NOTES ON WATER HAMMER. 55

Transcription

1 NOTES ON WATER HAMMER. 55 NOTES ON WATER HAMMER. By A. B. Robison. When the flow conditions of a liquid in a pipe line are varied by the opening or closing of a valve or the equivalent, a change in the pressure conditions is made. The more rapid the change, the more violent the pressure reaction. It has been determined by many authorities that if the change of velocity of flow is made in a time equal to, or less than, the time taken for a sound wave to travel from the point of velocity variation to the end of the pipe, either end, there is a tendency for the column of liquid to break on the downstream side and bank up on the upstream. The broken column of liquid eventually falls back on to the valve with a hammer blow often causing considerable damage. The higher the initial velocity of flow, the greater the hammer blow. Reciprocating type pumps are more prone to " Water Hammer" troubles than centrifugal, but neither type is immune. A. practical demonstration of the creation deliberately of water hammer, and its subsequent utilisation, is given by the hydraulic ram commonly used in hilly districts for water raising. A flow of water through the drive pipe is suddenly stopped. A pressure wave is created which forces open a valve, allowing the water to rush into an air vessel and be gradually brought to rest. As soon as this occurs, the valve to the air vessel closes, isolating the supply of water. The difference in pressure between the water and air in the air vessel, and the water in the delivery pipe, produces a fl ow. The escape valve is adjusted to close when the pressures in the air vessel and pipe line are about equal. This also explains the functioning of an air vessel in a pipe line subject to hydraulic shock. Several authorities have studied the capacity of the air vessel on an hydraulic ram, and its relationship to the size of the pipe line, the head against which it is to operate, and the number of beats per minute. As hydraulic rams with an efficiency of 80 per cent. and over are in existence, the determination arrived at are definitely proved, and are of great assistance in the study of air vessels, bearing in mind that an hydraulic ram has an action similar in principal to that of a single-acting reciprocating pump of the same displacement per stroke. For medium sized rams, the capacity of the air vessel in cubic feet is given by Molesworth as.0078d2h where D is the diameter of pipe line in inches. H is the delivery head.

2 56 VICTORIAN INSTITUTE OF ENGINEERS. This empirical formula gives results larger than necessary if used for mechanical pumps, but are reasonably right for single acting single cylinder pumps. Conditions where an Air Vessel is Indicated using Reciprocating Pumps: The time taken for a sound wave to travel twice the distance from the pump to the open end of the pipe line, or to an abrupt right angle bend, in conjunction with the time taken for the piston to read its maximum acceleration, determines the maximum length of pipe line which can be used without an air vessel. Sound travels at approximately 4000 ft. per second through water. The time taken for the piston to reach its maximum acceleration point each stroke is approximately No. of strokes per min. 240 seconds. If these times are approximately equal, water hammer will definitely be present, but not necessarily of sufficient intensity to vibrate the pipe. The intensity of the hammer or shock varies with the change of velocity, and is approximately 57 lb. per square inch for each foot per second change. This is modified by the pressure wave formula multiplied by the ratio Sound wave time Change of velocity time The effect of an adequately sized air vessel is to smooth out the variations of flow so that the change of velocity is kept low, thereby reducing the value of the shock pressure. The air vessel has only to be of sufficient capacity to take care of the variation in rates of flow above the mean value, consequently, single reciprocating pumps require larger air vessels than duplex, which in turn need larger than triple rain pumps. It is important to examine the suction pipe line for water hammer. One of. the most persistent cases of water hammer I have experienced was in a suction pipe 120 feet long, with a 10 feet suction lift. The delivery pipes vibrated severely and the pump gears stripped, despite an air vessel and a surge pipe. A small suction air vessel cured the trouble. Water hammer is often revealed only by excessive wear taking place on the pump bearings and gears, the rubber inlet and outlet valves being forced into their seats, and by similar signs. Water hammer frequently occurs in starting and stopping, there-

3 ROTES ON WATER HAMMER. 57 fore electric motors should be brought to speed slowly. This particularly applies to Repulsion induction types and motors with direct to line start. Large air vessels should be installed where these types of motors are used. With centrifugal pumps in pipe lines, water hammer is usually only experienced where the plant is shut down quickly. Despite the data given, it is very difficult to decide on paper as to whether an air vessel is essential. Two cases will reveal this where identical pumps were installed with the same pumping rates. (a) 90 ft. static head, 180 ft. of 2 in. piping ; very severe hammer, pump gaskets burst in quarter of an hour. (b) 45 ft. static head, 1100 ft. of 14 in. diameter pipe; plant in operation three years without air vessel before overhaul or renewals required. A third case- 30 ft. head, 40 ft. piping, 10 in. diameter 9 in. centrifugal pump, casing 9/16 thick. Belt broke, pump shattered by the hydraulic shock length of break being 45 in. In reply to questions raised during the discussion, the following information was given by Mr. Robison. Question : When was it advisable to look for water hammer troubles in centrifugal pump pipe lines? Water hammer in pump pipe lines is usually only encountered in stopping. When a quick start is attempted with a belt driven pump there is a tendency for the belt to slip, and it is improbable that the pump would be up to speed sufficiently quickly to induce a hammer wave of any intensity. The same argument applies to an electric motor drive, with a star delta or compensator starter. With the stopping, however, the rate of deceleration is very much faster. In general, except in very long pipe lines where the friction head is a large percentage of the total head, pump delivery has ceased when the speed has dropped to per cent. due to the inherent characteristics of the pump. Accordingly, in larger units deceleration relays are fitted to prolong the period over which the speed decreases, and in the latter installation these were adjusted on site. At the Wangaratta Water Works Trust installation, made about 1929, an air vessel 6 ft. long by about 2 ft. 6 in. diameter, was fitted in a pipe line 10 in. diameter by approximately 1000 yards long.

4 58 VICTORIAN INSTITUTE OF ENGINEERS. The pressure wave in starting was brought to rest in approximately one minute after the push button starter was pressed. On stopping, through deceleration relays, the pressure wave lasted approximately three minutes before being completely damped. Re Rate of Air Absorption. In reply to this question, it was stated that on actual test the following data were obtained. Capacity of air vessel, 40 gallons. Operating pressure, 40 lbs. per square inch. Temperature of water, 50 F. Volume of air N.T.P., 40 gallons, compressed by water rising until balance obtained. First 12 minutes sufficient air absorbed to lower pressure 5 lbs. Following 12 hours absorption produced a further loss of 5 lbs. These figures were obtained in a water supply system where the static head would be 20 feet and all pipe lines were sealed. The air vessel acting as a pressure reservoir.

5 Library Digitised Collections Author/s: Robison, A. B. Title: Notes on water hammer (Paper) Date: 1943 Persistent Link: File Description: Notes on water hammer (Paper)