GB2164421A - Device for venting gaseous and/or vaporous media - Google Patents

Abstract

A device for venting a gaseous and/or vaporous medium into the atmosphere comprises a tubular framework defining a vent 1a for flow therethrough, and valve means associated with the vent 1a to provide a vent orifice of variable cross-sectional area. The valve means comprises at least one flap 7b, 8b pivotally mounted on said framework for varying the volumetric rate of flow. Biassing means, such as the counterweight 38, is associated with the flap 7b, 8b for urging the latter into a position to close the vent orifice and prevent opening of the orifice until a predetermined first pressure differential is established on opposite sides of the flap 7b, 8b. Further means, for example magnets 45 and associated armatures 47, is associated with the flap 7b, 8b for holding the flap in the orifice closed position until a second and higher pressure difference is established on opposite sides of the flap 7b, 8b, this further means becoming inoperative when the vent orifice opens. <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 particuiar, 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. Since 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, the ensuing description will be mainly directed to such use.

Venting devices of the kind referred to have been described in my earlier British Patent Specifications Nos. 1 241 170 and 1 575 618. These venting devices comprising 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 mean 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 0.25 p.s.i. (i.e. 0.0175 kg/cm2). When the venting device is operating at maximum flow rate of the medium being vented during the filling of a liquid storage tank, the pressure drop at the vent orifice gives rise to an overpressure within the storage tank, which is considerably higher than the pressure at which the vent orifice opens.

It is customary to provide a liquid storage tank on a sea-going vessel 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 above atmospheric pressure.

From the point of view of cost and ease of maintenance, it would be very desirable to replace the venting device and the relief valve of such a sea-going vessel with a single device which served the purposes of both. However, if a venting device were to be provided which had a minimum opening pressure of 0.14 kg/cm2, in order to serve as a relief valve, it would not operate satisfactorily as a venting device. The reason for this is that, when employed as a venting device during filling of the liquid storage tank, an overpressure of 0.3 kg/cm2 or more could easily develop within the storage tank. Such a high pressure could easily damage internal bulkheads of the tank.

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 without causing damage to the tank.

According to the invention, a venting device for venting a gaseous and/or vaporous medium into the atmosphere comprises 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 at least one flap 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, biasing means associated with said flap for urging it into a position in which it closes said orifice and prevents opening of said orifice until a predetermined first pressure difference is established on opposite sides of said flap, and further means for holding said flap in the orifice closed position until a second and higher pressure difference is established on opposite sides of said flap, said further means becoming inoperative when the vent orifice opens.

Said further means for holding the flap in the orifice closed position may comprise one or more permanent magnets associated with the flap. The or each magnet would be arranged to exert its maximum attractive force on the flap when the latter is in its orifice closed position. When said second pressure difference is established on opposite sides of the flap, the attractive force of the magnet or magnets is overcome and the flap becomes free to open the orifice. Instead of using one or more permanent magnets, said further means for holding the flap in the orifice closed position may be an electromagnet. However, the use of an electromagnet for this purpose is not recommended in cases where the venting device is employed for venting inflammable media.

Instead of employing magnetic means for holding the flap in the orifice closed position, it is possible to employ purely mechanical means for this purpose. For example, in the case where the flap is biased into its closed position by a counterweight joined to the pivoted flap, it is possible to provide the counterweight with a movable member, for example a ball constrained to move along a track in the counterweight. This movable member would be arranged to augment the effect of the counterweight in the orifice closed position of the flap, whereas as soon as the flap moves away from its orifice closed position the movable member would move to a position in which it reduced the effect of the counterweight. Of course, an arrangement of this nature could not be employed in a situation where the venting device is liable to be tilted during use, as for example on a seagoing vessel.It could, however, be employed with venting devices installed on land-based liquid storage tanks.

Preferably, the venting device in accordance with the invention comprises two pivoted flaps which define the vent orifice, as in the case of the venting devices described in my aforesaid earlier British Patent Specifications.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a partly sectioned end view of a first embodiment of a venting device in accordance with the invention, Figure 2 is a plan of the venting device of Fig. 1, partially in section and with parts remoed to reveal the underlying structure, Figure 3 is a partly sectioned end view of part of a second embodiment of a venting device in accordance with the invention, and Figure 4 is a plan corresponding to Fig. 3.

Figs. 1 and 2 show the venting device which is illustrated in Figs. 4 and 5 of my British Patent Specification No. 1 575 618, and the same reference numerals are used in Figs. 1 and 2 as in Figs. 4 and 5 of the aforesaid Specification to designate the same items. The construction and mode of operation of this venting device will be clear from a reading of Specification No. 1 575 618 and therefore no further detailed description will be given here. It is sufficient to say that the device comprises a tubular vent 1 a of rectangular cross-section on which the two flaps 7b and 8b are pivotally mounted by means of respective arms 31, spindles 9a, 1 0a and bolts 37. The two flaps 7b and 8b are biased to the orifice closed position of the device by means of respective counterweights 38, 39.

The flaps 7b and 8b remain in the orifice closed position, shown in the Figures, until the pressure in the duct or vent 1 a of the medium it is required to vent is at least 0.0175 kg/cm2, and preferably at least 0.035 kg/cm2, higher than the pressure of the atmosphere bearing on the outwardly-facing surfaces of the flaps 7b and 8b.

The venting device shown in Figs. 1 and 2 differs from that shown in Figs. 4 and 5 of Specification No. 1 575 618 in that it is provided with further means for holding the flaps 7b and 8b in the-orifice closed position until the pressure inside the vent 1a a is about 0.14 kg/cm2 higher than the atmospheric pressure acting on the outwardly-facing surfaces of the flaps 7b and 8b.This further holding means takes the form of two permanent magnets 45 supported on brackets 46 secured to the end walls 4a and 5a of the duct 1a. These magnets are so positioned that, when the flaps 7b and 8b are in the orifice closed position, they attract and engage respective armatures in the form of plates 47 of ferromagnetic material secured to the counterweights 38, 39. The magnets 45, which may be mounted with a limited degree of freedom of movement on the brackets 46 so that they engage the armatures 47 accurately, have such a strength that, in combination with the counterweights 38, 39, an excess pressure of 0.14 kg/cm2 has to be established inside the duct la before the flaps 7b and 8b pivot away from the orifice closed position in the direction indicated by the arrows C.When the venting device is in this condition it serves as a pressure relief valve.

When liquid is introduced into the storage tank with which the venting device is connected, as soon as the aforementioned pressure difference of 0.14 kg/cm2 is established, the flaps 7b and 8b move away from their orifice closed positions and the magnets 45 no longer have any appreciable attractive force on the armatures 47 once a small air gap is established between each magnet and its associated aramture 47. This can occur when the two flaps have turned through a very small angle, for example 1 or 2 degrees, from their orifice closed positions. The flaps 7b and 8b can then provide a venting facility in exactly the same way as in the venting device of Figs. 4 and 5 of Specification No. 1 575 618.

In a modified form of the venting device shown in Figs. 1 and 2, one or more additional permanent magnets, similar to the magnets 45, is or are associated with each of the end walls 4a and 5a, each of these additional magnets being closer to the pivoting axis of its associated counterweight 38 or 39 than the magnets 45 shown in Figs. 1 and 2. A corresponding number of additional armatures 47 are also provided on the counterweights 38, 39. The strength of the magnets would be chosen so that the flaps 7b and 8b again commence to pivot away from their orifice closed positions as soon as the aforementioned pressure difference of 0.14 kg/cm2 is established. In this modified embodiment, the magnetic attractive force on the armatures 47 will then exist through a greater opening angle of the flaps 7b and 8b than in the case of the device shown in Figs. 1 and 2.In this way, it is possible to arrange for the magnets to have an appreciable attractive force on the counterweights 38 and 39 until the flaps 7b and 8b have moved through an angle of say 3 or 4 degrees, or more, from their orifice closed positions.

It will, of course, be appreciated that magnets similar to the magnets 45 could be provided in the venting devices shown in Figs. 1 to 3 of Specification No. 1 575 618 or in the venting devices shown in Specification No. 1 241 170 in order to convert these venting devices into combined venting devices and relief valves in accordance with the present invention.

In a preferred form of venting device in accordance with the invention, based on the construction shown in Figs. 1 and 2, the arms 31 on which the flaps 7b and 8b are pivotally mounted are replaced by rectangular plates having approximately the same width as the flaps. These plates define, with their associated flaps, channels through which a current of cooling air can flow in the event that the medium being vented ignites. These cooling channels help to cool the flaps 7b and 8b and decrease the risk of the gaskets 44 becoming damaged if the medium being vented ignites.

Figs. 3 and 4 show part of a modified form of the venting device of Figs. 1 and 2. These Figures show part of the counterweight 39 of Figs. 1 and 2 which, instead of the armature 47, is provided on one side with a hollow casing 50, the floor 51 of which is slightly inclined with respect to the lower edge 52 of the counterweight. Inside the casing 50 is a heavy steel ball 53. When the flaps 7b and 8b of the venting device are in the orifice closed position, the counterweight 39 is in the position shown in Fig. 3 and the ball 53 is in the position shown in full lines, where it augments the effect of the counterweight. When the flap 7b moves away from its orifice closed position (towards the position 7c shown in Fig. 1), the ball 53 rolls along the floor 51 of the casing to the position shown in dotted lines and designated by the numeral 53a. In this position of the ball, it reduces the effect of the counterweight. In this embodiment, the counterweight 38 of the device of Figs. 1 and 2 would also be provided with a casing 50 and ball 53. Of course, in this modified embodiment of the venting device the permanent magnets 45 of the device of Figs.

1 and 2 would not be provided.

Claims (5)

1. A venting device for venting a gaseous and/or vaporous medium into the atmosphere comprising 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 at least one flap pivotally mounted on said framework for varying the volumetric rate of flow for the medium being vented, said cross-sectional area increasing with decreasing flow rate of the medium, biasing means associated with said flap for urging it into a position in which it closes said orifice and prevents opening of said orifice until a predetermined first pressure differential is established on opposite sides of said flap, and further means for holding said flap in the orifice closed position until a second and higher pressure difference is established: on opposite sides of said flap, said further means becoming inoperative when the vent orifice opens.

2. A venting device according to claim 1, in which said further means for holding the flap in the orifice closed position comprises one or more magnets associated with the flap.

3. A venting device according to claim 3, in which the or each magnet is a permanent magnet.

4. A venting device according to any one of the preceding claims, in which there are two of said flaps pivotally mounted on said framework, each of said flaps being influenced by said biasing means and by said further means.

5. A venting device for venting a gaseous and/or vaporous medium into the atmosphere, constructed and arranged substantially as herein described with reference to Figs. 1 and 2 or Figs. 3 and 4 of the accompanying drawings.