Compressed air provides an important form of power for railway and train operations. It controls braking, door, pantograph and air-suspension systems, plus a variety of auxiliary equipment from air horns to air-conditioning units.
Railway applications use many types of air compressor, some of which are oil-free. They include scroll, screw and piston, and all must meet the same criteria – safety, reliability, low weight, small footprint and minimal maintenance requirements. These factors apply equally to other key compressed air system components, such as dryers and devices for air treatment and pressure regulation.
One of the main challenges in developing compressed air systems for railway applications is maintaining the quality of the compressed air feed to downstream equipment – especially if this is safety critical.
Atmospheric air is full of contaminants: dust, pollen, bacteria, vaporized oil and moisture. The concentration of each may vary depending on the operating environment and, under normal conditions, be relatively harmless. However, the process of compression dramatically increases the levels of concentration. For example, the concentration of oil vapor in ambient air might typically be around 0.1 mg/m3; when compressed to 10 barg, this will increase to 1.0 mg/m3.
The same issue applies to moisture vapor suspended in atmospheric air. Clearly, this will vary considerably depending on climatic conditions in the region where the railway is operating. Typically, however, a cubic meter of air at 35 °C and at 60 % relative humidity will contain around 23 grams of water in the form of vapor. Raising the pressure to 7 bar will amplify the concentration of vapor by a factor of eight; each cubic meter of compressed air will therefore contain 184 grams of water vapor.
The process of compression will increase the temperature of the air. As the air then passes downstream, through pipework or to other devices, it will cool. This reduces its ability to hold the same volume of water, causing vapor to condense-out as liquid.
If the compressed air system is to function safely and efficiently, these contaminants need to be removed using a combination of filtration and, for moisture, compressed air dryers. Failure to do this effectively will lead to the formation of condensation in pipework, with subsequent risk of corrosion, formation of ice, deterioration of lubricating oils and, worst case, the failure of critical rail braking, door opening or suspension systems.
Compressed air-drying units for use on board trains generally remove water vapor using membrane, specialized absorbent media, or pressure-swing adsorption technologies. In each case, it is crucial to install dew-point sensors. As a minimum, these should be fitted to the dryer outlet to monitor dry air quality and, by association, the efficiency of the dryer itself. Further sensors can be added downstream of the dryer to protect the integrity of, for example, braking or door opening and closing mechanisms.
One of the principal advantages of installing active dew-point sensors is the ability to reduce the maintenance intervals and operating costs of compressed air equipment. Our range is the only rail-approved (EN50155 and 50121-3-2) moisture and dew-point sensors and transmitters being manufactured. Lightweight, compact, robust, and based on proven technologies, these devices are easy to install and maintain, yet offer outstanding accuracy and repeatability.
Active sensors enable switching between a traditional time-based maintenance program – where dryers, for example, are serviced at regular intervals regardless of their operating condition – and a preventative maintenance strategy. Using real-time operating data to identify equipment wear or faults at an early stage enables maintenance teams to assess the rate of change and plan work schedules for the most convenient time. This plays an important role in keeping rolling stock in service for as long as possible, without affecting passenger safety.
Our online SF82-Rail transmitter has a measuring range of -60…+60 °C dew point, is accurate to within ±2.0 °C dew point, and has customizable alarm outputs, a standard M12 connector and a rugged 316 stainless steel IP66 housing. The transmitter has EN50121 EMC/RFI approval, plus shock and vibration certification under IEC16373.
For spot-moisture measurement checks, the MDM50 is an easy-to-use, lightweight portable and fast-response hygrometer. Our railway safety and efficiency range extends to a choice of fixed humidity and temperature measurement probes for use in station and tunnel air-conditioning systems, and intelligent liquid level switches for water tanks.
To learn more, visit our rail industry page.
With almost 50 years’ experience in the development of innovative precision sensors, we are the application experts in dew-point and moisture measurements for all railway applications. If you would like to discuss your requirements, please contact us today.
Sign up to one of our Industry newsletters and you’ll receive our most-recent related news and insights all directly to your inbox!Sign Up