GB2286240A - Glove box ventilation - Google Patents

Abstract

A glove box ventilation system, for use with one or more glove boxes (12) each having a casing (15) defining a working space (11) and a glove (14) secured to the casing (15) and protruding into the working space (11), includes; a header (30) connected to an extract fan (36), the connection (31) between the header (30) and the fan (36) being such as to provide substantially no pressure drop over the design working range and the fan (36) being designed to operate over a substantially flat part (32) of its characteristic, and means for connecting the header (30) to one or more glove boxes (12). <IMAGE>

Description

GLOVE BOX VENTILATION SYSTEMS The present invention relates to glove boxes ventilation systems.

Glove boxes are used in processes involving dangerous substances, and consist of closed boxes into the shells of which are built gloves which can be used by an operator to manipulate the contents of a box without danger of contact therewith.

The nature of glove boxes is such that there is always a danger of a breach caused by damage to a glove or by detachment of a glove from the shell. It is essential that provision be made to prevent the contents of a box leaking to atmosphere in the event of a breach. As part of this provision the space within a glove box is kept at a slightly reduced pressure relative to atmospheric. This is usually effected by a self acting control valve in a purging circuit, the space being kept at a depression of about 500Pa. However, the usual standard for such boxes requires that a minimum inlet containment velocity of 1 metre per second be established through a breach in the shortest possible time- conventionally accepted as two seconds- and this is beyond the capability of the purging circuit.The box is therefore kept permanently connected to a high pressure extract header through a valve which is normally kept closed. A pressure sensor is connected to the space and to valve operating means such that when there is a breach and the pressure within the space starts to rise towards atmospheric the rise is sensed and the valve opened to allow an airflow to be established through the breach to the header. The ducting (header and associated pipework) is usually sized to provide a duct velocity of 6 to 8 metres per second therethrough.

This arrangement is complicated and expensive. In order to ensure safety many components, such as the pressure sensor and valve operating means, have to be duplicated in order to ensure that a single failure does not result in a breach causing a leak to atmosphere. Also the components have to be regularly tested.

According to the present invention a glove box ventilation system, for use with one or more glove boxes each having a casing defining a working space and a glove secured to the casing and protruding into the working space, includes; a header connected to an extract fan, the connection between the header and the fan being such as to provide substantially no pressure drop over the design working range and the fan being designed to operate over a substantially flat part of its characteristic; and means for connecting the header to one or more glove boxes.

In one form of the invention the header is connected directly to the working space of a single glove box.

In another form of the invention the header has a connection to each one of a plurality of glove boxes, each connection consisting of a sub-header with a restrictor therein.

In operation the fan is operated to produce a pressure drop within the system such that there is a depression of, for example, 500 Pa within the or each working space. In the event of a breach the fact that the fan is operating over a flat part of its characteristic and that there is substantially no pressure drop in the ducting means that an inflow of the required velocity is quickly established. The use of restrictors in the connections to the working spaces when a plurality of glove boxes are connected to the header minimises the pressure rise in unbreached boxes when there is a breach in one of the boxes.

The fan may be a paddle type fan, which is well known to have a substantially flat operating characteristic, or may be a centrifugal fan having excess capacity such that over the the required operating range it uses a part of its operating characteristic which is substantially flat.

It will be realised that, compared with the conventional ventilation system, the present invention requires only a single connection for both operational and for emergency purging and that there is only one moving part, namely the fan. (In practice, of course, there will usually be a number of ganged fans so operated that a single fan failure will not result in failure of the system). It has been found in practice that using a system according to the invention, to the same standards of operational vacuum and breach purging flow as a conventional system, the breach purging flow is established within 0.2 seconds compared with 2 seconds for the conventional system.

A conventional ventilation system and a ventilation system according to the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, of which: Figure 1 shows a conventional ventilation system, Figure 2 shows a ventilation system according to the present invention in normal operational conditions, Figure 3 shows the system of Figure 2 operating in breach conditions, Figure 4 shows fan operating characteristics,and Figure 5 show a typical ventilation system according to the present invention as used in practice.

A conventional ventilation system (Figure 1) has a high pressure extract header 10 connected via filters (not shown) to a fan (not shown) which is constantly running. The header 10 is connected to a working space 11 of a glove box 12 via a valve 13. A glove 14 in a shell 15 of the glove box 12 projects into the working space 11. The working space 11 is also connected to a pressure switch 16 which is connected, via relays 17, to a solenoid pilot valve 18 in a compressed air supply 19 which is connected to the valve 13.

The working space 11 is also connected to a purging circuit having a supply 20 and an exhaust 21 which contains a pressure regulator 22.

In use the fan is operated to establish a potential pressure drop in the header 10 and the purging circuit is operated to establish a pressure drop in the working space 11. The pressure drop is set, usually at about 500 Pa, by the pressure regulator 22 and the purging gas is typically nitrogen.

In the event of a breach in the integrity of the shell 15, which might typically be caused by a rupture in the glove 14 or by detachment of the glove 14 from the shell 15, there will be an inflow of atmospheric air through the breach and the pressure within the working space 11 will start to rise towards atmospheric. The rise is sensed by the pressure switch 16 which operates the solenoid pilot valve 18 to connect the air supply 19 to open the valve 13. The header 10 is thus connected to the working space 11 and an inflow of air is established through the breach, drawing the contents of the working space 11 into the header 10 and hence through the filters.

Usual operating standards require the velocity of the air through the breach to be at least 1 metre per second, and it has been found that it typically takes 2 seconds for this velocity to be established.

A ventilation system according to the present invention (Figure 2) has a header 30 connected by a large diameter pipe 31 (of the order of double the diameter of the comparable pipe in a conventional system) to a filter system 32 downstream of which is a fan system 33.

The filter system 32 contains four filters 34 in two parallel branches 35 and the fan system 33 contains three fans 36 in parallel.

The fans 36 are either paddle type fans or are centrifugal or axial fans having capacity for flows much greater than they will be required to provide in practice. For example a fan designed to run over a region 50 of a characteristic 51 in Figure 4 will instead be run over a region 52 where the maximum capacity expected is significantly smaller than the normal operating capacity.

The header 30 is connected to a plurality of glove boxes 15 by sub-headers 38 in each of which is a restrictor 39. Each glove box 37 has a purging gas inlet 40.

In use two of the fans 36 are run at one time, each at 50% full power, to establish a purging gas flow (air or nitrogen) from the supply 40 through working spaces 11 in the glove boxes 12 at a pressure below atmospheric, nominally 500 Pa below atmospheric. To establish this each fan 36 will be operating at a point A of its operating curve 51.

In the event of a breach in one of the boxes 12, for example box 12a in Figure 3, the effect of operating the fans 36 over the regions 52 and of the oversized diameter of the pipe 31 such that there is substantially zero pressure drop therein is such that an increase of flow to a point B occurs automatically over a very short period of time so that an inflow through the breach at the required velocity of 1 metre per second is established, this typically occurring within about 0.2 seconds of the breach. The presence of the restrictors 39 in the sub-headers 38 to the un-breached glove boxes 12 ensures that the increase in pressure due to the increase in flow through the header 30 is within acceptable limits.

To allow for the inevitable increase in pressure in unbreached boxes when one box is breached the system may be designed to have a tolerance, for example of + 65Pa, so that under normal operation (purge condition) there will be a depression of 565Pa with a pressure rise in unbreached boxes to 435Pa in a breach condition.

The restrictors 39 will usually be locked, for example by bolts, into position after having their degree of restriction set during commissioning of a particular ventilation system.

In a typical system (Figure 5) three glove boxes 112a, 112b and 112c were attached to a single ventilation system. One, 112a, was used for processing swarf under a nitrogen atmosphere with a purge rate of two litres per second. Another, 112b, contained furnaces and was air purged at a rate of 47 litres a second. The third, 112c, contained a blender and was air purged at a rate of 1.4 litres per second.

It will be realised that many alternative forms of the present invention are possible. For example an ventilating system might have only a single glove box attached, in which case there will be no requirement for a restrictor 39.

With the system as described above with reference to Figures 2 and 3 the positioning of filters 34 in two branches 35 enables filters to be changed with ease. Also the arrangement with three fans 36, operated two at at time, gives a large margin of safety. If one fan fails, the other will be automatically switched to operate at 100% power until the third fan can be brought into play. The detective fan can be replaced without taking the system out of operation. In the unlikely even of there being two failed fans in the system at one time, of course, it will be expedient to close the system down until at least one can be replaced to restore a safety margin.

Claims (7)

1. What is claimed is; 1. A glove box ventilation system, for use with one or more glove boxes each having a casing defining a working space and a glove secured to the casing and protruding into the working space, including; a header connected to an extract fan, the connection between the header and the fan being such as to provide substantially no pressure drop over the design working range and the fan being designed to operate over a substantially flat part of its characteristic; and means for connecting the header to one or more glove boxes.
2. 2. A ventilation system as claimed in Claim 1 wherein the header is connected directly to the working space of a single glove box.
3. 3. A ventilation system as Claimed in Claim 1 wherein the header has a connection for each one of a plurality of glove boxes, each connection consisting of a sub-header with a restrictor therein.
4. 4. A ventilation system as claimed in any one of Claims 1 to 3 wherein the fan is a paddle fan.
5. 5. A ventilation system as claimed in any one of Claims 1 to 3 wherein the fan is an excess capacity fan acting over its low capacity range where its characteristic is substantially flat.
6. 6. A ventilation system substantially as herein described with reference to Figures 2 to 5 of the accompanying drawings.
7. 7. A ventilation system as claimed in any one of Claims 1 to 6 with at least one glove box attached thereto.