The Largest Security-Cleared Career Network for Defense and Intelligence Jobs - JOIN NOW

Military

Chapter 4

Pumps

4-1. Fundamentals.

a. Pump Types. Pumps may be classified according to use (for shallow or deep wells), design (variable or positive displacement), or method of operation (rotary, reciprocating, centrifugal, jet, or airlift). This chapter deals with shallow- and deep-well pumps. Shallow-well pumps (suction-lift pumps) are normally installed above ground, on or near the top of the well casing. Deep-well pumps are installed in the well casing with the pump inlets submerged below the pumping level. These inlets are always under a positive head and do not require suction to move or pump the water.

b. Selection Criteria. Consider the following items when selecting a pump:

  • Size of the well.
  • Quantity of water to be pumped.
  • Drawdown and pumping levels.
  • Type of available power.
  • Yield of the well.
  • Estimated total pumping head.

Well yield is frequently overlooked when selecting a pump for small wells. Installing a pump that can handle a large discharge capacity can either temporarily drain a small well or exceed the maximum possible suction lift. Therefore, pumping requirements and well characteristics must be matched to determine the optimum pump for each installation. Table 4-1 provides a general guide for use in pump selection. Military well drillers deploying with well-completion kits will normally use the deep-well submersible pumps that are supplied with the kits.

4-2. Shallow-Well Pumps. These pumps are limited to the depth from which they can lift water. At sea level, the practical limit is 22 to 25 feet for most pumps. This value decreases about 1 foot for each 1,000-foot increase in elevation above sea level. The operative principle of a shallow-well pump is similar to drinking through a straw. A partial vacuum is created, and the difference between the pressure inside the straw and the liquid outside the straw forces the liquid upward to a new equilibrium.

A pump exhausts air from the intake line, thus lowering the pressure on the intake side below atmospheric pressure. The atmospheric pressure on the water in the well then forces the water up through the suction line into the pump. The atmospheric pressure is the only force available to lift water to the pump. At sea level, the force is about 14.7 pounds per square inch (psi) (about 34 feet of water). The maximum is never reached because pumps are not 100 percent efficient and because other factors (water temperature and friction or resistance to flow in the suction pipe) reduce the suction lift. Since a partial vacuum is required in the suction line, the line must be airtight if the pumps are to function properly. Threaded joints must be carefully sealed with pipe-joint compound and all connections to the pump must be tight.

a. Pitcher Pump. This is a surface-mounted, reciprocating or single-acting piston pump (Figure 4-1). The pump has a hand-operated plunger that works in a cylinder designed to be set on top of the well casing. The suction pipe screws into the bottom of the cylinder. The plunger has a simple ball valve that opens on the downstroke and closes on the upstroke. Usually, a check valve at the lower end of the cylinder opens on the upstroke of the pump and closes on the downstroke. Continuous upstroke and downstroke actions result in a pulsating flow of water out of the discharge pipe. By lifting the pump handle as high as possible, the check valve (lower end of the cylinder) will tilt when the plunger is forced down on top of the valve. Tilting the check valve allows the pump and suction line to drain.

To reprime the pump after draining, pour water in the cylinder from the top of the pump. To maintain the pitcher pump, renew the plunger, check the valve leathers, and clean the suction pipe. Clean the suction pipe when it becomes clogged with sand, gravel, or other material. The pump will be noisy and the pump handle may fly up when released during the downstroke.

b. Rotary Pump. These pumps use a system of rotating gears (Figure 4-2) to create a suction at the inlet and force a water stream out of the discharge. The gears' teeth move away from each other at the inlet port. This action causes a partial vacuum and the water in the suction pipe rises. In the pump, the water is carried between the gear teeth and around both sides of the pump case. At the outlet, the teeth moving together and meshing causes a positive pressure that forces the water into the discharge line.

In a rotary gear pump, water flows continuously and steadily with very small pulsations. The pump size and shaft rotation speed determine how much water is pumped per hour. Gear pumps are generally intended for low-speed operation. The flowing water lubricates all internal parts. Therefore, the pumps should be used for pumping water that is free of sand or grit. If sand or grit does flow through the gears, the close-fitting gear teeth will wear, thus reducing pump efficiency or lifting capacity.

c. Centrifugal Pump. These are variable displacement pumps in which water flows by the centrifugal force transmitted to the pump in designed channels of a rotating impeller (Figure 4-3). A closed case, with a discharge opening, surrounds the impeller. The case has a spiral-shaped channel for the water. The channel gradually widens towards the outlet opening. As water flows through the channel, speed decreases and pressure increases. The hydraulic characteristics of the pump depend on the dimensions and shape of the water passages of the impeller and the case.

The centrifugal pump works as follows:

  • Water enters the pump at the center of the impeller and is forced out by centrifugal force. (You may have to fill the pump and suction pipe with water before starting the pump.)
  • The expelled water forces the water in the casing out through the discharge pipe, producing a partial vacuum in the center.
  • Atmospheric pressure acts on the surface of the water in the well and forces more water up the suction pipe and into the impeller to replace the expelled water.

(1) Head. Head is the pressure against which a pump must work the suction-lift and friction losses and the system pressure that the pump must develop. If the head is increased and the speed is unchanged, the flow rate will decrease. To increase the flow rate, you must increase the speed or decrease the head. If you increase the head beyond the pump's (shutoff head) capacity, water will not be pumped. The impeller only churns the water inside the case; the energy expended heats the water and the pump. If such action continues, enough heat may develop to boil the water and generate steam causing the impeller to rotate in vapor rather than water. With no coolant, the bearings seize, resulting in severe pump and possible motor damage.

(2) Connections. You may have to use several pumps to meet head or flow requirements. You can connect the pumps either in series or in parallel. If you connect two centrifugal pumps in series (the discharge of the first connected to the suction of the second), the discharge capacity stays the same. However, the head capacity is the sum of both pumps head capacities. The increased head capacity is only available as discharge head. You will not gain any appreciable increase in suction lift. You can obtain the same effect by using a multistage pump that contains two or more impellers within one casing.

If you connect two centrifugal pumps in parallel (both suctions are connected to the intake line and both discharges connected to the discharge line), the discharge head is the same as that of the individual pumps. The discharge capacity is close to the sum of the capacities of both pumps. The increased flow rates result in extra friction losses that prevent the combined flows from being the exact sum of the two pumps.

d. Self-Priming Pump. This pump has a priming chamber that makes repriming unnecessary when the pump is stopped for any reason other than an intentional draining. The pump is mounted on a frame with and driven by a two-cylinder, three-horsepower military standard engine (Figure 4-4). The unit is close-coupled. The impeller is secured to an adapter shaft that is fastened and keyed to the engine stub shaft. A self-adjusting mechanical seal prevents water from leaking between the pump and the engine. The pump is designed for optimum performance with a suction lift of 10 feet. You can operate the pump at greater suction lifts, but the capacity and efficiency of the unit are reduced proportionately.

(1) Installation. Install the pump as close to the source of water supply as possible to minimize the required suction lift. Install full-sized suction piping and keep friction losses as low as possible by using the least possible number of pipe fittings (elbows, bents, unions). To ensure that joints do not leak use pipe cement or teflon tape on all joints. If you use a suction hose, try to ensure that the hose is as airtight as possible. If you have to remove the suction or discharge piping or hose frequently, you should make the connections with unions to reduce wear on the pump housing.

(2) Priming. To prime the pump, remove the priming plug on top of the pumping case, and pour water into the pump case to the discharge-opening level. Failure to fill the priming chamber may prevent priming. If the pump takes longer than 5 minutes to prime, a mechanical problem exists. A self-priming pump is normally primed from a 10-foot suction lift in 2 minutes or less, depending on the length and size of the suction pipe. If you use a valve in the discharge line, you must open it wide during priming.

If the pump fails to prime, look for the following:

  • Plugged priming hole.
  • Air leak in suction pipe or hose.
  • Collapse of lining suction hose.
  • Plugged end of suction pipe or suction strainer.
  • Lack of water in pump housing.
  • Clogged, worn-out, or broken impeller.
  • Worn or damaged seal.

4-3. Deep-Well Pumps.

a. Submersible Pump. This is a centrifugal pump closely coupled with an electric motor that can operate underwater. The pump is typically multistage containing two or more impellers (depending on head requirements) housed in a bowl assembly. Because the system is designed for underwater operations, it has a waterproof electric motor, watertight seals, electric cables and connections. The motor is located beneath the bowl assembly with the water intake screen between the two units.

Military well-completion kits contain the submersible pump (Figure 4-5). The pump produces 50 GPM at 600 feet and is powered by a 15-horsepower, 460-volt, 3-phase electric motor. The pump comes with 700 feet of electrical conductor cable and 660 feet of 2-inch drop hose that supports the pump and brings the water to the surface distribution system. Currently, the submersible pump is the standard in deep-well, high-production systems.

DANGER

Do not handle live electrical wires when
wet or while standing in water. Do not
step on exposed electrical cables.

The following improvements have made the submersible pump a reliable pump:

  • Motors, cables, and seals have very low maintenance requirements.
  • Noise levels are reduced because the motor is located in the well.
  • Motor operates at a cooler temperature because it is submerged.
  • System does not require long drive shafts and bearings, so maintenance problems and deviations in vertical well alignment are not critical factor when using this pump.

The main disadvantage with the pump is that the entire pump and motor assemblies must be removed from the well if repairs or services are required.

b. Turbine Pump. The turbine (line shaft) pump is a shaft-driven, centrifugal pump. The pump is hung in a well at the lower end of a string of pipe called the column pipe. The shaft, which drives the pump, runs through the column pipe and extends from the pump to the ground surface where it is connected to a pump-head assembly. Bearings in the column pipe are used to stabilize the shaft. The turbine pump (Figure 4-6) is a multistage pump containing several impellers or bowl assemblies. The main advantage to the turbine pump is the accessibility to the power source. The power source is either a hollow-shaft electric motor or a reciprocating engine connected by a right-angle drive and is located above ground. The main disadvantages are maintenance requirements for the shaft and bearings and the requirement that the well be vertical with no deviations for installation.

c. Helical-Rotor Pump. This pump is a positive-displacement-, rotary-screw-, or progressing-cavity-type pump (Figure 4-7). The pump is designed for relatively low-capacity, high-lift wells that are 4 inches or larger in diameter. The main elements of the pump are a highly polished, stainless-steel helical rotor, a single-thread worm; and an outer rubber stator. The rotor is located in the stator. During the rotation process, the rotor forces a continuous stream of water forward along the cavities in the stator producing a uniform flow. The helical-rotor pump is designed to produce 50 GPM at 1,800 revolutions per minute (RPM) against a 250-foot head.

d. Jet Pump. This pump is a combination of a surface centrifugal pump, down-hole nozzle, and venturi arrangement (Figure 4-8). It can be used in small diameter wells that require a lift of 100 feet or less. The pump supplies water, under pressure, to the nozzle. The increase in velocity at the nozzle results in a decrease in pressure at that point, which in turn draws water through the foot valve into the intake pipe. The combined flow then enters the venturi where the velocity is gradually decreased and the pressure head recovered. The excess flow is discharged at the surface through a control valve, which also maintains the required recirculating flow to the nozzle.

A jet pump's efficiency is low compared to an ordinary centrifugal pump. However, other features make the jet pump a desirable pump. They are--

  • Adaptability to wells as small as 2 inches in diameter.
  • Easy accessibility to all moving parts at the ground surface.
  • Simple design resulting in relatively low purchase and maintenance costs.

4-4. Air-Lift Pumps.

a. Principle. Water can be readily pumped from a well using an air-lift pump. There are no air-lift pumps in the Army supply system; however, in the field, you can improvise and make a pump using compressed air and the proper piping arrangement. The assembly consists of a vertical discharge (eductor) pipe and a smaller air pipe. Both pipes are submerged in the well below the pumping level for about two-thirds of the pump's length. The compressed air goes through the air pipe to within a few feet of the bottom of the eductor pipe and is then released inside the eductor pipe. A mixture of air bubbles and water forms inside the eductor pipe. This mixture flows up and out the top of the eductor pipe. The pumping action that causes water to rise as long as compressed air is supplied is the difference in hydrostatic pressure inside and outside the pipe resulting from the lowered specific gravity of the mixed column of water and air bubbles. The energy operating the air lift is contained in the compressed air and released in the form of bubbles in the water. Figure 4-9 shows the operating air-lift principle.

WARNING

Air and fluids under pressure can cause injury.
Make sure all air couplings are tight and that
lines and hoses are in good condition.

You should arrange an air lift with the air pipe inside the eductor pipe (Figure 4-10). You can use this arrangement for test pumping wells and for well development. You can use the well casing for the eductor pipe. However, to pump sand and mud from the bottom of a well during well development and completion, use a separate eductor pipe. This type of pump is also useful in wells that, because of faulty design, produce sand with the water. This condition will quickly create excessive wear on most pumps. By setting the educator pipe to the bottom of the screen, sand will be removed before it fills the screen.

b. Installation Design.

(1) Submergence. Submergence is the proportion (percentage) of the length of the air pipe that is submerged below the pumping level. Use the following formula and Figure 4-11 to determine submergence percentage:

(2) Air Pressure. To calculate the required air pressure to start the air lift, you must know the length of air pipe submerged below the static level. See Figure 4-11, area from point B to point D, for the starting air pressure. Divide the area from point C to D by 2.31 (constant/conversion factor) to get the required air pressure (psi).

(3) Compressors. The 350 cubic feet per minute (cfm) compressor on military drilling rigs, such as the LP-12, is sufficient for operating an air lift. With a submergence of 60 percent, a lift not exceeding 50 feet and the compressor delivering 350 cfm of air, a well can be pumped at over 200 GPM. If you need more air, use another compressor in parallel. The maximum pressure that the compressor will produce is 200 psi, which is enough to start an airlift with about 420 feet of air pipe submerged.

CAUTION

Operate compressors upwind of the drilling
rig. If you do not, dust could damage the
equipment.

(4) Correct Air Amounts. For efficiency, the compressor must deliver the correct amount of air. Too much air causes excessive friction in the pipe lines and waste of air from incomplete expansion in the discharge pipe. Too little air results in a reduced yield and a surging, intermittent discharge. To calculate air-compressor requirements, see Table 4-2.

(5) Performance and Efficiency. The performance and efficiency of an air lift vary greatly with the percent of submergence and the amount of lift. Generally, a submergence of 60 percent or more is desirable. If a well has a considerable pumping-level depth, you will have to use a lesser submergence percent. However, if the submergence is too low, the air lift will not operate. See Table 4-3 for performance data for air-lift pumps corresponding to different submergence conditions and lifts. The values are for properly proportioned air and eductor pipes with minimum frictional losses. The efficiencies indicated in terms of gallons of water per cubic foot of air probably cannot be fully attained in military field operations.

(6) Foot Piece. For best efficiency, the end of the air pipe should have a foot piece (Figure 4-10). This device breaks the air into small streams so that the bubbles formed will be as small as possible. You can make a foot piece by drilling numerous small holes in a short section of pipe.

(7) Discharge Pipe. You can approximate the discharge-pipe length from Table 4-3. Lower submergence than those shown result in a lower pumping efficiency. The planned pumping rate must not cause an excessive drop in the water level, reducing the submergence. The two chief losses in the discharge pipe are air slipping through the water and the water friction in the discharge line. As the velocity of discharge increases, slippage decreases and friction increases. Eductor intake loss occurs at the lower end of the pipe due to friction and to the energy required to accelerate the flow of water into the pipe.



NEWSLETTER
Join the GlobalSecurity.org mailing list