GB1586751A - Detection of liquid leaks - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO THE DETECTION OF LIQUID LEAKS (71) We, JUNKOSHA COMPANY, LTD., of 42-1, I-Chome, Gotokuji, Setagaya-Ku, Tokyo, Japan, a Japanese Body Corporate, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a device for detecting liquid leaks, in particular leaks from storage tanks or pipelines. The invention is applicable for example in chemical plants which contain dangerous liquids such as fuels, solvents or poisonous liquids.

According to the present invention there is provided a liquid leak detecting device for detecting leaks from holding means for said liquid, said device being connectable electrically to a resistance measuring sensor and display means, said device comprising at least two electrical conductors separated from each other by a continuously porous polytetrafluoroethylene material having an electroconductive substance impregnating the pores of said porous material.

The present invention also provides a method of manufacture of a liquid leak detecting device which comprises providing a continuously porous polytetrafluoroethylene material having an electro-conductive substance impregnating the pores of said porous material the material having been formed by extrusion and rolling of a mixture of polytetrafluoroethylene fine powder, said electro-conductive material and a liquid lubricant, and locating said material between a pair of conductors, and which includes the step of removing said lubricant.

The invention will now be particularly described with reference to various embodiments of a leak detecting device in accordance with the present invention which are illustrated in the accompanying drawings. In the drawings: Fig. 1 shows a leak detecting coaxial cable; Fig. 2 shows a leak detecting flat cable possessing a plurality of conductors, and Fig. 3 shows a leak detecting device covered with conductive material and jacket material, together with a second, separate conductor.

The liquid detecting device comprises at least two parallel conductors separated from each other by continuously porous PTFE containing an electroconductive material within the pores of the porous PTFE.

The filled, porous PTFE having electro-conductivity which is used in the liquid detecting device of the present invention is prepared according to the following procedure: First, a mixture of PTFE fine powder and electro-conductive powder (e.g. graphite) and a liquid lubricant (e.g. liquid hydrocarbons such as kerosene, naphtha, etc.) is shaped by conventional compression, extrusion and/or rolling.

Second, the liquid lubricant in the shaped article is removed by evaporation or extraction to give an unsintered, shaped article of PTFE containing electroconductive material, thereby providing the material usable in the present invention. This unsintered, shaped PTFE article may then be heated for stabilization at a temperature between about 300 and 360"C. The melting point of PTFE is about 327"C. It is desirable that the heating for stabilization be carried out for a time period such that the article is not completely sintered.Also, the unsintered, shaped PTFE article (preferably containing no lubricant) may then be stretched at least in one direction to 1--15 times its original length, in accordance with the methods described, for example in JPP Sho 4844664and JPP Sho 51-18991. The specific gravity of the starting material is decreased and the porosity increased by such stretching. The stretched PTFE material may be heated to a temperature between about 300 and 360"C, i.e., around the melting point of the PTFE, so that the stretched product is stabilized dimensionally.

The electro-conductive material to be added to PTFE fine powder to produce the covering material of the device may include electro-conductive powders such as graphite, carbon black, fibrous carbon, powdered metals (e.g. Pt, Au, Ag, Ti, Ta), cermets (e.g. TiC-Co), nitrides (e.g. TiN, BN), borides (e.g. Tit2, ZrB 2), silicides (e.g. molybdenum silicide), metal oxides (e.g. Cu2O, TiO, VO, MnO, CoO, NiO, ZnO, CaO, the oxygen content being varied to provide a suitable electroconductivity) and semiconductive metals or organic substances. In addition to the electro-conductive material, pigments and rubbers such as fluororubber may also be added for the purposes of coloring and reinforcement, respectively.

The electro-conductive material to be added to PTFE fine powder is used in an amount such that the PTFE mixture has a desired electro-conductivity and sufficient plasticity so as to enable extrusion, rolling or stretching of the filled material. The amount of the conductive material ranges from 5 to 70 weight %, preferably from 10 to 50 weight %, based on the total weight of the mixture.

The electro-conductive material and PTFE fine powder in an amount as above are first uniformly mixed in a rotational mixer with the addition of the liquid lubricant in an amount of about 20 weight % based upon the weight of PTFE.

Alternatively, a mixture of the conductive material and liquid lubricant is added to PTFE fine powder or dispersion placed in a V-shaped blender. Still another alternative is to impregnate a dispersion of the conductive material into the shaped PTFE article after shaping (e.g. sheeting, etc.) by the extrusion and/or rolling process described above, or into an expanded PTFE article obtained by the stretching described above. A still further alternative is to incorporate a liquid precursor of the conductive material into PTFE fine powder or into a shaped PTFE article. The liquid precursor can then be changed by chemical and/or physical means, such as by heating, hydrolysis, etc., to deposit an electro-conductive material within the pores of the PTFE.In either of the above methods where the incorporated electro-conductive material may run out of the pores, it may be fixed to the carrier material by an adhesive material.

The conductors in the device at least two in number, are in parallel relationship and are separated by the above electro-conductive material and can be made from any metal having a high electro-conductivity. Examples thereof include copper, silver, gold, iron, aluminum, an alloy of the above or any combination (e.g.

cladding, plating)of the above which can be formed in the shape of a line or sheet conductor.

The conductors, at least two in number, are separated by the electroconductive material described above. In principle, the conductors can be separated in any fashion by the material, but in practice they are preferably separated in the following manner. Referring to Fig. 1, one conductor is placed as a center conductor I and the other conductor is placed as an outer shielding conductor 3 by braiding, the conductors being separated from each other by the electroconductive material 4, resulting in a coaxial cable structure. As shown in Fig. 2, many conductors 2 may be embedded in the conductive material 4 in a separated, parallel relationship as in the case of a flat cable. Alternately, in Fig. 3, conductor 1 is covered with the conductive material 4 and further with jacket 5 in a desired configuration, and the other conductor 1 is placed in a separate remote location.

In addition to the above structures, the liquid detecting device may be supplied as a parallel or twisted pair cable consisting of two wires insulated with the filled PTFE material, the cable having a jacket with a stripped portion(s) in a desired place(s), or two electrodes with the conductive material located between conductors, or any combination of such structures.

At least two conductors, held in a separate, parallel relationship, may be sandwiched between two conductive, filled PTFE sheets 4 or between a filled sheet and an unsintered, unfilled PTFE sheet (stretched or unstretched) or the foregoing assembly may be used with narrow PTFE tapes covering only the conductors and not covering between conductor areas, this assembly being compressed together into one body. The PTFE covering the areas adjacent the conductors can be heated from outside or inside to a temperature over 327"C, thus sintering the PTFE. This heating above 327"C from inside is accomplished by applying electric current to the desired conductors, thereby generating Joule heat.By sintering the PTFE material surrounding the cOnductors, the adhesion between the PTFE and conductors is increased, so that the contact resistance between these materials is reduced. Thus the sensitivity and mechanical strength of the device are increased.

The conductors of the device which are brought into contact with or covered with the conductive PTFE material may further be covered by sandwiching or enveloping them with an unsintered or partially sintered PTFE sheet, stretched or unstretched, which allows gases or liquids to be detected to pass through, but does now allow liquids having high surface tension (e.g. water) to pass through, followed by sintering of desired portions as described above.

The liquids to be detected by the device of the present invention include, for example, liquid hydrocarbons such as kerosene and gasoline, organic solvents such as carbontetrachloride and methylethylketone (MEK), vapors from liquids having high vapor pressure (e.g. MEK, gasoline, ammonia).

In use, the liquid leak detecting device of the present invention can be installed continuously or intermittently in places with desired spaces along or under holding means or pipelines of the liquids or vapors mentioned above. When liquid or vapor leaks from the holding means or pipings, the leaking liquid or vapor comes in contact with and enters into the continuously porous filled PTFE containing the electro-conductive material, and the electro-conductivity of the covering material is greatly reduced (or the electrical resistance of the material is remarkably increased). This increase in resistance can be detected by a sensor connected through a lead wire to the leak detecting device, thus indicating the occurrence of the leak.

The liquid detecting device of the present invention may also be fitted onto a float with the portion between the electrodes of the device being positioned on the surface of water. When an oil film resulting from an oil leak spreads on the surface of the water, the oil is absorbed in the conductive material of the device, thus changing the resistance of the device, enabling the oil leak on the surface to be detected.

Methods of detection included within the scope of the present invention include direct measurement of electrical resistance, a comparison method using a standard resistance, a bridge method, measurement of reflection from an impedance irregular point (TDR method) and like methods. A plurality of the devices of this invention can be connected in series or parallel. Parallel connection is suitable for the TDR detecting method. Measurement of the device in a viscous liquid (e.g. heavy oil) can be facilitated by applying a high voltage to the device, the device being self-heated due to the Joule effect thus lowering the viscosity of the oil around the device.

In the embodiment shown in Fig. 3, insulated conductor 1 and bare conductor 1 are placed with a specified distance therebetween in a location where the liquid to be detected enters.

The resistance change of the device of this invention due to the intrusion of the liquid to be detected is more than 50% of the initial resistance. This resistance change is not caused by the resistance change on the surface, but by the volumetric change of resistance of the porous material, so that the leak detector is intrinsically stable for severe environmental conditions, high temperature, humidity, etc.

Moreover, since the liquid detecting device of the present invention is made mainly of PTFE which is inherently repellent to water, it is little affected by water and moisture, as well as having excellent heat and weather resistance.

The present invention is further explained by the following examples which are presented for illustration and are not intended to limit the scope of the invention in any way.

Example 1.

A mixture of PTFE containing 15 weight % of graphite and about 20 weight % of liquid lubricant was preformed, extruded and rolled into a tape of 0.1 mm in thickness. The tape, 4 mm wide, was wrapped around a 1.5 mm O.D. copper conductor, to an O.D. of 3 mm. The resultant core was then covered with a braiding of thin copper wires, 0.1 mm in O.D. to give a coaxial cable assembly. This assembly was then cut to a length of 3 cm, and connected with a lead wire for each of the inner and outer conductor to provide the leak detecting device shown in Fig.

1. One half of this device was then placed in a constant temperature chamber set at a temperature of 350"C for one minute to partially sinter the PTFE, thus increasing the contact strength between the conductor and PTFE and the dimensional stability of the device. The second half of the device was not heated.

These leak detecting devices were immersed in MEK, gasoline, kerosene and heavy oil C, and at the same time the resistance change of the devices was measured. The- results in kQ are shown in the table below. At the time of measurements, the temperature of the liquids and devices was 20"C and the relative humidity of the ambient air was 56%. The devices used in this experiment were reusable after the impregnated liquid was evaporated. There was a certain relationship between the extent of evaporation and conductivity of the devices during these experiments so that these devices could provide an ability to differentiate between the kinds of liquid detected depending on the difference in their volatility.

Resistance Change of the Device having Unsintered PTFE in the Covering

Time Liquid Initial 5 sec 10 sec 1 min 1 hr 5 hrs 24 hrs 5 days MEK .701 4.2 5.0 5.0 5.0 5.0 5.0 5.0 Gasoline .750 4.1 5.5 5.5 5.5 5.5 5.5 5.5 Kerosene .800 4.0 6.0 7.3 7.3 7.3 7.3 7.3 Heavy Oil C .310 .320 .320 .330 .340 .400 .560 1.9 Water .440 .440 .440 .440 .475 .500 .540 .600 Resistance Change of the Device having Partially Sintered PTFE in the Covering

\ Time Liquid Initial Ssec lOsec 30 sec 1 min 1 hr 5 hrs 24 hrs 5 days MEK .152 .700 .850 .950 1.0 1.0 1.0 1.0 1.0 Gasoline .150 .650 .800 1.0 1.1 1.1 1.1 1.1 1.1 Kerosene .160 .600 .800 1.1 1.3 1.3 1.3 1.3 1.3 Water .210 .210 .210 .210 .210 . 255 .320 .420 .590 Example 2.

The leak detecting device of Example 1 was produced with the exception that the amount of graphite introduced was 50 weight %, and the shaped PTFE article was expanded and not sintered. The electrical resistance of this device in Q was measured and is tabulated below.

Time Liquid Initial 5 sec 10 sec 30 sec 1 min 1 hr 5 hrs 24 hrs 5 days MEK 6.0 6 5 7 0 7 0 7 0 7 0 7 0 7 0 7 0 Gasoline 7.5 7 5 8 5 90 90 90 90 90 90 Kerosene- 5.5 45 5 5 60 6 0 6 0 6 0 6 0 60 Water 6.0 6.0 6.0 6.0 6.0 6.5 6.5 7.5 9.0

Example 3.

A lubricated PTFE mixture containing 15 weight % of graphite was ram extruded and rolled into a 0.1 mm thick sheet having electro-conductivity and continuous pores. The sheet was placed in a constant temperature chamber set at 300"C for 30 seconds, then stretched in the lengthwise direction to 1.5 times the original length, and slit into a tape 4 mm wide. A copper conductor, 1.5 mm was wrapped with the stretched PTFE tape to an O.D. of 3 mm and then over braided with thin copper wires each 0.1 mm in O.D. This coaxial cable assembly was then cut to a length of 30 mm and connected with lead wires as shown in Fig. l to a resistance sensor.In one type of the device, the PTFE layer was left unsintered, and in the other type of the device the PTFE layer was partially sintered by placing it in a constant temperature chamber held at 3500C for one minute. These devices were immersed in MEK, gasoline, kerosene, heavy oil C and water, and the change in the resistance was measured. The results in kQ are shown in the following table. The liquid and device temperatures were 200C and the relative humidity of the ambient air was 56%. The devices used for the experiment were reusable after the impregnated liquids were evaporated.

Resistance Change in the Device using Unsintered PTFE

Time Initial Initial 5 sec 10 sec 30 sec 1 min 1 hr 5 hrs 24 hrs 5 days MEK 1.02 65.0 66.0 66.0 66.0 66.0 66.0 66.0 66.0 Gasoline 1.00 13.0 18.0 20.0 20,0 20.0 20.0 20.0 20.0 Kerosene 1.28 4.5 8.0 11.2 12.0 ::12.0 12.0 12.0 12.0 Heavy Oil C .540 .540 .540 .545 .550 .580 .600 1.1 4.0 Water .950 .950 .950 .950 .950 1.05 1.07 1.10 1.15 Resistance Change in the Device using Partially Sintered PTFE

Time Liquid Initial 5 sec 10 sec 30 sec 1 min 1 hr 5 hrs 24 hrs 5 days MEK .520 3.0 3.3 3.4 3.4 3.4 3.4 3.4 3.4 Gasoline .400 2.7 2.9 3.0 3.0 3.0 3.0 3.0 3D Kerosene .323 .900 1::5 2.7 2.7 2.7 2.7 2.7 2.7 Water .360 .360 .360 .360 .360 .410 .440 .490. .590 Example 4.

Using 12 copper conductors (0.254 mm O.D. and 1.12 mm spacing between conductors) and 2 electro-conductive PTFE sheets (0.1 mm thickness) containing 15 weight % of graphite, a leak detecting device in the form of a flat cable was prepared as shown in Fig. 2. The cable was cut to a length of 30 mm and the conductors were exposed at one end and assembled one after the other into two electrodes.

The resultant device was exposed to various liquids and vapors from the liquids as shown in the following table, and the resistance change of the device was measured. The results in Q are summarized in the table below. The liquid and ambient temperature was 250C and the relative humidity was 71%. The vapors to be detected from the liquids were held at their saturated vapor pressure.The devices were restored to their initial states after the absorbed vapors were evaporated.

Time LiquidN & Vapour Initial 5 sec 10 sec 30 sec 1 min 1 hr 5 hrs 24 hrs 5 days MEK 55 210 220 220 220 220 220 220 220 Gasoline 30 180 190 195 195 195 195 195 195 Kerosene 44 198 210 230 240 ' 240 240 -240 240 Heavy Oil C 42 42 42 42 42 90 150 400 600 Water 41 41 41 41 41 36 36 38 38 MEK Vapor 42 80 80 80 80 80 - - Gasoline Vapor 42 78 78 78 78 78 Kerosene Vapor 42 43 43 43 43 43 - -

Example 5.

Two copper conductors (each 0.29 mm in O.D.), held in parallel with a 25 mm spacing, were sandwiched between two electro-conductive, unsintered PTFE sheets (0.1 mm thick) containing 15 weight % graphite, and this assembly was sandwiched between two unsintered, extruded, unfilled PTFE sheets (0.3 mm thick) produced by extrusion and rolling. These members were pressed together by a pair of compression rolls, thus obtaining a liquid detecting device in a flat shape.

Each conductor of the device was heated by applying a l0A electric current for one minute, and the conductive PTFE and unsintered PTFE sheets in the region around the conductors were sintered, thus increasing the mechanical contacts between the conductor and the conductive PTFE and PTFE sheets.

The device was then cut to a length of 40 mm, and an end of each conductor was pulled out by 10 mm to which an insulated lead wire was connected. Both end sections of the device were sealed by epoxy molding in order to prevent water from penetrating into the device from the end sections. The resultant device was used for the experiment to detect an oil film resulting from an oil leak on the surface of water.

The testing procedure was as follows: The device was held vertically at a position in a water trough where the water surface was at the center of the device, i.e., the upper half of the section was out of water and the lower half was submerged in water. Onto the surface of the water was spread various liquids to be detected to a thickness 0.5 mm or less. Existence of the liquids was detected as a resistance change of the device which was caused by the absorption of the liquids into the conductive PTFE.The water and liquid temperatures were 250C, and the relative humidity of the ambient air was 56%.

The Resistance Change (in kQ) between Conductors in the Device Time Liqui2\ Initial 3 sec 1Q sec 30 sec 1 min 5 min 10 min 20 min 1 hr Initial 3 sec 10 sec 30 sec 1 min 5 min 10 min 20 min 1 hr Gasoline 5.3 13.3 41.1 41.1 41.1 411 1 41.1 41.1 41.1 Kerosene 5.1 5.1 24.5 37.0 39.5 39.5 39.5 39.5 39.5 Heavy Oil A 5.5 4.8 4.8 5.5 8.4 30.0 37.2 37.2 37.2 37.2 Crude Oil 5.1 5.1 5.1 5.1 5.6 12.8 20.9 31.9 37.7 (Arabia) Heavy Oil B 5.0 5.0 5.0 5.0 5.0 5.3 10.0 20.0 34.5 Heavy Oil C 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.6 6.1

WHAT WE CLAIM IS: 1. A liquid leak detecting device for detecting leaks from holding means for said liquid, said device being connectable electrically to a resistance measuring sensor and display means, said device comprising at least two electrical conductors separated from each other by a continuously porous polytetrafluoroethylene material having an electro-conductive substance impregnating the pores of said porous material.

2. A device according to claim 1 wherein said electro-conductive material is carbon powder.

3. A device according to claim 1 or claim 2 wherein the impregnated polytetrafluoroethylene material has been obtained by extrusion and rolling of a mixture of polytetrafluoroethylene fine powder, said electro-conductive material and a liquid lubricant, followed by removal of said lubricant.

4. A device according to claim 3 in which the impregnated polytetra fluoroethylene material has been obtained by the further steps of stretching the extruded and rolled lubricant-free material.

5. A device according to any preceding claim in which the polytetra fluoroethylene is unsintered.

6. A device according to any one of claims l to 4 in which the polytetra fluoroethylene is partially sintered.

7. A device according to any of claims 1 to 4 in which the polytetra fluoroethylene is sintered.

8. A device according to any preceding claim wherein said conductors are held in substantially parallel relationship.

9. A liquid leak detecting device substantially as herein described with reference to any one of the embodiments illustrated in the accompanying drawings.

10. A method of manufacture of a liquid leak detecting device which comprises providing a continuously porous polytetrafluoroethylene material having an electro-conductive substance impregnating the pores of said porous material, the material having been formed by extrusion and rolling of a mixture of polytetrafluoroethylene fine powder, said electro-conductive material and a liquid lubricant, and locating said material between a pair of conductors, and which includes the step of removing said lubricant.

11. A method according to claim 10 in which the impregnated polyetetrafluoroethylene material is obtained by the further steps of stretching the extruded and rolled lubricant-free material.

12. A liquid leak detecting device made by the method according to claim 10

Claims (1)

1. or claim 11.

   13. A liquid leak detecting device according to any one of claims 1 to 9 and 12 in which the conductors are connected electrically to a resistance measuring sensor and display means.