You work in a large manufacturing facility that has grown over time. The plant air system has grown along with it so that it now has six different air compressors of varying sizes and manufacture. It’s been a couple of years since any new process equipment was added or any new air tools added to the production areas. About a year and a half ago, some equipment had to be shut down due to low air pressure system-wide, and over the past year, this has happened with increasing frequency.
The maintenance group that normally handles the plant air system hasn’t been able to figure this out, so the plant engineer assigned you to get to the bottom of it. What are some things you should do?
Answer to quiz
The usual culprit in a situation like this is the air distribution system has become leaky. That’s not an electrical problem, but if you toss that back to the plant engineer, you will come across as an excuse-maker instead of a problem-solver. So begin with this, and get a small crew (in-house or contractor) to just start somewhere and use a leak detector to flag any leaking fittings. Then have another crew go through during shift changes and other windows of opportunity to fix those. This will take time, but eventually it will dramatically reduce the cost of plant air while increasing its reliability. Make sure this activity gets put into the CMMS as a form of ongoing maintenance.
Now, let’s address those compressors. Typical areas of neglect include both non-electrical and electrical areas. Check for these:
- Vibration due to problems with brackets and/or vibration dampeners on the air distribution piping, especially near the compressor outlet piping. Not only is this wasted energy that should go to compressing air instead of vibrating the motor and everything connected to it, but it’s also a cause of leaks.
- Water in the system. Check the air dryers and purging devices.
- Air filters. Debris blown into the air intakes can choke the compressor. Ensure there’s a differential pressure detection system to provide an alert when the filters need attention.
- Room vents. If an actuator went bad or manually-operated vents have been closed, you may have inadequate cooling air provided to the compressor motors. This can cause a compressor motor to kick out on overload. If the motor has an automatic restart, it will seem as if the system can’t keep up with the demand when in reality part of it was simply offline for a while due to an overlooked problem that is easily remedied.
- Motor lubrication. If this is done incorrectly, and many motor lubrication jobs are, the motor will overheat. Same issue with overload and automatic restart.
- Voltage imbalance. Voltage imbalance will cause the motors to run hot. You want it as low as is practical. For the compressors, perhaps 2% is a good target. How do you calculate voltage imbalance? Measure between phases and record the largest difference. Then divide that by the average voltage, and multiply by 100. The usual cause in a manufacturing plant is the way the lighting is powered. Lighting is a single-phase load. Go to the involved panels and breakers, and record the load and phase of each breaker. You’ll be able to see where the problem is.
- Low voltage. This means more current and thus more heat. If the voltage at the compressor motors is low, you will need to determine the reason and implement the repairs. If you have voltage loss across bad connections, those connections will appear hot in a thermographic image.
- Power factor. This hodgepodge collection of motor-driven compressors means a large inductive load and thus low power factor. You could install capacitors at the motors to bring power factor back up, or perhaps you already have power factor capacitors and one or more has gone bad. Consider installing a variable frequency drive (VFD) for each compressor motor, but make sure it is one that is already corrected for power factor and harmonics. If you go with the VFD, you can’t also have power factor capacitors at the motor.
- Voltage transients. Another aspect of this system is you may have several large motors starting across the line throughout the day. Over time, the resulting voltage spikes can severely damage winding insulation. A VFD will solve this problem.
You also should look at what’s happening out in the factory. At an appliance plant in Arkansas, the low plant air problem was traced back to the dozens of air-powered tools used by the operators. These had never been maintained, and all of them leaked. It happened over time, so nobody noticed the gradual increase in the noise level. But a newly hired operator who had worked at plant with similar tools remarked on the noise and she said she could feel the air leaking from her tool. There was another operator who left his tool resting on its back so the valve was slightly open; this is how he left that tool at the end of the day and it trickled out air all night. Look for not just air leakage but also air wastage.
This is a lot to look at, and much of it is not even electrical in nature. But all of it affects the electrical equipment in one way or another. If nobody is looking for these problems, then nobody will find them. If you don’t also address the non-electrical problems, then finding and fixing the electrical problems could still leave you with a system that cannot do the job it is intended to do. Be a problem-solver by addressing the entire system.
