Driving growth in technology maturity

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Train operators need air compressor technologies that combine high levels of operational efficiency and high in-service availability in all operation conditions, including very low duty cycle. To answer this need, Wabtec has undertaken the extensive field trial and development of the next generation of air compressors. Wabtec's Buran CSS with Covered / Sector Sealing piston is a state-of-the-art solution, capable of withstanding all operating conditions, including extreme temperature and humidity, and low duty cycle applications.

Shifting from lubricated to oil-free compressors

For decades, trains have been equipped with air compressors to supply their brake and suspension systems. Compressor technologies for rail were based on existing industry compressor technology, predominantly lubricated reciprocating piston or lubricated screw compressors.

However, customers found that lubricated compressor technologies had some serious drawbacks, including the need for:

• Daily oil verification
• Refill upon compressor consumption
• Regular oil and filter replacement and recycling
• Storing the oil-containing air dryer condensates onboard
• Regular condensates tank emptying and recycling

The use of lubricated compressors is also made complex by the required duty cycle. When designing a train’s air generation system, a balance must be found between two contradictory requirements: The duty cycle and the time to fill the train during initial preparation. Naturally, train operators are looking for shortest possible filling time.

For the duty cycle, there are two factors to consider here. Firstly, compressors are thermodynamic machines that are designed to run hot. Secondly, during the process of compression, water is separated from the air and mixes with the oil when it condenses. A short duty cycle may not allow the compressor to run hot enough to completely evaporate the water. Allowing this water to mix with the oil could lead to serious lubrication issues and critically damage the compressor.

The first steps to oil-free

About 15 years ago, many in the rail industry started to seriously consider oil-free compressor technology. The most suitable option for use on trains is the reciprocating piston, which can cover the whole air flow range required. This ranges from 300 l/min for short trains up to 2,500 l/min for long trains or locomotives.

At Wabtec, our main focus in developing and validating oil-free compressor technology was the new friction and compression technology between the piston and cylinder. Following in-depth high duty cycle wear analysis, we reached a solution that was adopted by most market players. This consisted of two compression segments and a PTFE-based friction material on the piston body.

This solution worked well in reasonably high duty cycles. However, in low duty cycles where the compressor did not reach its normal operating temperature, we encountered premature wear in the friction and compression material. In duty cycles as low as 5%, with lengthy parking mode, or when the compressor’s cooling being exacerbated by a roof-mounted arrangement, we sometimes found severe wear.

Using duty cycle regulators allows compressor operation to be maintained at an acceptable level. This is achieved using a valve on the main pipe circuit that creates a controlled leak to guarantee a minimum compressor duty cycle. However, this is more of a stopgap than a complete solution, and not ideal in view of the demand for increased energy efficiency.

Going beyond market requirements

Wabtec engineers have gone to great lengths to build up resilience to low duty cycles in reciprocating oil-free compressor technology. To validate a solution in a reasonable timeframe, we developed a unique test bench to create an accelerated simulation of the use of friction and compression materials in all kinds of conditions. It has been a long journey to develop and validate a new solution, and our efforts have paid off.

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It was clear that some applications would remain with low duty cycle applications and therefore would require a solution capable of withstanding such conditions and reaching an acceptable lifespan without a duty cycle regulator. This is precisely what our newly patented CSS piston assembly and cylinder now offers.

Our first step was a test bench demonstration. With a duty cycle as low as 5% and at a permanent low temperature, the CSS solution easily reached the equivalent overhauls periodicity with a large margin.

The second step was on-train validation. We now have more than 500 compressors in service with the CSS solution. We have chosen to implement CSS in priority on applications with low duty cycles and temperatures, and regularly conduct measurements on these compressors. The wear rate we observe in the field is even better than in similar simulated conditions on our test bench. Even under these worst conditions, we know that the CSS solution can reach more than 12,000 pumping hours, which corresponds to usual compressor MTBO.

Our next challenge will be to push MTBO beyond the usual market requirements.

We are grateful to our customers for their trust throughout this process and we are very happy with the result obtained. We have reached a major step in oil free reciprocating compressor technology maturity.

Wabtec’s new generation of Buran compressors, Buran CSS, is the new market reference, highly resilient to extreme operation conditions and state-of-the-art.