GB2215436A

GB2215436A - Pressure limitation method

Info

Publication number: GB2215436A
Application number: GB8804716A
Authority: GB
Inventors: Matthew Brady
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1988-02-29
Filing date: 1988-02-29
Publication date: 1989-09-20
Also published as: GB8804716D0

Abstract

The pressure in a glovebox 6 is maintained, for safety, between predetermined negative and positive limits relative to atmospheric pressure by means of a lute 1 comprising a closed vessel 2 having a tube 4 sealed through its upper region and extending downwards towards its lower region and having an open end thereat. The other end of the tube is connected to the glovebox and the upper region of the vessel is vented to atmosphere via an extract system. The vessel and tube are filled with liquid (13) to such a level, sealing the open end of the tube, that (a) at the said negative limit the liquid level in the vessel falls to unseal the tube and allow atmospheric air to enter the glovebox, and (b) at the said positive limit the liquid level in the tube falls to unseal the tube and allow gas from the glovebox to vent to atmosphere. A catch-pot 7 is provided between tube 4 and the glovebox to prevent any entrained liquid entering the glovebox, and similar means may also be provided to prevent any entrained liquid entering the extract system. <IMAGE>

Description

Pressure Limitation Method This invention relates to pressure limitation methods and in particular to methods for maintaining the gas pressure in a chamber between predetermined limits above and below a reference pressure, usually atmospheric pressure. It has one application where the chamber is a glovebox for handling dangerous materials, and where the pressure therein is normally controlled at below atmospheric pressure so that any leakage is inwards rather than outwards. In order to avoid damage to the glovebox, it is important in such applications that any malfunction or accident should not produce a chamber pressure relative to atmospheric pressure, ie a water-gauge pressure (wg), which exceeds predetermined negative and positive values, eg minus 20 cm (8 inches) nominal and plus 5 cm (2 inches) nominal.

According to the present invention a method of limiting the gas pressure in a chamber between predetermined negative and positive values relative to a reference value comprises: providing a lute including a closed vessel having a port located towards its upper end connected to a source of gas at the reference pressure and a tube sealed through the upper region of the vessel and extending downwards towards its lower region, the tube having an open end thereat; connecting the other end of said tube to said chamber;; filling the tube and vessel of the lute with a liquid to such a level above the open end of the tube, with equal gas pressures in both tube and vessel that, in relation to the ratio of the liquid surface area within the tube to the liquid surface area in the vessel outside the tube, (a) at said negative value the liquid in the vessel falls to a level which unseals the open end of the tube and allows gas from the reference source to enter said open end and thence enter the chamber, and (b) at said positive value the liquid in the tube falls to a level which unseals the open end of the tube and allows gas from the chamber to leave said open end and flow to the reference source; and providing means for trapping any liquid entrained with the gas in the tube under condition (a) above in order to prevent it entering the chamber.

The method may comprise connecting a catch-pot between said other end of the tube and the chamber to effect said liquid trapping. The trapped liquid may be caused to flow back into the vessel.

The chamber may be a glovebox and the reference gas source may be the atmosphere.

The invention will now be described, by way of example, with reference to the accompanying drawings wherein: Fig 1 is a diagrammatic vertical section of a lute showing liquid levels at the negative pressure limit.

Fig 2 shows the lute of Fig 1 with liquid levels at the positive pressure limit.

Fig 3 is a diagram showing the interconnection of the lute with a glovebox.

In Figs 1 and 2 a lute 1 comprises a closed cylindrical vessel 2 having a port 3 at its upper end. Extending downwards through the top of vessel 2 is an open-ended tube 4 reaching nearly to the bottom of the vessel. The vessel 2 and tube 4 are filled with a suitable liquid 13, eg water or oil, to equal levels 5 above the lower end of tube 4 with equal (atmospheric) pressure in both.

Fig 3 shows the lute 1 connected to a glovebox 6 via a catch-pot 7 which comprises a closed vessel having inlets 8, 9 at its upper end connected to the lute and glovebox respectively.

At its lower end a pipe 10 and valve 11 connect the catch-pot to the lower end of vessel 2 (this connection is not shown in Figs 1 and 2). The glovebox 6 has a vacuum connection 12 for maintaining its internal pressure at a controlled value below atmosphere. The conventional control means are not shown. The lute port 3 is connected to atmosphere via a conventional plant extract system which may include filters, scrubbers, etc.

In most modes of operation the glovebox 6 is normally controlled at a small negative pressure, typically minus 5 cm (2 inches) wg, via connection 12, and the liquid 13 is drawn partway up the tube 4 such that the difference in the liquid levels in the annulus between tube 4 and vessel 2, and in the tube itself will be 5 cm, assuming the liquid is water. Let it be assumed that, for safety, this negative pressure must not exceed minus 20 cm wg and a positive pressure must not exceed plus 5 cm wg. Then, referring to Fig 1, it is arranged that when the water level in the tube reaches a height 14 of 20 cm, the level in the annulus has dropped to the lower end of the tube, unsealing that end. Air is now drawn in via port 5, as shown by the arrows 16, to enter the glovebox and prevent any further reduction in pressure.The catch-pot 7 is included to trap any water entrained in the airflow and prevent it entering the glovebox. After atmospheric pressure conditions are restored throughout, the valve 11 is opened to return the trapped water to vessel 2.

Similarly, referring to Fig 2, it is arranged that when the water level in the annulus reaches a height 16 of 5 cm, the water level in the tube has dropped to the lower end thereof, thereby again unsealing that end. Air can now flow from the glovebox to atmosphere via port 5, as shown by the arrows 17, preventing any further rise in glovebox pressure.

The calculation of parameters for a lute system to meet a given requirement is best described by taking a particular example, suitably taking the pressure limits mentioned earlier, viz minus 20 cm wg and plus 5 cm wg, and assuming water (specific gravity = 1) is the liquid. Assuming also that a tube 4 of internal diameter 5 cm and external diameter 5.7 cm is to be used, then: At the -20 cm limit, the volume of water in tube 4 = nx 52 x 20 = 392 cc 4 At the + 5 cm limit, this volume of water is transferred to the annulus between tube and vessel.

Hence, if the internal diameter of the vessel is D cm, n (D2 -5.72) x 5 = 392 4 ie D2 ~ 5.72 = 392 x 4 = 100 5s D2 = 100 + 32.5 = 132.5 ie D = 11.5 cm The ratio of the negative and positive pressure limits can be shown to be the ratio of the liquid surface area within the tube to the surface area of the annulus. However this equality is only approximate in practice, because it neglects the wall thickness of the tube.

The height 5 to which the liquid must be filled above the open end of the tube, with atmospheric pressure in the tube and vessel, is given by (from Fig 1, and neglecting the wall thickness of the tube) 2 n x 11.6 x x h = 392, where his the height 5.

4 ie h = 392 x 4 = 3.8 cm iT x 132.5 If oil (specific gravity = 0.8) is used instead of water, h = 3.9 = 4.75 cm 0.8 and the negative and positive pressure limits remain unchanged.

A catch-pot 7 or equivalent trapping means may also be found desirable to prevent entrained liquid entering the extract system when excess positive pressure in the glovebox unseals the end of tube 2.

Although described with reference to a system where the glovebox is normally held slightly below atmospheric pressure, the invention is equally applicable where the glovebox is normally held slightly above atmospheric pressure.

In the described lute the tube and vessel are circular in cross-section and arranged coaxially, but these configurations are not essential.

Claims

1. A method of limiting the gas pressure in a chamber between predetermined negative and positive values relative to a reference value comprises: providing a lute including a closed vessel having a port located towards its upper end connected to a source of gas at the reference pressure and a tube sealed through the upper region of the vessel and extending downwards towards its lower region, the tube having an open end thereat; connecting the other end of said tube to said chamber;; filling the tube and vessel of the lute with a liquid to such a level above the open end of the tube, with equal gas pressures in both tube and vessel that, in relation to the ratio of the liquid surface area within the tube to the liquid surface area in the vessel outside the tube, (a) at said negative value the liquid in the vessel falls to a level which unseals the open end of the tube and allows gas from the reference source to enter said open end and thence enter the chamber, and (b) at said positive value the liquid in the tube falls to a level which unseals the open end of the tube and allows gas from the chamber to leave said open end and flow to the reference source; and providing means for trapping any liquid entrained with the gas in the tube under condition (a) above in order to prevent it entering the chamber.

2. A method as claimed in Claim 1 comprising connecting a catch-pot between said other end of the tube and the chamber to effect said liquid trapping.

3. A method as claimed in Claim 1 or Claim 2 wherein the trapped liquid can be caused to flow back into the vessel.

4. A method as claimed in any preceding claim wherein the chamber is a glovebox.

5. A method as claimed in any preceding claim wherein the reference gas source is the atmosphere.

6. A method of limiting the gas pressure in a chamber between predetermined negative and positive values relative to'a reference value substantially as hereinbefore described with reference to the accompanying drawings.