ARRANGEMENT OF SUCTION AIR CHAMBERS ON PUMPS.
What is the advantage in using a large air-chamber placed upon the suction pump? Mr. F. Meriam Wheeler, discussing this subject in a paper read before the American Society of Mechanical Engineers, calls attention to several arrangements of such chambers, good and bad. with which he has had experience, and points out how few appreciate that it is quite as important to provide them on the suction connection as it is on the discharge side of a pump, whereby water hammer is prevented, with its attendant evils.
“It must be remembered, however (he continues), that the moving column of water has considerable dynamic energy, and this should be utilized to improve the efficiency of the pump and not be a detriment to it. To avoid the noise and serious effect of water hammer, a suction air-chamber should not only be used, but it is most important that it is properly located and of sufficient size. Experience shows that water or other liquids, passing under or across the opening of an air-chamber placed at right angles to the flow, will cause the pump to pound about as much as if no air-chamber were used—except at a low rate of speed. Therefore, in arranging suction airchambers I always urge that they be so located that the energy or momentum of the column of water can be expended directly upon the confined air in them. In other words, it is very necessary to get the proper cushion effect for the column of water while the piston of the pump is reversing at each stroke, when running at anything but very slow speed.
“I recently tested a small Blake ‘simplex,’ compound, steam pump, not only to demonstrate the advantage of the suction air-chamber, but also to show the respective merits of two arrangements of such suction air-chambers. As shown in Fig. No. 140, one arrangement was to have the suction air-chamber on the opposite side of the pump to where the supply entered, and placed on an elbow. The other arrangement was the location of the suction airchamber in a direct vertical line with the suction pipe, the air-chamber being placed on a tee. Gate valves were provided so that either or both suction air-chambers could be shut off and opened’ at will. At a slow speed, with both chambers out of use, the pump ran quietly enough, but when the number of strokes was increased to a fair rate of speed water hammer was the result. To give an idea of the serious effect water hammer has on the piping as well as on the pumps themselves, I would call attention to the fact that this pump (intentionally left unbolted to its foundation, with the piping entirely free So move), at eighty double strokes per minute, produced water hammer sufficient to cause the suction pipe to vibrate at each stroke of the pump, at least one-half inch horizontally. When either suction-chamber was opened there was no perceptible movement in the piping, and the pump ran absolutely without jar. The pump drew its supply from a tank below, the total suction lift being about five feet, while the length of horizontal suction pipe was about twenty feet. The indicator cards taken, and submitted herewith, are quite an interesting study. All the cards were taken while the pump was running at about eighty double strokes per minute, with a water pressure of seventy-five pounds per square inch, the pressure in the steam chest of the high-pressure cylinder being about sixty pounds. Fig. No. 141 is an indicator diagram taken when the pump was running with both suction air-chambers cut off, while Fig. No. 142 is an indicator card taken at the same time from the suction pipe at a point close to the pump. This latter card graphically demonstrates what “water hammer” means. Fig. No. 143 shows an indicator card taken from the water cylinder of the pump with one suction air-chamber in use—the one iocated on the tee connection. Fig. No. 144 represents an indicator card taken at the same time from this suction air-chamber. The gate valve on the firstnamed chamber was then closed, and the valve on tlte other suction air-chamber, placed on the elbow at the opposite side of the pump, was opened. Fig No. 145 shows an indicator card taken from the water cylinder with this elbow style of suction-chamber; while Fig. No. 146 shows a card taken at the same time from the suction air-chamber itself. It will be seen from these indicator cards that the suction airchamber located on the elbow was more efficient than the other (tee style) suction air-chamber. The gate valves were wide open when the cards were taken from the suction air-chambers, but it was noticed that when the gate valve on the elbow chamber was opened it required only about one turn to stop the water hammer, while in the case of the chamber placed on the tee, it required nearly two turns of the valve to get the same quiet effect. The suction pipe of the pump was a two-inch size, hence had a cross section area of 3.14 square inches. With the gate valve one turn open it was found, by careful measurement, that the area of the opening was about 0.114 square inch. With the valve two turns open the actual opening was 0.78 square inch. Before completing the lest the pump was worked up to the extreme of 120 double strokes per minute, and at this speed it continued to run quietly, there being no vibration of the pump or pipes.*
“The illustration 1 (have already! referred to in the discussion (on another] paper, and which I desire to repeat here, was the case of the installation of a certain 1,500-horse power compound stationary engine, where the circulating steam pump, which supplied a surface condenser, had nearly 400 feet of fourteen-inch suction pipe. I urged the use of a suetion air-chamber, and understood it would be arranged as shown in Fig. No. 147. When visiting the place later on 1 was not surprised at the complaint about the noise made by the pump, as I found they had not properly located the suction air-chamber, having placed it at right angles to the horizontal suction pipe, as shown in Fig. No. 148, explaining that, for certain reasons, they could not approach the pump with the suction pipe on a vertical line, as was originally intended. The trouble was corrected by removing this suction air-chamber from the position in which they had put it, and placing it on the opposite side of the pump with a suitable elbow, as shown in Fig. No. 149. Thus the impact of the water was received over and across the water barrel of the pump into the suction air-chamber. It is hardly necessary to say that after this change was made the pump worked with perfect freedom from water hammer. In marine practice it is often very difficult to locate properly a suction air-chamber, owing to limited space. When not convenient to arrange suction airchambers on either plan as shown in Fig. No. 140, as, for instance, when the suction approaches the pump horizontally, then the arrangement shown by Fig. 150 is very satisfactory.
“If the manufacturers of pumps would take the trouble always to recommend the use of suction airchambers, and the pipe fitters were made to locate such air-chambers properly, then there would be less complaints about jar and noise in pumps, to say nothing about the saving in wear and tear. This remark applies to pumps of all types, whether single or double-acting, vertical or horizontal; whether receiving water under a pressure or drawing the supply by suction-lift; and especially to pumps which are liable to run at the higher speeds, as, for instance, in the case of fire pumps, ash-ejector pumps, wrecking pumps, etc. There are thousands of cases of noisy pumps which could be entirely relieved from the ill effects of water hammer by the use of properly located suction air-chambers.”
The Identical pump upon which those experiments were made was afterwards shown in operation under similar conditions, in the mechanical laboratories of Columbia Univer, site, it having been connected up especially for this occa sion.
The leaders in the movement at New Haven. Conn., to secure municipal ownership of public-service corporations have brought out the first number and prospectus of a small weekly paper, to advocate city ownership of the water, light, telephone, and trolley properties. The first number contains an article upon the water company, a new contract with which, involving the purchase power of the city, is now before the city council.