GB2220494A

GB2220494A - A system for the detection and location of leaks

Info

Publication number: GB2220494A
Application number: GB8816526A
Authority: GB
Inventors: Hugh Macpherson
Original assignee: WL Gore and Associates Inc
Current assignee: WL Gore and Associates Inc
Priority date: 1988-07-09
Filing date: 1988-07-09
Publication date: 1990-01-10
Anticipated expiration: 2008-07-09
Also published as: GB2220494B; GB8816526D0

Abstract

A system is provided for the detection and location of liquids and in particular for liquids leaking from apparatus such as a pipeline comprising a sensor in the form of three conductors (110, 112 and 114) arranged to extend in side-by-side relationship. Two of the conductors (110, 112) are isolated from each other by insulating means that will permit the establishment of a conductive path between the two isolated conductors in the presence of a liquid and create at least two current loops in which current may flow, the presence of current flow in one of the current loops being used to activate an alarm. At least one of the conductors (110, 112) connected by the conductive path has a resistivity that is predetermined throughout its length and a resistance that is a multiple value of the resistance of the or each remaining conductor so that the relative magnitudes of the currents flowing in each current loop established by the conductive path is largely dependent on the position of the leak. If R6 and R7 are equal and of negligible impedance in the loops, the position of a leak (represented by RF) can be determined from the ratio of the voltages across R6, R7. Digital dividing circuitry is envisaged. <IMAGE>

Description

A SYSTEM FOR THE DETECTION AND LOCATION OF LEAKS The present invention relates to a system for the detection and location of liquids and in particular to a system for the detection and location of liquids leaking from liquid handling apparatus.

It is often requird to detect the presence of liquids by remote sensing. Liquid detectors are commonly used to detect small leaks from pipelines, liquid storage tanks and other liquid handling apparatus so that any leak may be rectified at an early stage before a major accident occurs. Typically detectors of this kind are employed because the liquid in question is hazardous or would present a hazard if allowed to escape to the environment.

In the past such detectors have taken the form of two conductors typically several hundred meters in length extending parallel to each other separated by a plastic webbing and exposed at intervals along their length.

The conductors are interconnected at one end by means of a resistor while at the other end the conductors are connected to an electronic circuit that registers any change in the resistance of the conductive loop thus created. The value of the resistor is chosen to be large relative to the resistance of the conductor loop so as to present a known and characteristic loop resistance. Liquid falling on to the detector at any point along its length and bridging between the conductors will establish a conductive path between the two conductors altering the overall loop resistance.

Systems of this type are very reliable and are in common use but do suffer from some inherent disadvantages.

The above system only registers the presence of a leak and gives no indication as to its distance from the monitoring circuit. In some embodiments of the system where the loop is particularly long the detection of a leak is of limited value without knowing its exact location. Furthermore, detectors of this kind employ alternating currents and therefore radiate electromagnetic energy which may be detrimental to surrounding equipment. Similarly the detector is itself susceptible to local electromagnetic sources which may effect its own performance. In addition the electronic circuit that registers any change in loop resistance is relatively expensive and represents a significant proportion of the cost of the system especially in applications in which the loop is comparatively short, i.e. less than 100 metres in length.

Some of the disadvantages inherent in the above detector may be overcome by using a three conductor system such as a Murray Loop. The prior art will now be further described with particular reference to the accompanying drawings in which: Figure 1 is a schematic drawing of a three conductor system known in the prior art.

In the typical embodiment shown in Figure 1 three conductors 10,12 and 14 are arranged to extend parallel to each other, the first and second conductors, 10 and 12 respectively, being separated as before by a plastic webbing (not shown) and exposed at intervals along their length. The second and third conductors, 12 and 14 respectively, are interconnected at one end by connecting link 16 while all three conductors 10,12 and 14 are interconnected at the other end by a bridge circuit 18 comprising variable resistor Rx, fixed resistor Ry and galvanometer 22. Any liquid falling on the sensor and bridging conductors 10 and .12 will create a conductive path of fault resistance Rf and will draw a current if from a battery 20 connected between the bridge circuit 18 and the first conductor 10.This current if will be divided into two portions ifl and if2 at the point of the leak, the relative magnitudes of the portions depending upon the distance of the fault from the bridge circuit 18 the difference between the two resulting in a flow of current through the galvanometer 22. Variable resistor Rx may be adjusted so that the galvanometer 22 registers a Null and the distance to the leak calculated assuming that the second and third conductors 12 and 14 are of equal and uniform resistance per unit length, since: Distance to the leak = 2 Rx x L Rx +R y where Rx and Ry are the values of the resistors comprising the bridge circuit 18 and L is the length of the conductors as shown.

The above prior art system, although allowing the leak to be located, suffers from a number of drawbacks. The accuracy in locating the fault is dependant upon the resistance of the first and second conductors and any variation in the resistivity of either conductor along its length will contribute to an inaccurate result. In addition the bridge circuit requires high quality components if reasonable accuracy is to be achieved making the components expensive, this cost being a particularly large proportion of the overall expense of a system in which the loop is comparatively short.

Furthermore, the above system is intended for manual operation and is not easily converted for automatic monitoring.

The present invention is directed towards overcoming these disadvantages, the invention being particularly intended for applications where the loop is comparatively short. The leak detection and location system to be described overcomes the need to use expensive components and provides a system that is both economical to manufacture and capable of automation.

According to the present invention there is provided a system for the detection and location of liquids comprising a sensor in the form of three conductors arranged to extend in side-by-side relationship, two of the conductors being isolated from 'each other by insulating means that will permit the establishment of a conductive path between the two isolated conductors in the presence of a liquid creating at least two circuit loops in which currents may flow, at least one of the conductors connected by such a path having a resistivity that is predetermined throughout its length and a resistance that is a multiple value of the resistance of the or each remaining conductor.

When the system is in use the sensor may take the form of a cable, and in particular a coaxial cable, to be laid underneath a pipeline or storage tank where it may come into contact with any leaking liquid.

Alternatively the cable could be wrapped helically around the liquid containing apparatus.

The present invention will now be described with particular reference to the further accompanying drawings in which: Figure 2 is a schematic drawing of a system for the detection and location of liquids in accordance with the present invention; Figure 3 is a partial schematic drawing of an embodiment of a system for the detection and location of liquids in accordance with the present invention incorporating an alarm, and Figure 4 is a partial schematic drawing of an embodiment of a system for the detection and location of liquids in accordance with the present invention incorporating an automatic leak location display.

The system for the detection and location of liquids shown in Figure 2 comprises a sensor in the form of three conductors 110,112 and 114 arranged to extend in side-by-side relationship. The first conductor 110 is isolated from the second conductor 112 by insulating means (not shown) that will permit the establishment of a conductive path of fault resistance Rf between the two conductors in the presence of a liquid while the second and third conductors, 112 and 114 respectively, are interconnected at one end by connecting link 116.

At the opposite end the second and third conductors are interconnected by means of resistor R6 and the first and second conductors by means of resistor R7 and battery 120. The establishment of a conductive path between the first and second conductors divides the resistance of each conductor into two portions, the portions being defined as the resistance of the conductor between the point at which the conductive path is established and the respective ends of the conductor. Thus the resistance of the first conductor 110 is divided into two portions R4 and R5 and the resistance of the second conductor 112 is divided into two further portions R2 and R3, the relative magnitudes of R4 and R5 and of R2 and R3 being dependent upon the position of the conductive path. The third conductor 114 has a resistance R1.The first and third conductors, 110 and 114, are standard conductors and may be chosen to be of any convenient size for example 16 AWG copper cable which is 1.4 mm in diameter and has an electrical resistance of 15.45 ohms/km. Therefore for a sensor 100 metres in length R1 and (R4+R5) are typically of the order of 1.5 ohms. The second conductor 112 is chosen to have a resistivity that is predetermined at all positions along its length and which is preferably substantially uniform, the total resistance of the second conductor 112 being a multiple of the resistance of each of the other two conductors. For example the second conductor 112 may be chosen such that (R2+R3) is 1.5 Megohms, i.e.

(R2+R3) = 106 x (R4+R5) Any liquid coming into contact with the sensor and bridging the first and second conductors will create a conductive path completing a current loop and causing current to be drawn from battery 120. The current drawn from battery 120 will be split at the point of the leak into two portions il and i2 that will flow in each of the respective current loops as shown, the relative magnitude of each portion being almost totally dependent upon the relative magnitudes of R2 and R3 and therefore on the position of the leak. The magnitude of the current portion flowing in each of the current loops may be calculated by measuring the voltage across the each of the resistors R6 and R7 and hence the distance along the sensor to the leak may be obtained.

The present invention will now be illustrated by means of electrical circuit theory and in particular by the application of Rirchhoff's and Ohm's laws to the circuit shown in Figure 2.

Applying Kirchhoff's second law to the first loop the following equation is obtained: 0= i1(R1+R2+R3+R6) - i2(R3) Equation A Applying Kirchhoff's second law to the second loop the following equation is obtained: V = -il(R3) + i2(R3+R5+R7+Rf) Equation B Combining equations A and B yields the following expressions for the currents flowing in each loop: il = O - VR3 (R1+R2+R3+R6) (R3+R5+R7+Rf) - (R3)2 and i2 = V(R1+R2+R3+R6) O 0 (R1+R2+R3+R6) (R3+R5+R7+Rf) - (R3) since in accordance with Ohm's law:: V(R6) = ilR6 and V(R7) = i2R7 then V(R6) = - VR3R6 V(R7) VR7(R1+R2+R3+R6) V(R6) = - R3R6 V(R7) R7 (Rl+R2+R3+R6) If R6 = R7 then: V(R6) = - R3 V(R7) (R1+R2+R3+R6) For the purposes of calculating the position of any leak the significance of the negative sign may be ignored.

An advantage of the present invention is that the above expression is almost totally dependent upon the values of R2 and R3 since R2+R3 R1 Typically the second conductor 112 is chosen so that: R2+R3 > 10 x R1 In addition R6 and R7 are chosen to be of equal value and preferably should form a matched pair such that: R6 = R7 < 1 ohm Hence by neglecting the contribution to the loop resistance of R1 and R6 the ratio of the voltages measured across R6 and R7 approximates to: V(R6) R3 V(R7) # R2+R3 i.e. VR ~ The fractional length along the V(R7) N sensor at which a leak occurs Referring to the typical resistance values of the 100 metre sensor described above the present invention will now be further illustrated by means of two examples in which it is assumed that R6 and R7 have the value of 0.1 ohms.

EXAMPLE 1 Assuming that a liquid creates a conductive path between the first and second conductors 95 metres from the matched pair of resistors the ratio of the voltages across R6 and R7 is measured to be: V(R6) = 1425 x 10 V(R7) (1.5x106)+1.6 V(R6) = 0.94999 V(R7) By neglecting the contribution to the loop resistance of R1 and R6 the above measurement can be equated to the ratio of the distance to the leak compared to the total length of the sensor. Therefore the distance to the leak is found by multiplying this ratio by the length of the sensor: Distance to the leak = 100 x 0.94999 = 94.999 metres As the example illustrates the above approximation is accurate to less than 1 mm in 95 metres.

EXAMPLE 2 Assuming that a liquid creates a conductive path between the first and second conductors 5 metres from the matched pair of resistors the ratio of the voltages across R6 and R7 is measured to be: V(R6) = 75x103 V(R7) (1.5x106)+1.6 V(R6) = 0.04999 V(R7) Therefore: Distance to the leak = 100 x 0.04999 = 4.999 metres Again the assumption that the ratio of the voltages measured across R6 and R7 is equal to the fractional length along the sensor at which a leak occurs is sufficiently accurate to yield a result with an error of less than 1 mm.

The function of the system of the present invention is twofold. Firstly the system is required to register when a leak has occured and secondly to indicate its position. In one embodiment of the present invention the occurrence of a leak is noted by monitoring the current flowing within the second loop. This may be most easily achieved by measuring the voltage across a resistor and comparing this voltage with a predetermined level using a comparator integrated circuit. Since the second loop of the circuit is required to be current limited to allow for faults at extreme ends of the line and since the actual resistance of the second loop is irrelevant to the measurement accurcacy, it is convenient to measure the current across an additional resistor in series with R7.

Such an embodiment is shown in Figure 3 where similar features to those already described have been denoted by corresponding references. An additional resistor R10 is connected in series with the first conductor 110 the voltage across the resistor serving as an input to comparator IC1. The comparator is configured to compare the voltage across R10 with a threshold voltage set by a pair of fixed resistors R8 and R9. As the voltage across R10 exceeds the threshold voltage IC1 will change state giving rise to an output signal which may be fed to a leak alarm 124.

The second function of the system, that of indicating the position of the leak, has already been described.

This function may be best achieved however, by converting V(R6) and V(R7) to a digital format and then performing the division automatically using a digital electronic circuit.

Figure 4 shows an electronic circuit suitable to perform the above operation in which IC2 and IC3 are analogue to digital convertors connected across R6 and R7 respectively, 1C4 is a digital divider connected to the outputs of IC2 and 1C3 while IC5 multiplies the output of IC4 with a value determined by the length of the sensor and drives a digital display 126 indicating the distance along the sensor to the leaking liquid.

All the components employed may be commonly used electronic items, with the exception of the matched pair of R6 and R7, and this contributes to the system being economical to manufacture.

There are no specifically preferred materials other than that of the high resistance conductor which must be chosen to give the correct order of resistance for any given diameter. For example, in the 100 metre system used to illustrate the invention the standard conductors are 1.5 mm in diameter. To obtain a resistance of 1.5 Megohms and maintain the same diameter the conductor requires a resistivity of approximately 2.6 ohm-cm. This order of resistivity is not commonly found among metals and therefore it is preferred that this conductor be made of a semiconductive plastics material. Such Such'materials, for example carbon filled polytetrafluoroethylene can be easily made with the required resistivity.

Claims

1. A system for the detection and location of liquids comprising a sensor in the form of three conductors arranged to extend in side-by-side relationship, two of the conductors being isolated from each other by insulating means that will permit the establishment of a conductive path between the two isolated conductors in the presence of a liquid creating at least two current loops in which currents may flow, at least one of the conductors connected by such a path having a resistivity that is predetermined throughout its length and a resistance that is a multiple value of the resistance of the or each remaining conductor.

2. A system according to claim 1 wherein the resistivity of at least one of the conductors connected by the establishment of the said conductive path is greater than a thousand times that of the or each remaining conductor.

3. A system according to claim 1 wherein the resistivity of the said one conductor is substantially uniform throughout its length.

4. A system according to claim 1 wherein the said one conductor is of semiconductive plastics material.

5. A system according to claim 1 wherein the presence of a liquid is indicated by an alarm that is activated when a current flowing in a current loop causes the potential across a resistor within the current loop to exceed a preset threshold value.

6. A system according to claim 1 wherein the location of a liquid is determined by measuring the relative magnitudes of the currents flowing in each of the current loops created by the establishment of the said conductive path.

7. A system according to claim 1 wherein the sensor is in the form of a cable.

8. A system according to claim 1 wherein the conductors are arranged coaxially.

9. A system substantially as and for the purpose as herein described with reference to one or each of the embodiments depicted in Figures 2, 3 and 4 of the accompanying drawings.