Hot, Humid, and Ammonia: HELP!

I'm putting together a control system for a semi-autonomous diesel powered machine that will be operating in a compost environment not suitable for humans:

- 55 deg C (130 deg F) ambient outside the control panel
- 99% relative humidity
- up to 1000ppm ammonia in the air
- extremely dusty
- high vibration

So not only do I not want to EVER have to service this thing, but it needs to stand up to the environment for at least a year or two before electrical components need replacing.

Existing older machine have had their tek cable sheathing disintegrate and other plastics crumble in under a year, maybe from the ammonia, I don't know. I also don't know what type of plastics fell apart and which ones withstood it.

I need to put a basic little 24in-24out PLC in a box complete with an operator's interface on the cover. Current thoughts are to seal everything inside a fibreglass enclosure (like Hoffman ULTRX series) with a windowed door, and put the OI on swing panel behind the door so nothing is exposed. No fans, no filters. I think I can isolate the panel good enough such that screw terminals would be fine, but how does one know when to switch to spring-clamp terminals? What PLC's have these as standard?

And my biggest concern is the exposed wiring and connections outside the machine? What wires/insulation/plastics will take this environment? Or am I worried about nothing?

Just out of curiousity, why does the PLC need to be in that environment?

It'd be a lot easier (from a component selection standpoint) to mount it in a different area, or even outside in the rain for that matter. You would probably still need IS barriers and seals, though. Any added cost on the installation would probably be offset by the lower cost of servicing the equipment.
AK

Just a thought, this is something that I am tempted to do for another reason,...make all your connections, test everything six ways from sunday, and the pot everything except the HMI face in epoxy... I want to try this to solve a vibration problem, but it would keep the controls clean,too.

David

With that much humidity you are bound to have condensation problems. Will there always be power available, if so I would look into pressurizing and/or enviromentally controlling the panel with "external air"...ie air that does not have the ammonia and you can "control" the temp and/or pressure.

Ammonia goes by different names.

Materials Handbook (Brady & Clauser, 11th edition):

Ammonia - A gas of the formula NH3, originally called 'alkaline air' and 'volatile alkali' and later in water solutino called 'spirits of Harthorn'. It is a by prooduct in the distillation of coal, but is easily made by passing nitrogen and hydrogen and a catalyst through an electric arc.

Ammonia is readily absorbed by water, which at 60°F takes up 683 times its own volume of the gas, forming the liquid commonly called ammonia, but which is ammonium hydroxide, a colorless, strongly alkline, and pungent liquid of the composition NH4OH with a boiling point of 38°C.

At 80°F it contains 29.4% ammonia in stable solution. It is also known as ammonium hydrate and aqua ammonia, and is used for cleaning and bleaching, for etching aluminum and in chemical processing.

5th edition Chemical Engineer's Handbook, (1973) pg.23-20:
rating for materials in contact with an Aqueous Ammonia solution:

Polyethylene rates "complete resistance" at 0-100% concentatration from 0-150°F
Polyvinyle Chloride (unplasticized) rates "decomposition" at 150°F at 100% concentration.
Rubber (natural GR-S) rates satisfactory at 25-100% concentration at 75- 150°F
Butyl rubber rates satisfacotry only at 150°F at 25% concentration
Nitrile rubber rates satisfactory at 30% concentration at 200°F
Epoxy resins rate satisfactory at low temperature (room) at 100% concentration
316 stainless (enclosure) rates satisfactory at room temperature up to 100% concentration (meaning <0.002" corrosion rate/year)
Polyesters are rated unsatisfactory, so don't wear your leisure suit out to the site.

I have the Compass Publications Chemical Resistance Guide for Elastomers II, which has a

couple dozen elastomers, like teflon rated for
ammonia gas (cold or hot)
ammonia (aqueous liquid),
ammonium hydrate
ammonium hydroxide

but the tables run over 12 pages, far too much to type. And I'm not sure that a plastic, whether wiring insulation or PLC housing is an elastomer that would be listed.

The 3 recommended elastomers for aqueous ammonia or ammonium hydroxide are

isobutylene isoprene (IIR), ethylene proplylene (EP), Chloroprene (CR).

I like the potting idea.

You might consider posting on the Eng-tips forum, under chem engineering or corrosion to see if you can get a referral on what materials work in an ammonia atmosphere.

http://www.eng-tips.com/index.cfm
Dan

I have to go back to what akreel mentions: Why is this a concern? With today's technology, you could have the PLC literally miles away if you had to. At a minimum, I would stick a remote IO module in there, which would help out a bit. Or, you could just put your sensors in there (they can handle just about any environment) and have the entire control system outside. It just seems the logical think to do.

I don't understand the benefit of a remote PLC. Any remote I/O module is essentially identical: terminals, wires, plastic box, and full of boards'n chips. (Or as we say up in the Great White North, "same sh*t, different pile".)

Oh yeah, the machine drives itself around too, so remote cabling is a no-go. Besides, the external wiring and connections (like to engine and hydraulic sensors) is more of a concern to me than is the PLC.

I found some cool pellet-based filter bags from D-Mark which I might stuff into the sealed enclosure, knowing that no enclosure stays totally sealed for long. The pellets come in a variety of compositions depending on the flavour of the air.

I also have learned about some vaseline-like goo that heavy-duty mechanics like to put on the exposed engine electrical connections. The stuff seals very good but can be wiped off with a rag for servicing.

Thanks for the sweet info danw. I gotta get me some of them books. Most everything I found to this point deals with liquid or pure gaseous ammonia for ammonia processing, which is a different animal all together.

Thanks for the polyester suit advice, I'll remember that. Maybe I'll hang some polyester suits & ties from the rafters of the place and watch what happens over time. (I'll tell 'em that's all that is left of the last salesman to come into the place...)

Which reminds me, it's time to phone up some cable salesmen.

Fred,

I once worked in a chemical plant that made fertilizers. Ammonia nitrate is on of the main ingredients. The plant had a lot of free aqua ammonia (ammonia hydrates) and also ammonium nitrates floating around in the air and settling out as dust. I think this would be similar, and probably more severe, to the middle of a compost heap, regarding the ammonia concentration.

An important point is that you will probably not ever have a 100% concentration of ammonia gas. I guess that it could not ever go above 10% by volume in air. A 10% concentration is a heavy ammonia ratio, and you would not be able to breathe in this much ammonia. About a 5% concentration is about as I could stand without breaking into a run to get away! Based on my experience with compost piles and compost turning, your ammonia concentration level will vary from heavy at times to pretty light most of the time.

I found over a 20-year period that normal cable insulation held up well. But galvanized steel conduits were rapidly eaten away. After 10 or 12 years, the tops of the conduits, in the most exposed areas, were gone, and the cables were left lying in the bottom part of the conduits. What was left of the conduits looked more like cable trays!

The type of conduits and metals that did survive included PVC-coated steel conduit, stainless steel tubing, stainless steel boxes, and surprisingly, aluminum cable trays. The chemist told me that aluminum would never last in that environment, but one of the electricians ran out of stainless steel tray and put in one section of aluminum tray. It was still in good condition 20 years later.

The thing to learn about the art of materials science is that "every condition is local and has local effects". What works 10 feet away may not work here, and vice versa. Also, every point of corrosion creates a voltage potential between the metal being eroded and the metal being deposited, and a current loop ("galvanic corrosion"). If you can interrupt or reverse the current loop, you can mechanically stop the corrosion. This is the same technique used on large metal water storage tanks.

Stainless enclosures may hold up better than fiberglass.

You can spray the bejeesus out of all circuit boards with confromal coating.

You can use a purged enclosure, so there is a consant supply of air from outside the environment into the enclosure. that keeps contaminants out.

The best bet, as suggested above, is to use corrosion resistant conduit and explosion proof wiring glands to seal the electrical connections. Use a corrosion resistant terminal junction box. Put the PLC in an enclosure in a "safe" area.