GB2055771A - Draining and refilling system for steel pickling tanks - Google Patents

Abstract

The invention relates to a system for and a method of draining and refilling a tank such as an acid- containing steel pickling tank. A safe system is provided in which the tank and a pressurised storage vessel are isolated from one another during normal operation by a syphon tube. As shown a pickling tank T is connected via a syphon pipe 6 to a storage vessel 1. The syphon pipe 6 may be evacuated by an ejector device 11, or connected to atmosphere. The storage vessel may be pressurised from compressor 25 via an air receiver 24 and secondary receiver 16 which also provides pressure for the ejector device 11. To empty tank T syphon pipe 6 is evacuated via ejector device 11, vessel 1 is pressurised so that acid flows up pipe 6 into tank T until an "extra-high" level is reached. The ejector device is then cut off and the vessel 1 connected to atmosphere so that acid flows from tank T into vessel 10 until tank T is empty. Air then enters syphon pipe 6 which isolates tank T from vessel 1. <IMAGE>

Description

SPECIFICATION Draining and refilling system for steel pickling tanks.

This invention relates to a draining and re-filling system for steel pickling tanks.

Due to the corrosive nature of the pickling liquor, i.e. acid, and the abrasive nature of the steel strip treated in pickling tanks, such tanks are of steel and are first lined with rubber to resist corrosion and are then lined with bricks to protect the rubber lining.

In the event of any problems arising, for example strip breakage within the tank, it is necessary to drain the tank as quickly as possible in orderto restrict down time of the whole installation, or in the case of tank or other leakage, to avoid spillage of the highly dangerous liquor.

The necessary drainage of the tank could of course be effected by fitting a simple drainage connection to the bottom of the tank. This however, is not satisfactory because, in the event of failure due to corrosion or human error, the tank could be drained accidentally. In addition, dangerous leaks could result from corrosion of such a bottom drain.

Furthermore, the use of a bottom drain would require a hole to be made in the tank and the tank linings and sealing problems may result therefrom.

It is among the objects of the present invention to provide a draning and re-filling system for pickling tanks which avoids, or substantially reduces, the aforementioned problems.

It is further among the objects of the present invention to provide a draining and refilling system for pickling tanks which, after manual initiation, is completely automatic and therefore provides maximum safety and avoids the possibility of human error.

According to the present invention, there is provided a draining and refilling system for a pickling tank, which comprises a storage vessel positioned below the level of the pickling tank, a siphon pipe extending vertically between the storage vessel and the pickling tank, and ejector device for evacuating the air from the siphon tube during a draining operation, a compressor unit providing air under pressure to the ejector device and to the storage vessel, and automatically operated pressure control valves for controlling the movement of pressurised air through the system.

Preferably, the compressor unit includes a compressed air receiver, a secondary receiver being provided to receive air therefrom and serving to smooth out the air flow, and to prevent corrosive vapour from entering the compressor unit.

The invention is illustrated by way of example in the accompanying drawing which shows a schematic layout of a draining and re-filling system according to the invention. In this respect, the drawing shows two in-line pickling tanks but, since the system is the same for each tank, details relating to the second tank will be omitted from the description.

Referring to the drawing, there is shown a first pickling tank T and a second pickling tank T' . Each tank has associated therewith a closed storage vessel 1 which is sized to receive an amount of acid in excess of that to be held in the tank T. The vessel 1,which is positioned below the level of the tan k T, is provided with a level switch 2 indicating "high" and "low" levels, a pressure indicator and relief valve 3, and a fill and drain circuit 4. The level switch 2 is connected to a control panel 5.

A siphon pipe 6 extends from the bottom of the storage vessel 1 and upwardly, the free end thereof, which is provided with a vortex breaker 7, being bent over the wall of the tank T. The highest point of the siphon tube 6 is connected, via a suction pipe 8 having a level switch 9 and an automatic pressure control valve 10, to an ejection device 11. The free end of an ejector device discharge pipe 12, which is fitted with a baffle box 13, extends downwards towards the tank T. A compressed air line 14 incorporating an automatic pressure control valve 15 extends between the ejector device 11 and a secondary air receiver 16, the valve 15 being connected to the control panel 5.

Afurther compressed air line 17 extends between the top of the storage vessel 1 and the secondary air receiver 16 and incorporates a pair of spaced-apart automatic pressure control valves 18 and 19 which are also connected to the control panel 5. From a position in the line 17 between the valves 18 and 19 an air vent line 20 extends to atmosphere, the line 20 incorporating an automatic pressure control valve 21 which is connected to the control panel 5.

The secondary air receiver 16, which is provided with a pressure indicator and relief valve 22, is in communication, via line 23, with a primary air receiver 24 forming part of an electrically driven compressor unit 25. The unit 25 is also provided with pressure switching means which ensures an adequate supply of compressed air when required by starting/stopping the compressor unit according to the pressure inside the receiver 24. It will be appreciated that, by allowing the compressed air to pass from the receiver 24, via a non-return valve to the secondary receiver 16, the air flow tends to be smoothed out, and corrosive vapour is prevented from entering the compression plant.

A level switch 26 is also associated with the tank T and gives a "low", "high" and "high-high" indication of the level of the acid in the tank T. The switch 26 is also connected to the control panel 5.

The reference numerals 14', 15' and 17' indicate part of a duplicate system which serves the tank T'.

In this respect, it will be obvious that the second tank T' may be dispensed with.

In normal operation of an installation incorporating the system of the present invention, the tank T is filled with acid to normal working level corresponding to "high" level on the level switch 26; the storage vessel 1 contains acid up to a level below the "high" level on level switch 2 indicating that sufficient space is available in the vessel 1 to accept the contents of the tank T should it be required to be emptied; the siphon pipe 6 is empty of acid; and the automatic pressure control valves are positioned as follows; valve 18 is closed, valve 19 is closed, valve 21 is open, valve 10 is open and valve 15 is closed.

When it is required to transfer acid from the tank T to the storage vessel 1, an EMPTY push-button on the control panel 5 is depressed to initiate the operation. This effects, automatically, a sequence of operations as follows:- valve 15 is opened to provide motive air to the ejector device 11 and thereby cause evacuation of the siphon pipe 6; valve 21 is closed and valves 18 and 19 are opened thereby admitting compressed air to the storage vessel 1 so that acid is forced up the siphon pipe 6 into the tankT; when the acid in the suction pipe 8 of the ejector device 11 reaches level switch 9, the valve 10 is closed and the siphon pipe 6 is then full of acid; acid continues to flow into the tank T until the "high-high" level is reached on level switch 26.

When the "high-high" level is reached on level switch 26, valves 19 and 15 are closed thus cutting off the air supply to the vessel 1 and the ejector device 11; valve 21 is opened allowing air in the vessel 1 to escape to atmosphere; and the acid in the tank T is siphoned into the vessel 1 until the siphon is broken by air entering the pipe 6; the acid in the siphon pipe 8 then flows away causing the level switch 9 to open the valve 10.

At the end of the EMPTY operation, the automatic pressure control valves are positioned as follows:valve 18 is open, valve 19 is closed, valve 21 is open, valve 10 is open and valve 15 is closed. The valves will remain in this position until a tank fill operation is carried out.

In order to transfer acid from the storage vessel 1 to the tank T, a FILL push-button on the control panel 5 is depressed to initiate the operation. This effects, automatically, a sequence of operations as follows:valve 21 is closed and valve 19 is opened thereby admitting compressed air to the storage vessel 1 and causing acid to be pumped up the siphon pipe 6 into the tank T; and acid continues to flow into the tank T until the "high" level on level switch 26 is reached.

When the "high" level is reached, valve 19 is closed and valve 21 is opened whereby air pressure in the vessel 1 is released to atmosphere and air enters, via the valve 10, the discharge pipe 12 of the ejector device 11 thereby breaking the siphon action and preventing it again emptying the tank T. At the same time the siphon pipe 6 is emptied of acid.

Bearing in mind the hazardous nature of the fluid being handled, the automatic nature of the system provides various safety interlocks which ensure maximum safety for personnel, and reduce the possibility of human error. Such safety features are as follows: 1. During normal operation with acid in the tank T, valves 15 and 19 are closed and valves 18,21 and 10 are open. Thus, the tank T is completely isolated from the storage vessel 1.

If valve 19 leaks, compressed airwill pass, via valve 21 to atmosphere. Thus, there is no danger of accidentally pumping more acid from the vessel 1 to the tank T.

If valve 15 leaks, motive air will be supplied to the ejector device. However, there will be insufficient air to create suction in the siphon pipe 6.

2. The operation of transferring acid from the tank T to the storage vessel 1, i.e. an emptying operation, is carried out in two stages, the first stage being to prime the siphon by pumping acid from the vessel 1 to the tankT, and the second stage being to allow the siphon to transfer the contents of the tank Tto the vessel 1.

In order to start the emptying operation by depressing the "EMPTY" push-button, the level in - the vessel 1 has to be below the "high" level on level switch 2 otherwise the emptying operation is automatically prevented from proceeding, and a visual indication of this is given. This ensures that there is sufficient space in the vessel 1 to accept the entire contents ofthetank T.

In addition, there also has to be sufficient acid in the vessel 1 to prime the siphon. If there is insufficient acid, when the "EMPTY" push-button is depressed, compressed air will be blown up the siphon pipe 6 and will be pumped to atmosphere by the ejector device 11 which is operating during the siphon priming stage.

If the siphon is not primed, the level of acid in the tankTwill not rise to the "high-high" level on level switch 26, and therefore the emptying operation will not proceed further.

The ejector device 11 serves to evacuate air from the siphon pipe 6 when the priming operation takes place. If acid flows into the suction pipe 8 of the device 11, this is detected by level switch 9 which prevents acid being passed to the ejector device 11 itself.

When the emptying operation is complete, air enters the siphon pipe 6 thus breaking the siphon and isolating the tank T from the vessel 1.

3. When the emptying operation is completed and the acid is retained in the vessel 1, valves 19 and 15 are closed and valves 18,21 and 10 are open.

If valve 19 leaks, compressed air will pass, via valve 21,to atmosphere thus eliminating the danger of pumping acid back up to the tank T.

If valve 15 leaks, motive air will be drawn, via the siphon pipe outlet 7 and the valve 10, into the ejector device 11. However, even at full motive air flow, the ejector device will have insufficient suction to cause acid to flow from the vessel 1 to the tank T.

4. The operation of transferring acid from the storage vessel 1 to the tank 2 is effected by depressing the "FILL" push-button on the control panel 5. However, there must first be sufficient acid in the vessel 1 tofillthetankTtothe required level. If there is insufficient acid, this is detected by the "low" level on level switch 26 and the fill operation is prevented from proceeding and a visual indication will be given of this. If there is insufficient space the tank T to accept the contents of the vessel 1, the tank Twill only fill to the "high" level on level switch 26 and, at this point, the valve 19 will close and the valve 21 will open thereby venting the vessel 1 to atmosphere. As the acid in the siphon pipe 6 flows back to vessel 1, air enters the pipe via the discharge pipe 12 of the ejector device 11, and breaks the siphon.

Preferably, the siphon pipe 6 is sized so that emptying of the tank 1 can be effected in less than four minutes.

Claims (12)

1. A draining and refilling system for a tank, which comprises a storage vessel positioned below the level of the tank, a syphon pipe extending vertically between the storage vessel and the tank, an ejector device for evacuating the air from the syphon tube during a draining operation, compressor means providing air under pressure to the ejector device and to the storage vessel, and automatically operated pressure control valves for controlling the movement of pressurised air through the system.

2. A system according to claim 1 and in which the compressor means includes a compressed air receiver, a secondary receiver being provided to receive air therefrom and serving to smooth out the air flow, and to prevent corrosive vapour from entering the compressor unit.

3. A system according to claim 2 or claim 3 and in which the automatically operated pressure control valves include valves for supplying compressed air to the storage vessel or venting it to atmosphere, and a valve for supplying compressed air to the ejector or venting it to atmosphere, said valves being operated inter alia by level control devices in the tank and in the storage vessel.

4. A system according to claim 3 and in which the level control devices in the tank include a low level indicator, a high level indicator, and a "highhigh" or extra-high level indicator for use during emptying of the tank.

5. A system according to any of claims 1 to 4 and in which the tank is an acid pickling tank.

6. A system according to any preceding claim and in which means are provided to ensure that the tank is, in normal operation completely isolated from the pressure vessel.

7. A system according to claim 6 and in which said means includes arranging the syphon pipe so that, when the tank is emptied, air will enter the syphon pipe to cut off the tank from the storage vessel.

8. A method of filling and emptying a pickle tank using a system according to any preceding claim.

9. A method of emptying a tank containing liquid in which a storage vessel is pressurised so that the liquid is driven up a syphon pipe from the storage vessel by an ejector device, to prime the syphon pipe; the liquid containing tank is then filled to "extra-high" level; and the ejector device is then automatically cut off and the storage vessel opened to atmosphere so that the syphon withdraws liquid from the tank and deposits it in storage tank; until the tank is empty when air enters the syphon pipe and thus cuts off the tank from the storage vessel.

10. A method of emptying a tank containing liquid comprising the steps of opening a valve to provide air supply to an ejector device so that the ejector operates to evacuate a syphon pipe; closing a vent valve and opening an air supply valve to admit compressed air to a storage vessel to force liquid up the syphon pipe into the tank; automatically closing a valve between the ejector device and the syphon pipe when the liquid reaches a predetermined level such that the syphon pipe is full of liquid; allowing liquid to continue to flow up the syphon pipe until liquid in the tank reaches a predetermined extra-high level; automatically closing valves supplying air to the pressure vessel and to the ejector device and opening a valve to allow air to escape from storage vessel; allowing syphon operation to withdraw liquid from the tank and deposit liquid in the storage vessel, until the tank is empty when air enters the syphon pipe and thus cuts off the tank from the storage vessel.

11. A draining and refilling system for a tank substantially as hereinbefore particularly described and is illustrated in the accompanying drawings.

12. A method of draining and refilling a tank substantially as hereinbefore particularly described and as illustrated in the accompanying drawings.