IS Device Question 1

[jcfoley]

I have an application for an oil flow meter in a Class I, Div II, Grp D envirnoment. We have always used a REN Slow Flow meter for this lube oil measuring task. However, this meter has no electrical approvals at all. I am told by the manufacturer that it can be used in hazardous locations with the use of a zener barrier.

How can I prove to an inspector that this is an aceeptable practice? Is this an acceptable practice? The flow meter is a purely resistive switching device. It cycles between 1 kOhm and 1000 MOhm as it switches. Used with a 24VDC input, this device generates a pulse train that we use to measure the oil usage. If this is acceptable, what code can I point to?



Thanks,
Chris Foley
Midland, TX

 

[itsmoked]

Zener Barriers generally called, "Barriers" limit the amount of energy allowed into a hazardous space to a value so low that it cannot initiate an explosion. With this in mind the only thing that can violate this is if you have a way to store energy in the device in the hazardous space. This means capacitors, inductors, and of course batteries. If your device has none of these elements then an appropriate barrier, installed correctly, isolated from other unlimited wiring and circuitry is sufficient.

I am surprised you need further proof. Sorry I do not know what code to point at but DO go to the web sites of barrier makers as they work pretty hard to explain their products and the necessity and may point out the code aspects.

Pepperl-Fuchs is probably the biggest supplier of barriers. Unfortunately their web site truly stinks. I do believe they have a great catalog loaded with info you could get from them. A sales guy could probably spout "codes".

http://www.pepperl-fuchs.com/company/presence/europe/greatbritain/05764/index4_e.html

Gems is a good company too smaller but a better web site.

http://www.gemssensors.com/SpecTemplateStandard.asp?nProductGroupID=154

There are several other makers too. Google "zener barriers".

 

[ScottyUK]

You could add MTL in the UK as another useful source of IS barriers. And the website isn't bad. their techincal people in the UK are very good.

http://www.mtl-group.com/index.htm

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If we learn from our mistakes,
I'm getting a great education!

 

[itsmoked]

Thanks ScottyUK; Yes, nice site. Nice products too. I have used them many times.

 

[jcfoley]

itsmoked/ScottyUK,

Thanks for the info. This pointed me in the right direction. Both datasheets for both of these mfgs referred to switches as simple devices. This lead me to search my new NEC CD for simple switches. It landed me in Section 504.2 Definitions.

Simple Apparatus. An electrical component or combination of components of simple construction with well-defined electrical parameters that does not generate more than 1.5 volts, 100 milliamps, and 25 milliwatts, or a passive component that does not dissipate more than 1.3 watts and is compatitible with the intrinsic safety of the circuit in which it is used.

FPN: The following apparatus are examples of simple apparatus:

[ol a]
[li]Passive components, for example, switches, junction boxes, resistance temperature devices, and simple semiconductors devices such as LEDs [/li]

[li] Sources of genrated energy, for example, thermocouples and photocells, which do not generate more than 1.5V, 100mA, and 25 mW [/li][/ol]

The device that I am using is a totally passive device as it has been explained to me. I can now use a barrier for this device to create an instrinsically safe circuit as long as the inductance and capacitance of the wiring and the device do not exceed the limits of the selected barrier. I am aksing the flow meter mfg for an electical print to make sure that there are no signal filters or other capacitive or inductive components.

Thanks for your help.



Chris

 

[itsmoked]

Our pleasure. By the way you can have things like caps and inductors in EXP environments but there are certain limits like 10uF or less in 5V limited circuits. 1uF in 10V limited circuits. (this is just an example not the actual values). Same thing for inductors except uH/current.

Also the actual environment Hydrogen (worst), Acetylene:Ethylene Oxide (next), etc., all effect the allowed values. As in the above example 10uF @ 5V for Hydrogen, 10uF @ 7V for Acetylene.

I tell you this for the education and so if your instrument isn't simple realize there are options.

 

[jcfoley]

Where would the this info be located? I work with Natural Gas and everything we do is Group D. A lot of the equipment used in the oil patch is not rated for this envirnoment. Legal issues with this are catching up to the industry really fast. Having a reference to this would be great for problems in the future.

Thanks,
Chris

 

[itsmoked]

Ack! Man.. I had a book that had a chapter that had a few pages that explained this. This was at a company that got a lot of work in oil fields and Ethylene Oxide(ETO) Sterilizer plants. We built high powered battery devices that ran inside sterilizer chambers so we had to be conversant in the subject.
I will have to ask someone back at the company for the name of that book.

Maybe someone else will have another recommendation. I will try to get you the info. (may take a few days).

BTW if you have ever eaten a candy bar with nuts in it those nuts were sterilized in an ETO chamber controlled by a controller I designed. mmmmm nuts...