Static Electric Discharge Hazard 9

[MotoGP]

I'm rapidly getting in over my head on this subject.

My specific concern pertains to Bulk Oil Tank Vessels, but, I'm rapidly becoming interested in the "any given piping" scenario, to wit, and "borrowing" from the disconcerting language of NFPA 77, "...under certain conditions, particularly with liquid hydrocarbons, static (electricity) may accumulate in the liquid...The generation of static electricity can not be prevented absolutely, because its instrinsic origins are present at every interface". Oh gee, that's swell...

So, the pipe material, the physical properties of the fluid contained within the pipe and the velocity that it travels, the pipe dimensions, the action of fluid, let's say discharging through an open end of piping (and at a given distance over a fluid level within a tank, if a fluid is present),etc, should all play a role, in contributing to a "certain condition"...maybe?

I have looked through all of the "McGraw-Hill" Handbooks on Piping and done internet searches "ad nauseum", albeit with some "near misses"...any help to steer me in the right direction will be greatly appreaciated.

I am in search of a "recipe", or a "flowchart" or something similar to understand when, where and why there exists a need to ground a piping span in order to remediate a "misadventure" due to a hazardous electrostatic discharge.

 

[psafety]

MotoGp,
I work in a manufacturing environment and occasionally perform hazard analysis on flammable liquids. Some guidelines are:

-restricting fluid velocity, based on its charge capacity

-use downcomers to prevent free-fall of liquids in vessels etc. by terminating the fill pipe against a side wall or using a submerged dip-tube. Dip-tubes can present siphon issues, use a drilled hole in the pipe side-wall just inside the vessel to break vacuum in the event of a siphoning scenario.

-ground all equipment, verify bond/ability to dissipate the charge accumulation

-charge is generated by splashing, free-fall, filtering, general disturbances.

-use conductive equipment, watch-out for insulating instances, eg; gaskets, hoses, etc. you may need jumpers to create a good bond.

 

[Thomasjl]

MotoGP,

As long it is within the liquid hydrocarbons there is nothing to worry about static electricity or any other electricity (e.g. lightning) because it can´t ignite anything. What is combustible is the vapour of your hydrocarbons mixed with a little oxygen, not too much and not too little oxygen, more technical - it is explosive when the percentage oxygen is above LEL (Lower Explosion Limit) and below UEL (Upper Explosion Limit).
Liquid hydrocarbons are above UEL (no oxygen!). As long no sparks can occur above the liquid level, which is basically the case inside a piping system there will be no explosions.
Piping systems with 150# flanges can withstand explosions with supersonic speeds, it’s the equipment at the end that pays the price. So in case there is an accidental vapour trap inside the middle of your liquid hydrocarbons piping system would occur, which also happen to be between LEL and UEL and on top of that a spark would occur it would just give a little `woof` that perhaps nobody even would notice.

Out the top of my head the API 650 for storage tanks says that the entrance of vapour velocity should not exceed 7 m/s. Danger always remains, but below that speed the risk is brought back to an acceptable level.
Storage tanks has their liquid fill at the bottom and under normal operations they are never empty, so the entrance of the liquid is always submerged, so not to worry about static electricity.
There are little rules for tank vessels. The US Coast Guard might give some answers. What I do know it that, at least nowadays, the tanks on board are filled with a vertical pipe to the bottom which causes some splashing in the beginning (as these tanks are often completely empty) until the open end is submerged. That is one of the reasons they start loading slowly. Unless you are designing tank vessels I wouldn’t worry of static electricity on board because they do that for you. It is a major threat and they are scared like hell and have applied complicated insulating and earthing systems between sea/river bottom, jetty, loading arms and vessel.

 

[Montemayor]

MotoGP:

The overshadowing menace of all liquid organic (especially hydrocarbons) is the physical fact that they generate static electricity when introduced into a storage tank (or any tank, for that matter) as "free-fall" liquid. The obvious hazards related to the generation of static electricity in a hydrocarbon or organic chemical environment is well-known to experienced chemical engineers. I say experienced, because this wasn't taught in school. I learned it working DuPont projects and have never forgotten what they know about this phenomena. I know they wouldn't mind my telling you that their standard is a Dip-Pipe feed line where liquid hydrocarbons or organics are introduced into tanks. That, in my book, is the only safe way to do it.

But that's not where the safety engineering starts and ends. That's only where it starts; the technique also must involve the selective drilling of a "breather hole at the top of the dip pipe, just under the top (or roof) of the tank's shell. This must be done in order to avoid another hazard: the creation of a back-syphon effect. Through the years I've noticed that not that many people are aware of this hazard, although I believe DuPont has shared this information with others through the years. I usually allow a 3/8"-1/2" breather hole - although other, younger (& more naive) engineers use a 1/4" size. I just want to make sure that the hole is never plugged, especially when handling slurries or heavy crudes. As some of the respondents have said, once you know of the hazard it's common sense to deal with it in a practical and safe manner. I have participated in numerous HazOps and Management of Change meetings where this subject has been discussed to exhaustion and assurance that the operation is safe.

The important thing is to pass on the knowledge of the possible hazard: the creation of static electricity in free-falling hydrocarbon liquids or organics. There is even a standard DuPont dip pipe directive and drawings on the subject. Other companies surely must have similar standards. I hope this information is of service to you.

Art Montemayor
Spring, TX

 

[25362]

I've found that Crowl and Louvar's Chemical Process Safety: Fundamentals with Applications (Prentice Hall) can be very helpful in understanding static electricity risks, brings examples and gives multi-layered recommendations.

 

[katmar]

The use of dip pipes with anti-syphon holes below the roof are common in the light organic industries I have worked in. However, I have always worried about the anti-syphon hole.

Isn't there a risk of liquid coming out here and going into the free-fall situation we are trying to avoid? Or if the velocity in the pipe is too high for liquid to come out, maybe it would suck air in like a venturi, and we again have a dangerous situation with air mixing with the liquid.

It's not the sort of thing to do experiments with, so if anyone has the answers I would be grateful!

Katmar

 

[25362]

When using dip legs or dip pipes for tank filling I think the syphon-breaking hole should be preferentially located -on the upper vertical portion of the the dip pipe- inside the tank so, if overflows or suction effects take place, they will both happen within the tank space which should be anyway filled with an inert gas blanket.

An alternative to avoid using the hole, would be cutting the dip pipe (within the tank space) near the top tank inlet and attaching it to an angle iron, or half pipe, that could serve to direct the entering liquid towards the bottom thus avoiding splashing, free fall and the accumulation of static charges.

Enlarged pipe sections prior to enetering the tank may help in adding some relaxation time for charge build up to dissipate. Of course, bonding and earthing are sine qua non steps to protect the system from static electricity hazards.

 

[Latexman]

MotoGP,

An excellent reference is "Avoiding Static Ignition Hazards in Chemical Operations" by Larry Briton.


Good luck,
Latexman

 

[MotoGP]

Thanks to all of you for your great words of advice. As I am assigned to work in a shipyard in South Korea, I do not take any of the time that you all use to respond for granted. Thanks again for getting me started in the right direction...you are all such sharp people, but you don't need me to tell you that.

 

[TD2K]

I worked on one bulk gasoline terminal where we loaded trucks taking the fuel to your local service station. The client had a requirement that we provide downstream of the filter (which they considered to be a static creating device) that you allow 30 seconds residence time in the piping before the fuel entered the truck. That in their experience gave sufficient time for the static charge potentially created by the fuel to dissipate.

 

[TD2K]

I found some more information on the 30 second relax specification. In the information from the client, there was reference to "Per NFPA 77 section 4-5.3.9 & API 2003 section 2.19, at least 30 seconds relaxation time should be in piping systems".

You might want to pursue those standards.

 

[jmw]

When working on some flow metering applications i was made aware of an 11m/s velocity limit(as i recall. i.e. don't use this figure).

This, and the recomendations re tank entry were all allegedly from a proposed standard.

What is most remarkable is that though I thought that it was the basis of a proposed standard, i never was able to discover the standard. Interestinbgly enough, none of the members has so far been able to refer to any industry standards.


JMW

http://www.viscoanalyser.com

 

[jte]

MotoGP-

I think you have had plenty of guidance from an engineering perspective.

For some references at a consumer (dare I say "manager")level, check out

http://www.pei.org/static/

and

http://api-ec.api.org/media/index.c...=display_body&er=1&bitmask=001007000000000000

jt

 

[quark]

Is there any reason for not (anybody)mentioning about grounding(ofcourse to a separate earth pit) and neutralizing the pipe across its length? This is a standard practice we follow even for glass lined piping in our bulk pharmaceutical plants and tank farm areas as well. (An earthing plate is inserted at each flange joint in case of glass lined piping)

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[25362]

To Quark, in my posting of April 8, I wrote bonding and earthing, which should be equivalent -if I'm not mistaken-to your above recommendation.

 

[Montemayor]

All:

At the risk of lengthening this thread, I'm going to add a "qualifier" -or explanation- to address the important related issues brought up by Katmar, jmw, and quark. I think it important that the originator of this thread, MotoGP, does not get the idea that we are throwing more curves out at him or that the responses are in disagreement on how to approach the problem. I believe we are all in total agreement. Some may not have been exposed on methods believed to reduce static charge buildups - but all are certainly in agreement to eliminate them as much as possible.

The use of dip pipes is to reduce the basic reason given for the static electricity build-up: free-fall of organics through the air space in a tank. By using a dip pipe, this effect is reduced drastically - as opposed to splashing against a side wall. The vent hole required to avoid back-syphon is drilled on the actual side of the dip pipe, immediately after in enters the tank; i.e. the orientation of the hole is parallel to the direction of the flow stream. If the velocity head(v^2/2g) and the residual pressure in the dip pipe are low, we were taught that the amount of liquid exiting the hole would be minimal. Here, I'm going to stress that this dip pipe method in no manner or way guarantees that static electric charges will not be built-up. Please understand that I am not offering a disclaimer to confuse everyone; rather, I would point out that it is our duty and obligation as engineers to reduce the creation of static electrical charges to a minimum and also apply recommended practices like positive grounding. There are NO recipes or flowcharts to my knowledge that will positively eliminate the creation of static electrical charges on any/every fluid transfer operation. We engineers have to approach every potential application with the open mind and experience available to us to eliminate their possible creation and make sure that we have also taken steps to ensure the safe disposal of any subsequently created charges. Katmar's questions are well put. There is no 100% answer that I know of. The dip pipe standard in some process companies is an effort to reduce the static creation. I would not assume that it reduces it 100% and Katmar's concern should be well taken. Certainly bottom filling curtails the free-fall effect 100%; however, the containment tradeoff of confronting gravity backflow is another hazard to confront. When one has no other option except to top fill - as in tanker ships or barge vessels - the dip pipe is almost mandatory.

quark rightly points to a positive and standardized method to safely dispose of created charges. I have assumed this type of standard is already in place and working. As 25362 states, "bonding and earthing are sine qua non steps to protect the system". This should always be done when dealing with a potential static build-up and its safe disposal.

I believe that the industry standards that jmw refers to are the above-mentioned bonding and earthing methods. I don't have the specific codes or recommended standards at hand, but perhaps others can add these as they come across them or remember them.

Static electrical charges in a Hydrocarbon environment are a serious hazard that must be addressed by all engineers working in such an application and I think this thread has high-lighted that importance.

Art Montemayor
Spring, TX

 

[psafety]

Being the first resonder to MotoGp's question, my input was somewhat uncharacteristic as it was not related to the petroleum industry, but manufacturing dealing primarily with alcohol systems and combustible dust atmospheres. I understand this industry is different, but also very much alike in the safeguards we use.

As Art points out there are several design practices that are important to point out, but are not specifically dealing with reduction of charge accumulation.

MotoGp I hope this has been of value. I have throughly enjoyed this thread, seeing others involved with the assuring safe design concepts, and their ideas and methods is a learning for all.

Don Coffman

 

[Thomasjl]

To my opinion this threat is getting very confused because it swerves away from the purpose.
When talking about static electricity it is very important that we first distinguish whether we deal with an explosive mixture or not. If we do not deal with a vapour between LEL and UEL (see my threat of 7 April) there is no hazard for neither fire nor explosion nor whatsoever.
So, liquid lines or blanketed tanks are no subject to hazard. (Although tanks are grounded anyway, standard practice)

Now the dip pipe. Dip pipes should be avoided. A liquid fill line of a tank should be connected as low as possible, at the bottom. Only if this is not possible you do it from the top with a dip pipe. Always with a dip pipe!
Why a dip pipe? In case there is an explosive vapour above the liquid it avoids static electricity in the vapour. Static electricity in the dip pipe it self is not relevant. There likely will be static electricity in the dip pipe but this will be conduced away respectively through the steel of the pipe through the tank wall to the earthing as this is the easiest way.
In case there is no explosion hazard we still use a dip pipe but for other reasons, it is neater, quieter, better control of the loads, less splashing, less turbulence, less vibration etc.

So I would say if you do not deal with a explosive mixture LEL-UEL, whatever the configuration (Explosive mixture outside the pipe can also be a subject to earthing!!) keep your bonding and earthing money in your pocket.

Further I would like to learn something from Quark, threat Apr 10, which states that it is a standard practice with his company to insert earthing plates at each flange joint in case of glass lined (steel??) piping in the bulk pharmaceutical plants and tank farm areas as well.
That is a very expensive standard practice as glass lined systems are generally build up from relatively small flanged pipe spools and has therefore many flange connections and so will require many insulating plates.
All kinds of questions are coming up:
1. Is the content of the pipe always a vapour between LEL-UEL?
2.Is the earthing plate in contact with the content?
3. Is the plate unprotected? If so, why is the pipe glass lined if you have steel that can withstand the chemicals of the content?
4. I s this a plate between the flanges with a gasket on either side?
5. Do they realize that each single gasket is a subject to leakage? Is it worth it to double the risk?
6. Is this plate to avoid bridging between the flanges? (I don’t think so)
7. But if so, why is this not done with an earthing strap on the outside?
I just like to learn.

Thanks, Thomasjl.

 

[25362]

Art Montemayor is absolutely right when saying I quote: "Static electrical charges in a hydrocarbon environment are a serious hazard that must be addressed by all engineers..."

At this point I thought some clarifying remarks would be in order:

1. It was clear from the start that the thread referred to hydrocarbons which are potentially flammable.

2. The static electricity hazard is not only related to the LFL-UFL normally recognized range. If small drops are formed, as in a mist, the LFL may drop to less than 1/10 of the normal value.

3. Many hydrocarbons ignite with very low energies, their minimum ignition energies, MIE, are about 0.25 mJ. Electrostatic discharges resulting from fluid flow have energy levels exceeding this MIE. Where flammable vapors may be present, any charge accumulation exceeding 350 V and 0.1 mJ is considered dangerous.

4. Common industrial examples of "static" buildup include pumping nonconductive (in particular pure or refined) liquids through a filter an orifice or a pipe, whether entering a tank from below or through a dip pipe, free fall filling, mixing immiscible liquids, leaking steam impacting an ungrounded condutor, the use of non-metallic hoses, and pneumatic conveying of solids, to name just a few. Fluid entering a tank from below, as Thomasjl says, is preferable to dip legs for the above-mentioned reasons. Even then it is sometimes advisable to extend pipes into the liquid to prevent static buildup. Agitation and mixing may also develop static charges.

Books bring as a reference F.G. Eichel's "Electrostatics", Chemical Engineering, March 13, 1967, pp. 153-167 for a detailed explanation of the basics in the eletrostatic relationships involved in static buildup.

 

[quark]

25362!

You did mention and I am sorry for overlooking it.

Thomas!

1. As far as I am concerned, the flammability limits of the solvents are not checked and earthing is done as a general practice. (perhaps they don't want to keep no stone unturned and I am habituated to it)

2. Yes, the earthing plate is in contact with the content.

3. The plate is unprotected (I mean no need to). It can be stainless steel or any other superior material that can with stand the piped fluid. A glass lined pipe is rather cheaper.

4. This is a plate with gaskets on either sides.

5. This system does increase the chances of leakage but I don't want to comment on this issue.

6. The plate is to provide continuity to electric charge.

7. Earthing strip outside the piping provides continuity to the piping only but it can't take care of the static electricity generated with in the fluid because of glass lining(which is our main concern).

More clarification

Though I am a member of Engineering Language/Grammar Skills forum, I do make mistakes because english is other than my native and national languages. I didn't mean glass lined piping in plants and tank farms but it means "earthing for piping in tank farms and also for glass lined piping in plants".

As you already noted, this system may not be foolproof. But it requires more than engineering to open the blindness of thought which is prevalent in Pharmaceutical Industry (atleast in India).

Your comments on the original post are well taken.

Regards,


Eng-Tips.com : Solving your problems before you get them.