Electrical and pneumatic systems in hazardous areas

Until recently, combining electrical and pneumatic devices in control cabinets in zone 1/21 environments was impossible due to the wet and oily compressed air, which could cause corrosion and short circuits. Today, however, compressed air systems usually work with air qualities with a pressure dew point below -20degC (residual moisture of 0.88g/m3) and an oil concentration of less than 1mg/m3. Moreover, in the pharmaceutical and food and beverage sectors much better compressed air quality can be expected. Indeed, the quality of the exhausted compressed air is so good that it can be channelled into the control cabinet in order to prevent humidity or dust within the ambient air from entering the cabinet. With this arrangement a pressure release facility or exhauster is, of course, necessary.

While working with dry and oil-free air reduces the problem of corrosion and short circuits, there is still a detonation risk that remains due to the process conditions. In zone 1/21 hazardous areas, the pneumatic control of cylinders and actuators is generally achieved with either discrete explosion-proof valves directly mounted to the actuator or Ex-proof valve banks wired to a remote I/O system. Installing the pilot valves in a safe area is a further option - though this is often not possible, as the large distances between valves and actuators lead to unacceptably long switching times.

However, connecting sensors and actuators to bus systems substantially reduces the wiring effort. The use of advanced Profibus DPV1 communications relieves the control engineer of the task of installing the many special expensive electrical junctions and terminations required by conventional field wired systems in ATEX areas. It also offers additional major benefits, including a more compact control system, a more 'close-to-the-process' installation, simplified documentation, minimal power consumption and, therefore, significant cost savings.

Certified components

In addition, the new ex-proof electro-pneumatic automation system from Burkert offers another important advantage: all valves and other elements are harmonised and certified. This means that users do not have to provide evidence of the safety of the system. Instead, Burkert provides a certificate covering the complete control cabinet, enabling users to guarantee that all devices and elements are consistent to EN 60079 part 0/1/7/11/26 and EN 60529.

As Burkert's AirLINE Ex 8650 unit is fully certified, it can be used in explosion-hazard areas with gas or dust atmospheres. The system is particularly suited to decentralised process control tasks concerning fine chemicals, pharmaceuticals and cosmetics; and it can be employed anywhere where solvents, alcohol or lacquers are utilised. It can even be installed for direct factory automation - for example, for the automatic filling of solvents, alcohol or lacquers. In these application areas, the new system is claimed to be unique in that it is the only compact electro-pneumatic automation system allowing integration of EExia pneumatic valves without additional wiring.

Standardised interfaces and a highly flexible and modular design result in a fast and easy system-setup and allow pneumatic, electric and electronic modules with diverse functions to be combined with each other. The system allows up to 48 Burkert EExia solenoid valves to be combined seamlessly with Siemens ET200iSP EExia/ib electronic digital and analogue I/O modules in single distributed I/O assemblies. These offer extensive facilities for remote parameter setting and diagnostics, the ability to 'hot-swap' valves and electronic I/O modules under Ex- conditions, and the facility for easy system expansion at a later date. The pilot valves integrated in the pneumatic modules allow a wide range of different actuators to be controlled in the field (for example, process valves or pneumatic cylinders), explosion protection being guaranteed through electronic limitation of voltage and current.

In conjunction with the intrinsically safe Simatic ET 200iSP system, all options for combined electrical and pneumatic systems within explosion-hazard zones 1/21, including extensive maintenance and diagnostic functions, are completely achievable and open. The Simatic ET 200iSP communicates via Profibus DP using IS couplers, providing maximum openness and flexibility, and guaranteeing high availability and investment security due to international standardisation of the bus protocol.

At Burkert's Menden production unit, construction of 8650-based control systems and cabinet based system building has been audited by PTB, and all control cabinets leaving the factory are approved by KEMA.