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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