Detection of Leaks in Underground Pipes
Description of Methods of Detection — Use of Sounding Rod —Aquaphone — Leak Locator— Pulsograph—Water Hammer and Hydraulic Gradient Characteristics Helpful
H. E. BABBITT
Assistant Professor Municipal and Sanitary Engineering
University of Illinois
THIS subject is constantly before the average waterworks man and seldom fails to arouse interesting accounts of experiences in detecting leaks. In spite of the widespread experience of waterworks men in this line the publications of the American Waterworks Association are strikingly barren of information concerning it. We must turn to waterworks periodicals, the publications of other associations, and to our personal experiences to find accounts of methods in use.
It is the aim of this paper, originally read before the annual convention of the Illinois Section, American Water Works Association, to describe only a few of the more common methods for the location of leaks in underground pipes. In order to reduce the extent to which this subject can be expanded, the ground covered will be restricted to methods for the exact location of leaks after their existence and their approximate location are known. The advantages to be gained by leak and waste surveys and methods for the determination of the existence of leaks and waste will not be included.
Observation Simplest Method
The simplest method for the location of leaks is by observation, if that can be called a method. It could scarcely seem to take much experience to say “there’s a leak” if water is seen bubbling up in a city street, but while the visual evidence may be sufficient to demonstrate the existence of the leak, the exact location of the break in the pipe may not be immediately below the point at which the water is appearing. However, the first thing to do is to dig at the point, and if the water is found coming in from the side of the hole to follow along until the leak is found or to use one of the methods subsequently described. A water works superintendent who observes a luxuriant growth of vegetation along the line of his water pipes, in the midst of a hot dry spell, is safe to assume that his pipes are leaking nearby, and that an appreciable saving can be made by stopping the leak located there. If a sewer or drain pipe is seen to be discharging continuously even during the driest weather, suspicion should be directed to the line of water pipe that the drain crosses, as contributing the flow through a leak.
Use of Sounding Rod
Leaks discovered by observation in this fashion may follow along the pipes for some distance before making an appearance. If the leak is not located in the excavation made at the place of appearance, a pointed steel rod about to an inch in diameter and five to six feet long can be driven down to the pipe through most soils If on withdrawal the rod is found to be moist the indications are that the leak is further up grade, and on withdrawing the rod in a dry condition the leak has been passed and lies between the last two points of driving the rod. Such a rod is called a sounding rod.
Sounding rods have no relation to the detection of leaks by sound which is the most common manner in which leaks are located which is worthy of the name method. The sound caused by a leak from water pipe is a function of the velocity of flow, the size and condition of the opening, and whether the leak is discharging under water or into the atmosphere. A high velocity through a small aperture will make more noise than a large discharge through a large aperture. A free discharge will be more noisy than a submerged discharge. It is a well known law of physics that sound will travel faster and further through a solid body than through gas. If a leak is taking place in a pipe the sound will be transmitted to the pipe and carried for long distances. The sound can be picked up by placing the ear against the pipe or some fixture connected thereto, or better by the assistance of some of the instruments made for the amplification of this sound.
The aquaphone or water phone is the simplest instrument and the most commonly used. The parts which go to make up an aquaphone are shown in Fig. 1. The device consists of a solid metal rod soldered to the center of a thin metal diaphragm as used in an ordinary telephone receiver. The point of the rod protrudes from the end of the receiver and is placed in contact with the pipe or fixture. The leak is located by following along the pipe as the sound increases. If the pipe is buried, a sounding rod may be driven down to make contact with it, and the aquaphone placed in contact with the sounding rod. Considerable experience is necessary for the successful use of this instrument.
The detectaphone, the sonograph, the sonoscope, the geophone, etc., are somewhat similar instruments. The volume of sound in some of these instruments is increased by an electrical amplifier placed in the line between the point of contact and the receiver. The receiver is similar to the ordinary telephone instrument with electric coil and magnet. The sound waves set up in the pipe are converted to electric waves in the amplifier ana transmitted to the receiver much more loudly and clearly. It is to be noted that all of the instruments described so far require direct contact with the pipe. This is an undesirable feature as the exact location of the leak in a buried pipe requires that excavations be made in order to reach the pipe, as the sounding rod is not always satisfactory.
The Leak Locator
Darley’s Leak Locator, shown in Fig. 2, is a sound detecting instrument that does not require direct contact with the pipe. A very delicate sound detector is set up on a four-legged brass table which rests on the surface of the ground and is protected from air currents by placing the box containing the electric cells over it when in use, as shown in the figure. The vibration set up in the detector are converted to electric waves which are transmitted to a specially sensitive telephone receiver. The instrument is so sensitive that it will detect the sound of a leak through the pavement and ground at the ordinary depths to which water pipes are buried. The instrument cannot be used in a high wind or where other noises may interfere. Satisfactory results have been obtained by its use.
Water Hammer Useful in Locating Leak
The phenomenon of water hammer is useful in locating a leak in a water pipe. It is well known that if a valve in a pipe line is closed quickly a pressure is suddenly built up in the pipe. This pressure is called water hammer. It differs from the effect caused by a blow in that the pressure is maintained for some time. A wave of high pressure travels from the closed valve up the pipe to a point of relief and a wave of low pressure then returns to the original starting point. The pressure at the valve fluctuates above and below normal until the disturbance has become stilled by the friction of its passage through the pipe, and the pressure returns to normal. Intermediate discharges between the valve and the point of relief will cause fluctuations in the pressures at the valve after the hammer has been created, and if many of these intermediate discharges exist the fluctuations of pressure will be too confusing to be of value. The speed at which the wave travels is affected by the diameter and material of the pipe and other factors and varies between about 3,600 and 4,200 feet per second. In the application of this phenonenon to the location of a leak, the approximate location of the leak should be determined and the line of pipe on which the leak is located should be isolated by the closing of valves so that it is connected to only one large service main, preferably much larger than the pipe in question. It may be necessary to shut off service connections or to appply the method when the services are not being used. A quick shutting valve is attached to a hydrant so located that the leak is between it and the water main, as shown in Fig. 3. The distance from the hydrant to the connecting main along the pipe line should be accurately determined. A delicate recording pressure gage is attached to another nozzle on the hydrant as shown. In the Fig. 3 a Pulsograph is shown. The dial is revolved by clock work. Time is recorded by the vibrations of a tuning fork and variations in pressure are recorded by a delicate pressure gage. After the instruments are connected up and the clock work is running, the valve is opened and water allowed to flow from the hydrant. The valve is then suddenly closed. On the closing of the valve the pressure line on the diagram will jump up as shown in Fig. 4, marked point of closing valve. It will remain here until a slightly relieving wave has returned from the point of the leak when it will drop a little as shown at Point of Location of Leak. The pressure will then remain constant until the low pressure wave has returned from the point of relief. The exact location of the leak is then determined by proportion from the diagram.
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Use of Hydraulic Gradient Characteristics
The characteristics of the hydraulic gradient are useful in the location of a leak. Fig. 5 represents the profile of a long pipe line or a portion of a distribution system so isolated as to result in similar conditions. It is suspected that a leak exists somewhere between station 0.00 and 35.00. If the pipe line is shut off completely except at 0.00 and no leak exists, the gradient will lie along the line A and the pressures throughout the pipe line will indicate this height. If the pipe is opened and no leak exists the hydraulic gradient will be represented by line C, provided the diameter and other conditions are constant. If however a leak exists and the pipe is closed, the pressure at station 35.00 will be less than that at 0.00, it will be horizontal up to the point of the leak and will slope uniformly upwards from there to station 0.00. These lines can be determined by locating two or more points on them, their position plotted on the chart and the leak located at their intersection. This method was used with success by the writer in locating a leak in a five mile pipe line at Nogent-en-Bassigny, France, during the war.
Other Methods of Leak Detection
An ingenious one, based on volumetric displacement, was used for the location of a leak in a pipe line during the construction of the Field Museum in Chicago. A piston was made which fitted the inside of the pipe closely. A Y branch was inserted in the pipe line, the piston was put in the pipe and a cable attached to it was passed through a packed joint in the plug closing the Y branch. The water was then gently turned into the pipe just sufficiently to keep the piston moving. When it had reached the leak it stopped, and the length of cable paid out measured the distance to the leak.
A simple method for the location of a leak in a submerged pipe was to dump a quantity of blueing into the line. The appearance of the blueing located the leak. Other equally difficult and complicated methods are sometimes more effective than those described in greater detail.
If the location of the pipe line itself is unknown there is an instrument available known as the Wireless Pipe Locator by which its exact location can be fixed with little trouble. The apparatus shown in Fig. 6 is manufactured by the Clark Co., of Mattoon, Ill. An electric circuit is made by connecting two points on the pipe line by an electric wire. A battery and vibrator are put in this circuit, the vibrator serving to interrupt current rapidly. An induction coil and a detecting coil connected to a telephone receiver are carried in the hand. When the induction coil held in a horizontal position is brought into the vibrating electric field set up by the electric circuit through the pipe and wire a singing noise is heard in the telephone receiver. The volume of sound increases until directly over the pipe when it ceases altogether. If the coil is then turned into a vertical position the loudest sound is heard.