GB2271439A - Gas pressurising and purging apparatus - Google Patents

Abstract

An apparatus for pressurising and purging with gas the interior of an enclosure comprises an electric control circuit (2) housed in a casing mounted in or on the enclosure and having an internal space (A) communicating freely with the enclosure interior, the control circuit (2) being connected to a remotely located control unit (1) by an electric cable with only two conductors (3A, 3B). The control circuit (2) includes a low pressure switch (5), high pressure switch (6) and a flow rate switch (7) for sensing flow of gas through an exit orifice leading from the internal space (A) of the casing, the switches being connected electrically in series and having respective resistors connected in parallel therewith whereby operation of the individual switches influences the level of current flowing in the circuit. The casing also houses a valve (37) for providing a low flow of gas under pressure to the enclosure, and a solenoid valve (8, 36) for providing a higher purging flow of gas to the enclosure, the solenoid (8) being connected in the control circuit (2). The control unit (1) includes a voltage control device (13) for setting the voltage applied to the control circuit (2), a current level discriminator (15) for sensing the current flowing in the circuit and providing outputs according to the prevailing condition of the pressure and flow rate switches (5, 6, 7), and a logic/timing device (18) for controlling the voltage control device (13) to increase the voltage to actuate the solenoid valve (8, 36) when a minimum pressure is sensed by the low pressure switch (5), to maintain the increased voltage until a predetermined purge flow rate detected by the flow rate switch (7) has been established for a certain period of time, and then to reduce the voltage again.

Description

GAS PRESSURISING AND PURGING APPARATUS This invention is concerned with gas pressurising and purging apparatus.

It is known to pressurise and purge with gas, generally but not always air, an enclosure housing equipment which requires prctection, such as electrical equipment installed in areas where there is a risk of an explosive atmosphere being ignited by an electric spark, or equipment which requires a stream of gas to be passed over it for cooling purposes and/or to dilute and flush away potentially hazardous gases or vapours which may be released from the equipment during use. The apparatus described herein is specifically adapted to protect electrical equipment and to enable automatic control of the connection of the equipment to the electrical power source.

The technique of pressurising and purging an equipment enclosure is well known. Air (or other inert gas) is passed through the enclosure at a comparatively high flow rate and for a time period of sufficient duration to purge the interior of the enclosure of any hazardous gases which may have collected in the enclosure during a period of non-use of the housed equipment in the enclosure. At completion of purging the air flow rate is substantially reduced, but a small air flow is maintained to keep the interior of the enclosure at a pressure a little higher than ambient atmospheric pressure, to prevent flammable gases passing into the enclosure from outside. Electrical power can then be connected safely to the electrical equipment.

A pneumatically operated pressurising and purging apparatus is disclosed in our UK Patent No.

2225457. The present invention is directed to a pressurising and purging apparatus with electrical control. With such an apparatus, the sensors, valves and/or other electrical devices mounted in the enclosure for control and operation of the apparatus must be connected by electric cables to a control unit often located several meters away. The need for several cables running over long distances is inconvenient and costly, especially if the cables require barrier devices providing galvanic isolation.

The present invention addresses this drawback of the prior art apparatus and broadly stated resides in an apparatus for pressurising and purging with gas the interior of an enclosure, comprising an electric control circuit for mounting in or on the enclosure and including means sensitive to the gas pressure in the enclosure and the rate of flow of gas therefrom through an exit orifice, a control unit to be located outside the enclosure and coupled to said control circuit by electric cable means including only two conductors.

Said control unit and control circuit are preferably arranged to control automatically a purge device to establish a purge flow through the enclosure when gas pressure in the enclosure reaches a predetermined level and to interrupt said purging flow after sufficient purging of the enclosure.

In a preferred embodiment, the control unit includes means for sensing the level of electric current flowing through the control circuit, the pressure and flow rate sensitive means being arranged to influence the current in accordance with the pressure and flow rate conditions sensed.

More specifically, the present invention provides an apparatus for pressurising and purging with the gas the interior of an enclosure, comprising a control system for controlling operation of purging means to establish a purging flow of gas through the enclosure, said control system including a control unit, and an electric circuit including means for sensing the gas pressure in the enclosure and influencing the current flowing through the circuit in accordance with the pressure sensed, and means for sensing rate of flow of gas from the enclosure through an exit orifice and influencing the current flow in the circuit in accordance with the flow rate sensed, the control unit being adapted to sense the current flowing in said circuit.

The control unit may control operation of a fan which produces the purge flow. Alternatively the control unit controls operation of a purge valve which is opened to establish the purge flow. In an especially convenient arrangement the purge valve is a solenoid valve, and the coil of the solenoid is connected in said electric circuit whereby the control unit is able to monitor the condition of the purge valve by the current level in said circuit, and the said two conductor cable can be used to deliver the electric energy to operate the solenoid valve.

The means for sensing the pressure may comprise high and low pressure means, e.g. high and low pressure switches which change condition upon a predetermined high pressure being exceeded and a predetermined low pressure being exceeded, respectively. The means for sensing the gas flow rate may also comprise a pressure switch for changing condition when the pressure differential across a gas orifice leading to the exit orifice exceeds a given level indicating the correct purge flow has been established. The flow switches may comprise only components suitable for intrinsically safe circuits.

In accordance with a second aspect the invention provides a combined sensor and valve unit for an apparatus for pressurising and purging an enclosure with a gas, the unit comprising a casing having an aperture for freely communicating the interior of the casing with the interior of the enclosure, an exit orifice for conducting gas from the casing to the exterior of the enclosure, preferably via a spark arrestor, a gas inlet for connection to a supply of gas under pressure, valve means mounted in the casing connected to the gas inlet and to gas outlet means communicating with the enclosure interior, the valve means being selectively operable for providing a relatively low flow of gas into the enclosure and providing a substantially higher purging flow of gas into the enclosure, and sensors mounted in the casing for sensing the gas pressure in the casing and sensing the rate of flow of gas from the interior of the casing through the exit orifice, and a cable entry in the casing for passage of an electric cable for connecting the sensors and valve means to a remotely located control unit.By having all the sensors, valves, etc. housed in a common casing, they can form a compact unit, and installation of the gas pressurising and purging apparatus is facilitated.

A full understanding of the invention in its different aspects will be gained from the following detailed description in which reference is made to the accompanying drawings, in which: Figure 1 is an electric circuit diagram illustrating the control system of an apparatus in accordance with the invention; Figure 2 is a schematic and plan view of a combined sensor and valve unit forming part of the apparatus illustrated in Fig. 1; Figure 3 shows the sensor/valve unit in elevation; Figure 4 is a schematic flow circuit illustrating the pneumatic circuit included in the sensor/valve unit; and Figure 5 shows schematically the pneumatic circuit associated with the pressure sensors incorporated in the sensor/valve unit of Figures 2 and 3.

Figure 1 illustrates a control system for a pressurising and purging apparatus. It includes two main parts, namely a control unit 1 incorporating those components to the left of the broken line in Fig. 1, and a sensor/valve unit 2 shown on the right of the broken line. These two units are interconnected by a single cable 3 including just two conductor wires 3A, 3B. The sensor/valve unit comprises an electric circuit wherein there are connected in series between the cable conductor 3A, 3B, a normally open low pressure switch 5 arranged to sense when the pressure in an enclosure being protected by the apparatus is above a predetermined low pressure level, a normally closed high pressure switch 6 arranged to sense the pressure in the enclosure and to open when the pressure exceeds a predetermined high pressure level, a resistor R2, and a normally open flow switch 7 arranged to sense the gas flow rate through the exit, e.g. by sensingthe pressure differential across an orifice leading to an exit from the enclosure, and to close when the flow rate is above a predetermined level. Connected in parallel with the resistor R2 and the flow switch 7 are a resistor R1 and the coil 8 of a solenoid valve.The solenoid valve is the purge valve of the pressurising and purging apparatus and is opened by energising the solenoid to establish a purging gas flow through the enclosure with which the sensor/valve unit is associated. The resistor R1 provides an additional current path through the circuit when the flow switch 7 is open, but it could be omitted and the coil of the solenoid valve relied upon to carry all the current in this circuit condition.

Connected in parallel with the low pressure switch 5 is a resistor Rp and connected in parallel with the high pressure switch is a resistor Rh. Either or indeed both of the resistors Rp and Rh could be omitted, i.e. have an infinite resistance value, but it is preferable that at least one of these resistors be present so that an open circuit condition (zero current) in the circuit indicates with certainty whether the pressure level is below the predetermined low level or above the predetermined high level. A light emitting diode is shown connected in series with the flow switch 7, but it is not essential.

The control unit 1 includes a limiting resistor RL connected in series with the cable conductor 3B to limit the maximum current through the circuit of the sensor/valve unit 2 in the event of a fault occurring in the unit 2 or in the interconnecting wiring. To safeguard further against excessively high current levels in the circuit a barrier device (10), such as a D.C. current transformer, is shown included in the exemplory embodiment. The input side of the barrier device 10 is connected by conductors 11, 12 to a voltage control device 13 which is connected to a D.C. supply. A current level sensing device 15 is connected to sense the current flowing in the conductor 12, and hence the current flowing through the cable conductors 3A, 3B and in the circuit of the sensor/valve unit.The device 15 discriminates between current levels and provides signals at a series of outputs 16A, 16B, 16C when certain current levels are detected. These outputs are connected to respective inputs 17A, 17B, 17C of a timing/logic device 18 having an output 19 connected to an input 20. The voltage control device 13 and to a control input 21 of the current level sensing device 15, and having several outputs connected to indicators 24 for indicating the condition of the pressurising and purging apparatus.

Before explaining the operation of the control circuit shown in Fig. 1, the sensor/valve unit will be described further with reference to Figs. 2-5. It comprises a casing 30 adapted to be mounted on a wall of the enclosure housing the equipment to be protected. An aperture 31 in the casing provides free communication between the interior of the casing, generally denoted by "(A)", and the interior of the enclosure. Partition walls within the casing separate from the space (A) an exit chamber 32 and a through flow chamber (B) which is connected to the space (A) through a control orifice 33 and is connected to the exit chamber 32 through an opening controlled by a spring-loaded valve 34. The chamber 33 communicates with the exterior of the enclosure through an exit orifice 35 which is fitted with a spark arresting mesh.

Mounted within the casing 30 are the solenoid valve 36 and a flow control valve 37. The valve has an inlet (E) connected to a gas inlet 38 of the casing which is connected to a pressurised supply (D) of inert gas such as air, and an outlet (F) connected to a gas outlet 39 of the casing for delivering gas into the interior (K) of the enclosure. The solenoid valve 36 also has an inlet (G) connected to the gas inlet 38, and an outlet (H) connected to the gas outlet 39. The flow control valve is operable manually by means of a member which protrudes from the casing 30 and is accessible from outside the enclosure.

When opened it provides a limited flow of gas from the supply to enter the enclosure. The solenoid valve 36 is normally closed but opens upon energisation of the solenoid to allow gas to enter the enclosure at a substantially higher flow rate for purging purposes.

Also mounted in the casing is a circuit board 40 carrying the pressure and flow rate switches and associated resistors depicted in Fig. 1. The cable conductors 3A, 3B are connected to respective terminals 41A, 41B also carried on the circuit board. Wiring between the circuit board and the coil of the circuit board has been omitted for sake of clarity. As illustrated in Fig. 4, each of the low and high pressure switches 5, 6 is a differential pressure device connected on one side to the interior (A) of the casing and on the other side to ambient atmosphere (C) outside the enclosure. The flow switch 7 is also a differential pressure device and is connected to the interior space (A) of the casing on one side and to the interior of chamber (B) on the other side.The orifice 33 is designed to produce a certain pressure drop in response to a minimum acceptable flow of gas through the orifice during purging of the enclosure, and the flow switch is operated when the pressure drop associated with this minimum flow rate is detected. The pipework associated with the valves, pressure switches and flow switch has not been illustrated in Figs. 2 and 3 in the interest of clarity.

Operation of the apparatus will now be described. When it is desired to put into operation electrical equipment housed in the enclosure, after ensuring all access doors and the like to the enclosure are properly closed, the control valve 37 is opened to allow gas to pass at a low flow rate into the interior of the enclosure. The inlet opening to the exit chamber 32 is closed by the spring loaded valve, and provided the access doors etc. have been closed the pressure in the enclosure interior (K), and hence in the interior space (A) of the casing gradually rises. Before the pressure rises high enough to open the valve 34, it reaches the level at which the low pressure switch 5 closes.The voltage control device 13 (Fig. 1) is initially controlled to deliver to the control circuit of the sensor/valve unit 2 a low level voltage, say about 5 volts, insufficient to actuate the solenoid coil 8 of valve 36. Until the switch 5 closes, therefore, the first current level, which is sensed by the device 15, is determined by the resistance values of the resistors RL, Rp and R1 and the solenoid coil 8. When switch 5 closes, resistor Rp is short circuited, and a second current level determined by resistors RL and R1 and the coil 8 flows in the circuit.

The device 15 senses the new current level and sends a signal from output 16A to the timing/logic device 18 which responds by emitting a signal to energise a "pressure OK" indicator 24A, and by emitting at outlet 19 a control signal which is fed to the voltage control device 13. On receipt of this control signal, the device 13 increases the voltage to a level, e.g. 16 to 20 volts sufficient to actuate the solenoid of the valve 36. Thus, the purge valve 36 is opened and gas is allowed to flow at a relatively high rate from the pressurised supply into the interior of the enclosure.

Of course, the increased supply voltage inevitably means a different, higher current level in the control circuit, but as the control signal from the timing/logic device is also fed to the current level sensing device, the latter is expecting and does not react to this increase.

As a result of the increase gas flow, the pressure rises further and forces open the valve 34 so that gas starts to leave the enclosure via the interior space (A) of the casing 1, the chamber (B)1 and the exit chamber 32. The rate at which gas flows into the chamber (B) is detected by the flow switch 7 and when the minimum purge flow rate is established this switch closes. As a consequence the current in the control circuit is modified and assumes a fourth level determined by the resistances of the resistors R, R1 and R2 and the coil 8. This new current level is detected by the device 15 which produces a signal at its output 16B. The timing/logic responds to this signal by emitting a signal to energise a "purge in progress" indicator 24B, and to commence timing.

After a certain time period programmed into the timing/logic device 18 and calculated to be the time for which gas must flow through the enclosure at the increased gas flow rate to ensure adequate purging, the device 18 interrupts the control signal supplied to the voltage control device 13 and to the current level sensing device 15, and the supply voltage is lowered again so that the solenoid is deactuated. The solenoid valve therefore closes, interrupting the purge flow into the enclosure, and hence the flow rate out through the exit orifice 35 diminishes so that the flow switch 7 opens and once again the current level in the control circuit is changed. This change in current level is detected by the device 15 and the signal from its output 16B is interrupted.At the end of the timed purging period the timing/logic device 18 also emits a signal to energise a "pressure OK and purge complete" indicator. It is now safe to start up the electrical equipment housed in the enclosure, and the timing/logic device 18 can be adapted to provide a signal to bring this about automatically. Gas continues to enter the enclosure at a low flow rate via the valve 37, and a positive pressure set by the valve 34 is maintained in the enclosure. The control system continues to monitor the pressure level. If at any time the pressure falls below a level set by the low pressure switch 5, e.g. due to an enclosure door being opened or the valve 37 being shut, the switch 5 opens and the resulting change in current level in the control circuit is detected by the device 15 which interrupts the signal at its output 16A.The timing/logic device responds by emitting an alarm signal e.g. to illuminate a "low pressure fault" indicator 24D and possibly shut down the electrical equipment if it is in operation.

During normal operation of the described apparatus, the high pressure switch 6 is not actuated.

This switch is included to provide an additional safety feature whereby excessive pressure in the enclosure is detected before it reaches a dangerous level, for example at which the enclosure is at risk of rupturing and suddenly exposing the enclosed equipment to a flammable atmosphere. In the event of a predetermined high pressure level being reached, the switch 6 opens, whereby the current level in the control circuit is changed by introduction into the circuit of the resistor Rh normally short-circuited by the switch 6. The change in current level is detected by the current level sensing device 15 which produces a signal at its output 16C. The timing/logic device 18 reacts to this signal by emitting an alarm signal e.g. to activate a "high pressure fault alarm" device 24E.If the high pressure fault occurs during purging, the device 18 may also shut off the solenoid purge valve by emitting an appropriate signal to the voltage control device 13.

It will be appreciated that the values of the resistors R1, R2, Rp1 Rh and E will be chosen, taking into account the resistance of the coil 8 so that unique current levels will be established for the different control circuit conditions as described above, so that the control unit can recognise these conditions and respond accordingly.

From the foregoing it will be seen that the apparatus of the invention exhibits all the usual operational features of pressurising and purging apparatus, yet does so by means of a control system with a single two conductor cable 3 connected between the control unit 1 and the sensor/valve unit 2. As a result, installation of the apparatus is simplified and costs are reduced. Installation is further facilitated by the gas valves, and the pressure and flow switches being incorporated in one compact unit mountable in or on the enclosure as a single item.

Claims (29)

CLAIMS:

1. An apparatus for pressurising and purging with gas the interior of an enclosure, comprising an electric control circuit for mounting in or on the enclosure and including means sensitive to the gas pressure in the enclosure and the rate of flow of gas from the enclosure through an exit orifice, and a control unit for location outside the enclosure remote from the control circuit, the control unit being coupled electrically to the control circuit by electric cable means including only two conductors.

2. An apparatus as claimed in claim 1, wherein the control circuit includes means for actuating a purge device to establish a purge flow through the enclosure, and the control unit is arranged to operate said actuating means to establish a purge flow when gas pressure rises to a predetermined level in the enclosure, and to interrupt said purge flow after sufficient purging of the enclosure.

3. An apparatus as claimed in claim 2, wherein the control unit is arranged to sense the level of the electric current flowing through the conductors of the cable means and to vary the current level to operate said means for actuating a purge device.

4. An apparatus as claimed in claim 3, wherein the control unit includes timing means for the purge device actuating means to be operated to interrupt the purge flow after the purge flow has been established for a predetermined period of time.

5. An apparatus as claimed in claim 4, wherein the means sensitive to the rate of flow of gas from the enclosure is adapted to activate to change the current level flowing through the conductors of the cable means in response to a predetermined gas flow rate through the exit orifice produced by actuation of the purge device, and the control unit is responsive to said current level change to initiate operation of the timing means.

6. An apparatus as claimed in claims 3, 4 or 5, wherein the purge device and the actuating means thereof comprise a solenoid valve.

7. An apparatus as claimed in any one of claims 1 to 6, whereinthe means sensitive to the gas pressure in the enclosure comprise a low pressure switch arranged to change condition when the pressure is above a predetermined minimum pressure.

8. An apparatus according to claim 7, wherein the means sensitive to the gas pressure in the enclosure comprises a high pressure switch connected in series with the low pressure switch and arranged to change condition when the pressure exceeds a predetermined maximum pressure.

9. An apparatus according to claim 8, wherein the low pressure switch has a normally open condition, the high pressure switch has a normally closed condition, and at least one of the pressure switches has an electrical resistance connected in parallel therewith.

10. An apparatus as claimed in any one of claims 7 to 9, wherein the means sensitive to the rate of flow of gas from the enclosure comprises a normally open flow switch sensitive to the pressure differential across a control orifice for gas exiting the enclosure, said flow switch being connected electrically in series with the low pressure switch and having an electrical resistance connected in parallel therewith.

11. An apparatus as claimed in any one of claims 1 to 10, wherein the control unit comprises a voltage control device for varying the voltage applied to the conductors of the cable means and to the control circuit, current level detecting means for sensing the current flowing in the conductors of the cable means and producing a first output in response to the current level changing upon the pressure sensitive means detecting a predetermined gas pressure in the enclosure, and a second output in response to a change in current level due to the means sensitive to the rate of flow of gas from the chamber detecting a predetermined gas flow through the exit orifice, the second output ceasing upon the current level changing due to the gas flow rate sensitive means detecting the gas flow through the exit orifice falling below the predetermined flow rate, and timing/logic means coupled to receive the outputs from the current level detecting means and coupled to provide control inputs to the voltage control device and the current level detecting means so that in response to the first output the voltage applied to the conductors is altered to initiate delivery of a purging gas flow into the enclosure, and after a predetermined time following receipt of the second output the voltage is changed once more to terminate delivery of the purging gas flow.

12. An apparatus for pressurising and purging with gas the interior of an enclosure, comprising a control system for controlling operation of purging means to establish a purging flow of gas through the enclosure, said control system including a control unit, and an electric circuit including means for sensing the gas pressure in the enclosure and influencing the current flowing through the circuit in accordance with the pressure sensed, and means for sensing the rate of flow of gas from the enclosure through an exit orifice and influencing the current flow in the circuit in accordance with the flow rate sensed, the control unit being adapted to sense the current flowing in said circuit.

13. An apparatus as claimed in claim 12, wherein the control circuit includes means for actuating a purge device to establish a purge flow through the enclosure, and the control unit is arranged to operate said actuating means to establish a purge flow when gas pressure rises to a predetermined level in the enclosure, and to interrupt said purge flow after sufficient purging of the enclosure.

14. An apparatus as claimed in claim 13, wherein the control unit is arranged to sense the level of the electric current flowing through the control circuit and to vary the current level to operate said means for actuating a purge device.

15. An apparatus as claimed in claim 14, wherein the control unit includes timing means for the purge device actuating means to be operated to interrupt the purge flow after the purge flow has been established for a predetermined period of time.

16. An apparatus as claimed in claim 15, wherein the means for sensing the rate of flow of gas from the enclosure is adapted to activate to change the current level flowing through the circuit in response to a predetermined gas flow rate through the exit orifice produced by actuation of the purge device, and the control unit is responsive to said current level change to initiate operation of the timing means.

17. An apparatus as claimed in claim 14, 15 or 16, wherein the purge device and the actuating means thereof comprise a solenoid valve.

18. An apparatus as claimed in any one of claims 12 to 17, wherein the means for sensing the gas pressure in the enclosure comprise a low pressure switch arranged to change condition when the pressure is above a predetermined minimum pressure.

19. An apparatus according to claim 18, wherein the means for sensing the gas pressure in the enclosure comprises a high pressure switch connected electrically in series with the low pressure switch and arranged to change condition when the pressure exceeds a predetermined maximum pressure.

20. An apparatus according to claim 19, wherein the low pressure switch has a normally open condition, the high pressure switch has a normally closed condition, and at least one of the pressure switches has an electrical resistance connected in parallel therewith.

21. An apparatus as claimed in any one of claims 18 to 20, wherein the means for sensing the rate of flow of gas from the enclosure comprises a normally open flow switch sensitive to the pressure differential across a control orifice for gas exiting the enclosure, said flow switch being connected electrically in series with the low pressure switch and having an electrical resistance connected in parallel therewith.

22. An apparatus as claimed in any one of claims 12 to 21, wherein the control unit comprises a voltage control device for varying the voltage applied to the control circuit, current level detecting means for sensing the current flowing in the control circuit and producing a first output in response to the current level changing upon the pressure sensing means detecting a predetermined gas pressure in the enclosure, and a second output in response to a change in current level due to the means for sensing the rate of flow of gas from the chamber detecting a predetermined gas flow rate through the exit orifice, the second output ceasing upon the current level changing due to the gas flow rate sensing means detecting the gas flow through the exit orifice falling below the predetermined flow rate, and timing/logic means coupled to receive the outputs from the current level detecting means and coupled to provide control inputs to the voltage control device and the current level detecting means so that in response to the first output the voltage applied to the control circuit is altered to initiate delivery of a purging gas flow into the enclosure, and after a predetermined time following receipt of the second output the voltage is changed once more to terminate delivery of the purging gas flow.

23. In or for an apparatus for pressurising and purging with gas the interior of an enclosure a combined sensor and valve unit comprising a casing having an aperture for freely communicating the interior of the casing with the interior of the enclosure, an exit orifice for conducting gas from the casing to the exterior of the enclosure, a gas inlet for connection to a supply of gas under pressure, valve means disposed in the casing and connected to the gas inlet and to gas outlet means communicating with the enclosure interior, the valve means being selectively operable for providing a relatively low flow of gas into the enclosure and providing a substantially higher purging flow of gas into the enclosure, and sensors mounted in the casing for sensing the gas pressure in the casing and sensing the rate of flow of gas from the interior of the casing through the exit orifice, and a cable entry in the casing for passage of electric cable conductors for connecting the sensors and valve means to a remotely located control unit.

24. A sensor and valve unit as claimed in claim 23, wherein the valve means comprises a solenoid valve operable to establish the purging gas flow.

25. A sensor and valve unit as claimed in claim 24, wherein the valve means includes a further valve for establishing the low flow of gas, the further valve and solenoid valve being connected in parallel between the gas inlet and gas outlet means.

26. A sensor and valve unit as claimed in claim 23, 24 or 25, wherein the casing has internal partitions separating a main space accommodating the valve means and sensor means, and an exit chamber, the main space and exit chamber communicating through an opening controlled by a normally closed pressure responsive valve adapted to open upon the gas pressure in the main space exceeding the pressure in the exit chamber by a predetermined amount.

27. A combined sensor and valve unit according to claim 26, wherein a through flow chamber is formed between the main space and the wall of the exit chamber having said opening, the through flow chamber communicating with the main space through a control orifice, and the sensor for sensing the rate of flow of gas from the casing interior being responsive to the pressure differential across the control orifice.

28. A sensor and valve unit as claimed in any one of claims 23 to 27, wherein the exit chamber communicates with ambient atmosphere through an exit orifice equipped with spark arresting means.

29. An apparatus for pressurising and purging with gas the interior of an enclosure substantially as herein described with reference to the accompanying drawings.