In order to be an efficient pump operator it is necessary that the operator first have a thorough knowledge and understanding of the two sources of water supply, namely water supply from draft and water supply from a head pressure.

Water Supply from Draft
In order to understand the theory of drafting water we must start with the facts that:

1. Normal atmospheric pressure may be considered to be approximately 14.7 lbs. per square inch at sea level.
2. Water is practically incompressible.
3. Water is of a constant volume, therefore it may be said that a given weight of water may occupy the same space at all times and also that a given volume of water will have the same weight at all times.

Atmospheric pressure decreases as the elevation above sea level increases at the rate of one-half pound for every 1000 feet.

The average height for Lincoln above sea level is 1167 feet.

Water Supply From a Head Pressure
Head pressures are mainly received from fire hydrants, but can also be received from relay lines and other possible means where by a pump receives the supply from some force and at some pressure.

We are fortunate in having fairly good hydrant pressure and flow of water from our hydrants here in the city of Lincoln and as a result will find that in most cases our pumps can all deliver considerably more water than their rated capacity.

When working from a head pressure, both the volume of water and the pressure created as it enters the pump intake must be considered. The maximum GPM at the maximum pump pressure discharged from any pump supplied from a head pressure is determined by the size of the intake placed in service and the pressure recorded on the compound gauge WHILE THE LINES ARE DISCHARGING WATER.

It is very important when a company arrives at a fire incident that has already, or may develop into one where it will require many lines, that the pump be connected to the large intake.

The type of pump operated must be considered for the reason that with a centrifugal pump the capacity may be increased to a figure above the rated capacity of the pump depending largely upon GPM and the pressure of the water flowing into the pump, regardless if the pump is engaged or disengaged.

When working a centrifugal pump from draft, whenever the pump pressure exceeds the rated pump pressure the GPM discharged will be less than the rated capacity of the pump, but when connected to a head pressure, energy is added to the pump energy, and the rated capacity can be discharged at a lower pressure. Should the pump pressure exceed the sum of the rated pump pressure plus the head pressure the GPM will decrease below the rated capacity. From this it should be understood that a centrifugal pump receiving the supply of water from draft uses a part of the pump energy to get the water into the pump before it can discharge any water. But a pump receiving the supply from a head pressure can divert the entire pump energy to the discharging of water and is aided in doing this by the pressure of the water entering the pump.

When connecting any centrifugal pump to a head pressure or a hydrant, it is advisable to engage the pump before discharging any water. This head pressure will revolve the impellers possibly at such a speed that it may be necessary to stop discharging water before engaging the pump.

Never “pull a vacuum” on the compound gauge when working from a hydrant or head pressure. There should always be at least twenty pounds registering on the gauge.

We can see the need for every pump operator to have a knowledge of the main sizes within there area, this is an important factor in determining the water availability.

Centrifugal Fire Pumps Theory, Functions and Purposes of Parts
A general description of the course of water through a two-stage centrifugal fire pump and the functions and purposes of the different parts will be explained.

Before going into that, everyone should have an understanding of the theory of a centrifugal pump.

The centrifugal pump is a paddle pump. No doubt every firefighter has at sometime seen someone paddle or row a boat, or has seen a motor boat operate. The paddle moves in and against the water and pushes the boat ahead. If the boat were tied off, it is evident that the water would be set in motion rather than the boat. Even when a motor boat is skimming through the water, it is easy to see the current set up behind it by the propeller. The motion of the propeller becomes an impelling force against the water, and being a liquid, it is set in motion. The faster the impeller moves, the more velocity will be developed.

Let us apply this same principle to an enclosed stream of water. If in a water line some mechanical means can be devised to paddle the water, the water will be set in motion, its speed or velocity depending upon the speed or paddling. A centrifugal pump is designed to produce this paddling operation to a water stream. So as to be able to paddle the stream hard enough to put it under pressure, the entire mechanism must be enclosed.

It is evident that, being a paddle pump, the centrifugal pump will not create enough vacuum to produce suction unless the case is full of water. For this reason it must be primed (filled with water) before drafting.

Since the water is thrown outward from the center of the impeller by centrifugal force, the pump is called a “centrifugal pump”. However, if the discharges are closed for a long period of time with water “churning” in the pump, heat will be created which would damage the pump. Therefore a pump cooling valve is applied and this valve should be opened.

Pump Stages
A stage consists of a circular impeller that whirls within a casing. Water enters the center of the impeller and is whirled, creating a centrifugal force which throws the water off the impeller at its outer diameter. The water is gathered in the casing, which gradually gets larger as it follows the circular curve of the impeller, and ends in an opening that allows the water that has been collected to go into the discharge manifold.

(Click on images for larger view.)

The speed or RPM of the engine is what governs the speed of the impellers which in turn governs the velocity and volume of water discharged.

Single Stage: A single stage pump has only one impeller and housing. The impeller is larger than the one’s in a two stage pump.

Two Stage: A two-stage pump is one with two impellers with a control valve called a transfer valve that permits the water to pass through both impellers at once for volume or from one to the other for pressure.

From a supply line connection, water enters the pump at the intake manifold. When operating in pressure, the water enters the impeller of the first stage, passes through it being propelled into the impeller of the second stage, then into the discharge manifold. When operating in volume, the water enters directly into both stages and is propelled from them into the discharge manifold.

Transfer Valve
The changing of the pump from pressure to volume, or vice versa, is controlled by the transfer valve. The operation of this valve either allows the water to enter both stages at the same time, and is propelled from them to the discharge manifold, or it allows the water to enter only one stage, and pass through it to the other stage then out into the discharge manifold. When operating in series, the part of the intake manifold that allows the water to pass into the other stage of the pump is closed automatically by the pressure generated in the first stage of the pump, and flowing backward in this part of the manifolds thus forcing a gravity type clapper valve shut against the incoming water.

In other words, by operating the transfer valve, the water coming from the first stage of the pump is directed either to the discharge manifold or, directed back into the impeller of the second stage.

Transfer valves may be either mechanically or hydraulically operated. The hydraulic transfer valve is changed from one position to the other by the pressure of the water. This should be done while the pump is working at a low pressure merely by moving a small lever to the position desired. Changing over at a low pressure will eliminate the possibility of mechanical damage or injury to the fire fighter operating a hose line. Changing the position of the transfer valve MUST be done at idling speed and no pressure.

Pressure Gauge
The water that is collected in the discharge chambers passes from the pump through the discharge gate valves, and the valve to the booster reel. The pressure gauge is connected to the discharge manifold and registers the pressure of the water within the pump. This pressure gauge may be a compound type, or it may be a straight pressure gauge. On most pumps, individual pressure gauges are installed on each discharge gate on the discharge side of the valve. This eliminates guess work when two or more lines are operated from the same pump, and at different pressures.

Compound/Vacuum Gauge
This gauge is connected to the intake manifold of the pump, and the intake pressure from a water source registers on this gauge. It also registers the inches of vacuum created when operating from draft.

Pressure drop as an indicator of hydrant capacity
Static pressure is the pressure of the hydrant water at rest; that is, with the hydrant open to the pump and no water flowing through the pump. Residual pressure is the pressure in the hydrant with water flowing from the hydrant through the pump. Both are read on the compound (intake) gauge. Together, they can be used by the FAO who has connected to a hydrant to obtain an accurate indication of supply conditions within the hydrant. At the beginning of the operations, the FAO reads the gauge with the hydrant open and all discharge gates closed. This is the static pressure. Then one line is charged to the proper discharge pressure and is read again. This is the residual pressure.

The difference in the readings allows the FAO to determine how many more lines can be supplied by the water main system; a drop of about 5 percent from the static pressure to the residual pressure indicates that 3 equal lines equivalent to the amount being discharged can be supplied by the water main system. A drop of about 10 percent indicates that 2 more equal lines can be supply. And a drop of 20 percent indicates that only 1 more equal line can be delivered from the system.

Even after all lines are charged, the FAO must watch the compound gauge closely. This is to assure that immediate action can be taken if other pumps operating nearby cause the residual pressure to decrease below the 20 psi minimum.

Prime Pump
A centrifugal pump must be primed, that is, filled with water before it will draft water of its own action. The centrifugal pump relies on the imparting of velocity to the air or water enters the pump by a whirling action or motion of the impellers in the pump stages. Anything in motion has forces therefore the water and air being whirled moves away from the center of the impeller by a centrifugal force. The faster the impeller whirls, the faster the water or air moves away from the center of the impeller to its outer edge. The faster anything moves the more force it has. The more weight a moving object has, the more force it will have over a lighter object of the same size moving at the same speed. Force creates pressure.

Water has many times the weight of air so it can very easily be moved fast enough by the whirling motion or a centrifugal water pump impeller to create one pound pressure. When moving away from the center of the impeller, this one pound pressure causes a degree of vacuum behind it measured as 2.04 inches of mercury. This degree of vacuum within the suction hose would cause an atmospheric pressure on the surface of the water to force the water to rise 2.31 feet within it. Air being so light could not be whirled fast enough by the impellers of a centrifugal pump to create even one pound of pressure. Once a centrifugal pump has a solid volume of water from the surface of the water to the pump impellers, it can proceed with the problem of drafting water from as great a lift as any other type of pumps disregarding pump slippage.

Due to the fact that a centrifugal pump will not start to draft water because of its inability to create enough vacuum it must be primed by the use of a small rotary gear primer pump.

Where the small rotary gear primer pump is used the intake of the primer pump is connected to the centrifugal pump casing at the highest point so that all the air can be expelled from the centrifugal pump before attempting to discharge any water. The discharge from the rotary pump is to the outside below the pump. The action of this pump is positive; it takes small quantities of air from the centrifugal pump casing into the space between the rotating teeth and forces it through the discharge of the priming pump. This causes a partial vacuum to be created within the centrifugal pump casings and allows the atmospheric pressure to force water into the pump casing. The clearance between the rotary gears and the gear casing is very small, and is so designed to prevent water or air from slipping back past the rotary gears.

Since the clearance must be great enough to prevent actual contact, some slippage cannot be eliminated but in order to reduce this slippage to a minimum the primer pump is provided with a means of lubrication (oil) to prevent wear on the parts, and also act as a seal in this clearance.

The vacuum prime method used for priming a centrifugal pump is to connect the case of the pump to the intake manifold of the motor with a tube. The running motor pulls a vacuum on the pump casing that fills with water from the suction intake. A float valve set in the line to the manifold prevents water being drawn into the motors and a check valve in the line prevents a reversed action when the pump reacts. All the air in the centrifugal pump casing must be expelled before the use of the primer is discontinued. A steady flow of water from the primer pump is an indication that the pump has been primed. All openings leading to and from the pump must be closed.

Lubricating Primer Pumps
On all pumps using the rotary gear primer, lubrication of the primer pump is automatic. Each time the primer is placed in operation oil is drawn from an oil reservoir connected to the priming pump. The reservoir should always have a sufficient quantity of oil in it, and should be inspected frequently.

Pump Pressures
To regulate pressure on a centrifugal pump, a hand operated pump throttle is provided. Opening or closing the hand throttle increases or decreases the engine speed which in turn increases or decreases the speed of the pump, impellers. As the speed of the impellers increase, the centrifugal force increases, increasing the pressure.

If a centrifugal pump has been discharging water at a certain pump pressure and all the discharges are shut off, the pump pressure will rise in proportion to the engine speed, and the amount of water it has been discharging. The speed of the engine will increase because the flow of water through the pump impellers has been stopped, due to having no place to go, the water is merely being whirled by the impeller.

As the flow into the impeller is stopped, the motor is relieved of the labor of imparting a velocity to it, or starting it to whirl. As the labor is relieved, the engine “picks up” in speed, having only the frictional pressure of the water trapped in the impeller against the water trapped in the volute or turbine casing to overcome.

If a centrifugal pump has been discharging water at a certain pressure through 2 or more discharges and 1 of these discharges is shut off, the pump pressure will rise in proportion to the percent of volume as a whole that the closed discharges had been discharging. The increase in pump pressure will result in an increase in the volume of water flowing through the open discharge. This increase in volume will result in an increase in the nozzle pressure that may be undesirable and may cause accidents or damage.

Closing the nozzle has the same effect on the pump pressure as closing the discharge gate. When the nozzle or discharge gate is opened again, the engine pressure will return to its original pressures.

When a pump is discharging water through 1 or more discharges, and an additional discharge is opened, the pump pressure will decrease, due to the water being able to escape faster through the greater discharge area.

This would mean that the pump operator must be at their pump controls constantly, ready to operate the throttle to increase or decrease the pressure as the nozzles are opened or closed for any reason.

Volume and Pressure
The manner in which the pump pressures are built up by the stages of the pump are (for example, pressure position – 200 lbs. pump pressure desired) as follows:

When at draft the water enters the first stage at no intake pressure, then in the first stage it is built up to 100 lbs. and passed on to the second stage, here it is increased another 100 lbs, discharging the second stage, discharging the water through the discharge manifold at 200 lbs. pump pressure.

Each stage of a pump is designed to discharge a certain amount of water at a certain pump pressure. For example – a 2 stage pump with each stage capable of discharging 500 GPM at 150 lbs. pump pressure from draft, when set in volume, the amount of water discharged would be 1000 GPM at 150 lbs. pump pressure.

Should the transfer valve be in pressure, 500 GPM of water flowing from the first stage would enter the second stage at 125 lbs. pump pressure and would be discharged from the second stage at 250 lbs. pump pressure therefore, discharging 500 GPM of water at 250 lbs. pump pressure. Again it must be remembered that this is only an example, the figures are merely used to illustrate this statement, and they are only approximately correct.

If a pump is working at a head pressure, and the water entering the first stage with 30 lbs. flowing pressure, then the pressure leaving the different stages as described above would be increased by 30 lbs. In the above description, it must be assumed that the pump is operating at a certain RPM, and if this RPM were increased or decreased, the pressure added to the water by each stage would increase or decrease in proportion to the RPM of the pump.

While the pressure added by the work of each stage of the pump would not be the same in actual practice, this description is the general principle of how the pressure is increased as it passes through the different stages and the pressures used must be considered as approximately correct.

It should be understood then, that when working from a head pressure, the greater the intake flow pressure, the more water will be discharged per RPM. The more water that flows into a centrifugal pump, the more will flow out of one; and the higher the flow pressure the faster it will flow, therefore the faster it flows, the more water will pass a certain point at a given time. This accounts for the increase in discharge from a centrifugal pump when working from a head pressure. Centrifugal pumps will discharge more water when working from a head pressure, than their rated capacity, depending on the pressure or velocity of the water flowing into the pump intake. When working from an unlimited source of head pressure, the amount of water a centrifugal pump will discharge is limited only by: (1) the peak speed of the engine, (2) the peak speed at which the impellers could revolve without harm, and (3) the pressure for which the pump and its accessories have been designed and constructed to withstand. After a certain point is reached the increase in discharge would hardly be noticeable due to the terrific friction loss caused by the water passing through the pump. While the above statement is true, the operator is not likely to encounter such a set-up, but will be chiefly concerned with the head pressure entering the pump, and to stay within the safe limits of the peak speed of the engine so far as the discharge capacity of the pump is concerned.

When to Operate in Pressure and Volume
In order for any operator to give the proper pressures at the correct engine speeds, they must determine the volume of water to be used and from this will be determined whether the pump will be operated in either pressure or volume position.

When operating any centrifugal pump set the transfer valve in volume for large volumes of water (over half the rated capacity of the pump). Set the transfer valve in pressure for lesser amounts of water (less than half the rated capacity of the pump). This policy for operating in pressure or volume will be strictly adhered to in all cases.

At this point a complete knowledge of the pump being operated is necessary, if the desired pressure cannot be obtained, or the operator believes that the engine is running too fast, or is laboring, (learned through actual practice and by studying the sound of the engine) the position must be changed; that is from volume to pressure or from pressure to volume.

By reading the pressure gauge and the tachometer the operator determines the correct position of pumping, obtaining the desired pressure at the lowest engine speed possible. Sometimes there will be little or no difference in the readings of the pressure gauge or the tachometer after a change has been made because with a centrifugal pump, there is not any positive division as to when to pump in volume or pressure position. If the engine speed increases and the pressure drops, change from volume back to pressure; but if the engine speed drops and the desired pressure can be retained, leave it in volume.

Classification of Fire Pumps
When fire pumps are manufactured they have a given rated capacity, depending on the possible RPM of the engine operating them, the size of the pump, the number of stages, etc.

When purchased and accepted by the city, they are given a test as set forth by the National Board of Fire Underwriters, which requires each pump to produce a certain capacity for a given number of hours at various pressures. This test determines the classification of a pump, whether it is a class A, B, etc.

All class "A" pumps produce (from draft) their:
full rated capacity at 150 lbs. Pump pressure 70% rated capacity at 200 lbs. Pump pressure 50% rated capacity at 250 lbs. Pump pressure

Example: class "A" - 1000 GPM pump:
1000 gal. discharged at 150 lbs. Pump pressure 700 gal. discharged at 200 lbs. Pump pressure 500 gal. discharged at 250 lbs. Pump pressure

Example: class "A" - 1250 GPM pump:
1250 gal. discharged at 150 lbs. Pump pressure 875 gal. discharged at 200 lbs. Pump pressure 625 gal. discharged at 250 lbs. Pump pressure

These figures are the minimum required by the underwriters from draft, all of our pumps will produce above these figures to some degree.

Refer to the IFSTA Fire Department Aerial Apparatus 1st edition, #35 in the station library for:

• positioning aerial apparatus
• stabilizing aerial apparatus
• operating telescoping aerial equipment
• aerial apparatus strategies and tactics