Receiving Water Under Pressure.
In Vol. XIV. of the “Transactions of the American Society of Mechanical Engineers” may be found reports of tests made on pumps which were receiving water under pressure. These are well worth reading, but as they are probably not accessible to many of our readers, we briefly mention the conclusions arrived at. The pumps tested were so piped up that they could take water under pressure or from wells. It appears that although the water could be delivered to the pumps under pressure it had never been done, but was allowed to run into the wells, from whence it was pumped out and delivered into the service pipes.
Now it requires something of a bold nature to enable an engineer on taking charge of a plant of any kind to make such a radical change as to connect the receiving pipe directly to the pump cylinder, for while he may know that such a course will result in a saving of steam, provided the plan works well, still on the other hand if it does not, it may do more damage in ten minutes than the saving in steam would amount to in ten years, and the loss of his position would probably be the result.
The possible trouble to be guarded against in such a case as this is that a heavy body of water will be put in motion by the water piston of the pump, and when the piston stops at the end of the stroke the water will continue to travel on and will strike a heavy blow called “ water hammer,” and the result may be a wrecked machine.
The remedy is to provide an air cushion that will bring the water to rest without shock or jar, and to so arrange it that the air chamber shall never become inoperative. To put an air chamber or a suction chamber on a pump is a very simple thing, but it is also very simple and very easy to get it on in such a way as to make it practically useless.
Suppose that we are running a horizontal suction pipe to our pump and put a T in this pipe, looking upward, and screw an air chamber into it. Will this give us an air cushion ? It will not, because the water has no tendency to diverge from its direct course and go up into the air chamber so formed. If we cannot get a chamber on so as to receive the impact of the water in any other way, we may put it on the opposite side of the pump, as nearly all pumps are provided with inlets on both sides of the water cylinder, and so, when the water rushes in on one side it will strike the air cushion on the opposite side and there will be no jar.
In the test of the pumping engine referred to it was started up in the ordinary way, taking water from one of the wells and running at twenty-five strokes per minute. Then without varying the steam pressure or changing the throttle valve, the ater was turned on under pressure and the valve in the suction pipe to well closed. The result was that the speed was increased to thirty-three strokes per minute, using the same amount of steam, and the machine ran quietly. From this it will be seen that the experiment was a success and a large saving in steam effected, as it amounted to about twenty per cent when running the pump at the same rate of speed as it was customary to when taking water from the well.
Now a saving of twenty per cent in the amount of steam that it requires to run an ordinary boiler feed pump is a small item, but if we can in all cases arrange our suction chambers that our pump will run quietly, it will be a great improvement over what some of us are now doing. It is claimed that the suction chamber is under pressure it will fill solid with water, but this claim is not realized in practice, for the air goes to the highest point in such a case and will remain there a long time, provided the chamber is tight.
It is a good idea to have a small valve in the top of the air chamber, and when after running a long time the air is exhausted, it may easily be replaced by partially or wholly shutting off the water supply and opening this small valve, which will admit the air. On large pumps it is customary to have a water gauge with a glass tube to show the water level it.
We have said that if the air chamber is simply connected to the opposite side of the pump from where the suction pipe enters, it will be effective, but this statement must be qualified, for in some small pumps the passage from one side of the water cylinder to the other is so tortuous or crooked that it counteracts in a large measure the good effect that otherwise would be realized from the air chamber. If this passage is a straight one, then the air in the chamber will act as a cushion and bring the column of water quietly to rest.