GB2287817A

GB2287817A - Monitoring vessel integrity

Info

Publication number: GB2287817A
Application number: GB9505306A
Authority: GB
Inventors: Lee Forbes
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-03-19
Filing date: 1995-03-16
Publication date: 1995-09-27
Also published as: GB9505306D0

Abstract

An integrity monitoring system for use with a vessel, e.g. a tank or pipe (1), adapted to contain an electrically conductive liquid and having an electrically insulating lining, (7, Fig. 2 not shown) comprises an electrode (14) in the vessel and arranged to contact the said liquid, a second electrode (15) or conductor arranged between the lining and the structure or shell of the vessel, and means (2) monitoring electrical conductivity between the electrodes. <IMAGE>

Description

TITLE: MONITORING VESSEL INTEGRITY DESCRIPTION The invention relates to the monitoring of the integrity of vessels, more particularly holding or storage tanks or vessels such as pipes or tubes containing electrically conductive liquids or through which such substances are passed.

With the advent of fibre reinforced plastics tanks and vessels becoming commonplace as containers for highly corrosive liquids, e.g hydrochloric acid, it is necessary from time to time to check the integrity of the corrosion barrier in the tank or vessel whether it is of thermoplastics material, rubber or otherwise. This is for the reason that a leak can cause catastrophic failure of the tank or vessel. Even with the traditional constructional materials used in the past such as rubber linings in steel tanks and vessels, it is desirable that the integrity of the corrosion resistant barrier is checked from time to time to avoid any major damage occurring to the external structural material of the tank or vessel.

It is known in such a tank or vessel to apply carbon loaded resin or carbon fibre or conductive or metal foils externally to the corrosion barrier at all joint lines, nozzle penetrations and so on, to act as an electrical conductor to enable high voltage spark testing from the inside of the vessel to take place as part of a regular tank inspection schedule. This conductor is permanently installed as part of the structure, to allow the corrosion barrier of the tank, vessel or fabrication to be checked for pin holes or suspected breaches in this barrier at any time during the vessel's life. Often such regular inspection occurs too late to recover the tank as the contents percolate into the mechanical supporting structure and cause degradation which in many materials such as glass reinforced plastics is rapid and can cause catastrophic failure.

It is an object of the invention to provide means which will give a continuous monitoring of the integrity of a tank or other vessel containing an electrically conductive liquid whether corrosive or non corrosive, where the tank or vessel inner (corrosion barrier) shell or lining or coating is an electrical insulator, e.g. of plastics, rubber or glass, and which monitoring means will give a signal in the event of a breakdown of the electrical insulation of the inner shell or lining or coating. Being electrical insulators, thermoplastics, glass, rubber and fibre reinforced plastics are good materials to be used with this monitoring system.

According to the invention a tank or vessel, e.g.

pipe, integrity monitoring system for use with a tank or vessel containing an electrically conductive liquid and having an electrically insulating lining or coating, e.g.

of rubber or plastics, comprises an electrode in the tank or vessel and arranged to contact the liquid, a second electrode or conductor arranged between the lining and the structure or shell of the tank or vessel, and means monitoring the electrical conductivity across the lining.

Preferably the monitoring system comprise warning means actuated by an increase in conductivity across the electrodes.

In the case of a tank or vessel made from a plastics material, e.g. fibre reinforced plastics, the inner surface of the tank or vessel may be provided with one or more elongate electrical conductors e.g. of carbon fibres or metal foil. Where the tank or vessel structure is of metal, the structure itself can form the said second electrode or conductor.

There now follows a basic description of the monitoring system as applied to different kinds of tank or other vessel:a) Fibre, e.g. glass, reinforced plastics tanks or vessels with a thermoplastics inner corrosion barrier.

A network of carbon fibres, e.g. continuous fibres or tissue, can be placed behind all welds, nozzles on the shell and behind any other penetrations through the thermoplastics sheet. The carbon fibres can be integrally bonded within the GRP structure immediately behind the thermoplastics barrier. The network of carbon fibres (or other conductive materials) can be linked together to terminate in a metallic boss to accept a cable termination to enable connection via a single cable to a monitoring unit.

An electrode can be inserted in the liquid at the base of the tank or at the lowest part of the vessel through a flanged nozzle and will be constructed in a material compatible with the tank or vessel's contents. This might range from stainless steel, Hastelloy, carbon or other materials. The electrode can be mounted on a plate with an encapsulated termination box with a termination to allow electrical connection to the monitoring unit. The position of this electrode should be as low as possible in the tank or vessel to ensure that it is immersed when there is liquid in the tank or vessel. When applied to a pipe the electrode can be fitted in any suitable position in the liquid flow.

b) Tanks or vessels constructed throughout in fibre reinforced plastics, e.g. glass reinforced plastics, incorporating an integral polyester resin glass fibre laminate type corrosion barrier with a suitable resin rich surface incorporating either glass or synthetic tissue veil material as the face of the corrosion barrier, or other types of corrosion barriers incorporating polyester and epoxy resins.

After the laying down of the corrosion barrier, continuous strands of carbon fibre can be incorporated on the back of the corrosion barrier in continuous form over the entire surface. These fibres are not required to touch one another, but the closer they are to one another the more immediate the response on any deterioration of the corrosion barrier. Where nozzles, manways and other types of penetrations are made within the shell or structure of the tank or vessel, carbon fibre should be incorporated behind the corrosion barrier and brought out with an appropriate form of carbon fibre attachment to the carbon fibre layer or alternatively with a separate electrical connection to the termination block. A similar system for the in-tank or vessel electrode will be used as in (a) above.

c) Metallic tanks or vessels with electrically insulating linings or coatings. The same principle applies as in (a) above. However, the metallic shell or structure of the tank or vessel can act as an overall conductor behind the insulating lining. The monitoring unit can be connected to the shell of the tank or vessel with a similar in-tank electrode insulated from the shell of the tank or vessel in a flanged nozzle.

The monitoring unit may comprise a control box provided with an isolation switch, indicator lights and a test button. The heart of the unit is an electronic conductivity sensing device which is adjustable, measured in microsiemens per cm. Suitably the lowest conductivity sensed by the unit may be 1 microsiemen/cm. The unit may be powered by 110 volt single phase mains power. However, this can be altered to suit conditions. The unit may comprise a 24 volt transformer providing the operational voltage. The associated circuitry and protection fuses etc. can usually be incorporated within the panel.

The indicator lights will preferably show white for power on, which can be tested by the isolator switch, green for normal condition and satisfactory test, and red for breakdown in insulation when there is a rise in conductivity. Where the insulation factors are not so high as the normal thermoplastic materials (i.e. some conductive material present within the lining material), the sensitivity can be adjusted accordingly at the time of manufacture of the unit.

Where there is concern with tank or vessel contents of an inflammable nature and the possible danger of sparking, Zener barriers can be provided accordingly and the cabling taken out to a non-flameproof explosion area.

Alternatively the panel can be made flameproof.

Where a conductive metallic, e.g. carbon steel, tank or vessel is used there should be no problem in checking the conductivity of the steel tank or vessel to establish the electrical integrity of the monitoring system since the tank or other vessel is the major supporting structure.

However, where there are built-in conductive fibres or materials within a glass reinforced plastics structure there is a danger of discontinuity of the network of fibres either during manufacture or in service.

It is normal to select a material for the conductive layer which is chemically resistant to the contents.

Therefore in the event of a failure there is no danger of the conductive layer being destroyed. Fortunately carbon fibre is a highly chemically resistant material and in the majority of cases would not be attacked.

The integrity of the fibre network may be checked during manufacture on each strake or section of tank or vessel built by the use of a meter to check the continuity of fibre from the terminal point across a meter to the bottom edge of each strake as it is manufactured. This may be verified during each manufacturing process to ensure that every section is fully connected within the system.

All nozzles connections can also be verified on this basis and will incorporate a carbon fibre strip down the pipe of the nozzle connection onto the back of the face of the flange thus to enable monitoring of the entire shell including nozzles and flange faces. These too can be checked and verified during manufacture.

Finally the entire network can be checked by connecting to the carbon fibre terminal point across a meter onto a test point at the base of the tank or vessel or opposite end of the tank or vessel. This will indicate whether there is an electrical continuity throughout the length or height of the structure.

For in-service checking the same procedure can be carried out. It would be possible to bring a third cable back from a continuity check terminal to enable the continuity to be checked automatically on test from the control unit. The test button on the control unit is preferably arranged to test the entire circuitry within the box and to verify that all items are operating correctly.

In the event of a breakdown or leakage through the corrosion barrier, or as soon as there is indication of increased conductivity across the corrosion barrier, an indicator light may be activated together with a volt free relay which will allow an alarm to be connected either internally within the box or externally to a control unit.

In the event of a failure occurring this demonstrates to the operator the existence of a leak or perforation through the corrosion barrier which requires urgent attention. At this stage the tank or vessel can be drained down and during the draining down process, as the liquid level drops, the position of the leak can be identified as soon as the alarm stops. In the event of an alarm being activated with a leak high up in the tank or vessel and not being noticed as the liquid level drops and switches the alarm off, a latch type relay may be provided in association with the control light to indicate a fault.

This light would require to be cancelled manually thus alerting the operator to a fault above the existing liquid level.

Continuous monitoring as provided by the apparatus of the present invention is of benefit with the advent of annual internal inspection of storage tanks or vessels e.g.

pipes for containing corrosive liquids and containment of hazardous materials, particularly in view of regulations implemented by the Health and Safety Executive with regard to entry into confined spaces and the problems with toxic fumes, washing out and certifying of a confined space etc.

coupled with the expense of training and the use of breathing apparatus. Thus the monitoring system of the present invention provides a relatively inexpensive alternative.

The invention is diagrammatically illustrated, by way of example, in the accompanying drawings, in which: Figure 1 is a general schematic view of a monitoring system for a tank containing an electrically conductive liquid; Figure 2 is a cross-sectional side view, to an enlarged scale, of a detail of the monitoring system of Figure 1, and Figure 3 is a cross-sectional side view, to an enlarged scale, of a further detail of the monitoring system of Figure 1.

In the drawings there is shown a vessel in the form of a tank (1) for containing an electrically conductive liquid (5) and a system for monitoring the liquid-tight integrity of the tank (1). The monitoring system consists of a pair of electrodes (3 and 13) disposed on opposite sides of a liquid-proof lining or barrier (4) for the tank (1), the electrodes being coupled to a monitoring unit (2) which monitors conductivity across the electrodes and which is activated by an increase in the conductivity which would indicate a breach in the lining (4).

Referring more particularly to Figure 2 of the drawings, there is shown a liquid probe unit (14) mounted at the base of the tank (1) and comprising an electrode (3) supported in a nozzle-like housing (10) in the wall of the tank and extending into the interior of the tank (1) so that the electrode (3) is in contact with the liquid in the tank. The tank comprises a glass reinforced plastics shell (6), an inner thermoplastic liquid-proof barrier (4) and an electrically conductive layer (7) interposed between the barrier (4) and the shell (6). The electrode (3) is connected to a terminal (8) which is mounted on the tank (1) through an interposed gasket (9) such that the electrode (3) is electrically insulated from the conductive layer (7). The terminal (8) is electrically connected to the monitoring unit (2) by means of a cable (16).

Referring to Figure 3 of the drawings, there is shown a leak probe unit (15) mounted high in the tank and comprising an electrical terminal (11) connected to a plate-like electrode (13) positioned between the barrier (4) and the shell (6) and electrically connected to the conductive layer (7) which may, for example, be of a suitable metal foil or of conductive fibres such as carbon fibres. The terminal (11) is electrically connected to the monitoring unit (2) by means of a cable (12).

Thus in operation the electrodes (3 and 13) are normally electrically insulated one from the other but on the occurrence of a leak in the barrier or lining the electrically conductive liquid will permeate through the barrier (4) into contact with the conductive layer (7) to cause a rise in the conductivity level between the electrodes so as to trigger an alarm in the monitoring unit (2). Thus appropriate remedial action can be taken earlier than might otherwise be the case.

The invention thus provides a simple and effective monitoring system for tanks or other vessels for containing electrically conductive liquids.

Claims

1. An integrity monitoring system for use with a vessel adapted to contain an electrically conductive liquid and having an electrically insulating lining, comprises an electrode in the vessel and arranged to contact the said liquid, a second electrode or conductor arranged between the lining and the structure or shell of the vessel, and means monitoring electrical conductivity between the electrodes.

2. An integrity monitoring system according to claim 1, comprising warning means coupled to the monitoring means and actuated by an increase in conductivity across the electrodes.

3. An integrity monitoring system according to claim 1 or claim 2, wherein the vessel structure or shell is of a plastics material, and wherein the inner surface of the structure or shell is provided with at least one elongate electrical conductor.

4. An integrity monitoring system according to claim 3, wherein the elongate electrical conductor extends over a substantial part of the surface of the vessel.

5. An integrity monitoring system according to claim 4, wherein the elongate electrical conductor extends over substantially the whole of the surface of the vessel.

6. An integrity monitoring system according to any one of claims 3 to 5, wherein the elongate electrical conductor is of carbon fibres or metal foil.

7. An integrity monitoring system according to claim 6, wherein the elongate electrical conductor is in the form of a network or web of carbon fibres.

8. An integrity monitoring system according to claim 1 or claim 2, wherein the vessel structure or shell is of metal, and wherein the structure or shell itself forms the said second electrode or conductor.

9. An integrity monitoring system for use with a vessel adapted to contain an electrically conductive liquid substantially as hereinbefore described.

10. An integrity monitoring system for use with a vessel adapted to contain an electrically conductive liquid substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

11. A tank or vessel adapted to contain an electrically conductive liquid and comprising an integrity monitoring system as claimed in any preceding claim.