Experience shows that pressure and temperature variations in a refrigeration circuit can provide information about the operating condition of the refrigeration plant. In particular, suction and condenser pressures as well as the temperatures of suction and discharge gases may provide important information as to the operating conditions of the plant.

It often takes only very slight modifications of variable pressures and temperatures to produce considerable changes in operating conditions. The following chart helps marine engineers for troubleshooting of the reciprocating refrigeration compressor plant on ships.

Compressor fails to start

Causes

1. Control current circuit open owing to activated: Pressure cut-outs, Thermostats, Motor protection device or Defrosting timer
2. High-pressure cut-out has cut
3. Low-pressure cut-out has cut
4. Oil-pressure cut-out has cut

Remedy

1. Pinpoint open switch and remedy cause of interruption.
2. Reset pressure cut-out and investigate cause of high condenser pressure.
3. Compressor cannot start before suction pressure has risen above set point for pressure cut-out restarting.
4. Compressor starts at reset. Check oil level. If oil foams in crankcase, see section below.

Compressor starts and stops too often

Causes

1. High-pressure cut-out cuts at high pressure
2. Low-pressure cut-out cuts at too low suction pressure
3. Low-pressure cut-out differential is too small between stopping and starting
4. Compressor capacity too high
5. Filter in suction line clogged
6. Solenoid valve in liquid line does not close tight
7. Discharge valves on compressor are leaky

Remedy

1. High condenser pressure – see section below. Check condenser cooling and adjust pressure cut-out to correct breaking pressure. Replace defective pressure cutout.
2. Low suction pressure – see section below. If low-pressure cut-out is set too high, adjust pressure cut-out.
3. Increase differential pressure.
4. Check operating conditions and, if necessary, reduce capacity.
5. Check suction filters in compressor.
6. Check direction of flow. Replace defective valve.
7. At compressor stop, pressure equalizes relatively quickly between suction and discharge side. Clean or change discharge valves.

Compressor starts, but stops again immediately

Causes

1. Motor protection cuts
2. Low-pressure cut-out has cut
3. Defective oil-pressure cutout
4. Oil charge insufficient
5. Oil pressure failing owing to formation of foam in oil.

Remedy

1. Look for cause of overloading. If star-delta start, set starting time at minimum.
2. Open any suction stop valve which is closed.
3. Replace cut-out.
4. Top up with oil and investigate cause of oil shortage.
5. Reduce capacity.

Compressor operates continuously

Causes

1. Thermostat or low-pressure cut-out does not cut at too low temperature/pressure
2. Restricted supply of refrigerant to evaporator. Compressor working at too low suction pressure.
3. Refrigerant charge insufficient.

Remedy

1. Adjust operating points.
2. Remove dirt in filters and check function of expansion device as per special instructions.
3. Top up with refrigerant of correct type.

Abnormal noise from compressor

Causes

1. Compressor capacity too high during start-up
2. Oil pressure too low
3. Liquid refrigerant in suction line
4. Incorrect alignment of motor and compressor. Loose bolts in coupling
5. Worn or defective bearings
6. Too much oil circulating through the plant, resulting in too low oil level in compressor
7. Capacity regulation oscillating owing to failing oil pressure

Remedy

1. Reduce capacity.
2. See section below
3. Slugging. Adjust expansion or float valves.
4. Check alignment as per special instructions. Tighten with torque wrench.
5. Overhaul or replace
6. Check oil level. Solenoid valve, filter or jets in oil return system may be clogged. Leaky suction valve ring plates, piston rings and worn-out cylinder may also produce such oil consumption.
7. Low oil pressure

Too little capacity on compressor

Causes

1.  Insufficient oil charge
2. Iced-up evaporator
3. Defective oil pump and hence failing oil pressure
4. Defective capacity regulating system

Remedy

1.  Top up with fresh oil of same type and make.
2. Defrost evaporator; adjust defrosting time if required.
3. Repair or replace oil pump.
4. Cause is most often failure in oil pressure or refrigerant in oil.

Slugging in compressor during start-up

Slugging in the compressor should not occur, because at worst this can cause rupture of the valve ring plates and damage to the built-in relief devices. Furthermore, it can result in damage to the connecting rod bearings and cylinders if the coolant degreases the faces and impairs the lubricating capacity of the oil.

Causes

1.  Adsorption of (H)CFC refrigerant in oil. Sudden reduction in pressure across the oil sump (suction pressure) produces foaming
2. Refrigerant has condensed in suction line or crankcase. Suction line has free fall towards compressor

Remedy

1. Reduce compressor capacity or start throttling suction stop valve.
2. Heating element in crankcase should be connected for 6-8 hours before starting, so that refrigerant dissolved in oil can be separated before starting compressor up. Start throttling suction stop valve – stop when hammering is heard. Liquid separator should be mounted in suction pipe.

Slugging in compressor during operation

Causes

1. Refrigerant gas in liquid line
2. Superheating of expansion valve is set too low

Remedy

1.  Expansion valve is oscillating.
2. Adjust superheating, which should normally be 5-8°C

Excessive condenser pressure

In the event of abnormally high pressures in the refrigeration system, there is a risk of damage to the compressor. At very high pressures, the risk of the components in the refrigeration plant exploding can constitute a threat to life. Abnormally high pressures may occur in the case of:

• Extreme heating of plant parts (fire, solar radiation or other abnormal heating)
• Volumetric expansion of fluids in sealed-off premises.

Causes

1.  Overfilling with refrigerant
2. Insufficient condenser cooling, e.g. if cooling water fails, fan/cooling water pump clogs, soiling, scaling or fouling of heat-transmitting surfaces
3. Presence of non condensable gases (especially air) in condenser

Remedy

1. Refrigerant fills condenser and reduces its effective area. Draw-off coolant.
2. Regulate water/air supply or reduce compressor capacity, if called for. Check condenser as per instructions for same.
3. Blow air out of condenser. Follow instructions for condenser.

Too low condenser pressure

Causes

1. Excessive condenser cooling
2. Defective piston rings or worn cylinders
3. Discharge valves are defective or leaky
4. By-pass between high pressure side and suction side of compressor
5. Compressor lacks capacity

Remedy

1. Regulate condenser cooling
2. Replace worn parts.
3. Check valve ring plates and piston rings
4. Check compressor for internal leakage by performing pressure-drop test.
5. Check whether compressor capacity corresponds to load on plant. Reduce condenser cooling.

Excessive suction pressure

Causes

1. Error in setting of liquid regulation valve
2. Leaky suction valves.
3. Open by-pass between suction side and high-pressure side of compressor. Safety valves leaky, or opens prematurely.
4. Compressor lacks capacity.

Remedy

1. Liquid refrigerant in suction line. Adjust, repair or replace expansion valve.
2. Remove cylinder covers; check valve plates. Renew if needed.
3. Check system for any by-pass detectable as for instance a warm connection. Adjust or repair leaky valves.
4. Regulate compressor capacity. Check whether all cylinders are operating. Check function of capacity regulator.

Too low suction pressure

Abnormal low pressure in the refrigeration plant will increase the compression ratio of the compressor with a subsequent risk of damage to the compressor. The danger of air being sucked into the refrigeration plant also increases at abnormal low pressure.

Causes

1. Oil in evaporator
2. Refrigerant charge of plant insufficient. Bubbles in liquid line sight glass and possibly a warm liquid line.
3. Freezing-up of expansion valve (HFC/HCFC plant)
4. Thermostatic expansion valve has lost charge
5. Excessive superheating of suction gas
6. Filter in liquid line clogged
7. Solenoid valve in liquid line fails to open
8. Compressor has excessive capacity

Remedy

1. Draw-off oil
2. Check refrigerant charge. Charge plant with refrigerant. Find and seal any leak.
3. Thaw out expansion valve with hot, wet cloths. Replace desiccant in drying filter.
4. Valve fails to open. Change valve.
5. Regulate expansion valves to higher capacity
6. Check and clean filter in liquid line
7. Coil may have blown. Control signal lacking.
8. Reduce compressor capacity. Check capacity regulating system.

Oil temperature too low

Causes

1. Too little oil in compressor
2. Oil foams in compressor
3. Oil pump defective
4. Bearings worn
5. Oil filter clogged

Remedy

1.  Top up compressor with oil and investigate cause of oil consumption. See foaming below
2. Repair or replace
3. Repair or replace
4. Change filter cartridge

Excessive discharge pipe temperature

Causes

1. Excessive suction temperature as result of reduced refrigerant supply to evaporator (extensive superheating) owing to insufficient refrigerant charge
2. Excessive suction temperature as result of reduced refrigerant supply to evaporator (extensive superheating) owing to incorrectly adjusted liquid regulating valves
3. Leaky discharge valves
4. Open by-pass between high and low-pressure side of compressor, e.g. leaky safety valve

Remedy

1. Check refrigerant charge
2. Check thermostatic expansion valves
3. Leaking discharge valves gives rise to generation of heat. Change defective valves.
4. Localize by-pass and remedy any leakages.

Too low discharge pipe temperature

Causes

Low suction temperature as result of overflow of liquid refrigerant from evaporator

Remedy

Adjust liquid regulating valve. Increase superheating.

Excessive oil temperature

Causes

During operation, the temperature of the compressor crankcase must be 40-70°C.

Remedy

When woking with R717 and R22, it may be necessary to supply the compressor with oil cooling.

Oil level in crankcase falling

Where HFC/HCFC refrigerants are used, there will be some blending of refrigerant and oil during the initial operating period. It may therefore prove necessary to top up the oil after initial startup of the plant. The oil level must always be visible in the oil level sight glass on the compressor.

Causes

1. Filter in solenoid valve or nozzle in oil return line clogged
2. Liquid in suction line and crankcase may cause foaming in oil and thus increase oil consumption
3. Worn-out piston rings or cylinders
4. Solenoid valve in oil return line defective

Remedy

1. Oil return pipe must be warm during operations. Clean filter.
2. Examine evaporator system and check superheating of suction gas.
3. Renew piston rings and, if need be, renew pistons and cylinder linings.
4. Coil in solenoid valve defective

• Replace coil
• Electrical control signal lacking

Remedy

Heavy oil foaming in crankcase

Causes

1. Liquid in suction line
2. Too much refrigerant dissolved in oil

Remedy

1. See liquid in suction line
2. Before starting compressor, heating element must have been on for at least 8 hours in order to boil refrigerant out of oil. During start-up phase, capacity should be connected at a slow rate to prevent a sudden drop of pressure on suction side with resultant foaming. Under normal operating conditions, compressor should operate under as stable pressure conditions as possible

Crankcase “sweating” or frosting up

Causes

1. Liquid in suction line
2. Expansion valve sensor misplaced
3. Liquid regulating valve or float valve producing too much liquid

Remedy

1. See liquid in suction line
2. Check positioning of expansion valve sensor – see instructions for expansion valve
3. Increase superheating on thermostatic expansion valve

Capacity regulation oscillating

Causes

Oil foaming in crankcase

Remedy

See Oil Foaming

Impossible to bleed plant

Causes

1. Solenoid valve leaky
2. Defective piston rings
3. Defective discharge valves
4. Defective suction valves

Remedy

1.  Pinpoint and seal leak, or change leaky component
2. Check and replace any defective parts
3. Check and replace any defective parts
4. Check and replace any defective parts

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

Related Links:

Pressure Testing Refrigeration Plant on Ships

Pumping Down of the Refrigeration Plant on Ships

Maintenance of Reciprocating Refrigeration Compressors on Ships

Troubleshooting of the Reciprocating Refrigeration Compressor Plant on Ships

Capacity Control or Regulation for Refrigeration Compressor on Ships