Suitability of a PLC

[archana_sharan 0]

Hi, In one of our automation projects for underground coal mine conveyors, we want to design a PLC based system. The system will comprise of three nodes, preferably all PLCs with all nodes interconnected through fibre-optic cable to surface HMI. I need to know the following things. Does any of the PLC suppliers manufacture intrisically safe modules (Processor as well as I/O) that we can use for the application. Who are the manufacturers? We would prefer stand alone controls and therefore want to opt for individual PLCs for each conveyor in place of RIO networked with a surface PLC. Secondaly, in case above is not available, can we use the ordinary PLC inside the mine by putting it in a flame proof enclosure? What are the hazards in this case? What are the design aspects we need to think of.

[TimWilborne 108]

You will usually use some type of barrier on your Inputs and Outputs. As far as a "flame proof enclosure", I don't think this will work though it worries me that an increasing number of people inquire about doing this. I used to work for a company that made enclosures for the mining industry. Probably more important than being "flame proof" is being able to withstand debris flying by them and roof collapses. The thickness of the steel will vary by the size of the enclosure but could be between 1/2" and 1". They will be stitch welded on the inside and solid welded on the outside. All mating surfaces such as the cover must be machined, The cover bolts on with bolts about every 6", all holes must be planned so fittings can be welded in. The larger boxes are even gusseted and ribbed on the inside for additional strength. Does anyone know if this is still the norm in the mining industry, or at least the coal industry are they pretty much letting anything down there now?

[JesperMP 2]

There are several intrinsically safe bus systems + intrinsically safe i/o. There does not exist an intrinsically safe PLC CPU. To put a regular PLC CPU inside an EX-d box is a baaaad idea. As you are already planning to pull network connectrions to the surface, what do you get from putting the CPUs in the hazardous area ?

[Ken Moore 23]

Never worked in a coal mine, but would assume it's a Class II, Div I electrical environment. You will need an enclosure designed for the environment, hopefully with third party certification. If it were my project, I would install the I.S. barriers in the main enclosure, and run multi-conductor cables to robust (but not explosion proof) marshalling enclosures. This way if you ever need to do trouble shooting etc... you can open the marshalling cabinets without fear of a spark, because the barriers are protecting everything down stream. In a mine I would recommend galvanic barriers vs. zener types, the zener barriers have very rigorous grounding requirements. The galvanic barriers are more expensive up front, but are pretty much maintenance free, the zener types require periodic ground testing. Just about any PLC will do, the barriers provide the spark protection, and as TW said, you have to plan all your penetrations of the main enclosure, I always recommend a couple of spares. Condensation is also a concern but there are special drain fittings available for these type of enclosures. I would also suggest you check the min. and max. temperature rating of whatever PLC you choose, these air tight enclosures can build up a lot of heat, for the low temperatures you can always add a small heating element. As you can see, a project of this type requires a lot of advance planning and forethought, it's not something you throw together on the fly. Good Luck. Ken

[rpraveenkum 5]

Refer to this post more relevant to your topic http://www.plctalk.net/qanda/showthread.php?t=33493

[gravitar 3]

I worked for a company that did gasoline fuel fill systems and have come to understand that controls in hazardous locations are not something to "ease your way into". you must either know it 100% or don't do it (yourself) at all. Maybe the best bet would be to work closely with an automation design house experienced with mining applications. You can learn from this project and apply what you've learned to future jobs that you may elect to keep in-house. Now with that said, as far as intrinsic barriers vs. PLC I/O, I've come to really respect the Allen-Bradley Flex-Ex product line. Keeps the wiring soooo much simpler than individual barriers and controls costs too. But, the I/O blocks may not be suitable for all your field I/O, you'll have to verify first.

[paulengr 44]

For those guessing, coal mines are Class I, Division I. Everything is surrounded by fuel and oxygen. Same as with a gas station. No point in even trying to get anything classified to anything less. Coal frequently has a lot of natural oils and hydrocarbons with it, something most folks don't realize, so the air and everything around it is saturated with fuel (solid, liquid, and gaseous) just like it is around a refineery. Flame proof is pointless. You really need to read up on your UL information here. A Class I/Division I box is designed to be "explosion proof". Fire proof and nonsparking are not really all that useful with those rules. Under those rules, the goal is that if an explosion occurs, fully contain it within the enclosure. The enclosures are huge and "beefy" simply because they have to withstand containing whatever blows up INSIDE and prevent it from getting OUTSIDE. In addition, intrinsically safe is nice in concept and extremely useful where it belongs. The trouble is that in a coal mine, explosion protection in reality is almost secondary to the bigger problem, which is coal miners. There are flying rocks and abrasive materials everywhere, never mind 5 pound sledge hammers used as a universal tool for fixing everything. I wouldn't even consider exposed wiring. It must all get run in conduit. About the only small advantage of IS is that you don't need to pack fire stop in all the conduit joints. If you can, try to visit a coal mine first, preferably an underground long wall mining operation, or at a minimum, an underground operation. Make sure to look closely at an underground primary crusher area as well. Have the maintenance crews show you around. Observe what the equipment looks like and what they do to it just from "normal use".

[JesperMP 2]

I dont understand why intrinsically safe and mechanical robustness are mutually exclusive of each other.

[Clay B. 1]

There a re exclusive because intrinsically safe means it will not create a spark. This is done thru isolators. Mechanical Robustness means it will take a considerable amout of abuse. One qualifies you for safe operation the other makes it run better.

[Ken Moore 23]

There's nothing that says it can't be safe & robust.

[JesperMP 2]

"Mutually exclusive" means you can't have both at the same time. And I do not understand why IS means I cannot also have a mechanically robust system.

[Clay B. 1]

I read what you posted wrong. I have seen and worked on systems that where pretty robust as well as IS. Generally the componets I have used in Class1 DIV1 rooms where pretty tuff and would take alot of abuse. Usually the cabinets gave me fits with the Diff presure switches shutting everything down. It really does depnd on what your doing to figure out what componets are going to work best for you. I know from working on Natural Gas rigs that if it was not Stainless Steel it was going to corode. Copper seemed to degrade overnight. All wires where soldered tipped before they terminated because of this. In the printing industry we try to avoid moving parts (instead of selector switches use pushbuttons with b sealing boots). The ink always managed to get into limit switches and the such and gum up the works. Tried to use proxes where ever possible. It seems like every process has its own set of obstacles. From paulengr's post coal mines sound like a real nightmare.

[paulengr 44]

Mechanical robustness in this instance means to protect the wiring mechanically. Almost by default, this means that everything will be run in rigid metallic conduit. This satisfies the mechanical robustness requirement. Class I/Division I rules for conduit runs require everything to be inside rigid metallic conduit with special fittings and the joints where the fittings go are usually specified to be packed with fire stop. Intrinsically safe rules allow you to use any wiring method without regard for hazardous location rules. This is extremely useful if for instance you want to run SO (or SOOW) cord, use cable trays, intermetallic, or even nonmetallic (PVC) conduit, or use exposed wiring where allowed such as network or coaxial cable runs. Any wiring method where Class I/Division I equipment was not available becomes usable. A great example of intrinsically safe rules are useful is making 4-20mA runs in a tank farm in hazardous locations. It is far less expensive and time consuming to simply wire tie a cable to an existing mechanical line up the side of a tank to a level sensor on top than it is to run RMC all the way up with explosion proof fittings. In a coal mine, intrinsically safe allows us to use wiring methods other than RMC w/ C1/D1 rules. However, the mechanical environment requires RMC. So the only significant advantage is that the hazardous location rules fall out, and there is the additional requirement of using only IS devices and using isolation barriers in the cabinet before the wiring exits the cabinet. So we've traded one extra requirement for another. Thus from a practical point of view, I stand behind my statement that there really isn't any reason to try to meet IS rules.

[Ken Moore 23]

There is one additional benefit of using I.S......maintenance/trouble shooting. With ExP devices you have to be concerned with the atomospheric conditions when you open a device in the field. With I.S. this concern goes away.

[paulengr 44]

OK, I see the problem here. IS is not mutually exclusive with being mechanically robust. They are two different aspects to specifying a wiring system. I'll give you two examples of why I would use IS (and conduit) or not. I work in a foundry. I have used intrinsically safe wiring in a tank farm storing asphalt paint. The atmosphere around the tank farm is literally saturated with paint fumes. This is clearly a Class I/Division I environment. However, it's off in a block building by itself with limited access. Fork trucks and the like can't go driving through the building. It "stinks" so this tends to keep people out of it. In this environment when we added level sensors to the tanks, intrinsically safe wiring made all the sense in the world. We could just wire tie the cords directly to the sensors and use ordinary wiring methods out of the building back to the PLC instead of using explosion proof rigid metallic conduit fittings and methods. Flexible wire looms are allowed and the last few feet up to the sensors can be ordinary flexible cord. The wiring is easy to run, low cost, and easy to change. This is a place where IS wiring rules are ideal. Running rigid conduit would be overkill and costly. Another part of the foundry has gas and fuel oil trains running through areas around a cupola. For lack of a better word, it's a foundry. Instead of flying rocks, it's flying liquid metal, occasional flying pieces of shredded scrap cars, and electrically conductive dust. The conduit gets sprinkled once in a while with a spray of liquid metal, but ordinarily it's not enough to burn the wiring inside (similar to welding slag). The wiring inside is usually "upgraded" (TPE or better instead of THHN). Obviously a stream of metal is just going to melt everything, but there is very little cable on the market which will survive those conditions (silicone or ceramic jacketted nickel/copper alloys at roughly 20 times the price of THHN per foot are the only options). In that part of the plant, there is almost no advantage in using intrinsically safe rules since we are going to be running heavy rigid metallic conduit anyways. We could spend the time in doing it but there's no time or financial benefit.

[JesperMP 2]

I can see many advantages of IS, even if there is already metallic conduits everywhere. IS allows you to make a maintainable networked system. I cannot see how you can arrange blocks of i/o, let alone a PLC, to sit in the hazardous area, and still be accessible for troubleshooting and modifications. As I understand it, in a coal mine you must consider the atmosphere to be constantly hazardous. Btw. I work in foundries too nowadays. In my previous job I supplied quite a bit of equipment to customers in refineries and such places.