GB2103926A - Tank ventilating method and apparatus - Google Patents

Abstract

Tanks can be gas-fed by means of a fan to force air into the tank and purge pipes (6) through which the gas/air mixture leaves the tank. The fan is a water powered axial turbine fan and the purge pipes have control means (7) therein to prevent flow of the gas/air mixture through the purge pipes (6) until the pressure in the tank is sufficient to cause flow through the purge pipes (6) at a speed of at least twenty metres per second thereby to prevent flame propagation down the purge pipes (6). The control means (7) comprises a butterfly valve plate (15) asymmetrically pivoted on an axle (14) and biased by a weight to a closed position. <IMAGE>

Description

SPECIFICATION Tank ventilating method and apparatus The invention relates to a tank ventilation method and apparatus.

When discharging inflammable substances such as petroleum products from tanks, there is the problem that as the level of the contents falls, air or other gas must be drawn into the tank to replace the volume previously occupied by the substance being discharged. Some oil tankers, particularly large ones, have inert gas installations whereby inert gas can be fed into the tanks as the cargo is discharged. Such installations are however expensive to instal and also to operate since, apart from the cost of the inert gas installation, the weight of the installation represents dead weight which reduces the amount of the cargo which can be carried. If inert gas systems are not installed then for safety reasons tanks which had previously contained petroleum products must be gas-freed.Regulations provide that, when gas-freeing tanks involves ejection of inflammable or toxic gases, the gases must be ejected at a required height, currently two metres, above the top wall of the tank, usually the deck of a ship, through purge pipes. For safety reasons, the inflammable gases must also be ejected through the purge pipes at an escape velocity such that a flame cannot propagate back down the column of escaping gas to cause combustion within the tank, which combustion could well take the form of an explosion.

According to one aspect of the invention, a method of gas-freeing a tank comprises impelling air into a tank and preventing escape of gas from the tank through a purge pipe until the pressure in the tank is such that the escape velocity of the gas through the purge pipe will be at least twenty metres per second.

Preferably air is impelled into the tank by a water driven axial flow fan mounted on the top wall of the tank, gas escapes through a generally upright purge pipe mounted on the top wall of the tank and an extendable pipe extends from the lower end of the purge pipe to a position adjacent the bottom wall of the tank whereby the escaping gas is drawn from the bottom of the tank.

According to another aspect of the invention, apparatus for gas-freeing a tank comprises an axial flow fan to impel air into the tank, the fan comprising stator blades and rotatable rotor blades, and a purge pipe to extend upwardly from an aperture in the top wall of the tank and having flow control means associated therewith.

Advantageously, the fan is driven by water flow by way of a water turbine or pelton wheel.

Preferably the fan is provided to impel air through one aperture in the upper wail of the tank and at a position or positions spaced therefrom, the purge pipe extends upwardly from the upper wall to a height of two metres above the upper wall of the tank. If desired, particularly where the gas to be removed from the tank is heavier than air, the purge pipe or pipes may include within the tank an extendable pipe to extend downwardly within the tank so that gases forced thereinto by the air impelled into the tank are drawn from lower portions of the tank rather than from the upper portion thereof.

The flow control means may be in the form of a pivotable valve member in the purge pipe. Thus the valve of which the pivotable valve member forms a part may be of the butterfly kind wherein a plate, of a shape corresponding to the section of the bore of the portion of the purge pipe in which it is mounted, is pivoted about an asymmetric pivot axis and is biased by a weight or by spring means to a position in which it prevents flow of gas through the purge pipe. When the pressure in the tank rises to a level such that it will cause gas to escape from the tank at an escape velocity of at least twenty metres a second, the pressure acting on the asymmetrically pivoted plate will overcome the closing bias to move the plate to a position such that it will allow escape of gas from the tank through the purge pipe.If for any reason the pressure in the tank falls below that which will result in an escape velocity of the gas through the purge pipe at a minimum rate of twenty metres per second, the closing bias will cause the plate to move towards its closed position thereby to restrict or prevent flow of gas through the purge pipe.

The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which: Figure 1 is a plan view of a tank to be ventilated by the method and apparatus according to the invention; Figure 2 is a corresponding elevation; Figure 3 is an elevation of a purge pipe of apparatus according to the invention; Figure 4 is a sectional plan view of the purge pipe of Figure 3 taken on line IV--IV of Figure 3; Figure 5 is a sectional view taken on line V-V of Figure 3; Figure 6 is a fragmentary sectional view taken on line VI--VI of Figure 4; Figure 7 is an elevation of an axial flow fan of apparatus according to the invention; Figure 8 is a similar view to Figure 7 but in section; and Figure 9 is a view taken in the direction of arrow IX of Figure 7.

Referring to the drawings, a tank 1 has six standard Butterworth openings 2a to 2f in its top wall 3, three on each side of a longitudinal centre line 4. A water driven axial flow fan 5 can be mounted to impel air through a middle one 2b of the openings on one side, thereby to act as a compressor raising the pressure in the tank 1, and two purge pipes 6 can be mounted at the openings 2dand 2f on the other side, the remaining openings 2a, 2c and 2e being closed off.

The tank 1 may have a height h of as much as fifteen metres and the fan 5 therefore should be so designed that it forces a column of air into the tank in a manner such that the column does not diverge unduly until it approaches the bottom of the tank. Alternatively or in addition, an extendable and/or flexible pipe (not shown) could be provided on the fan 5, to duct the air driven by the fan 5 right to the bottom of the tank, or on each purge pipe, so that an air/gas mixture within the tank is fed to the purge pipes from lower rather than upper portions of the tank.

The extendable and/or flexible pipe could be formed by a plurality of telescopic sections or by a peripherally corrugated pipe.

Each purge pipe 6 has a height h1 of at least two metres and contains a valve 7 of the butterfly kind.

With reference to Figures 3 to 6, each purge pipe 6 comprises a bottom casting 8, for example of resin coated aluminium, which mounts the valve 7 and an upper sleeve 9, for example of 1 8 swg galvanised steel or fibreglass tube, having its bottom end engaged in the casting 8 and having a handling ring 10 at a convenient height.

The casting 8 includes a flange 11 to rest on the edge of the top wall 12 (Figure 6) of a tank around a Butterworth opening therein, three cam locks 1 3 being provided for locking the purge pipe in position.

A chordal axle 14, off-set from a diameter.

mounts a valve plate 1 5 which, in the position shown. completely closes off the bore 8a of the casting 8 and thereby prevents gas flow therethrough. An external arm 1 6 on the axle 14 mounts a weight 17, preferably adjustable in position on the arm 16, to provide a bias to retain the valve plate 1 5 in the closed position.

As shown in Figure 6, the closed position is 5 off-set from the horizontal and in the closed position the plate 1 5 abuts a stop screw 1 8. Since the axle 14 is off-set from a diameter of the bore 8a of the casting 8, rise of pressure in the tank will cause a moment of rotation to be applied to the plate 1 5 tending to pivot it about the axle 1 4 and when such moment is sufficient to overcome the bias of the weight 17, the plate 15 will rotate through 800 to the open position shown in dotted lines in Figure 6 to allow gas to escape at a velocity of not less than twenty metres per second through the purge pipe.Since the weight 1 7 has a decreasing effect as the arm 1 6 moves away from the horizontal position, the valve plate will tend to snap open. Reduction of pressure in the tank due to failure or switching-off of the fan 5 will allow the plate to move back to the closed position, again, due to the increasing effect of the weight 1 7 as the arm 1 6 approaches the horizontal, with snap action.

Thus flame cannot propagate back down the flowing column of gas to cause combustion in the tank.

The arms 1 6 give a visual indication of the position of the valve plates 1 5 at any time.

The fan 5, shown in Figures 7 to 9, has a cylindrical casing 20 including a flange 21 such that the fan can be located over a Butterworth opening in similar manner to the purge pipes 6.

Handles and trunnions (not shown), to permit transportation on a trolley, are provided to facilitate location in position.

The top and bottom of the casing 20 are open and a central unit 22 is provided in a bore of the casing supported on spider webs 23 to form an annular air flow passage 24. A pelton wheel 25 in a water housing 26 is coupled by a shaft 27 to a bevel gear 28 meshed with a bevel gear 29 on a turbine shaft 30. A rotor 31 mounts rotor blades 32 and is secured on the lower end of the turbine shaft 30 and stator blades 33 extend between a lower central unit 34 and the casing 20. Water under pressure supplied to an inlet 35 of the water housing 26 is ejected through nozzles (not shown) to impinge on the buckets of the pelton wheel 25 and rotate it and thereby the rotor 31 to draw air through the annular flow passage 24 and force it into the tank. The water drains from the water housing 26 through an outlet 35.

Gas sensing means may be inserted into the tank 1 through one of the openings 2a, 2c and 2e not being used for purging to test the gas content of the tank. The fan 5 and purge pipes 6 can be changed over after a time, for example so that the fan 5 is at the opening 2e, the purge pipes 6 are at the openings 2a and 2c and the openings 2b, 2d and 2f are closed, or to other dispositions so that all parts of the tank 1 can be gas-freed.

Claims (11)

1. A method of gas-freeing a tank comprising impelling air into a tank and preventing escape of gas from the tank through a purge pipe until the pressure in the tank is such that the escape velocity of the gas through the purge pipe will be at least twenty metres per second.

2. A method according to claim 1, in which the air is impelled into the tanl < by a water driven axial flow fan mounted on the top wall of the tank, gas escapes through a generally upright purge pipe mounted on the top wall of the tank and an extendable pipe extends from the lower end of the purge pipe to a position adjacent the bottom wall of the tank whereby the escaping gas is drawn from the bottom of the tank.

3. Apparatus for gas-freeing a tank comprising an axial flow fan to impel air into the tank, the fan comprising stator blades and rotatable rotor blades. and a purge pipe to extend upwardly from an aperture in the top wall of the tank and having flow control means associated therewith.

4. Apparatus according to claim 3, in which the fan is driven by water flow by way of a water turbine or pelton wheel.

5. Apparatus according to claim 3 or claim 4, in which the purge pipe can extend upwardly from the top wall of the tank to a height of at least two metres above said top wall of the tank.

6. Apparatus according to any one of claims 3 to 5, including within the tank an extendable pipe to extend downwardly from the lower end of the purge pipe into the tank so that gases forced thereinto by the air impelled into the tank are drawn from lower portions of the tank rather than from upper portions thereof.

7. Apparatus according to any one of claims 3 to 6, in which the flow control means comprise a pivotable valve member in the purge pipe.

8. Apparatus according to claim 7, in which a valve of which the pivotable valve member forms a part is of the butterfly kind wherein a plate of a shape corresponding to the section of the bore of the portion of the purge pipe in which it is mounted is pivoted about an asymmetric pivot axis and is biased by a weight or by spring means to a position in which it prevents flow of gas through the purge pipe.

9. Apparatus according to claim 8, in which the bias in the tank is such that when the pressure in the tank rises to a level such that it will cause gas to escape from the tank at an escape velocity of at least twenty metres a second, the pressure acting on the asymmetrically pivoted plate will overcome the closing bias to move the plate to a position such that it will allow escape of gas from the tank through the purge pipe.

10. A method of gas-freeing a tank as claimed in claim 1 and substantially as hereinbefore described.

11. Apparatus for gas-freeing a tank substantially as hereinbefore described and illustrated with reference to the accompanying drawings.