Regulating Water Pressure with Balancing Reservoirs
The Night “Rest Period” Transformed Into One of Work to Equalize Pressure in the Daytime—Steel Tanks Are Used for This Purpose
THE plan described in the following paper will prove of especial interest to those superintendents who are wrestling with the problem of fluctuation in consumption and how to meet it.
Balancing reservoirs have an every-day name in Los Angeles. We call them “kickback tanks,” a self-explanatory name. That is just what they are. They fill up with water at night and kick it back out the next day. The principle of their action is not new. Any reservoir placed at the end of a water main farthest from its source of supply, acts as a kickback tank. By its use a main trunk or feeder line supplying a water distributing system along its length is made to work twenty-four hours a day instead of fourteen or fifteen hours as is usually the case when not double headed by reservoirs at each end.
Chart Resembling Cross-Section of Egg
A twenty-four hour pressure gauge chart of the circular type was clocked on a water main having a reservoir at one end only. Charts such as this almost invariably resemble the cross-section of an egg along its major axis. This, as we well know, is due to the fact that during the hours from 6 a. m. to 9 p. m. the draft on the main is heavy, and the resulting pressure is low, thus forming the small and pointed end of the egg. The larger and more rounded end is formed during the somewhat shorter night period of low consumption and corresponding high pressure in the main.
Complaints of Lack of Pressure
Let us assume that our trunk line has been designed to care for only a moderate increase in consumption. Then suddenly, as quite frequently happens, and especially so in California cities, it becomes burdened with an abnormal and unexpected demand upon its capacity. Inspection of our pressure charts show that the pointed end of the egg is becoming shorter and flatter and may in extreme cases become concave at the points of peak load. Our patrons begin telephoning, complaining of poor pressure, and insisting that we do something to relieve the annoying and unsatisfactory condition. Our first thought of course is to enlarge our trunk line, and we at once begin making estimates of costs. The existing main may be a perfectly good cast iron pipe, capable of giving many more years of service. The cost of enlarging or paralleling it may mean the expenditure of thousands or even hundreds of thousands of dollars, depending on its size and length. In water works, the same as in any other business, the financing of enlargements and betterments is an ever present worry. Engineers are constantly faced with the necessity of building their works not more cheaply, but more economically.
Making Rest Period One of Work
With this aim in view we endeavor to find some means of prolonging the usefulness of our trunk line without expending too much of our funds. We again consult our pressure charts, but instead of viewing with alarm the ever increasing flatness of the small end of the egg we discover that there has been practically no change in the large rounded end and we ask ourselves if that nine or ten-hour period cannot be made one of work instead of rest for our trunk line.
If from our charts, which have been made at several points along the length of the main, we plat hydraulic grade lines for different times of the twenty-four hour period, we find that during the periods, 6 to 10 a. m. and 5 to 9 p. m., the hydraulic grade lines have a very marked downward slope throughout the entire length of the main, and, in extreme cases may almost reach as low as the main itself at the end farthest from the reservoir or source of supply. During the night however, from 9 p. m. to 6 a. m.. the grade lines will extend nearly horizontal from the water surface in the reservoir at the head of the main. In other words we have static pressure. The main is idle. Plenty of water but no place for it to go. Here then is a tentative solution of our problem. Provide a place at the end of the main for the water to go during the night hours so as to be ready for use bright and early the next morning, at the points where it is most needed. Build a reservoir or tank. And, fortunately, it does not have to be large, a small one is usually sufficient. It is well to remark here of course, that this solution of our problem is not always possible. I he topography of the ground may not permit a tank to be built at the proper elevation. The main may be so extremely small in proportion to its duty as to make its enlargement imperative. In many instances however, the principle of the kickback reservoir may be utilized very economically.
“Speaking briefly of the success of such reservoirs in Los Angeles, I will say that they have not only been money savers, but have enabled us to fairly well keep pace with the unprecedented growth of the city. Fortunately the topography in our vicinity is adaptable to their use. High towers are unnecessary. For domestic use our practice is to construct riveted steel cylindrical tanks which permits of quick and reasonably cheap construction.”
(Continued an page 136)
Regulating Pressure with Balancing Reservoirs
(Continued from page 108)
Determining Elevation and Size of Reservoir
If, in the further study of our problem, we conclude that conditions are right for the construction of such a reservoir, the next step is to determine the elevation at which to build it and to what size. These two factors are a function of each other, and both are dependent upon the capacity of the main, and the length of the period during the night when water will be flowing into it. In Los Angeles it is common practice to locate the reservoir at such an elevation as will provide a friction head of about thirty feet in the main.
Alternatingly Fills and Empties Each 24 Hours
By taking a series of twenty-four hour pressure gauge readings on the main, we determine the average length of the period during the night when the main is idle. With a friction loss of 30 feet—or whatever allowable loss we may adopt as practicable —it is a simple problem of hydraulics to compute the amount of water which will flow into the reservoir during the idle period at night. The proper size of reservoir is that which will contain—or slightly more than contain—this amount.
Such a reservoir, properly designed, will alternately fill and empty each twenty-four hours, and if hydraulic grade lines are platted under the new conditions, we will find that, instead of sloping uniformly downward as before, they will assume more the position of a suspended cable or catenary, almost as high at the outer end of the main as at the head. Now the main is performing useful work twenty-four hours a day. Complaints of poor pressure and lack of water cease coming in. We breathe easy once more and feel justly proud of our accomplishment.
Have Proved Money Savers
Speaking briefly of the success of such reservoirs in Los Angeles, I will say that they have not only been money-savers, but have enabled us to fairly well keep pace with the unprecedented growth of the city. Fortunately, the topography in our vicinity is adaptable to their use. High towers are unnecessary. For domestic use our practice is to construct riveted steel cylindrical tanks which permits of quick and reasonably cheap construction. An altitude valve or pressure regulator is placed in the main near the tank which automatically shuts off the flow into the tank when filled. The capacity of the tanks is usually from 300,000 to 600,000 gallons. One, however, located at the southerly terminus of a 36inch main, eight miles in length, on Western Avenue, is 150 feet in diameter, 50 feet high, and has a capacity of about seven and one-half million gallons. Its elevation is such as to allow about eighty feet friction head in the main. It also, is of riveted steel construction, and was built in 1917. This tank has fulfilled its purpose exceedingly well, and is still doing so in spite of the increased demands made upon it. Its construction, which cost about $90,000 obviated the necessity of duplicating this eight-mile line of 36-inch pipe, the cost of which, as you can readily see would have been many times the cost of the tank.
(Excerpts from paper read before annual convention of the California Section of the American Water Works Association.)