EXPERIMENTS ON LEAKAGE IN MAINS.
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| | | | | | L P A |
| | | | Air Pressure | Loss of | o e i |
| | | | in Mains. | Pressure. | s r r |
| | | |---------------|-------------| s |
| | | | | | | | C D |
| |System of Mains | Length. | | | | | o e e |
|N| Tried. | | At | At | | | f n l |
|u| | |Begin- | End |During| Per | t i |
|m| | |ning of| of |Trials|Hour. | A . v |
|b| | |Trials.|Trials.| | | i e |
|e| | | | | | | r o r |
|r| | | | | | | f e |
| | | | | | | | d |
--+-----------------+---------+-------+-------+------+------+-------|
| | | yards. | atm. | atm. | | | |
|1|Southern reseau | | | | | | |
| | to Place de la | | | | | | |
| | Concorde. | 9,980 | 6.5 | 6.0 | 0.5 | 1.5 | 3 |
|2| Total reseau | 18,500 | 6.9 | 5.9 | 1.0 | 1.5 | 6.3 |
|3|To Place de | | | | | | |
| | la Concorde | 9,980 | 7.0 | 6.43 | 0.57 | 0.75 | 2.16 |
|4|Total reseau | 18,500 | 6.7 | 5.28 | 0.88 | 1.32 | 5.5 |
|5|Northern reseau | | | | | | |
| | to Rue de Belle-| | | | | | |
| | ville. | 1,530 | 6.0 | 5.0 | 1.0 | 0.6 | 2.3 |
|6|To the Rue des | | | | | | |
| | Pyrenees. | 600 | 6.1 | 3.7 | 2.4 | 0.56 | 2.2 |
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Fargeau station to the Place de la Concorde, a length of 9.142 kilometers; to the whole system of mains, 16.5 kilometers; to the northern mains running from St. Fargeau to the Rue de Belleville, 1.4 kilometers; and from St. Fargeau to the Rue des Pyrenees, 6.5 kilometers. It will be seen from the figures given in the table that the actual loss is small, and it is stated that this is due chiefly to the elastic joint employed throughout the system, excepting in the Rue de Belleville, where rigid couplings are used, and continual trouble is experienced from loss by leakage. In all cases the losses given are the maximum, which only occur under the most unfavorable conditions.
It was found, during the first, second, and fourth tests, that considerable leakage occurred between the St. Fargeau central station and the Rue de Belleville. During the trials two and four, an uncertain amount of loss occurred from the consumption of air required to work the pneumatic clocks, and also motors in the circuit, that could not be stopped. The tests two and four include all losses in the service pipes, as well as the mains.
The production of compressed air at the central station is assumed at 30,000 cubic feet per hour (atmospheric pressure), and in all cases the loss in the mains is taken as a percentage of the total production.
The losses due to resistance in the mains were also examined with great care, over independent sections, as well as through the complete réseau. During the early part of these trials, an unusual and excessive loss was recorded, the cause of which could not be at first ascertained. At intervals along these mains are placed a number of water reservoirs which receive the water injected into the mains; in addition to these the direct flow of the air is interrupted by numerous siphons, the stop valves to branches, etc. Investigation showed that the presence of these reservoirs created considerable resistance on account of an increased and subsequently reduced section. The exact loss from this cause was, therefore, carefully measured, as well as the losses existing in the mains not so interrupted. The results show that the loss by expansion at one reservoir, when the speed of the air flow was 23 ft. per second, was equal to 0.15 atmosphere; with a speed of 29 ft. 6 in. per second, it amounted to 0.2 atmosphere.
Therefore, the presence of five such reservoirs would cause a loss in pressure equal to one atmosphere. This very undesirable arrangement is not repeated in the new system, the sumphs being connected in such a way as not to modify the section of the tube, nor consequently the pressure of the air. The presence of the siphons and stop valves did not seem to affect the pressure to any measurable extent. The following table contains a list of the more important mains tested, and it may be mentioned that the resistance, due to the reservoirs, was at first partially included. The trials were carried out while the mains were not being drawn upon by subscribers.
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| |
Section of Mains Tested. | Length. |No. of
| |Tests.
| |
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| yards. |
From the central station to the end of reseau and | |
back to central station by return circuit | 18,100 | 7
From the central station to the Rue Fontaine au |\ 14,600 |/ 3
Roi |/ 9,900 |\ 4
From the central station to the Rue de la | |
Charonne | 9,490 | 5
From the Rue de la Charonne to Fontaine au | |
Roi | 4,770 | 3
From the central station to the Avenue de la | |
Republique | 1,860 | 8
Various trials on different lengths of mains |770 to 8,000| 11
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Over the whole system of 16.5 kilometers, which was also tested when no air was being taken off, there were four reservoirs of considerable size, and which offered a large resistance with a corresponding loss of pressure; on the line there were also 23 siphons and 42 stop valves.
These trials were repeated several times to secure accuracy, and the speed of the air was brought to 49 ft. a second. The results obtained in one of these trials may be taken as an example. The main between the Rue St. Fargeau and the Fontaine au Roi, on which there are no collecting reservoirs, but three siphons and eight stop valves, gave, with an average speed of 21 ft. 3 in., a loss in pressure of 0.05 atmosphere for each kilometer of main.
From these experiments it would appear that, assuming a speed of 21 ft. per second, a loss in pressure of one atmosphere would correspond to a distance of 20 kilometers; that is to say, a central station could extend its mains on all sides with a radius of 20 kilometers, and the motors at the ends of the lines would receive the air at a pressure 15 lb. less than at the central station. Professor Riedler states that as an actually measured result, the velocity of the air through the mains of the St. Fargeau system is 19 ft. 8 in. per second, and that the loss in pressure per kilometer is 0.07 atmosphere. From this it follows that including the resistances due to the four reservoirs, and other obstructions actually existing, an allowance of one atmosphere loss on a 14 kilometer radius is ample. By increasing the initial pressure of the air, much better results can be obtained, and future attention in practice should be devoted to this point. The amount of work required to compress air does not increase in the same ratio as the pressure, and for this reason considerable economy can be effected at the first stage, and the loss in the mains will be reduced.