By: Graham Pearl, CompAir, UK
In today’s power generation industries, compressed air is absolutely everywhere! Whether you are involved 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 demands placed on the earth’s valuable energy resources by a growing population will continue to escalate, and with this rise, comes a higher demand for power. In its 2007 World Energy Outlook, the International Energy Agency estimates that some $22 trillion will need to be invested in the power supply infrastructure to meet demand over the next 25 years. It is clear therefore that the power generation sector has a significant task at hand to keep pace with both consumer and industrial demand, and the need for continuous power production has never been more important.
The need for compressed air
The British Compressed Air Society claims that over 70 per cent of companies use compressed air for some aspect of their operations, and this is certainly the case in the power generation sector, where it is used extensively for many different applications.
Typical reasons cited for choosing compressed air are that it is a clean, safe, simple and efficient utility, producing 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.
Compressed air can be used where other energy types would pose a potential explosion hazard or fire risk, and it can also function at extremes of temperature, making it ideal for the demands of the power plant.
Within the power plant, compressed air will provide a constant power source to drive instrumentation, general plant processes and air blast switchgear, in which high-pressure air is used to interrupt the current flow.
In hydroelectric applications, CompAir has worked on a number of projects to supply compressed air to pressurize 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 1728 MW and reaching maximum power generation in 16 seconds.
Ontario Power Generation is using CompAir compressors in the drilling of a tunnel beneath the city of Niagra in a hydropower project
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.
Whether used for generating steam by nuclear fission or through the burning of fossil fuels, compressed air is fundamental to the continuous delivery of electricity to the grid. 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 minimize 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 supplied by CompAir’s high-pressure 5337 units from two large pressure vessels. The vessels are recharged routinely after a number of starts have taken place.
Quality is the key
As we have seen, 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.
Firstly, the compressors not only have to compress the air to a specific pressure at a certain flow, they also have to deliver air of the correct quality. Air is naturally contaminated with solid particles, such as dust, sand, soot and salt crystals, the quantities of which alter with environment and altitude. Water vapour is another substance which can be found in variable amounts in the air and if left untreated can cause significant corrosive damage under compression. An increasing concentration of moisture and contaminants has a detrimental effect on pneumatic equipment. This can result in production downtime and reduced equipment life. So to avoid unnecessary damage, international specification for power generation dictates 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 of damaging costly plant.
Compressed fun in the Spanish sun
A good illustration of this point is the use of a CompAir Iberia-engineered compressed air system at the AES Corporation’s new power plant in the port of Escombreras, Spain.
When AES opened the doors to its new power plant in 2006, its challenge was to guarantee a continuous power supply to the local community. The company chose a CompAir compressed air system, which plays a vital role in ensuring maximum productivity and offered significant advantages in its flexibility and economic performance over the other equipment considered.
The new plant consists of three power generators with gas and steam turbines that provide a combined net power rating of 1170 MW. After accessing several equipment suppliers, AES’s appointed contractor EPC decided that CompAir Iberia could deliver the best turnkey solution within a tight timeframe that would encompass a proven and robust range of compressors, with full engineering and service support.
The company placed an order for a 1200 MW compressed air system comprising Dryclon compressors for plant auxiliary services, two D37H units for instrument air and six Reavell H5236s for cleaning the turbines. The entire system is controlled by the Delcos 5000 monitoring system and includes all necessary air dryers, filters and storage tanks.
At certain times, the turbines and pipes require a huge volume of compressed air. The system was specified to produce a maximum air capacity at standard temperature and pressure of 3400 m3/h using both Dryclon compressors and more recently, the high-pressure H5236s operating at 24 bar.
To avoid any danger of contaminated air coming into contact with plant equipment, CompAir supplied a range of proven oil-free compressors that are inherently cost-effective, with fewer parts to buy and maintain. In its single-stage DH compressors, for example, 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 and no condensate to treat, bringing significant benefits to a plant where environmental performance is monitored closely.
In single-stage DH compressors, the traditional oil system has been replaced with more efficient water lubrication
Purified water injected into the compression element lubricates, seals and cools the compression process. The direct drive, with no gearbox, means that there is no additional lubrication required. In addition, low bearing loads allow the use of sealed-for-life bearings that require no oil lubrication.
Studies have proved that the largest cost component of a compressor during its lifetime is the power required to run it. The CompAir DH design incorporates numerous energy-saving technologies and was proven to consume significantly less energy than the other manufacturers’ models considered in the tender process.
The 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 optimize power transmission.
This technology means that the installation can generate the compressed air it requires at a significantly lower cost.
In other applications, particularly processes that need high air pressure, 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.
A dry time
Typically, the compressed air will be passed through an aftercooler, which lowers its temperature and reduces its ability to hold water vapour, removing around 70 per cent 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 the 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 before and after compression. Filtering the intake air reduces the wear on the compressor by removing large particles. In typical oil-lubricated machines, compression air can be contaminated with oil, which is removed by filters that are often set up in multiple stages. Fibre filters can only trap oil as droplets, while more efficient active carbon can trap oil as a vapour.
Just as important as the air quality is the compressed air system’s overall reliability. As highlighted, in all the applications, 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.
This was the case at AES, where CompAir Iberia engineered the system to use the Delcos 5000 system to balance air supply to meet fluctuations in plant demand. Delcos monitors the air station continuously and selects the best combination of compressors to provide the appropriate volume of air across the entire plant. Rotating compressor usage in this way minimizes 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 like CompAir, which are able to engineer and package a bespoke compressed air solution to meet their exact requirements. For example, Joaquin Sanchez, engineering manager at AES for example found this to be the case with the CompAir package. He says: “We considered compressed air solutions from several suppliers but found that the CompAir system offered the best overall solution, with economic air production, a guaranteed oil-free supply and compressor rotation to ensure equipment longevity à‚— all backed by comprehensive engineering, installation and service support from the experts at CompAir Iberia.”
Sanchez adds: “The equipment has now been operating reliably for six months and is helping us to ensure that the plant remains at peak productivity every day.”
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.
Power generation is one industry in which 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.
Almost all of CompAir’s power plant operators take advantage of the company’s full air service package, which gives them access to a global network of locally-based CompAir engineers.
The power sector relies heavily on the consistent performance of its compressed air systems, the performance of 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 to meet the challenges of supplying continuous power to industry and consumers alike for many years to come.