Aircraft refuelling is a demanding process when it comes to both efficiency and safety.
The explosive fuel mixture must be pumped into the aircraft tank quickly in order to prevent any disruptions to air traffic. To avoid hazardous situations and prevent operating errors during the refuelling process, explosion-protected control devices are installed and all safety-related functions are monitored by sensors. Slim Ex i switching repeaters with integrated line fault monitoring have proven to be particularly suitable for signal transmission in refuelling vehicles.

Isolators for galvanically separated signal transmission and conversion are used in the processing and manufacturing industry, as well as on ships and in special machines and vehicles; they ensure safe, reliable transmission of measuring signals to the control level. A particular challenge pose airfield refuelling vehicles that pump thousands of litres of kerosene into aircraft or helicopter fuel tanks in a short time. To monitor safety-related machine functions, ROHR Spezialfahrzeuge GmbH – a world-leading manufacturer of special vehicles and refuelling vehicles – equips its aircraft refuelling models with up to 24 sensors. The intrinsically safe transmission of sensor data to the control system is performed by extremely compact switching repeaters from the 9270 series manufactured by explosion protection specialist R. STAHL.

Air passenger numbers continue to climb

In 2018, air transport saw another global increase in passenger numbers compared to the previous year as the number of travellers rose by almost 7 percent to 4.4 billion. An average of 1400 flights take off or land at Germany’s largest airport, Frankfurt/Main, every day. Around 15 million litres of kerosene are pumped into aircraft tanks at this airport every day, in order to ensure that the aircraft are always ready to go. The tank volume of a commercial plane varies between 20,000 and over 300,000 litres, depending on the model. Flight times and turnarounds are so tightly scheduled that any malfunction in the refuelling systems can have significant consequences, bringing airline operation to an abrupt standstill for thousands of passenger and cargo planes. Fail-safe airfield refuelling requires compliance with strict safety provisions. The kerosene, which is pumped into the tank at a pressure of around 3.5 bar, poses a danger of ignition due to electrostatic charge or the formation of vapour, which can mix with air to form an explosive atmosphere. As a result, the machines must not be refuelled when the engine is running, in closed hangars or during storms. Additionally, the aircraft must be earthed and electrically connected to the refuelling system during the refuelling process. If refuelling is not carried out using a dispenser from the underground supply pipeline, airfield refuelling vehicles can be used to perform this task.

Robust pushbutton

At their Straubing-based factory, ROHR Spezialfahrzeuge GmbH–a subsidiary of Kässbohrer Transport Technik GmbH–produces around 40 refuelling vehicles and dispensers each year in a range of customer-specific variants. With delivery rates of 100 l/min to 4000 l/min, these vehicles are designed for efficiently refuelling a range of helicopters and aircrafts, including the largest Antonov AN-225 and Airbus A380 model series. The refuelling units, lifting platforms and safety-relevant functions of ROHR special vehicles are fitted with explosion-protected control devices and EMERGENCY STOP buttons from R. STAHL. Thanks to their compact dimensions, the installed pushbuttons from the 8040 series are optimally integrated into the control panel in order to control the pumping speed, lifting platform, hose reel and other hydraulic functions. The EMERGENCY STOP switch stops the refuelling process and switches the engine off. The weather- and temperature-resistant control devices in the robust GRP housing are designed to be modular and can be flexibly adapted to the required application using a range of needs-based actuators that can be combined. To provide lighting for their pump fittings, the special vehicle manufacturer ROHR has decided to use a compact version of R. STAHL’s LED tubular light fittings. The slim light fittings in the 6036 series are manufactured to comply with Ex d type of protection and designed to be long-lasting, impact-resistant and vibration-resistant–this makes them particularly low-maintenance and fail-safe.

Safety–monitored by sensors

The safety-relevant machine functions and statuses of the refuelling vehicles are monitored by sensors in accordance with the strict requirements for explosion protection and functional safety. Intrinsically safe sensors monitor the speed of the hydraulic pump. Two-stage overfill protection prevents the tank from being overfilled when reverse fuelling from the aircraft and filling via the refinery by switching off the pump and locking the bottom valve. The grounding clamp connection and valve positions are monitored, as are the retraction of the lifting platform and the position of the refuelling hoses and filling brackets once the aircraft has been refuelled. The values that are determined are transmitted to the machine control system and displayed for the driver to view. The control level automatically evaluates the signals in order to ensure that no damage or hazardous situations are generated as a result of operating errors. This means that no refuelling can take place if the fuelling system is connected incorrectly, and that the vehicle can only be moved if the lifting platform has been correctly retracted and the hose brackets and other equipment have been stowed away correctly.

Intrinsically safe signal transmission

ROHR uses the slim isolators from R. STAHL’s ISpac range to guarantee intrinsically safe signal transmission. The switching repeaters from the 9270 range are certified up to SIL 2; these conduct the sensor signals in a galvanically isolated manner via relays or electronic outputs to the machine control system. An additional contact detects any line faults that occur between the isolator and sensors and alerts the vehicle driver accordingly. No unnecessary delays are caused if these are accidentally triggered because the driver can immediately identify and react if a lock is incorrectly activated due to a wire breakage or short circuit. These systems also allow significant amounts of space to be saved: The ISpac modules, which are only 12.5 mm thick, require around 30% less installation space than conventional switching components. As a result, the switching repeaters can be installed in the switch housing behind the passenger seat in a way that optimises the use of space. If the switching layout is otherwise unchanged, this significantly improves the signal density and creates additional space for future functional expansions. Installation can be performed without the need for tools thanks to pac-Bus – this guarantees vibration-resistant installation, saves individual wiring steps and minimises maintenance work thanks to the use of collective error messages.

Slim series with long service life

Like the switching repeaters, almost every item in the ISpac isolator range from R. STAHL is available in a 12.5 mm version. Thanks to the slim isolating amplifiers, transmitter supply units and binary outputs, users can benefit from a range of incredibly flexible and reliable solutions for 1- and 2-channel signal transmission from field devices in hazardous areas at risk of explosions. Almost without exception, the ISpac modules that are certified according to ATEX and IECEx fulfil the functional safety criteria according to SIL 2 or SIL 3. Tested operating periods of 15 years ensure above-average availability over the long term as well as a high level of fail-safe reliability. What’s more, the modules are particularly energy-efficient and designed for use in a wide range of temperatures. They can be installed without the need for tools using pac-Bus or directly on the DIN rail. This allows a range of isolators, which are all compatible with conventional automation systems from leading manufacturers, to be combined regardless of the models and in a way that guarantees they will be future-proof.

In short

From lighting to operation to signal transmission, R. STAHL offers comprehensive system solutions for global use in industrial areas where explosion hazards are a fact of life. The explosion protection specialist’s product ranges are designed to be compact, long-lasting and robust, making them ideal for use in unique application areas such as aircraft refuelling. For this reason, special vehicle manufacturer ROHR places their trust in the compact control devices and maintenance-free LED tubular light fittings manufactured by R. STAHL, as well as their incredibly slim ISpac switching repeaters for intrinsically safe transmission of signal data to the machine control system. The long-lasting, SIL 2-certified switching repeaters in the 9270 range identify any incorrect fault messages in order to avoid any delays to precisely scheduled flight plans. Thanks to its incredibly slim 12.5 mm wide design increases the signal density and frees up more space for additional switching components.

■Contact: R. STAHL http://www.r-stahl.com