Grounding in hazardous atmospheres

[weco]

This came up at work recently and I'm not convinced it was dealt with
properly so I thought I'd pose the question to the group and see what you
guys thought....

We have a solvent reclamation machine that was recently installed. It is
technically not in an explosive atmosphere but there are explosive drums of
solvent connected to it...

The machine and the drums required grounding, and the commisioning
instructions were confusing. Additionally the Field engineer did not seem to
know what to do - he was more of a software guy...

We bonded the machine direct to a main ground with a #8 wire and the drums
have ground cables clamped to the fill pipes and then directly to ground as
well.

While this will serve to keep the system at zero volts it is a very low
impedance path to ground, and should a charged body approach any part of the
machine especially around the drums, there would probably be an arc
discharge.
Seems to me this is not what was intended by the grounding system

For electronics work (my background) we use mats with a relatively high
impedance to slowly bleed charge potentials down to prevent the arc....

I've looked through the NEC but have not found any rules specifically
relating to this problem....

I know from my compliance testing background that we used resitors in ground
paths to minimise the arc discharge in areas other than the equipment under
test, and there are standards that describe what and where they should
be....

I dont want to allow ignorance to create a dangerous situation here but none
of the plant electricians knew anything...

Any thoughts???

henry

 

[DarkMatter]

This came up at work recently and I'm not convinced it was dealt with
properly so I thought I'd pose the question to the group and see what you
guys thought....

We have a solvent reclamation machine that was recently installed. It is
technically not in an explosive atmosphere but there are explosive drums of
solvent connected to it...

The machine and the drums required grounding, and the commisioning
instructions were confusing. Additionally the Field engineer did not seem to
know what to do - he was more of a software guy...

We bonded the machine direct to a main ground with a #8 wire and the drums
have ground cables clamped to the fill pipes and then directly to ground as
well.

You could also core drill the floor, and drive a ground rod right at
the machine site.

While this will serve to keep the system at zero volts it is a very low
impedance path to ground, and should a charged body approach any part of the
machine especially around the drums, there would probably be an arc
discharge.
Seems to me this is not what was intended by the grounding system

For electronics work (my background) we use mats with a relatively high
impedance to slowly bleed charge potentials down to prevent the arc....

I've looked through the NEC but have not found any rules specifically
relating to this problem....

ESD is probably not too well covered by the power distribution boys.

What you need is an area around the machine that has a bona fide
conductivity to both the machine, and ground. say ten feet...

Then, ALL personnel approaching the machine must have heel straps on.

THEN, you need posts with ESD mat, grounded surfaces on them, near
the machine so that personnel can be doubly sure that they are
discharged before touching any metal on or near the machine.

I know from my compliance testing background that we used resitors in ground
paths to minimise the arc discharge in areas other than the equipment under
test, and there are standards that describe what and where they should
be....

The problem here is that ANY arc, however small, is likely to be
above the flash point temp of the gasses involved.

Who would risk it? Not me.

I dont want to allow ignorance to create a dangerous situation here but none
of the plant electricians knew anything...

Any thoughts???

Oooops... see above. Standard ESD procedures should work, if you
think of using high resistance mats to slowly discharge any
accumulation on personnel, as THEY are the charged element, when not
grounded. DO NOT use metal touch points as there would STILL be an
arc. The safest hedge is the heel straps as they ground the person,
even as they approach the machine, and with each step.

Merely walking across the room in insulated shoes can allow one to
pick up several hundred volts of charge, right out of the air.

 

[DarkMatter]

Merely walking across the room in insulated shoes can allow one to
pick up several hundred volts of charge, right out of the air.

Also... use good, flowing ventilation in the area to keep any whisps
of vapor from accumulating into prospective pockets of pain.

 

[Rowbotth]

For starters:

Generally, the thing that causes a concern with solvents is static
elctricity when unloading the material, or moving it from Point "A" to
Point "B".

Grounding shold be to bond the area to the same point, so that the
entire system rises as one as faults may occur. (For instance, a
lightning strike should see the entire area rise as one voltage level;
trying to stay at Zero is both difficult and also may be causing a
problem).

Put a cable which is tied to the main ground on the vehicle unloading
the solvent, and also ensure that any pipelines moving the solvent are
bonded well to your main ground system.

HR.

 

[weco]

this is for me where the confusion begins

the words "bonded well" ..does that mean a heavy guage conductor to ground?
for the main operating components of the machine where there are heaters or
motors this makes sense, but for areas like the solvent drums I'm not sure
it does. It almost seems like you have to evaluate which hazard is more
likely. A motor fault wont be an issue at the drum if the motor is well
grounded...static wont be an issue around the motor - assuming it's not in
the explosive atmosphere...
I'm going to look into the standard that has been suggested....hopefully
they will have an answer, but it reall seems like this is not a "one method
always works" situation....

do most of you guys use low impedance grounds in situations like this?- my
understanding of well bonded...
and if so what do you use as the rule for the ampacity of the ground
conducter?

thanks for your thoughts

henry

 

[Rowbotth]

Do you have a ground grid around your entire facility, or at least
around the part where you are dealing with the solvent? I know some
facilities do; I am assuming you would have this. (This would be so you
could provide a common ground reference point to your entire facility; I
thought it was common practice for the past 30 years or so.)

That is what I am talking about when I say you should be "bonding" to
your ground system; be it the vehicle off-loading the solvent or the
pipeline transporting it.

I suppose if you do not have a common ground grid for your facility, you
could create a triad with 3 10' rods in an eqiliateral triangle 10'
apart - or 3 20' rods in the same configuration 20' apart... And then
tie your static ground to this.

(If you want, go off-line with this and we can talk one to one.)

HR.