Modicon 984 Plc

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Guest
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Hi,
I am facing a disturbing problem here in 132 MW power plant. Maximum ambient temperature is +45C and humidity is about 55% to 70% and PLC pannels are in Engine room, some times engines trip with out any reason/alarm and restarted again, I feel the behaviour of the PLC power supply is not correct. If any of you have such experience please tell me the remedy.

Also there are about 117 instruments (sensors) such as thermocoples, PT-100s, pressure transducers are installed on each engine and there are total 9 engines. If any one have some calculations of persentage instruments malfuntions allowed with PLC please urgently send it to me. I shall be thankful.
Regards to all
Shahid Iftikhar
 
Two posibilities come to mind.
1. The consult the PLC manual Spec.s regarding its temperature and humidity range. It sounds high but may be OK.

2. Put a C.R.O on the power supply to the PLC. Check for noise on the DC supply lines, this could cause extraneous operation.
If Noise is present then check the AC Input also.

Install decent line filters if this is the case.

Also be aware of Notebook Power supplies.
Some will re-injest noise if the power supply is not clean.
 
Most temperature and pressure transmitters have higher allowable ambient temperatures than PLCs. You can also get these instruments with mil spec electronics that have even better specs. Don't worry about thermocouples - they are inherently immune since they are passive devices.

The allowable quantity of failures is dependent on function. If you loose jacket water temperature signals or lube oil pressure signals, for example, you can cook your engine, so the allowable percentage of failure for this measurement is 0%, and you should either get redundant sensors or use the appropriate logic in your PLC to shut down the engine if you loose the signal.

Strange things happen to microprocessors and power supplies as temperature goes up, so you should consider cooling your panel. Remember the panel interior temperature is going to be higher than the ambient. A quick cooling solution, if you have compressed air available, is a vortex cooler. AutomationDirect.com just added them to their product line, and they are only a few hundred dollars.
 
You should be good to about 60 deg C on the PLC's.
That doesnt mean you don't have flakey components
caused by heat. I have had to install vortex coolers
on enclosures in the past. It helps. A power blip,
flakey power supply or processor could cause the problem
that you desrcibe. Do you have several PLC's that
communicate global ESD's over a network? You could have network problems.
 
If you are having a problem with your power supply in can you not put a chart recorder/data logger on it? This would tell you if you were having power problems. Some scopes can be set up to measure max min voltages. I know we have a Fluke scope that can be set up in this way which is handy for testing your power supply when your not around for the weekend. If power dips are a worry have you thought of a UPS?

Temperature and pressure are best monitored via current loops. 4-20mA. This is because if your loop breaks you don’t have any current flow and if the current loop shorts out you get max current. This can all of course be set up in your plc program to tell you which current loop is faulty and therefore which instrument is on its way out, broken, intermittent fault, loose connection. Ideally you send this information to a panel view or some kind of SCADA package to give you the fastest diagnostic.

Digital inputs are tricky if they go of and on for that split second that stops your plant. I am sure you can set up some kind of "mouse trap" in your plc program to establish which is giving you a problem.

Heat and high humidity are not good for electronics so air conditioning is a good bet. It isn’t cheap to install and maintain though.... How critical is your process?
Putting fans in your panel doors may be another cheap option but if its hotter outside your panel you may be making things worse. We have used heat exchangers before on panels that were getting warm.

Hope this helps you out a little.
 

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