Ex PCs / Workstations

Ex measuring- and control devices

Pressurized enclosure systems (Ex p)

F830 F840 F850-SYST F850S F850U F860S F870S OEM SR852 SR853

Ex p control unit FS850U - Description



The use of pressurized enclosures allows the operation of ‘non explosion protected’ devices in hazardous areas. The protection type "pressurization" is based on the principle of maintaining a constant overpressure using air or a protec-tive gas to prevent an explosive mixture forming near the apparatus inside the pressurized enclosure.

The use of pressurized enclosures allows the operation of ‘non explosion protected’ devices in hazardous areas inside division 1 or division 2 areas according to NEC 500. The protection type ‘pressurisation’ is based on the principle of maintaining a constant pressure using air or a protective gas to prevent an explosive mixture forming near the device inside the pressurized enclosure.

Before start-up, the pressurized enclosure must be purged with air or protective gas to remove any explosive mixture that may be inside the enclosure.

After this purging period the pressurized enclosure system has to monitor the pressure inside the cabinet. If this pressure drops below (at least) 0.8 mbar the control unit switches off the line voltage to secure the devices inside the cabinet.

Pressurized enclosure system F850U

The pressurized enclosure system F850U contains at least the control unit FS850U and an inlet nozzle or solenoid valve. The FS850U can be mounted in- or outside the enclosure. Furthermore, an operator panel is available for remote control. It is also possible to connect intrinsically safe peripherals (e.g. temperature, pressure sensors, switches) to the control unit FS850U.

The pressurized enclosure system F850U can be configured in different modes.

  • Only nozzle
    • Manual mode: Manual operated valve
    • Simple mode: Simple inlet nozzle

  • Using solenoid valve and nozzle in line
    • Automatic mode: D-solenoid valve
    • Pressure control mode: P-solenoid valve

The integration of proportional working valve technology makes this purge system unique:

  • Pressure regulated purging phase
  • Integration of flow during purging
  • PID controlled cabinet pressure during normal operation phase with minimum purging gas consumption

Leakage compensation

After purging, the control unit FS850U holds the pressure inside the enclosure on at least 0.8 mbar. For that two different solenoid vale techniques are available: digital working solenoid valve (DSV) technique or proportional working solenoid valve (PSV) technique.

Digital solenoid valve technique

While purging, the DSV is activated and a big amount of purge medium flows inside the enclosure through a nozzle with a large cross-section. After purging, the control unit turns off the DSV. The leakage compensation is made by a bypass choke, with a very small adjustable cross-section (diameter 0.3 ...1 mm), inside the valve. The protective medium that flows into the enclosure now is adequate to maintain a pressure of at least 0.8 mbar. The pressure is monitored by the control unit FS850U. The maximum and minimum pressure of the enclosure is programmable.

Purging period

For purging, a common and a new integrating method is available:

  1. Using the traditional method, purge quantity is a product of a pre-set minimum of flow rate and time. The flow rate depends on the size of the internal nozzle (diameter 1 ...6 mm) of the valve and can be specified by matched charts. The common rule of purging must be considered: flow minimum is less than let in minus leakage loss. This purging method is named as time based purging method.
  2. In contrast to the common one the integrating purging method measures the real volume flow through the enclosure outlet and adds it up to get the real purge volume. Besides, the flow rate is monitored, dependent on the size of the plate orifice of the control unit. If the flow rate sinks below its minimum, it will be ignored and it will not contribute to volume integration. Therefore we achieve a safe and economic purging method. See also Figure 1

flow wasting

Figure 1: consumption of protective gas

The digital solenoid valve technique has a considerable disadvantage: while purging process and normal operation, a constant rate of protective gas is needed. Of cause of safety the rate must be bigger than leakage rate of the enclosure. But wasting protective gas causes high costs in many applications.

Proportional solenoid valve technique

sing proportional solenoid valve technique stops wasting protective gas. The internal proportional working sensory equipment and a proportional valve as actuator are combined to a

pressure feedback control system.

Advantages of pressure feedback control are:

  1. Considerable less consumption of protective gas - additional costs for proportional valve will be amortised soon
  2. Increased service reliability achieved by constant pressure inside enclosure - increasing leakage caused by e.g. ageing of the enclosure will be balanced and sudden failure will be prevented
  3. Almost no flow noise and only a small protective gas consumption using a solid enclosure

pressure feedback control while purging

Another advantage using a proportional solenoid valve is; that pressure control is used even during purging. A set-point pressure will be achieved in the enclosure, while the flow volume, that leaves the enclosure, will be recorded and integrated through time, until the required purge volume is achieved. Advantages of this method are:

  1. A definite pressure while purging - pressure sensitive parts of the enclosure, like membrane switch panels or windows, will not be overloaded.
  2. Purge volume accuracy is achieved by integration of the purge medium flow volume on the outlet. Wasting purge medium is no more a topic of today.

Operation mode continuous flow

The control unit FS850U has moreover the operation mode "continuous flow" implemented. This operation mode is necessary, for example, if an analyzer could produce an explosive atmosphere inside the enclosure (containment system). The operation mode continuous flow purges the enclosure permanently. After the (pre-)purging procedure (purging process) a set-point flow rate is adjusted during normal operation. The monitored flow rate minimum is adjustable. The operation mode continuous flow can be realized using 2 digital solenoid valves as well as using one proportional solenoid valve.

Peripherals: Operation panels

For the control unit FS 850 several operating and visualizing panels are available. The panels have the explosion protection class intrinsically safe and are very advantageous, particularly when the control unit is mounted inside the enclosure.


Control panel BT851

1. Common operating panels: BT 814.1 and BT 815.1 with

  • On/Off-Switch
  • Key-operated switch for bypass
  • LED-indicator for READY and ON

2. Intelligent operating panel type BT 851

This operation panel indicates operation and malfunction reports as text messages. Using the 4 membrane switches offers total command of the control unit. Status, momentary pressure and flow rate as well as remaining purge time are always available.

Pressure loss alarm

This alarm means an addition pressure monitoring on a programmable level. the alarm is indicated on terminal 21/22 on the FS850S. If the pressure falls spontaneously below the level the EEx e relay on te 21/22 opens.

  last modified: 07.09.2010