LOW WATER PRESSURE

Otago Daily Times, Issue 23223, 22 June 1937, Page 7

LOW WATER PRESSURE

TROUBLE AT PORT CHALMERS REPORT ON SERVICE Following complaints received recently on the lack of water pressure in some parts of Port Chalmers Borough, the Works Committee of the borough undertook to investigate the matter fully. At the monthly meeting of the council last night a report from the engineer was received, setting out the probable causes of the low pressure and alternative means of improving the situation. No action was taken, however, the chairman of the Works Committee (Cr H. Watson) stating that his committee was not prepared to make a recommendation until the position had been more fully investigated. ENGINEER’S REPORT The engineer (Mr F. J. Williams) reported that the water officer had spent some time in testing mains, and that he had found the following:— 1. That in Wickliffe terrace main there was a difference in quantity and pressure between the two last fire plugs with the Laing street valve closed. 2. In the Meridian street four-inch main there was a good flow at the top of the steps at the first fire plug, but a poorer flow at the fire plug past Fox street on the three-inch main. It was worse with the valve closed on the four-inch main from Wickliffe terrace.

3. The same result was found at the top of Laing street in Harrington street. It was apparent that no flow was obtained from Currie street, Grey street, and Wickliffe terrace in this main. These results pointed to the fact that cleaning of the mains would require to be undertaken if a better pressure and supply were desired. “I have made the necessary surveys,” the engineer’s report continued "to enable me to plot a longitudinal section of the trunk main from the reservoir to the band rotunda in George street. The total length is one mile 714 chains. On plotting the survey data the longitudinal section obtained showed that when the reservoir is full (which is the best condition for service) the main rises above the mean hydraulic gradient in two places, but for practicable purposes the first place —namely, the Dunedin-Pnrt Chalmers highway—governs the discharge of the trunk main. When any portion of a pipe line rises above the mean hydraulic grade line it results in a condition of unsteady flow with irregular turbulence and a tendency to set up water hammer.

“The maximum quantity of water that can be delivered in George street is governed by the capacity of the trunk main between the reservoir and the top of the hill. I am advised that beyond the outlet pipe the whole of the main is seven inches in diameter, and it follows that, whereas the supply is governed by the discharge of the seven-inch main at the point where it rises above the main hydraulic gradient, the line below this point is too large in comparison; in other words, there is a turbulence in the main at this point. “ Comparing the results which were submitted by the superintendent of the fire brigade and those as calculated from the information now obtained, we find that at this point the discharge by the testing apparatus showed 258 gallons per minute available, with a resulting head of 481 b to the square inch. My levels show that the actual pressure at this point is 62Jlb per square inch and that the discharge by cast iron pipes in ordinary condition would amount to 363 gallons per minute. Some of this difference is due, no doubt, to the disturbance at this particular point. This is further exemplified by the results obtained at the fire plug on Reservoir road, where the testing apparatus obtained 460 gallons per minute with a pressure of 93ib per square inch. The levels indicated that the pressure should have been 9741 b per square inch and the discharge 524 gallons per minute. It can, reasonably be followed that the testing apparatus depends upon certain conditions and combinations and can be accepted only in a general way. The conclusion, therefore, at which I have arrived is that the trunk main is in a fair condition with perhaps the exception of the nine-inch main which passes under the reservoir, and that the main rising above the mean hydraulic gradient distinctly restricts the quantity of water that can be delivered. “In all authorities I have been able to consult and in my own experience I have found that it is wrong in principle to have any portion of a pipe line rising above the mean hydraulic gradient. The difficult'”’ can be overcome in two wavs:

(1) By placing the line below the mean hydraulic gradient; and (2) By building a service reservoir at such a height as will bring the whole main above the mean hydraulic gradient.

“I believe the construction of a service reservoir is much the preferable course to adopt, for the reason that the main tan be discharged into this reservoir during the whole of the 24 hours and so no limit is placed above its capacity at any particular moment. “ I recommend that surveys be continued and that a site for a proposed service reservoir be investigated and that the cost of its construction, together with the cost of continuing the main to the reservoir and a new main from the service reservoir to the town reticulation be ascertained. NO RECOMMENDATION

Cr Watson said that so far only an investigation of the position was being attempted. At the next meeting they would have a further report. He assured the council that the Works Committee would not bring down a recommendation until the matter had been fully investigated. On the suggestion of the Works Committee the report was received without discussion