Breathing easy

Whether in the transportation and distribution of natural gas to the consumer, or in the production of electricity via fossil fuel burning, hydroelectric dams or even nuclear fission, air compressors are playing a vital role in ensuring plant reliability and round-the-clock operation.

The British Compressed Air Society states that more than 70% of companies use compressed air for some aspect of their operations and this is certainly the case in the power sector, where it is used extensively for many different applications.

Compressed air can be used where other energy types would pose an explosion hazard or fire risk and it can also function at extremes of temperatures, making it ideal for the demands of the power plant.

Within the power station, compressed air provides a constant power source to drive instrumentation and for general plant processes. It can also drive air blast switchgear, where high-pressure air is used to interrupt the current flow.

Typical reasons cited for choosing compressed air are that it is a clean, safe, simple and efficient utility that produces no dangerous exhaust fumes or other harmful by-products.

It is equally easy to store in purpose-built tanks, making it ideal for use in areas where no other power is available or practical.

Hydroelectric power

In hydroelectric applications, compressed air is used to pressurise an accumulator. This operates the inlet valves that regulate the flow of water from the reservoir through the turbines.

In one typical installation at an underground power station, compressed air is used with six turbines, offering a combined station output of 1 728 MW and reaching maximum power generation in 16 seconds.

Turbine dewatering, where compressed air is used to purge residue or water from the turbines and storage pumps, plays an important role in routine plant maintenance and safety inspection.

It can also help to avoid cavitation (the process in which a void in the water flow collapses, producing a shock wave), thereby reducing the risk of serious damage to the impeller and extending the life of the turbine.

Steam power

Compressed air is fundamental to the continuous delivery of electricity to the grid, whether it is for generating steam by nuclear fission or through the burning of fossil fuels.

In a coal-fired power station, compressed air is typically used to clean the fireside of boiler tubes, where a blast of air will remove the accumulated molten slag much more effectively than steam, ensuring high boiler availability.

Compressed hydrogen, rather than air is often used to cool turbine generators, as the gas is much less dense than air, helping to minimise the power losses that can occur as gas circulates through the fans and cooling passages.

The heat transfer capability of hydrogen is also up to twice that of air, giving a more effective heat removal process, which results in lower turbine temperatures and smaller coolers.

In another application, at a nuclear generating station in Canada, where safety is paramount, emergency generators are on standby to provide an immediate power source if the regular backup generators fail.

These engines are fitted with air-driven starter motors that take the compressed air from two large pressure vessels.

The vessels are recharged routinely after a number of starts have taken place.

Gas distribution

As North Sea gas supplies continue to decline, the UK’s National Grid has needed to increase its reliance on liquid natural gas (LNG) imports, which it receives into the Isle of Grain importation terminal in Kent.

As a result, compressed air suppliers have seen a significant increase in activity in the installation of gas compression stations for LNG along the pipeline, where nitrogen generators provide a secondary seal on the large, centrifugal compressors and prevent any gas from escaping.

Quality is key

The uses for compressed air are many and varied, but there is one key factor that applies across all applications: that correct equipment specification, in terms of the quality of air delivered and the absolute reliability of the compressor system chosen is essential to plant performance.

The compressors not only have to compress the air to a specific pressure and at a certain flow, but they also have to deliver air of the right quality.

Air is naturally contaminated with solid particles, such as dust, sand, soot and salt crystals, the quantities of which alter with differing environments and altitude.

Water vapour is also found in variable amounts in the air and if left untreated can cause significant corrosive damage under compression, as the concentration of moisture and contaminants increases causing a detrimental effect on pneumatic equipment.

This can result in production downtime and reduced equipment life.

To avoid unnecessary damage, therefore, international specifications for power generation dictate that completely dry and oil-free air be used, so that there is no risk of contaminating sensitive pneumatic equipment, such as controls, valves and actuators or damaging expensive plants.

Oil-free air

Oil-free air compressors have the added bonus of being inherently cost-effective, with fewer parts to replace and maintain.

The traditional oil system has been replaced with simple water lubrication, meaning that there are no oil filters to be changed or waste oil to dispose of, nor is there any condensate to treat, which brings significant benefits to a plant where environmental performance is monitored closely.

Purified water injected into the compression element lubricates, seals and cools the compression process and direct drives with no gearbox means that there is no additional lubrication required.

In addition, low bearing loads allow sealed for life bearings that also require no oil lubrication to be used.

Energy saving

Studies have proved that the largest cost component of a compressor during its lifetime is the power required to run it, so manufacturers are now incorporating energy-saving technologies into their design of new equipment.

Instrumentation plant compressors are water lubricated which reduces air temperature for near isothermal compression and, the direct drive motor, with no gears or belts helps to optimise power transmission.

This technology means that the installation can generate the compressed air it requires at a significantly lower cost.

In processes that demand very high air pressures, the air will be cleaned after compression. Usually a suitable system is engineered to include a variety of filtration and drying processes that remove water vapour and contaminants before the air reaches the point of use.

The compressed air will normally be passed through an aftercooler, which lowers its temperature and reduces its ability to hold water vapour, removing around 70% of moisture.

However, at this point the air is still saturated, so a refrigerant dryer is used to cool the compressed air further, whereby a large amount of the water condenses and can be separated.

The compressed air is then heated so that condensation does not form on the outside of pipe work system.

Another method, desiccant drying, works on the principle of absorption of water vapour through a bed of desiccant material, in a pair of chambers. Two types are used; heatless regenerative and heat regenerative.

The heatless type uses a percentage of the dried air for regeneration of the desiccant material, while the heat regenerative type uses an electric heat disk, which reduces the amount of purge air needed for regeneration.

Water separators are used for the efficient removal of bulk liquid contamination from compressed air.

Filters are used both prior to, and after compression. Filtering the intake air reduces the wear on the compressor by removing larger particles.

In typical oil-lubricated machines, compression air can become contaminated with oil, which is removed by filters that are often set up in multi-stage.

Fibre filters can only trap oil as droplets, while more efficient active carbon can trap oil as a vapour.

Dependable supply

Just as important as the air quality is the compressed air system’s overall reliability.

The compressor is a critical piece of equipment and must be able to perform consistently and continuously in round-the-clock operations.

For this reason, most operators choose to have full system redundancy, typically purchasing two, if not three machines for each plant application, which are rotated regularly to ensure consistent component wear on each unit.

Rotating compressor usage in this way minimises component wear, reduces maintenance concerns and extends equipment life to improve overall cost of ownership.

Rather than sourcing equipment from multiple sources, many operators favour a single source of supply and maintenance and are turning to equipment suppliers who are able to engineer and package a bespoke compressed air solution to meet their exact requirements.

Many smaller power plants have found that these packaged solutions, which are supplied in a weather-tight container can save significant costs as they can be sited outside and can eliminate the time and expense of building and civil engineering work to house the compressor station.

Total support

Power generation is one industry where professional maintenance is considered as important an aspect in plant performance as the actual equipment specification itself and most operators will build the cost of a full service contract into their capital expenditure.

The majority of compressed air equipment suppliers will provide scheduled on-site maintenance and repair, but it is worth considering the locality and expertise of the engineers and checking what services are included in the contract fee.

The power sector relies heavily on the consistent performance of its compressed air systems; performance which is driven by two essential ingredients – the quality of air provided and the reliability of the equipment chosen.

Get these basics right and you can be sure of meeting the challenges of supplying continuous power to industry and consumers alike, for many years to come.

Graham Pearl is industry sales manager for CompAir.