Maintenance, Overhaul, Inspection and Repair
Planned work, scheduled and recorded — the object is to prevent the fault, not to find it.
Key Principles at a Glance 8 points
- Planned or preventative maintenance is not necessarily best for every installation — but on a ship carrying perishable cargo it almost always is, because the service is essential and spares lead time can run to weeks or months.
- A layer of scale only 2 mm thick on a condenser tube can cause a power increase of 16 %, and the need to clean a condenser can usually be deduced from the condensing pressure alone.
- Under no circumstances should refrigerant be added to a leaking circuit without first making a repair — the only arguable exception is a continuous process plant where shutdown cost overrides the risk of a small continuous leak.
- Where gas is detected at the shaft gland of a stopped open compressor, run the machine briefly to re-lubricate the gland — the leak may then cease.
- Cold repair is preferred to arrest a leak temporarily, and for a short period only; renewing the whole section of copper pipe is the permanent answer.
- Piston clearance of 1.5 mm is the maximum — above it volumetric efficiency falls away.
- Safety controls must be checked at least once a year, using throttling valves, stopping pumps or removing the load to set up the unsafe condition deliberately and safely.
- The important line in the spares table is D — full duplication of the plant, to comply with Lloyd’s Rules, which is exactly why ships carry two refrigerating plants of the same capacity.
1. What the Engineer Must Understand
The maker's manual, the SMS and the class rules override. All major maintenance work and findings must be recorded in the plant running log as a guide to the reliability of components, the need for cleaning, and other indicators to future work.
The average user, unless of an industrial nature, will tackle only the simpler of these maintenance jobs and will entrust the major work to a specialist firm. Large concerns will have their own trained and skilled personnel and will do all work themselves.
User maintenance includes operation where not automatic, cleaning of filters and strainers, attention to oil and lubricant levels, belt tensioning, general cleaning, running standby equipment, and verification of control operation.
Planned or preventative maintenance is not necessarily the best for all installations. If the service is not essential, and spares are known to be available within a reasonable time, nothing is done beyond obvious routine servicing work. Then, when a breakdown occurs, it is repaired. This approach, although not widely accepted, is an option which should be considered.
That last option is rarely defensible — a reefer plant carrying perishable cargo is an essential service, and the spares lead time can be measured in weeks or months.
2. User Maintenance — The Daily and Weekly Routine
| Item | What to do |
|---|---|
| Air and water filters | Keep clean. Finned coils, especially outdoor condenser coils, must be cleaned frequently. Accumulated dirt on air filters will increase the resistance and lead to reduced air flow — by far the most frequent cause of malfunction of air-conditioning equipment |
| Water side of heat exchanger coils | Clean of accumulations of scale or algae as soon as a change in working conditions shows they are getting dirty |
| Water strainers | Cleanable type; indication of a dirty strainer is an increase in pump pressure — essential to have a pressure gauge on the pump discharge |
| Oil and lubricant levels | Most equipment is designed to run for long periods without addition of lubricant, and the dangers of adding too much should be noted. The user will not normally add oil to a refrigeration system, apart from an industrial R.717 plant which has a routine for draining parts of the circuit and replenishing the compressor sump |
| Belt tensioning | Replacement of broken or worn belts is a normal maintenance procedure, but may be missed if equipment is out of sight. A routine check will find these out |
| Standby plant | Run frequently, both to ensure it is in working order, and to keep items such as shaft seals oiled and run-in, and thus gas-tight |
| Changeover valves | Location and function of any changeover valves operated in conjunction with standby plant should be clearly marked |
| General cleanliness | The general cleanliness of plant is an indication of the care and interest taken by the maintenance staff. There is no reason or excuse for accumulations of dirt and refuse on or around any system |
| Control and safety instrument settings | Many malfunctions, and some dangerous situations, arise from incorrect setting of control and safety instruments. It is good practice to arrange that instruments are locked, sealed, or otherwise guarded from tampering — even to the extent of putting dummy controls in a conspicuous place |
| Safety controls | The function of safety controls should be checked at least once a year |
| Water treatment and corrosion inhibition | Require periodic attention; full instructions should be left on site by the supplier or installer |
| Bleed-off | Where water is evaporated from a circuit — cooling tower, evaporative condenser, humidifier — there is no way of avoiding a steady bleed-off or frequent flushing, to restrict the concentration of dissolved solids. Much trouble arises from efforts to stop the bleed-off to "save water" |
Even those with the most sophisticated separators have continuous oil migration and require attention every week or, in some cases, every day, to ensure maximum working efficiency. In larger plants the oil removed can be filtered and used again. It is useful to enter in the running log the quantities of oil removed and put in, since it has been known for large amounts of oil to accumulate in an evaporator without operators being aware of it.
3. Major Maintenance Work
3.1 The contract question
- The services of an outside maintenance contractor are usually on an annual contract basis and should clearly define the work to be carried out by the two parties. Day-to-day operation and the simpler tasks such as air filter cleaning are nearly always excluded.
- Various types of contract are offered and it is recommended that the original suppliers are approached during the commissioning period (or before) for their suggestions.
- In particular, there may be friction if a rival firm undertakes maintenance while the plant is under guarantee, and it is usual to let the first year's contract to the installer.
3.2 The major work items
| Work | Notes |
|---|---|
| Cleaning and refilling the oil sump of a viscous air filter | A major job, distinct from routine filter changing |
| Periodic draining, cleaning and flushing of water tanks | Particularly cooling towers |
| Leak testing (halocarbons) | A regular leak test should be part of the general maintenance schedule |
| Moisture control | Moisture in halocarbon circuits is indicated by the colour trace on the sight glass. Immediate action is required, especially with a hermetic or semi-hermetic compressor, before damage is caused. The drier should be changed and the sight glass watched for reversal of the colour to "dry". Bad cases of contamination may need a second change of drier |
| Drier/strainer pressure drop check | If the liquid line leaving the drier or strainer is colder than the inlet, there is a severe pressure drop within, indicating dirt. A new drier, or cleaning of the strainer, will cure this |
| Heat exchanger cleaning | Aqueous circuits can be cleaned with a chemical such as sulphamic acid, brushed, or subjected to high-pressure water jets. All traces of dirt and chemical need to be removed before the circuit is put back to work. A layer of scale 2 mm thick on a condenser tube can cause a power increase of 16 % |
| Safety control checking and readjustment | An essential part of periodic maintenance — possibly annually. A time should be chosen when temporary stoppage of the plant will not cause inconvenience. Unsafe conditions can be set up by throttling valves, stopping pumps, or removing the load. In each case the relevant safety control should function at the pre-set conditions |
| Fire damper safety checks | May be required from time to time by local authorities; these checks, together with the expert advice available from the testing officers, should be welcomed as proof of the inherent safety of the installation |
It is essential that all major maintenance work and findings are recorded in the plant running log as a guide to the reliability of components, the need for cleaning, and other indicators to future work.
4. Planned Overhaul of Compressors
Refrigerant compressors, air compressors and some other items of mechanical equipment might be subjected to a periodic part strip-down inspection and overhaul, as may be recommended by the manufacturer and indicated by running time meters or estimated running hours from the plant log sheet.
Such planned maintenance entails, as its name suggests, some planning:
- Manuals, diagrams and drawings should be obtained beforehand, and sources of possible spare parts located.
- Special tools or instruments may be required.
- The manufacturer should be able and willing to advise and guide in this work.
- Failed, worn and other replaced parts should be retained for later examination, and a post-mortem held in cases of doubt; records should be kept.
- In the case of a shut-down of a process plant, the major components should be tackled in rotation, lest a serious fault or shortage of spares prevents the process re-starting at the end of the closure.
4.1 Overhaul preparation checklist
4.2 What to inspect on a compressor strip-down
| Component | What to look for |
|---|---|
| Suction and discharge valves | Reed/disc/ring-plate condition: cracks, distortion, pitting, correct lift (about 2 mm in average sizes), seating on the stainless steel seats; damping springs intact; valve retainer spring-loaded so it lifts on liquid carry-over |
| Valve plate and gaskets | Erosion, fretting, correct gasket thickness (this sets the clearance volume) |
| Pistons and rings | Two or three compression rings and a lower oil seal ring; ring wear, gumming, broken rings |
| Loose liners | Scoring, wear step at the top of the ring travel, ovality |
| Piston clearance | 1.5 mm maximum — above this the volumetric efficiency falls away |
| Connecting rods and bearings | Aluminium or H-section steel forgings; white-metal-lined steel small-end bushes; bearing wear and embedment |
| Crankshaft | SG iron; journal wear, ovality, taper; oil drillings clear |
| Main bearings | White-metal-lined steel shells; wear, wipe, embedment |
| Shaft seal | Carbon ring face, metal facing ring, spring tension, bellows, neoprene/duprene sealing ring; the seal is lubricated from the compressor system and can give trouble if there is insufficient or contaminated oil |
| Oil pump | Rotor-type (inner rotor one tooth less than the outer) or gear pump; wear on the rotors; strainer condition |
| Oil relief valve | Setting about 2.5 bar above crankcase pressure |
| Suction strainer (felt-lined) | Scale and swarf — freons are searching liquids and clean the circuit, so the strainer does the catching |
| Bursting disc | Nickel, 0.05 mm; renew if ruptured |
| Crankcase heater | Function — prevents refrigerant dissolving in the oil and foaming on start |
| Crankcase flaps / oil return drillings | Clear, so returning oil drains back to the crankcase |
4.3 Reassembly and return to service
5. Leak Testing and Repair
5.1 Leak testing methods
| Method | Application |
|---|---|
| Leak detector lamp | Freon systems. Pale blue or colourless flame turns green when freon is drawn into it. A large leak burns violet — ventilate the space to clear excess gas before pinpointing |
| Soapy water brushed over joints and flanges | Universal fallback; bubbles indicate the leak |
| Electronic leak detector | Where available — the modern equivalent of the lamp |
| Pressure/vacuum test | After any major work, before charging |
| Sight glass colour change | Indicates moisture, and therefore an ingress path |
Where gas is detected at the shaft gland of an open compressor which is not turning, the compressor should be run for a short time to re-lubricate the gland. The leak may then cease.
Staff should be forbidden to smoke while leak testing or repairing. Many operatives are ignorant of the danger to their health if smoking in the presence of traces of the halocarbons.
Under no circumstances should refrigerant be added to a leaking circuit without first making a repair. The one exception to this rule may be a continuous process plant, where the cost of a shut-down may override the cost and inherent danger of a small continuous leak.
5.2 Repair methods
| Repair | Type | Notes |
|---|---|---|
| Renew the whole section of copper pipe | Permanent | The correct answer for a holed refrigerant line |
| Ferrule connection | Temporary | Cut the pipe with a pipe cutter, insert ferrule female and female pieces, apply thread seal tape, tighten. Then open the valves and confirm the leak has stopped |
| Cold repair (plastic steel putty, acrylic solutions) | Temporary, short period only | Cold repair is preferred to arrest the refrigerant leakage temporarily and for short period only |
6. Heat Exchanger Inspection and Cleaning
6.1 Condensers and evaporators
- Aqueous circuits (evaporator or condenser) can be cleaned with a chemical such as sulphamic acid, brushed, or subjected to high-pressure water jets.
- In each case all traces of dirt and chemical need to be removed from the circuit before it is put back to work.
- In cases of doubt, the manufacturer's advice should be sought.
- A layer of scale 2 mm thick on a condenser tube can cause a power increase of 16 %, and the need to clean a condenser can usually be deduced from the condensing pressure.
- Persons using high-pressure water jets should wear face masks to avoid inhaling aerosol droplets.
- Water side: swirl strips promote turbulence but interfere with maintenance cleaning and are not much in favour.
- Tube plates: galvanic protection blocks (zinc or iron) should be provided, and steel tube plates are best treated with chromium for corrosion resistance.
6.2 Air-cooling coils
- Finned coils, especially outdoor condenser coils, must be cleaned frequently.
- Fin spacing from 2 mm (compact air conditioner) to 12 mm (low-temperature coldroom coil) — the tighter the fins, the more readily they block with condensed moisture or frost.
- Frost on the evaporator coils is removed by washing with a hot water hose with the plant shut down, after clearing the drip tray drain if necessary.
7. Safety Control Testing — The Annual Routine
The checking and readjustment as necessary of all safety controls is an essential part of periodic maintenance — possibly annually. A time should be chosen when temporary stoppage of the plant will not cause inconvenience. Unsafe conditions can be set up by throttling valves, stopping pumps, or removing the load. In each case the relevant safety control should function at the pre-set conditions.
How to test each control without damaging the plant:
| Control | Method |
|---|---|
| High-pressure cut-out | Throttle the condenser cooling water (or stop the pump) with the compressor running and confirm the cut-out operates at its set point and requires a manual reset |
| Low-pressure cut-out | Close the liquid line / pump down and confirm the cut-out operates at its set point; check cut-in on the return |
| Oil differential pressure cut-out | Confirm the time delay and the trip at the set differential; check the setting has not been altered |
| Relief valves | Function of safety controls to be checked at least once a year; note the requirement for two valves on a vessel greater than 285 litres, and that no shut-off valve is permitted between the relief valve and the vessel it protects unless two such valves are fitted |
| Bursting disc | Visual; a ruptured disc is indicated by suction and discharge pressures being about equal |
| Thermostats / humidistats / room solenoids | Simulate the temperature change and confirm the solenoid opens and closes |
| Back pressure valve | Confirm it holds the evaporator at the required pressure |
| Defrost timer | Confirm the interval and that the heaters and solenoids change over correctly |
Record the settings and the results. The settings must be restored to the commissioning values, and the record is the proof that they were.
8. Guarantee Period
- Most equipment will be guaranteed by the supplier or contractor for 12 months from the date of supply, installation or commissioning, and these precise details should be agreed and noted to avoid arguments later.
- In particular, some items may have left the factory several months before commissioning and, if the manufacturer is advised, the guarantee may be extended from that date.
- Such a guarantee will probably cover the cost of repair or replacement of the item, but not labour charges in removal or refitting. Again, these details should be noted.
- The situation often arises where equipment is brought into service before it has been commissioned and accepted by the purchaser. Such operation can only be on the instructions of the purchaser and with the agreement of the supplier. A maintenance contract must come into force at this time.
- Many disputes arise in this first year if the installation is not maintained to the satisfaction of the supplier, and a split responsibility of this sort is to be avoided. As already stated, where possible, maintenance for the first 12 months should be by the original contractors or a firm recommended by them.
- There is a growing tendency to extend the guarantee period on small packaged plant to 3 years.
9. Spare Parts
Except in the case of a planned overhaul, spare parts will only be wanted in an emergency, and then in a hurry. Most manufacturers can guarantee supplies from their own stocks and will despatch quickly, providing they are given enough information to correctly identify the parts required.
There are two predominant problems:
The scale of spares to be held under these varying conditions must be judged by the user, taking into account the problems which might be met and seeking advice from the supplier. For remote installations the latter may be asked to recommend "Spares for one year's operation — a classification which is subject to the vagaries of mechanical breakdown".
Suggested scale of spares:
| Type of installation | Very good | Fair | Poor | Very bad |
|---|---|---|---|---|
| Air-conditioning — comfort, small | Nil | Nil | A | B |
| Air-conditioning — comfort, large | Nil | R | A | B |
| Air-conditioning — essential process | D | D | D + A | D + B |
| Cold stores — small, above 0 °C | Nil | R | D + A | D + B |
| Cold stores — large, above 0 °C | R | D | D + A | D + B |
| Freezer — small | R | D | D + A | D + B |
| Freezer — large | D | D | D + A | D + B |
| Industrial process | D | D | D + A | D + B |
Code:
- R — refrigerant
- A — refrigerant, drier, solenoid valve coil, compressor suction and discharge valves and gaskets, electrical controls, solenoids and overloads, any other items specifically recommended by the manufacturer
- B — as A, together with expansion valve
- D — duplicate equipment throughout plant, to comply with Lloyd's Rules, which say in effect that the process shall not be at risk in the event of the failure of any one component
It is the class-driven answer for any essential process plant: full duplication of the plant, so that no single component failure puts the process at risk. That is exactly the shipboard practice of two refrigerating plants of the same capacity, so that one is available during maintenance of the other.
10. Training and Records
- The nature of refrigeration and air-conditioning equipment requires specialised training for operating and service personnel.
- The basic skills of those entering the industry should be the ability to read an engineering drawing and to read and understand flow, circuit and electrical diagrams. When recruiting labour it will be an advantage if the candidate already has some knowledge of electrical circuits.
- A contractor supplying new equipment should be required to instruct the staff who will operate it, and to advise where further instruction may be had.
- If the senior operator is allocated to work alongside the contractor during the final erection and commissioning stages, he should pick up enough knowledge of the system to give him the confidence and skill to run it.
- It is not sufficient that one person only has this knowledge. A clear set of operating instructions should be posted in the main plantroom, enabling any authorized person to start, run and shut down the system in a correct, safe and efficient manner.
- All staff who may be required to operate the plant need to be instructed and have some practice.
- Another factor when taking on new staff must be their attitude to a changing technology. Advances are being made in the many aspects of engineering and application in refrigeration and air-conditioning. This situation requires adaptable personnel who are always ready to assimilate new ideas.
- It is usual to mark the grade of lubricant on each item which might need periodic attention.
It is essential that all major maintenance work and findings are recorded in the plant running log as a guide to the reliability of components, the need for cleaning, and other indicators to future work. Failed, worn and other replaced parts should be retained for later examination and a post-mortem held in cases of doubt; records should be kept.