Operation and Maintenance of Screw Compressors

Intermittent motion compressors, or non-continuous flow compressors, include screw compressors which are categorized as medium flow and medium pressure compressors when compared to other compressor types based on pressure/flow charts.

The screw compressor gets its name from the two screws it contains, one of which is mounted with the prime mover (motor, turbine) and the other is driven. The screws are mounted together by gears and rotate in opposite directions to each other, squeezing the compressed air with oil in the compression zone to raise its pressure according to the direction of rotation of the driver (counter-clockwise).

During operation, the compressed air is mixed with oil (flooded type) inside the compressor. The compressed oil and air are then separated in an oil separation unit, with the air being removed to the discharge line and the oil being returned to the oil filter for filtration before being returned to the suction of the compressor. This compressor type is known for its large axial displacement during operation, with vibration measurements showing higher axial displacement than any other type of compressor.

Oil and screws

Oil serves two main functions inside the compressor. Firstly, it lubricates the screws, bearings, and seals during operation and cools the screws to prevent overheating. Overheating of the screws can cause localized thermal stresses in the mid-length of the screw (hotspots), leading to screw rubbing against the casing or other screws. However, screws are manufactured from materials with low thermal expansion to avoid overheating or expansion due to poor lubrication or cooling.

Secondly, oil creates a protective layer on the surface of the screws to prevent erosion and scratches caused by dust particulates escaping from the air suction filter. Overheating of screws can cause over-expansion and increase the amount of run-out of the screw, which can be adjusted on a balancing machine. The position and angle of defection in the screw can be determined on the balancing machine, and then it can be restored by heating it. However, this procedure should be used only for a low percentage run-out as it may cause localized thermal stress in the screw for a bigger percentage.

It's worth noting that this procedure can also be used for oil screw pumps to maintain their screws. Screws that have been maintained using this procedure are marked with black spots at the place of heating.

Oil problems

To avoid oil problems in this type of compressor, the oil used should be able to withstand high temperatures, as the outlet temperature can reach up to 99°C, which is very high. Some types of oil cannot withstand this high temperature and will start to precipitate particulates in the system, causing fouling. These particulates can block oil filters and oil separator cartridges, leading to a rapid rate of cartridge change.

Filters and contaminates

Torn or blocked air suction filters can cause serious damage to the compressor. If the filter is saturated with dust or oil, the air flow will bypass it and enter the compressor with dust, causing scratches on the screws. Dust or sandy weather around the compressor suction can also affect filters badly, as sand particulates are very soft and can pass through suction filter holes. When mixed with oil, the sand gets precipitated in oil filters, contaminating them and increasing the rate of filter changes.

PCV internal leakage

If the pressure control valves (PCV) on the discharge line after the oil separator have any defects, such as in the springs or seals, or any internal cracks in the valve body, oil can return to contaminate the air suction filter. If the discharge valve is opened to discharge the system (if the valve is defective), the system pressure (discharge pressure) will create a backflow for air to return inside the compressor through the internal leakage of the PCV, pushing the oil backward to the suction of the compressor until it reaches the air filter. Symptoms of this include higher amperage of the running motor and lower discharge pressure.

High motor ampere (power)

There are other reasons for an increase in motor amperage (power), including higher temperature of suction air, malfunction of the oil cooler, decreasing voltage or torque of the motor (motor driven), blockage in system filters, and an increase in screw run-out or misalignment. Any dust or particulates from suction air or bad oil can reach the bearings, causing erosion and damage to the internal parts of bearings. Any defect in the bearings causes more motor amperage (power) and can even lead to motor overload due to friction (particulates in lubricating oil causing erosion and friction).

Increasing screw run-out will raise vibration levels as it generates induced force that increases with time, requiring power to increase it. This increase in power will appear as an increase in amperage and power consumption.

Screw scratches

Scratches or rubbing in screws will start at a small rate and increase with continuous running. Rubbing refers to friction between the screw and the static casing, leading to metallic deposits in the oil. Therefore, periodic oil samples are required from the compressor to inspect the oil and check for contaminants and ashes.

Higher amperage (power) not only means a higher noise level but can also indicate any defects in the screws, which can cause a higher noise level at the start of the problem.

Couplings and vibrations

Couplings are a crucial component of the compressor train as they are responsible for transmitting power from the prime mover to the driven machine.

There are different types of couplings available, including:

1.    Couplings consisting of rubber saddle-shaped pieces that mount between the motor hub (mover hub) and compressor hub.

2.    Jaws coupling that mounts between two hubs.

3.    Membrane coupling.

Coupling failure

Rubber couplings can get torn due to successive startup and shutdown (alternative loading), with the tearing usually occurring at the bolt-hole region. This is because the high stiffness of the material against loading makes it more prone to tearing. To accommodate more flexibility with the operation, lower stiffness materials can be used, such as rubber with the same properties as belts.

Belts are flexible and have the ability to sustain variable loading (tensile stress). Some high-load belts are reinforced with steel beams inside to be more yielding in sustaining tensile stress. This material can sustain variable loading of successive startup and shutdown, variable torques without any tearing problem for the coupling. Jaws coupling can break down from the jaws themselves with fluctuating loading.

Coupling and misalignment

If there is even a small percentage of misalignment, the membrane coupling can break down. This is because the misalignment causes the coupling to experience alternate loading in opposite directions, leading to fatigue and eventual failure of the membrane. As a symptom of failure, the torque transmission from the motor to the coupling will start to decrease as a part of it is lost in the figure of losses in the coupling (energy absorbed by the coupling).

This percentage of misalignment can cause bearings to become defective (breaking cage and friction in bolts) with continuous loading, as the induced force of vibration will be excited to reach higher values with continuous operation.

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