Compressor Blow-by Simply Explained

By Shaun Notley, May 2019

Without functioning dive compressors we would not have the ability to fill our SCUBA tanks and go diving. Monitoring and keeping high pressure breathing air compressors in good health is a priority for delivering safe air.

No matter how healthy a compressor system is, it will never be 100% efficient. Efficiency in pumps is sometimes split down in to mechanical and volumetric efficiency. For the purpose of this article we will exclude mechanical efficiency which includes factors like friction etc. Instead we will be looking at blow-by which influences volumetric efficiency. Volumetric efficiency is simply explained as how much gas is actually compressed vs. how much gas should theoretically be compressed. Both internal and external leaks will cause a drop in volumetric efficiency.

Let us firstly look at some of the basic components that we will be mentioning and their function.

Cylinder: The cylinder sits on the crankcase and houses the piston. On the top of the cylinder a head contains intake and exhaust valves which allow air to flow in and out of the cylinder for compression.

Piston: The piston is driven mechanically and moves up and down. It’s downward movement draws air into the cylinder via the intake valve. When the piston moves upwards in the cylinder, it compresses the volume of air above it and pushes it out via the exhaust valve. The piston is usually fitted with rings which help make a seal with the cylinder wall.

Crankcase: The crankcase houses the mechanical assembly that turns the rotational movement from the motor into an upwards and downwards motion for the pistons to move in the cylinder.

Blow-by is leakage past the piston / piston rings and cylinder wall down into the crankcase. As the piston makes its upwards compression stroke, if the piston, rings or cylinder are damaged or worn, an excessive amount of gas will leak internally down into the crankcase.

A certain amount of blow-by is normal however excessive amounts indicate worn components that should be replaced. If a compressor has excessive blow-by, the volumetric efficiency is reduced. At the user end, a reduced filling rate might even be noticed.

Some manufacturers publish standard and maximum blow-by values providing an easy reference. For those that do not have published figures, periodic checks can still be useful to monitor and compare compressor condition between routine services.

A flowmeter connected to the crankcase is used to get the readings. The whole process takes around 15 minutes. Being non-invasive it has a distinct advantage over a physical inspection of piston, ring and cylinder condition. Blow-by readings are included in our compressor services. They are also provided and tracked in our compressor health checks and taught during the compressor maintenance and repair course.