Practical Training for Auxiliary Firemen
IN this installment of the series of articles on “Practical Training for Auxiliary Firemen” is discussed drafting of water.
Q. What is meant by suction, in connection with drafting of water?
A. Suction may be termed the reverse of pressure, the same as pulling, the reverse of pushing. But the term suction is misleading. The conception of sucking water as the act of pulling or drawing water is erroneous; for to draw or pull an object prescribes the necessity of having some form of connection to it. In the case of sucking up water, what is really done is to reduce the air pressure in an airtight vessel in direct communication with a source of water supply, by removing the contained air, and thus to enable the atmospheric pressure outside to force the water into the vessel.
Q. Specifically, how does a pump raise water when drafting?
A. Reference to Fig. 1 herewith shows how a pump lifts water when drafting. A tightly fitting piston A in cylinder B is moving in the direction indicated by the arrow. For purposes of illustration we’ll assume that this piston is able to create a perfect vacuum in the cylinder. Thus the pressure within the cyinder, to the right of the piston, is zero pounds.
The cylinder is fitted with a pipe C, which has been submerged at its lower end in water in container D.
The container is open to atmospheric pressure, and therefore the atmospheric pressure over the surface of the water in container D is 14.7 pounds per square inch.
The pressure in the cylinder B is zero. Thus the atmospheric pressure will force water up through the suction pipe C into the cylinder.
This is the principle by which water is lifted when drafting.
Q. To what height theoretically can a pumper lift water by suction?
A. Several factors determine the maximum height to which a pumper can lift water by suction. First, the ability of the pump to create a vacuum, which depends upon the tightness of the parts of the pumps; next, the air tightness of all connections; third, the atmospheric pressure at the location at which the pump is being operated.
Q. Assume atmospheric pressure is 14.7 pounds per square inch, and that the pump can create a perfect vacuum; assume further that all connections are airtight. To what height can the pumper pump water under these conditions?
A. A pressure of 1 pound per square inch will raise water a height of 2.304 (this figure is based on a weight of 62.5 pounds per cubic foot of water. The actual weight of water is approximately 62.4 pounds per cubic foot, but the former figure is used because it simplifies calculations). Assuming that each pound pressure raises water 2.304 feet, an atmospheric pressure of 14.7 pounds per square inch will raise the water 14.7×2.304, or 33.8688 or 33.9 feet approximately.
Figure 2 illustrates a typical setup of pumper when taking suction. As will be noted in the sketch, the pressure on the surface of the water is 14.7 pounds per square inch. In this particular problem it is assumed that the pumper is able to create a perfect vacuum, and therefore the suction hose from the pump down to the water has zero pressure within it. The 14.7 pounds pressure will therefore force the water up into the hose, because there is no pressure to resist it, and the maximum height to which it will raise water is, as noted above, 33.9 feet.
Q. As no pumper can create a perfect vacuum, and as there is apt to be some air leakage in the connections, what would be considered a fair height to which water could be raised by suction?
A. 25 feet.
Q. Has water ever been lifted by suction to a higher elevation than 25 feet?
A. Yes. There are instances on record where pumps have operated lifting water 28 feet, and have continued to operate under these conditions for prolonged periods.
Q. What effect have changes in atmospheric pressure upon the ability of a pump to lift water by suction?
A. As the atmospheric pressure becomes less, the height to which water can be lifted by suction is also reduced. This is because the pressure on the surface of the water is less, and therefore the water can be forced to a lesser distance vertically through the suction hose.
Q. What effect has elevation of a city upon the ability of its pumpers to draft water?
A. The higher a city is. the lesser the distance a pumper can raise water by suction. The reason for this is that as higher elevations are reached, the atmospheric piessure lessens. For instance, at Denver, Colo., where the elevation is approximately one mile, the atmospheric pressure is in the neighborhood of 12.1 pounds per square inch.
Q. In what way does the air leakage into connections of suction hose or intake on pumper affect its ability to draft water?
A. Before a pumper can draw a supply of water to its pump, it must first exhaust the air in the suction hose. If serious air leaks occur in the suction hose or in the suction connection on the pumper, the pump may be unable to create the necessary vacuum to raise water. Under these conditions the pump may not be able to draw any water at all.
Q. What remedy would there be for this condition?
A. Tightening up of all connections to make sure they are airtight.
Q. If connections are tight, but pump fails to receive water, where may the trouble be located?
A. The trouble may be with the strainer, which may have become clogged by leaves, or other debris picked up beneath the surface of the water. An examination of the strainer will quickly disclose this condition. Incidentally, when putting a pumper at suction, a rope attached to the strainer is kept taut to prevent the strainer from touching bottom and picking up debris. It is important that the strainer be kept clear of the bottom of a body of water unless there is absolutely no danger of becoming clogged from debris, such as would be the case when drafting from concrete cisterns or other source having no debris along the bottom.
Q. Can all three types of pumps, namely, poison, rotary gear and centrifugal, lift water by exhausting the air in the suction line?
A. No. Only positive displacement pumps, as commonly found in the fire service, can establish a flow or lift water by exhausting the air from the suction line.
Q. If this is the case, how do centrifugal pumps take suction?
A. The majority of centrifugal pumps are fitted with small priming pumps of the positive displacement type, which create the necessary vacuum to bring the water up to the centrifugal pumps. Once the suction line and pump housing of a centrifugal pump are filled with water, and the pump placed in operation, it will continue to draft water.
Q. What types of positive displacement pumps are used to prime centrifugal pumps?
A. Two types are commonly used for this purpose, namely, the rotary vane pump and the internal gear rotary pump. The suction side of the priming pump is connected directly into the casing of the centrifugal pump so that, when the priming pump is operated, a vacuum is created within the centrifugal pump and water rises, filling the pump. Once the pump is primed, the prime is maintained by engaging the impeller of the centrifugal pump, and the priming pump is then disengaged. Usually the priming pump discharges air first, and then water, underneath the machine. When the water shows from this discharge, it is indication that the pump is primed and ready to operate.
Q. Is priming of centrifugal pumps accomplished by any other means than by small priming pumps?
A. Yes. Two methods have been used for priming centrifugal pumps in addition to the small priming pumps. They are: priming by vacuum and priming by utilizing the velocity of exhaust gases from the exhaust manifold.
Q. How is a pump primed by vacuum?
A. In the vacuum method of priming a centrifugal pump, advantage is taken of the natural vacuum created by the alternating compression and expansion stroke of the piston, and the valve action in the operation of the gasoline motor. The principle on which this is based is the. same as is involved in the operation of the vacuum type of windshield wiper. When the gasoline motor is operated, the action of the pistons and valves tends to draw air from the intake manifold into the gasoline mixture entering the cylinders. Thus the connection between the intake manifold and the pump casing is subjected to a vacuum, and since the centrifugal pump has a continuous waterway between the suction intake and the discharge outlet, the entire pump assembly and suction tubes are eventually subjected to a vacuum, causing water to rise within the tube and fill the pump. The priming connection is made to the intake manifold. From here a tube runs to a priming valve reservoir which is fitted with a float valve. A connection from the priming valve reservoir passes into the housing of the centrifugal pump. As water is raised by the vacuum created at the intake manifold, it enters first the pump and fills the pump housing and the overflow enters the priming valve reservoir. Here the float valve is actuated when water reaches a predetermined level in the reservoir and closes the port leading to the tube which reaches the manifold. Thus the water is prevented from entering the intake manifold, which would be troublesome enough to say the least.
Q. How does the exhaust primer work?
A. The exhaust primer utilizes the exhaust gas velocity in the exhaust manifold of the gasoline engine. These gases are directed through a device very similar to an ejector. The ejector consists of a venturi valve to which a connection is made from the pump casing. When the control valve is opened, the pressure of the exhaust gases escaping through the venturi causes a suction pull in the connection to the pump and eventually withdraws all the air from within the casing, thus priming, the pump. The exhaust manifold is so designed as to cut itself out as soon as water has filled the pump casing.
Q. What is the purpose of a churn valve on a positive displacement pump?
A. Each revolution of the gears in a rotary pump or each stroke of a piston in a piston pump discharges a definite amount of water, for these pumps are of the positive displacement type. When a pump is operating, should the nozzle be closed the pump would be instantly stopped and the engine stalled unless some means of taking care of the discharge were provided. To enable a pump to operate after the shut-off nozzles are closed on the hose line, a churn valve is provided. This valve regulates the flow between the intake and the discharge sides of a pump through a bypass. Thus if it is desired to have the pump continue to turn over while no discharge from the line is permitted, the churn valve is opened, and the water flows from the discharge side of the pump back to the intake side through the by-pass and thus flows through a closed circuit. The plain churn valve is simply a manually controlled valve placed in the by-pass between the suction and discharge sides of the pump.
Q. Is the churn valve used for any other purposes than that described in the previous question?
A. The by-pass and churn valve are used where a pumper is connected to a hydrant and the hydrant pressure is sufficient for the purpose immediately in hand. In this case the churn valve is opened and the flow from the hydrant under hydrant pressure passes to the intake side, then through the by-pass to the discharge side and on into the hose line.
Figure 3 shows a diagrammatic sketch of a churn valve and its relation to a rotary pump.
Q. Are churn valves supplied on centrifugal pumps?
A. No. Since a centrifugal pump is not of the positive displacement type, the churn valve is not a necessity on such pumps, although it may sometimes he provided. When a line on a centrifugal pump is shut off, the water within the pump is carried around by the impellers, even though there is no discharge.
Q. What is an automatic relief valve, and what is its purpose?
A. The pump operator seldom knows in advance when a line of hose is to be shut off. If a single line is operating from a displacement pump, a sudden closing of the line may result in stopping the pump before the operator can open the churn valve. Before this takes place, however, a momentary so-called “backing up” of the pressure in the pump may be sufficient to blow the hose line or even the pump casing or connection, and in any case may be injurious to the motor. If more than one line is operating and a portion of the lines are suddenly closed, the pump may continue to run as long as some lines are in operation, but the pressure built up may be sufficient to affect the operating lines, and the consequences are likely to be serious, particularly if a line is being operated from a precarious position. To prevent this, an automatic by-pass around the churn valve is provided. Such a valve is known as an automatic relief valve. There are several different types of modern automatic relief valves in common use, but all operate upon the same basic principle.
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Q. How does the automatic relief valve function?
A. Fig. 4 shows a typical automatic relief valve. The relief valve is connected to the discharge chamber of the pump. Chamber D is connected to the pilot valve by a small passage marked by the arrow. The pilot valve which is referred to by the arrow N is controlled by the springs shown in the sketch; this acts on the same principle as a safety valve. When the pilot valve is set at any pressure, for example, 120 pounds, and the pressure in the discharge chamber of the pump exceeds 120 pounds, the valve N rises. With the lifting of the valve, the water from discharge chamber by-passes as shown by the arrows into chamber D. The pressure then pushes down the valve H, and the water from discharge chamber then flows directly into the chamber marked E as shown by the curved arrow.
From chamber E the water passes down the return pipe to the suction chamber of the pump. The valve R acts as a drain from pilot valve and chamber D. All the time the relief valve is open, a steady stream will be emitted from the valve R. Immediately the nozzle is open, and the pressure in discharge chamber goes below the pressure at which the relief valve is set, which we are assuming is 120 pounds, the pilot valve N closes, the spring in question taking care of this. The pressure in chamber D and pilot valve is then relieved through the cock R, after which valye H is closed through the action of the spring beneath it and the pump resumes pumping. All movements as explained above take place in less than a second.
Q. What is the combination automatic relief valve and manually operated churn valve?
A. In most of the older models of pumps, the automatic relief valve and the manually operated churn valves were separate features and controlled through independent by-passes between the discharge and suction sides of the pump. Most of the modern pumps provided with these valves combine the two functions in a single by-pass control assembly. This combination is known as the combination automatic relief valve and automatically operated churn valve.
Q. What is an automatic pressure regulating governor, and what is its purpose?
A. Sudden shutting off of a discharge gate valve on a centrifugal pump will not cause the pump to stop nor the motor to stall, as it will with the displacement type pump, since there is a continuous passageway through the pump between the suction and the discharge side. However, shutting the gate valve will stop the flow of water through the pump and the pump impeller will churn the water in the pump casing. When the flow of water is stopped, the pump, and consequently the motor, will be relieved of a great portion of this load. When the load is relieved, the motor’s speed will increase greatly, and since the pump speed is proportional to the motor speed, the speed of the pump impeller will also increase. This can readily be illustrated by releasing the clutch of an ordinary automobile when the car is in gear and the throttle depressed. Theoretically the pressure will increase at a rate equal to the square of the speed increase of the impellers so that it may readily be seen that even a slight increase of speed would result in an appreciable increase in pressure.
If such pressure increases were permitted to take place every time a discharge valve were closed, difficulty in handling this speed, and possible danger to the men on the lines might result. Furthermore, a large increase of speed in the pump under appreciably no load would not be beneficial to the pump nor the motor.
For these reasons, centrifugal pumps are frequently provided with automatic relief valves similar to those used on displacement pumps. Lacking a relief valve, some other form of automatic pressure regulator is provided. Two common types of pressure controlling governors which serve the same functions as a relief valve are found on centrifugal pumps. Fundamentally, the governor operates in principle along the same line as a relief valve in that it may he said to relieve at a predetermined pressure, and its operation is actuated by the actual pressure at which the pump is discharging. Common types of governors differ from relief valves, however, in that instead of opening a by-pass between the suction and discharge sides of the pump, they control the setting of the carburetor valve on the motor, thus affecting directly the speed of the motor. Both types of governors referred to above are based upon the carburetor control operation. One, however, utilizes a pre-loaded spring to balance the pump discharge pressure, whereas the other utilizes the pressure created by the pump itself.