WATER WORKS FROM FIRE AND INSURANCE STANDPOINTS*
(Continued from page 212.)
Water Available for Fire Use From a Gravity Water Works:
Example No. 10—We will assume the same pipe line system as shown on Sketch No. I, and that instead of a pumping system, a reservoir of ample dimensions is located on a hill or formed by a dam across a valley at pumping station location hereinbefore considered and at such elevation as to give eighty pounds of static† pressure in the business center.
By Table D we find 80 pounds = 185 feet head, approximately. Therefore, the reservoir is 185 feet above the business center. Hydraulic Grade—
We will assume that the first 5,000 feet of the 16-inch main from the reservoir is laid across a flat country, on a grade of 0.15 per too feet = 1.5 per 1,000 feet = 7.5 feet in 5,000 feet.
By Table D we have 1.5 feet per 1,000 feet (the total fall in 1,000 feet) corresponds to 1.5 X 0.433 = 0.649, say 0.65 pound pressure lost per 1,000 feet when the line is delivering its maximum amount of water on this grade, without any head over pipe in reservoir except for proper influx of water.
Looking in Table H under 16 inches we find this loss of head opposite 1,600 gallons per minute, as the maximum amount of water that can be delivered per minute = 1,304,000 gallons per day.
Under example No. 3 we found the equivalent length of our main line to Wilson Avenue to be 64,250 feet of 16 inch. Therefore, in delivering to the business center the maximum capacity of the upper end of the 16-inch line, or 1,600 g.m., we would lose 64.25 X 0.65 pounds = 41.76 pounds, say 42 pounds which, taken from 80 pounds, our normal pressure, leaves but 38 pounds at the hydrants. This is too low a pressure for direct fire streams without use of engines.
We ought not to have less than 50 pounds net at the hydrants. This would allow 80 — 50 = 30 pounds for loss in main line friction in equivalent of 64,250 feet, or say in 65,000 feet = 0.461 pounds per 1,000 feet. Looking in Table H under 16 inches we find 0.44 opposite 1,300 g.m., or slightly in excess of r.300 g.m. is the maximum amount of water per minute we can deliver under 50 pounds in the business center under a static head = 80 pounds, with no allowance for loss in hydrant branches.
In the above we have 185feet fall from reservoir to the business center in a pipe line the equivalent length of which is 64.5 thousand feet of 16 inch; the rate of average grade or fall is therefore 2.87 feet per 1,000 feet. If there was no point of the pipe line above this imaginary grade line, our maximum delivery under no pressure head at the outlets or hdyrants would be as follows:
2.87 feet X 0.433 (by Table D) = 1.242 pounds loss in pressure per 1,000 feet. By Table H we have 0.945 pounds per 2,000 g.m. and 1.43 pounds for 2,500 g.m. By proportion we have, therefore, 1.242 corresponding with 2.305 gallons per minute as the maximum delivery of the line if laid below “hydraulic grade,” or 705 gallons per minute greater in maximum capacity than if the upper end is laid on flat grade of t.5 feet per 1,000 feet, as assumed. But it is assumed as laid on grade below that necessary to give discharge under 50 pounds pressure of 1,300 g.m. or below a (0.44 X 2.31) — 1.016 per 1,000 feet grade.
Suppose we assume we want a hydrant working pressure of 40 pounds at the Old Ladies Home (see Example No. 6. Fire No. 2) from the reservoir with the static pressure at 58 pounds, what quantity of water can we obtain ?
Static pressure . 58 lbs.
Working pressure .. 40 “
Difference . 18 “ to be lost in friction in a line the equivalent length of which was found to be 364,000 feet of 16 inch, or a loss of 0.0438 per 1,000 feet, or say 0.044 per 1,000 feet, and which we find by Table H to correspond with a flow of 350 gallons per minute, or two small fire streams, each through about 150 feet of 2 1/2-inch hose, with I-inch nozzles, with about 32 pounds nozzle pressure, with the streams being thrown about 50 to 60 feet, depending on the wind. If the supply is by gravity this is too small a delivery to properly protect; and the line on Congress Street should have been at least 8-inch. Supply By Gravity Without Use of Fire Engines.
Fire No. 6—Let us assume Fire No. 6 to occur in a church at the corner of Wilson and West Avenue and that hydrant No. 17 is in use, and that the only line feeding the same is 16 inch from reservoir to Congress Street, or 15,000 feet of 16 inch, then 8,500 feet of 8 inch on West Avenue, and that we want a working pressure of 50 pounds.
We have, by Table G:
15,000 feet 16 inch = 15,000 ft. 16 in.
8,500 feet 8 inch = 8,500 X
28.8 = 244,800 “ “
Equivalent line in 16 inch 259,800 “
Or say 260,000 ft. 16 in.
Assuming static pressure at 80 pounds, we have 80 — 50 = 30 pounds as loss in friction. Assuming delivery at 700 g.m. our loss in hydrants and branch, as per previous assumptions, would be 3.5 pounds, leaving 26.5 pounds as loss in pipe line, or say 26 pounds, 26 / 260 = 0.1 pound per 1,000 feet. Looking in Table H we find 0.11 for 600 g.m., or, with final corrections for flow of 600 instead of 700 g.m. through hydrant and branch, we have a flow of nearly 600 g.m., or about three fair fire streams through reasonable lengths of hose, as considered in Part 2.
Delivery From Reservoir and Standpipe Simultaneously.
Now suppose at this same fire we are having water delivered through the above considered 16 inch-8 inch line from the reservoir and from a standpipe 15 feet in diameter and high enough to give the same 80 pounds static pressure and situated 2,000 feet southerly (off the sketch) from hydrant No. 17; what would be the delivery under 50 pounds working pressure?
It is evident that the pipe line from the reservoir and the one from the standpipe will act as independent lines, each with a total loss in pressure of 30 pounds and which loss in each case will be divided between loss in main friction and in hydrant branches and. hydrants.
At first let us eliminate the loss in hydrant and branch in order to find directly an approximate—yet high discharge.
In the case of the 8-inch line we have a total loss of 30 pounds, or 15 pounds per 1,000 feet, and without loss in hydrant and branch, by Table H. we have a little over 1,400 g.m.
In the case of the 16 inch-8 inch line from the reservoir, we have 30 / 260 — 0.115 pounds loss per 1,000 feet, and by Table H this corresponds with a flow of 600 “
Total discharge 2,000 “
without considering loss in hydrant and branch.
As we see that the flow from the standpipe will be a little over double that from the reservoir, we can assume that the loss twothirded of the loss in hydrant and branch will be chargeable to the standpipe line and onethird to the reservoir line, or assuming a total flow of 1,800 g.m. under the conditions, a 0.5 per 100 gallons flow, we would have a total loss of 9 pounds, of which 6 pounds would be for standpipe line and 3 pounds for reservoir line. Deducting, we have: 30 — 6 = 24 pounds per 1,000 feet for standpipe line, and 27 pounds / 260= 0.104 for reservoir line per r,ooo feet.
By Table H we have:
Discharge for 8-inch line (use proportion) 1,155 g.m.
Discharge for 16 inch-8 inch line
(use proportion) 580 “
Total discharge under 50 pounds net pressure 1,735 g.m.
Or in as close agreement with our assumed discharge of 1,800 as possible, or prudent to suggest, for no two pipe lines are alike in discharging capacity, due to good and bad laying, different makes of hydrants, etc.
The solution of the problems presented by fires No. 7 at G, and No. 8 at H, are left to those interested, both with and without the local standpipe, assumed in various locations.
Proper use of the tables and data will permit of the solution of almost any problem of flow in pipe lines with a degree of accuracy quite sufficient in most cases.
* Examples showing some uses of Tables G and H, etc., on page 820. See map on page 212.
† Normal pressure is less than static, depending on amount of flow for ordinary uses, exclusive of fire stream flow.
Copyright, 1914, by J. B. Rider.