Maintain efficiency
September 1st 2009

For industries reliant on compressed air supply, nothing has a greater impact on the life and efficiency of a compressor system than the maintenance programme, according to Atlas Copco Compressors Maintenance makes up the smallest proportion of total operating costs for a compressed air system, generally in the region of 5-10% of the initial equipment investment value. By adopting a well planned maintenance programme it is possible to not only anticipate these costs but to extend the service life of the equipment and to achieve 100% operational efficiency.

While most plant operations will rely on the OEM's scheduled service plans, carried out by an accredited distributor or service centre, there will be instances where the level of engineering knowledge will enable the maintenance procedures to be undertaken in-house. In either situation there are practical issues to be considered.

One way to tell if the compressed air system is being well maintained and is operating properly is periodically to track and record power, pressure and flow readings. If, on later checks, power use at a given pressure and flow rate goes up, the system's efficiency is degrading. This indicates whether the compressor is operating at full capacity, and whether capacity is decreasing.

Key areas that require maintenance are the compressor itself, heat exchanger surfaces, the oil separator, oil levels, oil filter, and air inlet filter. The compressor and intercooling surfaces need to be kept clean; if they are dirty, compressor efficiency will be adversely affected. Fans and water pumps need inspection to ensure that they are operating at peak performance. The oil separator in a lubricant-cooled rotary screw compressor generally starts with a 0.2 bar pressure drop at full-load when new.Most manufacturers' maintenance manuals suggest changing them when there is about a 0.6 bar pressure drop across the separator.

Because compressor lubricant can become corrosive and degrade the equipment and the system efficiency, the oil and oil filter need to be changed as per manufacturer's specification which in our experience, and depending upon operating conditions, requires oil levels to be checked monthly and, in the case of oil-injected screw compressors the lubricant to be changed after 4000 running hours. As a rule, the oil filter should be be replaced annually. Oil-free screw compressor lubricants allow a longer interval between lubricant replacement – up to 16,000 running hours.

Air inlet filters and piping also need to be kept clean. A dirty filter can reduce capacity and efficiency, so filters should be maintained at least per manufacturer's specifications, taking into account the level of airborne contaminants at the system's operational site. It is our experience that a weekly check is advised and the filter unit replaced on an annual basis.

If the electric motor driving a compressor is not properly maintained, it will not only consume more energy, but be apt to fail before its expected lifetime. Important aspects are lubrication and cleaning.Over-lubrication can be as harmful as under-lubrication and is a common cause of premature motor failure.Manufacturer's typically recommend that motors should be lubricated anywhere from every two months to every 18 months, depending on operation and motor speed.

Since motors need to dissipate heat, it is important to keep air passages clean. For enclosed motors, it is vital that cooling fins are kept free of debris. Poor motor cooling can increase motor, temperature and winding resistance, which shortens motor life and increases energy consumption.

Motor belt drives also require periodic maintenance because belts stretch and wear.

Tight belts can lead to excessive bearing wear, and loose belts can slip and waste energy. A good rule-of-thumb is to examine and adjust belts after every 400h operation.

Fouled compressed air treatment equipment can result in excessive energy consumption as well as allowing poor-quality air to damage other equipment. Dryers, aftercoolers and separators should be cleaned and maintained according to individual specifications. In the main, that will mean a monthly inspection and appropriate action taken to maintain peak operational condition.

Regular leakage checks should be conducted as a 1mm hole in an air line supplying air at 6bar pressure can leak air at the rate of 1L/s, resulting in a power loss of 0.3kW at the compressor.

Automatic drain traps need to be inspected periodically to ensure they are not stuck in either the open or closed position.

It may be prudent to consider monitoring solutions such as Atlas Copco's AIRConnect, a remote monitoring service that links compressed air installations to a highly automated 24/7 Internet-based monitoring system. It offers a system status overview and notifications in case of irregularities, reports analyses and immediate response.

Equipment status is monitored in real time – including shock pulse monitoring that determines bearing wear as the basis for efficient condition monitoring. The system provides automatic warnings via remote diagnosis and service scheduling – interventions based on specific requirements rather than time related. Services are delivered rapidly, breakdowns can be prevented and downtime shortened.

Most manufacturers' aftermarket offering provides a choice of service plans. In Atlas Copco's case, these are with options of inspection, preventative maintenance or total responsibility.

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