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Maintenance of Reciprocating Refrigeration Compressors on Ships

Maintenance of Reciprocating Refrigeration Compressors on Ships

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Maintenance of reciprocating refrigeration compressors to be carried out as per the time intervals or running hours specified in the product manual.

The main causes of operating malfunctions of the plant are:

  1. Incorrect control of liquid supply to the evaporator
  2. Moisture in the plant
  3. Air in the plant
  4. Anti-freezing liquid is missing
  5. Congestion due to metal shavings and dirt
  6. Congestion due to iron oxides
  7. Congestion due to copper oxides
  8. Inadequate refrigerant charge

Before opening up the compressor for inspection and maintenance, the pressure inside must be lowered to slightly above atmospheric pressure. This can be done in the following way, depending on whether the compressor is operational or defective:

The Compressor is Operational

  1. Run the compressor at minimum capacity at normal operating temperature.
  2. Adjust the low-pressure control so that the compressor stops at a suction pressure of approx. 0.1 bar.
  3. Throttle the suction stop valve very slowly. Keep an eye on the suction pressure gauge. The suction pressure must be lowered slowly enough to give the refrigerant dissolved in the oil time to escape without the oil foaming. This is of great importance in compressors running on HFC/HCFC.
  4. Once the pressure is down to approx. 0.1 bar, stop the compressor and perform the following steps in the order specified.
  5. Close suction stop valve.
  6. Cut off power to compressor motor.
  7. Close discharge stop valve.
  8. Drain off last remains of refrigerant gas.
  9. Having ensured that power to compressor motor cannot be inadvertently connected, the compressor is ready for opening. For this purpose, remove all fuses to the electric motor.

The Compressor is Inoperative

  1. Leave heating rod in crankcase connected for a couple of hours before the compressor is due to be opened in order to heat up oil. Warm oil does not contain much refrigerant.
  2. Suction stop valve must be open while heating rod is connected.
  3. Keep discharge stop valve closed.
  4. Close suction stop valve and disconnect heating rod.
  5. Equalize the pressure in the compressor.
  6. Once pressure has been equalized to atmospheric pressure, the compressor is ready for opening. Remember to make sure that power cannot be connected inadvertently and start the motor. Consequently, remove all fuses to the electric motor.

Precautions in Overhauling Refrigeration Compressor

In order to prevent moisture penetrating into the refrigeration plant during any repair work, it is advisable to follow the rules below:

  1. No component should be opened unnecessarily.
  2. When dismantling the system, the pressure in the system should be a little higher than atmospheric pressure.
  3. Note: If the piping system is colder than the surroundings, there is a considerable risk of damp precipitation (condensation) on cold plant parts. Plant components to be dismantled must be warmer than the ambient temperature.
  4. Only one point should be opened at a time.
  5. Plug, close or at least cover opening with oiled paper or suchlike.
  6. Be aware of the possibility of filters being very moist.

Overhauling of Refrigeration Compressor

The following sections describe the individual components. When dismantling and assembling, parts should generally be fitted in the same position from which they were taken and should therefore be marked as they are removed. Further they should be
thoroughly cleaned, checked and lubricated prior to being reassembled.

Valve Intermediate Plate including Suction and Discharge Valves

The valves are plate valves, consisting of an intermediate plate, upon which suction and discharge valve plates are mounted. The intermediate plate is positioned right under the top cover. When the top cover has been removed this intermediate plate
becomes visible and can be removed, too.

The intermediate plate is delivered with suction and discharge valve plates as one spare part, including complete gaskets.

Mounting

  • Without causing any damage clean all gasket surfaces thoroughly of any gasket remains.
  • Lubricate the new gaskets in refrigerating machine oil and check that the gaskets are facing correctly in relation to the holes for the fixing screws.
  • Check that the intermediate plate is positioned correctly before fitting top cover.
  • Insert screws and tighten cross-wise with a torque moment as indicated in instruction on Torque moments for screws and bolts in the instruction manual.
  • After approx. 25 hours of operation screws and top cover should be re-tightened.

Shaft Seal or Mechanical Seal

The reciprocating compressors is fitted with high quality shaft seal. This consists of a rotating and a stationary unit. A routine inspection of the shaft seal is not normally necessary. With regard to increased operational reliability it is, however, recommended to make an inspection in connection with an oil change, faults in the oil supply and also at regular intervals when operating with high discharge gas temperatures and oil temperatures. Special attention should be given to cracks in the O-ring, as well as wear, scoring and material deposits, carbon and copper plating on the sealing ring. An oil leak rate of 0.05 cm3/h is within the tolerance.

Possible Causes of Failure of Mechanical Seal

  • Llack of lubrication (insufficient oil supply, high refrigerant concentration in the oil)
  • Heavy wear of driving parts (high proportion of dirt in the oil)
  • Axial play of crankshaft too large
  • Overheating (hardening and cracking of O-rings, oil carbon)
  • Strong vibration (insufficient fixing of coupling or drive pulley, drive not smooth enough, coupling or drive pulley displaced)
  • Belt tension too high

The pressure in the compressor must first be released. According to the drive system, the drive pulley, motor, coupling housing, coupling and key should then be removed.

Fitting of Mechanical Seal

  • When strong wear to the drive parts is suspected (contaminated oil, strong deposits) a precautionary compressor exchange or overhaul is urgently recommended.
  • The shaft, flange (gasket remains) and the shaft seal chamber should be cleaned very thoroughly. Any deposits on the shaft must be carefully removed. If necessary the surface can be smoothed with fine polishing cloth soaked in oil (not smoothing cloth).
  • Always exchange the complete shaft seal when possible
  • Never re-use old O-rings
  • Do not touch the sealing surfaces
  • Oil the rotating sealing surface, O-ring and shaft with clean refrigeration oil. Do not oil the asbestos-free gasket or the flange surface.
  • Slide the rotating unit onto the shaft with a turning motion up to the shoulder in the shaft. The drive pin must be located in the slot provided.
  • Lightly oil the sealing surface of the stationary unit, then mount the whole unit including the gasket over the shaft. The gap between the crankcase flange and the cover should be approx 5 mm (spring tension).
  • The fixing screws should be evenly tightened in a crosswise order with a torque wrench with torque as mentioned in the instruction manual.

Cleaning of Oil Filter

The oil filter should be cleaned at regular intervals. Please note in this connection that often the filter must be cleaned already
after a short operating period following the initial start-up. This is a consequence of the tiny dirt particles that will be coming from the plant during its first operating period. Clean the oil filter in a suitable dissolvent and blow clean with pressurized air before refitting.

Suction Filter

Between suction stop valve and compressor a fine-meshed filter has been fitted. The purpose of this filter is to prevent that impurities from the plant are conveyed with the gas flow into the compressor. Clean the suction filter at regular intervals as stated in the section on servicing the reciprocating compressor. On cleaning the filter dismantle suction stop valve by removing screws.
The filter and gaskets can be removed without the use of any tools. Clean the filter in a suitable solvent and blow clean with pressurized air.

Stop Valves

Suction and discharge stop valves are used to cut off the compressor from the plant. They are closed tightly by manual tightening. Hence, it is not advisable to use any tools in order to close the valve as this would just lead to overloading of the valve parts. The valve spindle is fitted with a maintenance-free gasket which needs no replacement. Further, the valve is fitted with a backsealing, which is brought into operation when the valve is completely open and the valve cone screwed back towards the cylinder head
(anticlockwise rotation).

Note:
In case the compressor is operating, the valve cone should not be screwed completely back against the cylinder head as any safety pressure controls connected to the valve housing will hereby be blocked.

REFERENCE: Instruction Manual for SABROE Reciprocating Compressors SBO 21, 22, 41, 42, 43

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