GB2480606A - Flame arrester - Google Patents

Abstract

flame arrester 10 comprises a vessel 12 defining an upper chamber 32 and a lower chamber 33 separated by a demister packing layer 30; a gas outlet 28 communicating with the upper chamber 32; and a multiplicity of inlet holes 36 for flow of gas in the vicinity of the base of the vessel 12. In use water 34 fills the lower chamber 32 to a level well above the inlet holes, so that all the in-flowing gas flows as bubbles through the water. This prevents flame propagation. If the number of holes, and the size of those holes, are selected in accordance with the flow rate of gas that is to be treated to ensure that the gas velocity through each hole 36 is greater than the flame velocity, this provides a further assurance against flame propagation.

Description

Flame Arrester The present invention relates to a flame arrester for preventing the propagation of flames along a duct that carries a flow of an inflammable gas mixture.

A variety of designs of flame arrester are known.

For example the gas may be caused to flow through a metal mesh or a metal honeycomb structure, which effectively conducts the heat away from any flame. However, where the gas mixture may contain droplets of liquid,, or indeed a vapour that can condense, then liquid can collect on the metal surfaces. This is a particular problem when the gas stream may contain tarry material, as this will restrict or block the flow paths, and is itself flammable.

According to the present invention there is provided a flame arrester comprising a vessel defining an upper chamber and a lower chamber separated by a demister packing layer; a gas outlet communicating with the upper chamber; and a multiplicity of inlet holes for flow of gas in the vicinity of the base of the vessel.

In use the vessel would be filled with water in the lower chamber, to a level well above the inlet holes, so that all the in-flowing gas flows as bubbles through the water. This prevents any possibility of flame propagation. Bubbling through the water also helps condense any remaining tar. The demister packing layer traps and removes any water droplets. It also provides a backup in that it would also prevent flame propagation.

Preferably the number of holes, and the size of those holes are selected in accordance with the flow rate of gas that is to be treated to ensure that the gas velocity through each hole is greater than the flame velocity. This provides a further assurance against flame propagation.

In a preferred embodiment the flame arrester also comprises at least one inlet duct in the vicinity of the base of the vessel, and the multiplicity of holes for through flow of gas are defined in the wall of the inlet duct or ducts.

Preferably the holes are on the underside of the inlet duct or ducts. Where there are a plurality of inlet ducts the flame arrester preferably comprises a gas inlet plenum that communicates with the inlet ducts.

If the water in the vessel were to leak out, the vessel would still be an effective flame arrester because of the speed at which the gas flows through the holes, and because of the provision of the demister packing layer.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings In which: Figure 1 shows a side view of view a flame arrester of the invention, partly broken away to show internal structure; and Figure 2 shows an end view of the flame arrester in the direction of arrow A of figure 1.

Referring to figure 1 a flame arrester 10 consists of a generally rectangular vessel 12 or tank, and at one end a rectangular inlet plenum 14 that communicates with four tubular inlet ducts 16 (see figure 2) . The ducts 16 extend the entire length of the vessel 12, each extending through the end wall and being closed and sealed by an end plate 18. In the side of the vessel 12 next to the bottom are two valves: an inlet valve 20 for introducing water into the vessel 12 and a drain valve 21 for emptying water from the vessel 12. A drain valve 22 is also provided at the bottom of the inlet plenum 14.

The top of the vessel 12 and the top of the inlet plenum 14 are both covered and sealed by a cover plate 24 in which are a gas inlet 26 communicating with the plenum 14, a gas outlet 28 communicating with the vessel 12, and an inspection hatch 29. At a distance of about 100 mm below the top of the vessel 12 is a demister packing 30 which extends the entire length of the vessel 12 and so subdivides it into an upper chamber 32 (which communicates with the gas outlet 28) and a lower chamber 33. The demister packing 30 may be of thickness between mm and 300 mm, for example 200 mm. The lower chamber 33 would, in use, be filled with water 34 to within about 50 mm above or below the underside of the demister packing 30, the vessel 12 being provided with a sight gauge 35 so that the water level can be checked.

In this example the vessel 12 is of length about 2.5 m and of width about 1 m. Along the length of each inlet duct 16 are equally spaced 12 mm holes 36, seventy-eight holes 36 along each side, at positions 45° above the centre of the underside of the duct 16 (the positions are indicated in figure 2) The number and the size of these holes 36 is selected in relation to the gas flow rate expected through the flame arrester 10 to ensure that the gas flow velocity through each hole 36 is greater than the flame propagation velocity in the gas. For example, for a stoichiometric methane/air mixture the flame velocity is about 39 cm/s, although the flame velocity is about 34 cm/s if the gas flow is humid. Hence if the gas flows in through the hole 36 at above 39 cm/s then (even in the absence of the water) a flame cannot propagate back through the holes 36.

The detailed structure of the demister packing 30 is not critical, although it may consist of a stainless steel wire knit-mesh pad, or alternatively a multiplicity of closely spaced metal plates, for example with gaps of 2-30 mm, preferably defining zigzag passages. Where the demister packing 30 consists of metal plates, then it will also inhibit flame propagation, at least if the plate spacing is less than 10 mm. As a gas flow carrying water droplets passes upwardly through the demister packing 30, the gas flow has to undergo changes of direction to flow past the items that form the demister packing 30, and the droplets, owing to their inertia, impact with the metal items and are removed from the gas flow. The collected water droplets fall down from the demister packing 30 back to the water 34.

Where gas is supplied from a number of different sources and so through a number of different gas flow ducts, these may all feed into the inlet plenum 14.

Alternatively, rather than being connected through an inlet plenum 14, each gas flow duct is instead connected to a separate inlet duct 16. As described above each inlet duct 16 is below the water in the vessel 12, so that the flame arrester 10 protects each of the different gas flow ducts.

Claims (7)

1. Claims 1. A flame arrester comprising a vessel defining an upper chamber and a lower chamber separated by a demister packing layer; a gas outlet communicating with the upper chamber; and a multiplicity of inlet holes for flow of gas in the vicinity of the base of the vessel.
2. 2. A flame arrester as claimed in claim 1 also comprising means to introduce liquid into the lower chamber, and means to drain liquid from the lower chamber.
3. 3. A flame arrester as claimed in claim 1 or claim 2 wherein the number of holes, and the size of those holes, are selected in accordance with the flow rate of gas that is to be treated to ensure that the gas velocity through each hole is greater than the flame velocity.
4. 4. A flame arrester as claimed in any one of the preceding claims also comprising at least one inlet duct in the vicinity of the base of the vessel, wherein the multiplicity of holes for through flow of gas are defined in the wall of the inlet duct or ducts.
5. 5. A flame arrester as claimed in claim 4 wherein the holes are on the underside of the inlet duct or ducts.
6. 6. A flame arrester as claimed in claim 4 or claim 5 wherein there are a plurality of inlet ducts, and the flame arrester also comprises a gas inlet plenum that communicates with the inlet ducts.
7. 7. A flame arrester substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.