Diary of engineer: the compressor is burning

Submitted by AWL on 30 September, 2020 - 8:05 Author: Emma Rickman
Engineering plant

Underneath the plant turbine is a concrete basement where most of the noisy machines operate. One of those is the air compressor housed inside a large blue box in a locked iron cage.

I‘ve been working with an electrician (A), the other apprentice (L), and a contractor for Simms, who manufacture the compressors. We can’t hear each other, but we can smell the compressor burning.

A explains to the contractor that the two air compressors operate in phases; one compressor will build up pressure in its cylinder and then release it onto the plant; while the air flows out of one compressor the other is building up pressure. In this way, the compressed air lines around the plant maintain a steady pressure.

Compressed air is used for cleaning, both in the workshops and the filters that collect pollutants from the furnace gases. It’s also used to operate pneumatic valves and switches, for cooling bearings and keeping instruments clear of ash. We probably shouldn’t, but we also use it to clean chemicals from our overalls in a hurry.

We isolate the compressor using the heavy circuit breaker on the wall, which thunks. Then we remove the cage, the box covers and dismantle the inverter. We use tools insulated with thick red plastic to do this, as the inverter contains coils that can store charge for up to ten minutes. The thick, heavy power cables carry a high voltage to run a motor the size of a small car, which is tucked behind the electrical panel. The Simms contractor squeezes round the back and sticks a screwdriver into the fins at the back of the motor, trying to make it spin.

He emerges with grease on his nose. “Looks seized. I’ve got some movement in there but, it should turn more freely than that.”

While Simms inspect the other compressor, we test the supply to the motor using a “megger”, which is a meter designed to test high-voltage circuits. We run 500volts through each line, and read 55mega-ohms resistance — which is good, it means there’s no damage to the cable insulation. A holds out the megger probes to L, and gives me the meter.

“You hold these in your hand, and you push test.”

“Ahhhh — no.”

With the motor disconnected, we run the compressor without a load. Charge builds back up in the inverter, then trips on a fault. A thinks this shows the inverter is working correctly, because it’s building up charge — it’s tripping because there’s nowhere for the power to go. The Simms contractor was right; the fault is mechanical, probably in the motor bearings.

We seek out the contractor and find him arm-deep in hydraulic oil lines at the other compressor. The oil is used to keep the machine lubricated and cool, but this stuff looks like lumpy custard. Simms curses as a line ejects over him and A drops his phone in the oil rag bin as we step in to help.

More contractors arrive to take the compressors off-site for servicing, and we dismantle the electrical connections once again. The cable is heavy and rigid — I scrape off some of the insulation trying to pull it out of the compressor casing, and wince.

One of the contractors releases the air pressure suddenly and we all jump, and laugh at ourselves.

• Emma Rickman is an apprentice engineer at a Combined Heat and Power Plant

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