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Author Topic: Gas plus Spark =?  (Read 1691 times)

BarryM

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Gas plus Spark =?
« on: November 22, 2009, 04:31:48 PM »

One for the Combustion Engineers if we have any on the Forum.

If a space contained a butane/propane mix such as is used for firing model boilers;
If the air/gas mix was within explosive limits, i.e. neither too rich or too thin;
If a 12V. DC electric motor consuming 2.16W was run in said atmosphere;
Would sparking at the brushes have sufficient energy to ignite the gas?

I realise that this is open to questions such as 'how much sparking is occurring'? To this I could only answer that normal suppression would be applied to the motor which would be driving a fan.

Obliged for any informed opinions.

Barry M
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dodgy geezer

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Re: Gas plus Spark =?
« Reply #1 on: November 22, 2009, 05:55:59 PM »

The safety answer is generally considered to be yes. I don't think there is any 'energy' limit on sparks - if the air/gas mixture is taken over its ignition temperature it will ignite. Piezoelectric gas igniters can't have mucg energy in their spark, but they still do the business.

Are you thinking of a paint extractor system? The usual thing to do would be to run the motor outside the explosive atmosphere, or use a brushless...
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BarryM

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Re: Gas plus Spark =?
« Reply #2 on: November 23, 2009, 05:14:51 PM »

But temperature of spark would be dependent on energy input to create it. I recall that after some VLCCs suffered cago tank explosions in the 70's, experiments to establish the cause focused on the static electricity generated by tank washing jets and whether this contained enough energy to create a spark which would detonate an explosive gas mixture.

I think that you are right and any spark generated at the motor brushes would be sufficient to ignite the air/gas mixture.

No, not a paint extractor system but a system connceted with Project X which is lurking in my workshop. I can't tell you what it is or the entire population of Mayhemmers would have to be shot. Understandably this might be construed as an ant-social act.  %% %%
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flashtwo

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Re: Gas plus Spark =?
« Reply #3 on: November 23, 2009, 07:25:08 PM »

Hi,

Many years ago I attended an Intrinsically Safe Equipment course regarding the design of equipment to be used in explosive atmospheres - it was a bit of an eye-opener.

The course started with a photo of the chard remains of a victim of the Flixborough explosion (just to grab your attention!).

It was pointed out that hydrogen only ignites above a surface temperature of 400C and it was demonstrated that a steel hammer hitting a piece of aluminium can ignite an explosive atmosphere.

Apparently to stop a spark causing an explosion you have to limit its energy measured in joules and for each type of gas mixture there is a different limit. Well, 1 joule/sec is 1 Watt and Watts equals volts x amps as we all know.

Any Intrinsically Safe Equipment has to be capable of (if I remember correctly) having three faults within it,  but still limit the available energy by means of resistors. This also means considering any devices that can store energy i.e. capacitors and inductors (motors).

If you can limit the available voltage to 1.2V it is then deemed "Simple Equipment" and is exempt from the regs. Also, if the equipment is enclosed in an air-purged enclosure it is also allowed in the explosive area, though if the air-purging fails the equipment has to be automatically isolated. Zener barriers, which limit the available supply voltage, can also be used.

You can have a metal mesh as a barrier, so if a flame developes it energy is sapped away by the mesh (remember the Davy Lamp).

To work out the available spark energy you would have to know the inductance value of the motor across the commutators, but thats getting a bit silly!

If you ensure that the motor ran in a pure butane/propane atmsophere that would be ok! Our generators ran at 23 thousand volts in a hydrogen atmosphere quite happily (except once!) and we used to make our own hydrogen and pump it up to over 400 psi.

In the early development of my computer controlled steam boat the "Vital Byte", I had trouble with the computer behaving erractically and finally traced it to a steam jet causing an electrical discharge much in the same way as those tank washing jets.

The final idea is to use a canary - if it stops singing you know you're in trouble.

Ian G.
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sheerline

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Re: Gas plus Spark =?
« Reply #4 on: November 23, 2009, 08:59:03 PM »

Your electric motor would indeed ignite your explosive mixture should a spark occur.  I think the present day, safe current limit for downhole electronics in the oil business is still 25mA. Your motor is drawing 180mA and could produce a hot enough spark to ignite your gasseous mixture. In any case, there is no guarantee your motor may run cleanly all the time, a bit of dirt, slight loading or simple intermittency could cause a spark.... It all sounds extremely dangerous and should be avoided at all costs if you wish to keep your eyebrows... or any of your anatomy! %%
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BarryM

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Re: Gas plus Spark =?
« Reply #5 on: November 23, 2009, 09:00:53 PM »

"If you ensure that the motor ran in a pure butane/propane atmsophere that would be ok! Our generators ran at 23 thousand volts in a hydrogen atmosphere quite happily (except once!) and we used to make our own hydrogen and pump it up to over 400 psi."

Ian,
B****y Hell!   :o  You must have liked to live dangerously; 6kV in atmosphere gave me the shivers.  

However, I do recall extracting a duff circuit breaker from the back of a live 440V switchboard in a very hot engine room with my arms next to the busbars. The sweat trickling down my arms was not entirely (?) caused by the heat. We did daft things in those days.

That said, the suggestion of the Davey Lamp mesh has given me pause for thought. You may just have given me the solution.

Thank you,

Barry M
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malcolmfrary

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Re: Gas plus Spark =?
« Reply #6 on: November 25, 2009, 11:19:58 AM »

Brushless motors are generally considered spark-free - you only have to worry about broken wires then.
The other way is not to evacuate by sucking but to purge by blowing, that way the explosive mixture should never get near the motor brushes.
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BarryM

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Re: Gas plus Spark =?
« Reply #7 on: November 25, 2009, 02:06:33 PM »

Malcolm,

Thank you. That gives me something else to have a think about.  :-))

Barry M
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