the hammer blinds you are swinging? Why are they so difficult to move? Is the piping exerting compression on the blind so that it sticks when you try to move it? If so spreader bolts may help.

RE: Positive Isolation - Blinds

Isolation for entry should be separated from isolation for product contamination.  The original question seems to be concerned with an economic hit if one grade of product is allowed to leak into and contaminate another, and not a safety item pertaining to vessel entry, or reaction of different products.

If this is accurate, a double block should prove adequate whether from two individual block valves or a single ‘twin seal’ type, and selection would be an economic one.  Bleeding the space between the valves would decrease the chance of contamination, but may not be necessary.  In the case of a twin seal valve, the center cavity requires a bleed, but can probably be bled to the lower grade material side if there is no place to drain the material. (Several valves can be purchased like this.)

With the current hammer blind set-up, you presumably already have block valves on both sides, and draining capability.  Not sure how your piping is set up, but if one of the existing block valves can be used in isolation capacity, then one new block valve can be added per station or this single valve can be replaced with the twin seal type; whichever proves less expensive.

If vessel entry is still a concern, the plant preference I am used to is to require a blank, or offset pipe as already mentioned above.   Double block and bleed might be used to ensure depressuring so that a thinner blank may be used, but not for entry.

RE: Positive Isolation - Blinds

What need to be seen is
What level of control is needed to prevent operation of that valve for any reason.  To make whoever is going to operate the valve, stop and again ask ‘Is this the correct valve for what I need to do?’
What is the level of control needed to protect Human safety? The list as I see it is.
1. Close the valve
2. Put a tag on the closed valve “do not open”
3. Have a rigorous procedure on how the “do not open” tags are controlled
    the rigorous procedure includes dismissal for inappropriate operation of equipment
4. Add a lock and chain to the tag (need a key or a bolt cutter)
5. Assign a person to stand next to the valve to prevent operation in addition to the lock and tag
6. Use the double block and bleed (add lock, tag, or persons)
7. Slide in a device that requires a tool to install and remove, (line blind) include the tag and lock
8. Remove a section of pipe so that the material flows elsewhere instead of into the tank

The last two items may have a craft separation element (such as operators do not touch wrenches and mechanics do not turn valves) so the establishment and release of a ‘clearance’ requires at least two chains of command to override.

All of the above items except the first two are acceptable to OSHA as a procedure for energy isolation.

This sliding scale should be used to match the level of control to the level of hazard

RE: Positive Isolation - Blinds

You can have a look at this website www.onislineblind.com

this company manufactures quick action lineblind (manual, gearbox and automated) which are used in oil, gas, and chemical environment.
I am sure that their product will fit.


RE: Positive Isolation - Blinds

Pricard, Thanks a lot

At the moment I am examining following alternatives

1. Conventional Double block and bleed arrangement
2. Twin design - as suggested by CRG as per above
3. Twin taper plug DBBs - as suggested by Jimcasey
4. And your suggestion .

We will see what is easy and cost effective and acceptable Techncially to our process guys

Thanks once again and am happy to get lots of responses and ideas !!

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