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Service Engineers’ Section
Bulletin No 16
DIAGNOSTIC GUIDE TO
OIL PRESSURE PROBLEMS
ON PUMP LUBRICATED COMPRESSORS
Low oil differential pressure on old
compressors is quite a common
occurrence and requires remedial
action.
The appropriate action is
dependent upon the causes of the drop
in pressure and to ascertain these,
certain facts have to be established.
1. System Operating Conditions
Should the crankcase pressure be close
to suction, as it is normally, the system
operating conditions, such as suction
pressure and suction temperature,
should be logged.
From this, the
suction superheat can be obtained
using a suitable refrigerant comparitor.
Under certain excessively low suction
superheat conditions, oil dilution and
foaming may occur, most often causing
flickering on the oil discharge gauge. In
this case, a check should be made of
system
evaporators
and
any
adjustment made to increase the
superheat setting of the expansion
valve, in order to increase suction
superheat
and
compressor/system
safety.
cylinder bore wear as well as the
condition of valve gear. Should the
crankcase pressure be too high for the
oil pressure regulating valve to increase
oil pressure over and above the
crankcase pressure, it is indicative of a
serious wear problem with the
compressor.
Replacement will be
required. If the compressor is fitted with
an oil float device, this should be
checked to see if the float is leaking
past, thus pressuring the crankcase
through the oil return.
3. Draining Compressor Oil Charge
and Checking The Oil Strainer
If the above is not applicable, it will then
be necessary to drain the compressor
oil charge and remove and check the oil
strainer. This is also a good opportunity
to carry out an acid test on the oil
draining out of the compressor. Should
the filter be choked this should be
cleaned and refitted prior to recharging
with an appropriate oil for the refrigerant
used.
4. Oil Pump Removal
2. Actual Crankcase Pressure
This could be higher than the suction
pressure depending on piston ring and
SES Technical Bulletin No 16
Only if any of the above actions are not
applicable should the oil pump be
removed and its porting checked. An
efficient oil pump has no score marks or
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April 2004
heavy wear on the outside of the rotor or
inside rotor barrel, as this causes recirculation of oil across the oil pump
and, therefore, lowers the oil differential
pressure.
However, this is not a
common problem due to the high
standard of filtration. An oil differential
pressure of 1.5 bar should be
achievable, even on a well used
compressor.
5. Internal Oil Pressure Regulating
Valve
The internal oil pressure regulating
valve is not easily accessible and further
advice is required regarding any
adjustments from the factory setting
1
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fromthe manufacturer. It is quite a rare
occurrence for a factory set oil pressure
regulating valve to require adjustment.
Therefore, all other actions described
above should be carried out before
embarking on such adjustments under
advice
from
the
compressor
manufacturer.
Following the above steps will solve the
vast majority of potential problems on
pump
lubricated
compressors.
However, if problems are still being
experienced, then further investigation
will be required in conjunction with
advice
from
the
compressor
manufacturer.
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Oil pressure switch mounted (1) and with engineer’s gauges fitted (2).
This Technical Bulletin is based on the Bitzer UK CPD Briefing Paper Number 001 by D.E.
James.
The Service Engineers’ Section has reproduced this Bulletin with the kind
permission of Bitzer UK.
SES Technical Bulletin No 16
Page 2 of 2
April 2004