Pressurized enclosure

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The type of protection pressurized enclosure (Ex-p) (from p for pressure) enables non- explosion-protected devices to be operated in potentially explosive areas. The type of protection pressurized enclosure is based on the idea of ​​keeping explosive gas mixtures away from the non-Ex devices used.

How the pressurized enclosure works

The explosion protection is p-Ex in the protection realized by the non-explosion-proof devices in a pressurized housing (Ex-p housing) to be operated. This housing is protected from the ingress of potentially explosive atmospheres in the surrounding area by permanent overpressure with air or an inert gas . Depending on the particular application and existing explosion protection zone is in a start-up pre-rinsed the Ex-p housing of the device. This ensures that any ignitable gas / air mixture that may be present in the housing is removed. This process is known as pre-flushing.

Construction of pressurized enclosures

Modern pressurized encapsulation controls consist of an integrated control device, which contains all electronic and pneumatic components such as control core, pressure sensors, flow meter, spark arrester, outlet valve, etc. In addition, an adjustable throttle or a solenoid valve is used on the inlet side of the Ex-p housing for the purge gas supply. Both components can be mounted inside or outside the Ex-p housing. A distinction is made between the operating modes "constant purging", in which the Ex-p housing is permanently flowed through with an ignition protection gas, and "compensation of leakage losses", in which the outlet valve is closed after the purging phase and only as much purging gas is introduced into the housing that a minimum overpressure is maintained. Most applications are based on the operating mode "Compensating for leakage losses".

Compensation for leakage losses

After completion of the purging phase, an overpressure in the mBar range is maintained inside the Ex-p housing in order to guarantee protection against explosion. On the inlet side, digital solenoid valves (open / closed) as well as proportional valves can be used for the purge gas supply.

Digital valve technology

The digital valve used is opened during the purging phase. After successful pre-rinsing, the digital valve closes; Leakage losses are compensated for by a mechanically adjustable bypass in the valve. The pressure in the Ex-p housing is continuously monitored. If the pressure falls below a minimum or a maximum pressure is exceeded, the pressurized enclosure control switches off or issues an alarm message.

Systems with digital valve technology are typically characterized by an increased consumption of protective gas during and after the purging phase. In order to ensure sufficient operational reliability of the system, the amount of purge gas supplied must be significantly higher than the leak rate of the Ex-p housing. Medium that is not required is drained into the Ex area via the outlet valve in the control unit.

Proportional valve technology

By using a proportional valve, this waste of protective gas is largely avoided. The underlying principle is based on a proportionally working pressure and flow control, in connection with the proportional valve as an input-side actuator. Only as much flushing medium is introduced as is required by the leakage rate of the Ex-p housing. In modern Ex-p control systems, this pressure control within the Ex-p housing is also combined with an integration of the real purging gas flow during the pre-purging phase. This purge gas flow is recorded and integrated with the aid of a proportional flow measuring device. In this way, the system recognizes which flush volume has actually flowed through the outlet and can end the pre-flush phase immediately after the required pre-flush volume has been reached. With this method, a fixed pre-purging time is no longer required, which is started, for example, when a flow rate threshold value is exceeded, but remains constant regardless of the actual flow rate (Q> Q min.). The pressure control described in connection with the integrated pre-purging phase was developed and patented by Gönnheimer Elektronic in 1993.

The advantages of this technique are

  1. Significantly lower air and inert gas consumption.
  2. Significantly increased operational safety through constant internal pressure in the Ex-p housing (higher leakage rates, for example due to aging of the housing seals, do not lead to a sudden failure of the system).
  3. Minimized flow noise.

Another advantage of using proportional valve technology is that pressure control can also be used during pre-purging. As a result, there is always a defined overpressure in the Ex-p housing, which causes pressure-sensitive parts of the housing, such as B. protects windows, membrane keypads.

literature

  • Heinz M. Hiersig (Ed.): VDI-Lexikon Maschinenbau. VDI-Verlag GmbH, Düsseldorf 1995, ISBN 978-3-540-62133-1 .
  • Carsten Hilgers: Interfaces in process automation. Oldenbourg Industrieverlag, Munich 2003, ISBN 3-486-27040-0 .

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