Explosion-proof and economical

20 October 2008

Working with volatile liquids – as it is in every day business in the pharmaceutical and chemical industry – often implies potentially explosive atmospheres. Previously in such zone 1/21 environments control cabinets which combined electrical and pneumatic devices could not be used. Too high were the risks of corroding electrical parts which could subsequently cause an explosion.

Explosion-proof and economical

New technologies permit the combination of electrical and pneumatic devices even in zone 1/21 environments achieving several cost- saving possibilities at the same time such as requiring less wiring, less planning and less documentation.

Hybrid solutions combining electrical I/O-systems with optional pneumatic outputs are state of the art for installations in safe environments and in zone 2. “They are well accepted by OEMs and end users”, said Werner Bennek, Segment Application Specialist at Bürkert Fluid Control Systems. Forming nodes, those electro-pneumatic systems connect numerous different electric signals and pneumatic outputs via a bus interface with higher automation systems. However, assembling electrical and pneumatic devices together in a single control cabinet for installation in explosion-hazard environments of zone 1 remains a challenge. Bürkert took that challenge and developed a hybrid solution suitable for installation in zone 1 combining their modular pneumatic automation system AirLine Ex 8650 with modules from the Simatic ET 200iSP system from Siemens. With this solution Bürkert offers a complete ATEX certified unit.

“When we started the development of our new electro-pneumatic automation system we had the ambition to take all the advantages of our successful 8644 AirLine system and make it available for applications in hazardous environments”, explained Bennek. Until some years ago combining electrical and pneumatic devices in such zone 1/21 environments control cabinets was hazardous due to the wet and oily compressed air, which caused corrosion and short circuits. Today’s compressed air systems usually work with air qualities with a pressure dew point below -20°C and an oil concentration of less than 1 mg/m³.

Especially in the pharmaceutical as well as in the food and beverage industry one can expect better compressed air quality again by several orders of magnitude. The quality of the exhausted compressed air is in fact so good that it can be channelled into the control cabinet in order to avoid humid, aggressive or dust-containing ambient air entering the cabinet. A pressure release by a climate socket or an exhauster is of course necessary. Whilst 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 explosion-hazard areas of zone 1 the pneumatic control of cylinders and pneumatic actuators is generally done with either single explosion-proof valves, directly mounted to the actuator, or ex-proof valve banks wired to a Remote-IO-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 bus systems substantially reduces the wiring effort. Therefore engineers and users of ex-proof I/O-systems welcome integrated pneumatics for the following advantages: Close-to-the-process installation; significantly reduced wiring; compact design; simplified documentation; proven intrinsic safety; little power consumption and therefore only little waste heat and cost savings.
Bennek added: “The new ex-proof electro-pneumatic automation system offers another important advantage – as all elements, valves etc. are harmonised and certified. This means that users do not have to provide evidence for the safety of the system. We give them a certificate for the complete control cabinet so they don’t have to worry about the interior.”

On the contrary: they can be sure, that all devices and elements are consistent to EN 60079 part 0/1/7/11/26 and EN 60529:

At first sight IP54 only means “protected against dust and splashes of water”. But as a control cabinet is defined as a category 1 device according to EN 60079-0, the test for dust protection has to be conducted with a defined low-pressure for a certain time like it is usually done for IP6x units only. In the case of non-certified components, like bulkhead fittings which are used in the context of AirLine Ex 8650, such components have to pass an impact test with seven Joule as well as the IP54 test.

According to EN 60079-11 every intrinsically safe connecting part like clamps or plugs has to be explicitly labelled to differentiate them from non intrinsically safe circuits. Usually they are marked in a light blue.

While assembling strip terminals with parallel intrinsically safe and non-intrinsically safe circuits it is mandatory that uninsulated voltaged elements of the different circuits are kept apart at a minimal distance of 50 mm.

Should a cabinet’s built-in components be operated at higher ambient temperatures than which they are approved for, it is possible they could work for years without affecting their performance, but they are actually a safety risk with regard to explosion protection. Considering this for the construction of a control system, it means to attune the evolving power loss to the surface size of the housing, the maximum ambient temperature of the weakest component and to the maximum ambient temperature of the cabinet location. Of course after assembling the cabinet a temperature measurement close to the weakest component is a must.
Even when ex-systems only contain certified and audited components, the whole system has to be finally tested with regard to potential new sources of danger. For instance what happens to the IP-protection of the cabinet’s housing if a tube breaks inside? Or can the valves still be operated under vacuum conditions after the user has lead its tube into zone 0?

The above mentioned examples make it obvious: certified subsystems gain more and more importance, for they help engineers, contractors and users to concentrate on their main business by making them exempt from having to design and engineer control cabinets.

Being wholly certified Bürkert’s AirLine Ex 8650 unit is designed for decentralised operation and provides full support to this concept. It can be used in explosion-hazard areas with a gas or dust atmosphere and is most valuable for process control in fine chemicals, pharmaceuticals and cosmetics and anywhere else whereby solvents, alcohol or lacquers are utilized. It can even be installed for direct factory automation, e.g. automatic filling of solvents, alcohol or lacquers. In these application areas, the new system is unique so far as it is the only compact electro-pneumatic automation solution allowing integration of EExi pneumatic valves without additional wiring. Thanks to integrating the pneumatic functions, process and production sequences can now be completely decentralised in an explosive environment. Standardised interfaces and a highly flexible and modular design guarantee a fast and easy system-setup and allow pneumatic, electric and electronic modules with different functions to be combined with one another. The components are connected by screwing and/or latching thereby establishing both the electrical and pneumatic connections.

The pilot valves integrated in the pneumatic modules allow a wide range of different actuators to be controlled in the field, e.g. process valves or pneumatic cylinders. The explosion protection is 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 system solutions within explosion-hazard zone 1 area, including extensive maintenance and diagnostic functions, are completely achievable and open. The Simatic ET 200iSP communicates via Profibus DP fieldbus using IS couplers. Thus offering maximum openness and flexibility and guaranteeing high availability and investment security due to international standardisation of the bus protocol.

All electrical connections are configured automatically when assembling the module in line within the application, whilst the technical software has to be configured during the initial operation. Accessibility from the front allows far better handling, particularly when exchanging a module or a valve. This can be done easily at any time, even under explosion-hazard conditions and during operation. The whole interior system complies with protection class IP30. And by combining fieldbus, I/O and pilot valves its virtually unlimited modularity saves space, wiring and time during validation and documentation.
In Bürkerts Menden production unit construction of control systems and cabinet based system building has already been audited by the PTB and all control cabinets leaving the factory are approved by KEMA – so the new modular electric and pneumatic automation system is not just explosion-proof.