How to save energy in hazardous area control systems

With valves, pumps and fans used widely in food and beverage production, the possibilities for energy savings are considerable. Take compressed air, for example; its generation is equivalent to about 10 per cent of industry's total electricity usage, rising to 30 per cent in some sectors. However, one area of pneumatic energy saving potential is still largely unexploited: improved process valve control techniques. These can lead to reduced compressed air use through the adoption of on-actuator or in-actuator pneumatic solenoid valves.

At present, the majority of pneumatically piloted valves on production and process lines centralise pneumatic control around valve islands in a control cabinet or enclosure. This arrangement means that the pipework carrying the pilot pressure to the actual valve from the valve island can travel for many metres before it reaches the valve head. As a result, the venting cycle of the valves operation will exhaust more air than is necessary.

One solution to this problem is Burkert's Type 8690 EEx valve control heads. Developed for use in the food, beverage and pharmaceutical industries, these units provide a decentralised method of opening and closing of process valves automatically, thereby eliminating the need for the wasteful venting of control air. The Burkert control heads integrate electrical and pneumatic control components as well as position feedback units and, optionally, fieldbus interfaces for AS-Interface or DeviceNet. They are mounted directly above the valve body and, because there is little or no distance between the actuator and the valve that it is piloting, there is no air bleed.

With this system the pressure feed goes directly into the valve head and the control signal is supplied either from a local closed-loop control sensor or switch; or from a PLC or other machine controller via a field; or multipole (parallel) directly into the valve.

Digital positioners

Similarly, adopting digital positioners with integral solenoid valve control heads for regulating/modulating process control valves will automatically lead to air savings, as these will normally ensure zero air use in their stable state. The traditional technique with process valve positioners is to incorporate pneumatic flapper-nozzle systems, meaning that air is being bled constantly, even when the valve is at rest. This can consume the equivalent of a 0.75kW (or one horsepower) in compressed air for every twenty valves in operation; a large process site can therefore be using a vast amount of energy unnecessarily.

An embedded PID process controller, working in conjunction with the positioner, provides a fast-acting, decentralised control loop in combination with the associated process sensor. This arrangement also reduces complexity and saves unnecessary components and wiring.

An extension of this integration concept, which also delivers considerable savings in energy, is intelligent valve systems. In the field of process valves, conventional automation solutions - including control cabinets with valve terminals, I/O systems and fieldbus connections - require extensive and costly tube connections and wiring.

With this technology, the valves at field level are connected to the control unit in the control cabinet through a large number of extended pneumatic control lines and discrete feedback connections. In many cases, long control air lines increase the air consumption and have a negative effect on the switching times of the valves. In addition, apart from the high costs for planning and installation of such systems, there are also concerns in relation to hygiene. This is because every additional control air and feedback line within the production plant is a potential source of contamination and risk, and must therefore be monitored, serviced and cleaned regularly - which is a costly undertaking.

The use of innovative intelligent process valves eliminates these problems, as automation functions - pilot valves, electrical and optical position feedback and fieldbus interfaces - are integrated directly into the actuators on the process valves. This approach, based upon the principle of decentralised automation, minimises the number of cables and compressed air lines, thereby reducing air consumption considerably.

Ex-rated valve islands

Where intelligent valve systems are not an option, another alternative is ATEX Ex-rated valve islands. In zone 1/21 hazardous areas, the pneumatic control of cylinders and pneumatic 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 - although often not possible, as the large distances between valves and actuators lead to unacceptably long switching times.

However, connecting sensors and actuators to fieldbus 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, minimal power consumption and, therefore, significant cost savings.

In addition, new Ex-proof electro-pneumatic automation systems, such as Burkert's Type 8650, offer 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, being the only compact electro-pneumatic automation systems allowing integration of EExia pneumatic valves without additional wiring.

Standardised interfaces and a highly flexible and modular design guarantee a fast and easy system setup and enable 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 seamlessly combined 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 - with explosion protection being guaranteed through electronic limitation of voltage and current.