GB2292639A

GB2292639A - Instrument housing for use in hazardous areas

Info

Publication number: GB2292639A
Application number: GB9416567A
Authority: GB
Inventors: Michael Jahn
Original assignee: Weatherford Oil Tool GmbH
Current assignee: Weatherford Oil Tool GmbH
Priority date: 1994-08-16
Filing date: 1994-08-16
Publication date: 1996-02-28
Also published as: GB9416567D0

Abstract

An instrument housing (102) for use in hazardous areas comprises a container (103) which accommodates an instrument (101) and a cylinder (117) of a non-combustible gas. Before use, the container (103) is purged with non-combustible gas introduced through an inlet (115). After purging is complete the container (103) is maintained at a small super atmospheric pressure by allowing gas from the cylinder (117) to trickle into the container (103) through a restricted passageway in a three way valve (108). <IMAGE>

Description

Instrument Housing for use in Hazardous Areas This invention relates to an instrument housing for use in hazardous areas and to an instrument provided with such a housing.

In the oil and gas industry it is often desirable to operate electronic instruments in the vicinity of the wellhead. Since highly combustible vapours can be present in this area it is usual to take elaborate safety precautions to inhibit a spark in the instrument initiating a fire and/or explosion.

One solution to this problem is to place the instrument in an instrument housing and to pass a supply of non-combustible gas, for example nitrogen or clean air to the instrument housing via a hose to maintain it at a pressure slightly above atmospheric. This prevents the ingress of combustible vapour into the instrument housing.

The disadvantage with this arrangement is that the hose, which has to be trailed behind the instrument housing at all times, not only makes it difficult for the operator to manoeuvre the instrument housing but also provides an obstruction over which other drilling operatives can trip.

The problem posed was how to provide an instrument housing which can be operated in hazardous areas and which does not need to be connected to a hose at all times.

Our initial (unpublished) solution to this problem was to connect a small cylinder of non-combustible gas to a container adapted to hold an electronic instrument.

This solution was not acceptable since, prior to use, it was necessary to purge the container to ensure that no combustible vapour was present. The volume of gas required for purging was such that a large cylinder of compressed non-combustible gas was required which, in turn, made the whole apparatus unacceptably heavy and difficult to manoeuvre.

The present invention solves the problem by providing means which enable the container to be purged from an independent supply of non-combustible gas but maintained at the required pressure after purging by a cylinder connected to the container.

According to the present invention there is provided an instrument housing for use in hazardous areas, which instrument housing comprises a container, characterised in that said instrument housing further comprises a cylinder connected to said container for maintaining said container at super-atmospheric pressure when said instrument housing is in use, and means allowing said container to be purged by a separate source of non-combustible gas.

Preferably, said cylinder is mounted in said container although it could also be mounted on said container.

The cylinder may either be a high pressure (200 bar) cylinder or a low pressure (20 bar) cylinder. The high pressure cylinder is capable of storing a larger mass of non-combustible gas although it is significantly heavier than a low pressure cylinder of the same internal volume.

Advantageously, the means allowing the container to be purged comprises an inlet in the container.

The inlet is preferably provided with a non-return valve which is advantageously biased to a closed position.

Preferably, the container is provided with a pressure regulator which will open to allow purge gas to leave said container.

Advantageously, said inlet communicates with said cylinder so that said cylinder can be replenished via said inlet.

Preferably, said inlet and said cylinder are disposed upstream of a pressure reducing valve which is preferably arranged to limit the outlet pressure to from 2 to 6 bar g and, more preferably, to 4 bar g.

Advantageously, said instrument housing is provided with a valve which, in a first position, will allow sufficient flow of clean air to purge said container and which, in a second position, will permit only a comparatively small flow of clean air into said container.

The present invention also provides an instrument provided with an instrument housing in accordance with the invention.

For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a simplified perspective view of an instrument provided with a known instrument housing; and Figure 2 is a block view of an instrument provided with an instrument housing in accordance with the invention.

Referring to Figure 1 of the drawings there is shown an instrument which is generally identified by reference numeral 1. The instrument 1 includes an electronic circuit board which could potentially create a spark.

The instrument 1 is mounted in an instrument housing 2 which essentially comprises a container 3. The container 3 comprises a base 4 and a lid 5 which is secured to the base 4 by four screws 6.

A seal (not shown) is located between the lid 5 and the base 4 and renders the container 3 substantially (but not completely) leak proof.

A pressure regulator 10 is mounted on the base 4.

The pressure regulator 10 and the three-way valve 8 are connected to a separate and distinct control unit 11 by intrinsically safe control lines 12 and 13.

Finally, power is supplied to the control unit 11 via a power supply 20. An armoured power cable 14 also extends from the control unit 11 to the instrument 1.

It will be appreciated that this arrangement is difficult to transport and that the armoured power cable 14 and hose 7 both provide hazards for rig personnel.

In use, hose 7 is connected to a supply of clean air or nitrogen. The container 3 is then purged by pressing a button (not shown) on the control unit 11.

This opens three-way valve 8 to admit clean air to the container 3. The clean air leaves the container 3 via the pressure regulator 10.

After purging is complete (which typically takes from 2 to 7 minutes and which is pre-set by the operator) the control unit 11 automatically sends a signal to to rotate the three-way valve 8 to admit a trickle of clean air to the container 3. At the same time control unit 11 enables power to be supplied to the instrument 1 via the armoured power cable 14. Provided that the container is well sealed the pressure in the container 3 will remain above atmospheric at a pressure predetermined by the pressure regulator 10. By maintaining the pressure in the container 3 above atmospheric (typically at 20 mbar) the ingress of combustible gases into the instrument housing 2 is prevented.

Only after all the above procedures have been completed can the instrument 1 be activated.

If, for any reason, the pressure in the container 3 drops below a certain level a sensor in pressure regulator 10 sends a signal to control unit 11 on control line 12. The control unit 11 immediately moves three way valve 8 to its purge position to restore the desired internal pressure and disables the supply of power to the instrument 1 via the armoured power cable 14.

Referring now to Figure 2 there is shown an instrument provided with an instrument housing in accordance with the invention.

The instrument 101 is mounted in an instrument housing 102 comprising a container 103 having a base and a lid screwed thereto.

The container 103 is provided with an inlet 115 which can be connected to a supply of compressed air or nitrogen.

A non-return valve 116 is mounted downstream of the inlet 115 and upstream of a cylinder 117 and a pressure reducing valve 118 which is arranged to close when the downstream pressure is at 4 bar g.

A three way valve 108 is situated downstream of the first pressure reducing valve and is mounted in the container 3 alongside pressure regulator 110 and control unit 111.

Power is supplied to the control unit 111 and the instrument 101 from accumulator 120 via power cable 114.

Prior to use of the instrument 101, the instrument housing 102 is connected to a source of non-combustible gas via a hose connected to inlet 115. This operation is conveniently carried out at a safe location from a source of non-combustible gas which is typically at 200 bar g. Typically, the non-combustible gas comprises clean air which flows into the cylinder 117 until it is full. While the cylinder 117 is being filled a button (not shown) on the control unit 111 is pressed to open a valve to allow clean air to enter the container 103 via three way valve 108. The clean air purges the container 103 and leaves the container 103 via pressure regulator 110.

After purging is complete the three way valve 108 is moved into a second position in which only a trickle of clean air passes into the container 103. Simultaneously, the control unit 111 enables power for the instrument 101. The hose is disconnected from the inlet 115 and the instrument housing 103 is carried to the point of use unencumbered by the trailing hose and armoured power cable of the prior art. The instrument 101 can then be activated for use.

The container 103 is maintained at super atmospheric pressure until the cylinder 117 is nearly empty.

At this time the pressure in the container 103 falls below a predetermined limit. At this time a sensor in the pressure regulator 110 sends a signal to the control unit 111 which electrically isolates the accumulator 120 thus rendering the apparatus safe.

If desired, the cylinder 117 can be replenished at regular intervals, for example every 2 to 3 hours, by temporarily connecting the inlet 115 to a source of high pressure clean air to refill cylinder 117. Provided that such refilling is effected while the pressure in the container 103 remains above the pre-set minimum additional purging is not required.

Claims

1. An instrument housing (102) for use in hazardous areas, which instrument housing (102) comprises a container (103), characterised in that said instrument housing (102) further comprises a cylinder (117) connected to said container (103) for maintaining said container (103) at super-atmospheric pressure when said instrument housing (102) is in use, and means (115) allowing said container (103) to be purged by a separate source of non-combustible gas.

2. An instrument housing as claimed in Claim 1, characterised in that said cylinder (117) is mounted in said container (103).

3. An instrument housing as claimed in Claim 1 or 2, characterised in that said means (115) allowing the container (103) to be purged comprises an inlet (115) in the container (103).

4. An instrument housing as claimed in Claim 3, characterised in that said inlet (115) is provided with a non-return valve (116).

5. An instrument housing as claimed in Claim 3 or 4, characterised in that said container (103) is provided with a pressure regulator (110) which will open to allow purge gas to leave said container (103).

6. An instrument housing as claimed in Claim 3, 4 or 5 characterised in that said inlet (115) communicates with said cylinder (117) so that said cylinder (117) can be replenished via said inlet (115).

7. An instrument as claimed in Claim 3, 4, 5 or 6, characterised in that said inlet (115) and said cylinder (117) are disposed upstream of a pressure reducing valve (118).

8. An instrument housing as claimed in any preceding Claim characterised in that it is provided with a valve (108) which, in a first position, will allow sufficient flow of clean air to purge said container (103) and which, in a second position, will permit only a comparatively small flow of clean air into said container.

9. An instrument housing for use in hazardous areas substantially as hereinbefore described with reference to and as shown in Figure 2 of the accompanying drawings.

10. An instrument provided with an instrument housing as claimed in any previous claim.