GB2195744A

GB2195744A - Valves for venting gaseous and/or vaporous media

Info

Publication number: GB2195744A
Application number: GB08722269A
Authority: GB
Inventors: William Stephen Martin
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-09-23
Filing date: 1987-09-22
Publication date: 1988-04-13
Anticipated expiration: 2007-09-22
Also published as: GB2195744B; GB8722269D0; GB8622854D0

Abstract

To prevent flashback through a venting valve, e.g. in a liquid storage tank of a vessel from an ignited plume of gas exiting from the valve, two pivoted cowl plates (17a, 17b) are provided which substantially completely surround first valve flaps (16a, 16b) controlling egress of gas from a vent (12). The gap (X) between the cowl plates (17a, b) when flaps (16a, b) are closed, nowhere exceeds 1.5 mm and is preferably less than 1.0 mm. The gap (X) width is adjustable by screw members 19 threaded into cowl plates (17) and which support flaps (16) via a universal mounting (20). <IMAGE>

Description

SPECIFICATION Device for venting gaseous and/or vaporous media This invention relates to a venting device of the kind used for venting a gaseous and/or vaporous medium into the atmosphere. In particular, but not exclusively, the invention relates to a venting device for venting a gaseous and/or vaporous medium from a liquid container, for example a storage tank for liquid hydrocarbon material, which may be land-based or on a sea-going vessel.

It is expected that the primary use of the venting device will be for venting a storage tank on a sea-going vessel, for example an oil tanker, as it is being filled with a hydrocarbon liquid, but as the ensuing description makes clear other useful applications are envisaged.

Venting devices of the kind referred to have been described in my earlier British Patent Specifications Nos. 1 241 170, 1 575 618 and 2 164 421. These venting devices comprise a tubular framework defining a vent for flow therethrough of the medium being vented, and valve means associated with the vent to provide a vent orifice of variable cross-sectional area available for the flow of medium from the vent, said valve means comprising two flaps pivotally mounted on said framework for varying the cross-sectional area of the orifice in response to the volumetric rate of flow for the medium being vented, said cross-sectional area increasing with increasing flow rate of the medium and decreasing with decreasing flow rate of the medium, and biasing means associated with the flaps for urging them into a position in which they close said orifice.In order for such a venting device to open, a pressure difference has to be established on opposite sides of the vent orifice, and in the case of the venting device described in Specification No. 1 575 618 the vent orifice remains closed until this pressure difference amounts to at least 0.0175 kg/cm2 (0.25 p.s.i.).

It is customary to provide a liquid storage tank (land-based or sea-going) with a pressure relief valve to prevent overpressures arising in the tank. For example, in the operation of an oil tanker, a rise in pressure in the storage tanks can occur due to evolution of gas from the liquid in the tanks and from a rise in temperature of the liquid due to the tanker sailing from a cool climate to a warmer one. Typically, such a relief valve is set to open when the pressure within the storage tank rises to 0.14 kg/cm2 (2 p.s.i.) above atmospheric pressure but lower and higher opening pressures are used in practice.

The present invention aims to provide a venting device for a liquid storage tank which can also serve as a pressure relief valve for the tank and in particular to provide a venting device with improved means to prevent flashback into the tank from an ignited medium being vented.

According to-the invention, a venting device for venting a gaseous and/or vaporous medium into the atmosphere comprises a framework defining a vent for flow therethrough of the medium being vented, and inner valve means associated with the vent to provide a first vent orifice of variable cross-sectional area available for the flow of medium from the vent, said inner valve means comprising a flap pivotally mounted on said framework for varying the cross-sectional area of the first vent orifice in response to the volumetric rate of flow of the medium being vented, said crosssectional area increasing with increasing flow rate of the medium and decreasing with decreasing flow rate of the medium, biasing means associated with said flap for urging it into a position in which it closes said first vent orifice and prevents opening of said first vent orifice until a predetermined pressure difference is established on opposite sides of said flap, and outer valve means defining a second vent orifice of variable cross-sectional area downstream of the first vent orifice in the direction of medium leaving the tank, the second vent orifice having a minimum gap size (measured in the closing direction of the second vent orifice) when the first vent orifice is closed which minimum gap size does not exceed 1.5 mm.

Suitably the minimum gap size of the second vent orifice lies in the range 0.3 to 1.5 mm, a preferred minimum gap size being in the range of 0.5 to 1.0 mm.

Preferably the outer valve means defines a two-part cowl which substantially completely surrounds the inner valve means when the former is closed, so that the outer valve means can serve as a radiation shield for the inner valve means should the plume of exiting medium ignite. Such a radiation shield could protect gaskets used to seal the flap(s) of the inner valve means.

Suitably there are a pair of flaps defining the inner valve means and a two-part cowl defining the outer valve means, one flap of the inner valve means being movably mounted (e.g. via a universal joint connection) below each part of the cowl defining the outer valve means, each part of the cowl being mounted to turn about a respective axis, the two axes being parallel.

Suitably the inner and outer valve means are each biassed by gravity into the closed/minimum gap positions and closing weights can be affixed (e.g. to the outer valve means) to ensure an adequate biassing force towards the closed/minimum gap position. A biassing weight can be affixed to the or each flap constituting part of the inner valve means to ensure that the or each such flap closes first at its upstream end (relative to gas flow through the vent).

The invention is expected to find applications in the venting of land-based storage tanks (e.g. for inflammable liquids), tanker holds or other marine-based liquid storage vessels and waste gas flare-off pipes (e.g.

such as found at oil wells and chemical processing plants). A venting device according to the invention can allow both large and small volumes of gas to escape efficiently and safely.

One embodiment of venting device in according with the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a partially sectioned side elevation of the device, Figure 2 is a plan from above of the device of Figure 1 with part of the outer cowl cut away, and Figure 3 is an end elevational view of the device of Figures 1 and 2 to the right of the centre chain line, and a section with the outer cowl removed to the left of the centre line.

The device illustrated is designed to vent gas from a land-based storage tank from the bunker of a marine vessel (e.g. during filling with liquid) or from a waste gas discharge pipe. The device is designed to prevent an excessive pressure rise occurring in the tank or pipe connected thereto but to close off the vent if the excess pressure in the vent does not exceed a designed safe value (e.g. 0.07 to 0.14 kg/cm2-1 or 2 p.s.i.).

The device comprises a frame 10 consisting of an apertured base plate 11 with a rectangular upstand 12 which defines the vent. In the illustrated case the vent is divided in two by a central wall 13 to leave vents 14a and 1 4b on each side thereof. The upstand 12 and wall 13 define an accurately coplanar valve seat 15 for each vent, the plane of each seat 15 being inclined at an angle of some 35Q to the vertical.

A rectangular closure flap 16a, 16b is provided for each vent 14a, 14b,the mating surfaces between the flap and respective seat 15 being designed to provide a good gas seal when the flap is pressed against the seat. In the illustrated case this seal arises without the need for a gasket, but it is not ruled out that a resilient (and suitably refractory and/or corrosion-resistant) gasket be provided on the seat and/or flap.

Each flap 16a, 16b is movably supported above its respective seat 15 by being adjustably supported below a respective cowl plate 17a, 17b. The cowl plates together envelop the flaps 16a, 16b when the latter are in their vent-closing positions leaving only very narrow gaps 18 between the two cowl plates. The gaps (shown at "X" in Figure 1) provide the important flame-trapping property of the venting device and the gap widths should not significantly exceed 1.5 mm at any point along mined by screw members 19 which are threaded into the respective cowl plate 17a, 1 7b and which support the respective flap 16a, 16b via a universal mounting 20.

Each cowl plate 17a, 1 7b is turnably mounted on a shaft 21 journalled in lugs 22 forming part of the frame 10.

The side portions of the cowl plates overlap each other, each cowl plate having one large side portion 23 and one small side portion 24. The large side portion 23 has an edge shape representing the outline of the cowl in its closed position whereas the small side portion 24 has an arcuate edge 25 shaped to avoid fouling with the other cowl plate as the gap 18 increases in size on the venting of gaseous medium through the vents 14a, 14b.

To provide the desired gravity forces to bias the flaps into their closed positions extra weight 26 can be provided on each large side portion 23. Additional weight (shown dotted at 27) can also be added to each inner flap 16a, 16b to ensure these open first along the upper edge of the seat 15.

As the pressure in the vents 14a, 14b rises to exceed the designed opening pressure of the device, at least one of the flaps 16a, 16b will move off its seat 15 to allow venting of gaseous medium. In the event of a considerable flow of gaseous medium (such as occurs on filling a storage tank) the flaps could open to a significant extent so that they barely obstruct the vents 14a, 14b at all. If, under these conditions, the plume of gaseous medium leaving the vents were to ignite there could be a risk of the flame travelling back into the vents when the medium flow rate drops. The cowl plates 17a, 1 7b prevent this since the gap 18 will close off the path of the flame before it can retreat back down the burning plume and enter the cowl.

When medium is venting off at a significant rate, there is a tendency for air to be drawn up between the flap 16 a, 16b and its adjacent cowl plate 17a, 1 7b and these air flows will assist in cooling the flaps (and providing some thermal protection for any gasket on the flaps) in the event of the plume igniting. The cowl plates also act as radiation shields for the flaps when the plume is burning.

The devices described in my British patent specification 2 164 421 to temporarily hold the flaps in their closed position prior to opening on a pressure rise may be incorporated in a venting device according to this invention.

Claims

1. A venting device for venting a gaseous and/or vaporous medium into the atmosphere comprising a framework defining a vent for flow therethrough of the medium being vented, and inner valve means associated with the vent to provide a first vent orifice of variable cross-sectional area available for the flow of medium from the vent, said inner valve means comprising a flap pivotally mounted on said framework for varying the cross-sectional area of the first vent orifice in response to the volumetric rate of flow of the medium being vented, said cross-sectional area increasing with increasing flow rate of the medium and decreasing with decreasing flow rate of the medium, biasing means associated with said flap for urging it into a position in which it closes said first vent orifice and prevents opening of said first vent orifice until a predetermined pressure difference is established on opposite sides of said flap, and outer valve means defining a second vent orifice of variable cross-sectional area downstream of the first vent orifice in the direction of medium leaving the tank, the second vent orifice having a minimum gap size when the first vent orifice is closed, which minimum gap size does not exceed 1.5 mm.

2. A device as claimed in claim 1, in which the minimum gap size of the second vent orifice lies in the range 0.3 to 1.5 mm.

3. A device as claimed in claim 2, in which the minimum gap size is in the range of 0.5 to 1.0 mm.

4. A device as claimed in any preceding claim, in which the outer valve means defines a two-part cowl which substantially completely surrounds the inner valve means when the former is closed, the outer valve thereby serving as a radiation shield for the inner valve means should the plume of exiting medium ignite.

5. A device as claimed in any preceding claim, in which there are a pair of flaps defining the inner valve means and a two-part cowl defining the outer valve means, one flap of the inner valve means being movably mounted below each part of the cowl defining the outer valve means, each part of the cowl being mounted to turn about a respective axis, the two axes being parallel.

6. A device as claimed in claim 5, in which a universal joint connection is provided between a flap of the inner valve means and the respective part of the cowl.

7. A device as claimed in any preceding claim, in which the inner and outer valve means are each biassed by gravity into the closed/minimum gap positions.

8. A device as claimed in claim 7, in which closing weights are affixed to the outer valve means to ensure an adequate biassing force towards the closed/minimum gap position.

9. A device as claimed in claim 8, in which a biassing weight is affixed to the or each flap constituting part of the inner valve means to ensure that the or each such flap closes first at its upstream end.

10. A venting device substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

11. A storage tank for liquid hydrocarbon material which includes at least one venting device as claimed in any preceding claim.