Reciprocating compressors come in a variety of sizes and some are lubricated and some are non lubricated. I’d like to pass on some of my thoughts and experience regarding common problems that I’ve experienced when asked to repairs these goliaths of the process industry.

First of all, may I say, the root cause can be excruciatingly difficult to pinpont. Let me begin by telling you of one such problem that I had with high interstage pressure that was causing the machine safety valves to pop (as designed to do!) and also causing the machine to trip. I had just overhauled the machine with a new set of reconditioned suction and discharge valves on both first and second stages. I had also replaced the piston rider and piston rings. I had replaced all of the packing in the stuffing box and intermediate packing. The compressor used to be lubricated, however it had been converted to run as a non-lubricated machine (and I must say that the mean time between failures had improved substantially). Although, the cylinder heads were now both non-lubricated the crank was still lubricated – so I had also changed out the oil in the frame and fitted new duplex filters. Finally, one of the actuating piston rods on the second stage inlet valves was bent, so I fitted a new oem (original equipment manufacturer) part that I got straight from the on site stores stock.

All was looking fine, I checked my piston standouts, crosshead shoe clearance, piston rod run out and clearance between the double acting piston head and the frame and outer ends (on both stages). So why was may pristine machine crashing out on high interstage pressure? When such a thing happens the first thing I do is head to the archive cabin and pull out the compressors manufacturer’s manual. A good manual is normally brimming with information regarding maintenance, operation and troubleshooting tips to help get one’s head into the operating characteristics of the machine – pinpointing exactly what makes the machine tick. Therefore, I browsed the common causes of high interstage pressure.

A higher than normal interstage pressure indicates problems in the cylinder that follows the interstage cooler. As this is a two stage compressor, the problem had to be with the second stage cylinder. If the compressor is loaded, then the problem normally lies with the suction valves. If the interstage pressure is high whilst the cylinder is unloaded – then the problem normally lies with the discharge valves. From my discussions with the Duty Shift Engineer, I was informed that the problem was occuring once the compressor was loaded. Therefore, I looked back at all the maintenance that I had performed on the machine and noted that I had changed out a second stage piston unloader. I quickly examined the piston rod with the piston rod that had been removed from the machine and there lied the problem. The new piston rod was 6mm (1/4″) longer than the piston rod that I had removed from the machine. Therefore, the piston was holding the suction valve open, allowing the pressure to flow back to the interstage and blow the safety valve. I cut the piston rod down to size, installed and started the compressor. This time all pressures were fine and the compressor ran as expected.