Compressor Knocking?

Reciprocating compressors make noise, even in the best conditions. The trick is to train your ears to pick up new, potentially damaging sounds before they silence your compressor. The ability to pick up changes in the noise is a skill that will make you a better troubleshooter.

You can develop this skill by spending time, each day, paying attention to the sounds of your compressors and learning what each compressor sounds like when it is running well.

You can buy a stethoscope or ultrasonic device to help you trace the source of the noises from a compressor. However, these tools take time to learn, and they pick up more sounds than you need.

An ‘old school’ technique is easier, and all you need is a long screwdriver (Fig. 1). You place the handle to your ear and the metal tip to any area on the compressor to listen to the sounds in that particular area. If you try this on each valve cover, you can quickly distinguish normal and abnormal valve operation.

Valve knocks: A loose valve (Fig. 2) will give a deep, hollow knock that is similar to the sound of a bongo drum. Here are some situations that can cause a valve to become loose:

• Issues that create a gap between the seat gasket and the valve: Debris or nicks on the sealing surface can break the gasket or the gasket could slip out of position during installation.

• Valve installation mistakes: Not making sure the valve seat was in good contact with the seat surface, or using the wrong torque and having the valve bolts vibrate loose from the pulsations of a running compressor.

Maintenance tip: Always check valve bolts/setscrews a few hours after startup, a day after startup and then once a month. And, if you find a loose valve, don’t just tighten the bolting and think you have solved the problem. Remove and inspect the valve and its internal parts, because it is common for parts to break when a valve rattles loose in the valve port.

Broken valve parts: A ticking or clicking sound will be heard when broken valve parts rattle around in the valve or in the valve port (Fig. 3). Valve parts can break without warning for a variety of reasons. This includes the following:

• mistakes during valve repairs

• installation errors

• normal wear and tear from old age

• a slug of liquid (water from condensation or oil from over-lubrication)

• large solid contaminates getting past the inlet air filter

• pieces that break away from other internal components (piston rings, coolers, etc.).

You can confirm which valve has broken internals by using the screwdriver to listen at each valve cover, or use a thermometer to do a quick comparison of valve temperatures to search for the hottest valve.

The piston: A loose piston will produce a loud, metallic noise once the nut becomes loose. Ignoring a knock in the cylinder can have a devastating effect (Fig. 4). The most common causes occur during installation.

• Torque: Using inadequate torque or backing up the nut after reaching the torque spec to install a cotter key.

• Contamination: If the components are assembled without attention to cleanliness, a single burr or solid contaminate can create a false sense of security. The nut can be installed with the proper torque, and trap a burr or debris particle between the piston and the nut. This particle will release as the piston contracts and expands through normal operation, creating a noisy and destructive gap between the piston and the nut.

Maintenance tip: A noise caused by a problem in the cylinder can be heard in the cylinder and in the frame. This is because the knock can travel through the piston rod.

Bearings: A knock in the frame can produce a wide range of sounds, depending on which bearing is causing the problem (Fig. 5). A knock can be caused by bearing clearances opening up, but this is usually the last place to look for the source of a knock.

As a general rule, the crosshead pin bushing will typically fail before the crankpin bearing or the main bearings. In fact, it is common for the precision bearings to last for decades.

Once a knock has been confirmed to be resonating from the frame, a lift check inspection will help you zero in on a starting point for bearing disassembly and more in-depth evaluations.

Maintenance tip: Bearing knocks typically do not travel up the piston rod and into the cylinder.

Dan Wise is the webmaster for the CompressorWise website located at http://www.compressorwise.com.