Positive displacement pumps
Positive displacement pumps were developed long before centrifugal pumps. Liquid is positively displaced from a fixed-volume container. Positive-displacement pumps are capable of developing high pressures while operating at low suction pressures. They are commonly referred to as constant-volume pumps. Unlike centrifugal pumps, their capacity is not affected by the pressure against which they operate. Flow is usually regulated by varying the speed of the pump or by recycle. Positive-displacement pumps are divided into two groups: rotary and reciprocating pumps.
Rotary pumps are normally limited to services in which the fluid viscosity is very high or the flow rate too small to be handled economically by other pumps. Rotary pumps are commonly used to circulate lube oils through engines, turbines, reduction gears, and process-machinery bearings. Rotary pumps displace a fixed quantity of fluid for every revolution of the driver shaft. They have different pumping elements such as vanes, lobes, gears, and screws. Fig. 1 illustrates three (internal gear, external gear, and screw) of the most commonly used rotary pumps in production operations.
Most manufacturers rate rotary pumps by capacity (i.e., throughout). Capacity is the total liquid displacement of the pump less slip. Slip is the quantity of fluid that leaks from the higher-pressure discharge to the lower-pressure suction. Slip occurs because all rotary pumps require clearances between the rotating elements and pump housing. These clearances provide a leak path between the discharge and suction sides. A pump with large clearances, because of machining tolerances or wear, exhibits a proportionally larger slip. Rotary pumps cannot move nonlubricating fluids such as water or fluids containing hard or abrasive particles. Rotary pumps can move large quantities of air or vapor for short periods of time without losing prime.
Rotary pumps are self-priming but are not designed to run dry for long periods. For best operation, there must be enough fluid at the suction port to keep the pumping chamber completely filled.
Fig. 2 illustrates the relationship between speed, volumetric efficiency, and displacement of a rotary positive-displacement pump. The principles of operation of some of the more common types of rotary pumps are described next.
A set of vanes is mounted in a rotor in which the vanes slide in and out of the rotor. The rotor is mounted off center in the casing. As the vanes rotate past the suction port, they slide out of the rotor while maintaining constant contact with the casing. Springs or sealer rings help hold the vanes against the casing, thus the vanes make a close seal, or fit, against the casing wall. Trapped fluid is forced from the suction port to the discharge port.
The sliding-vane design is capable of delivering medium capacity and head. They deliver a constant flow rate for a set rotor speed. They work well with low-viscosity fluids and are somewhat self-compensating for wear. They are not suitable for use with highly viscous fluids (thicker fluids interfere with the sliding action of the vanes). A large wear area results from the friction fit between the vanes and the cylinder.
Flexible Vane. The flexible vane is similar to the sliding vane except that the vanes are generally a soft, pliable material and are integral with the rotor. As the rotor turns, the vanes bend and conform to the eccentric shape of the cylinder. They are simple, inexpensive, and are capable of developing a vacuum. They should not be allowed to run dry and should be used only with low-temperature fluids and in low-head applications.
The external gear consists of two equal-sized meshing gears, one a driver and the other an idler, that rotate inside a housing. As the gears unmesh at the suction side of the pump, a vacuum is formed. Pressure forces the fluid into the pump where the fluid is carried between the gear teeth and the case to the discharge port. At the discharge, the meshing of the gear teeth creates a boundary that prevents the fluid from returning to the suction. Gear pumps operate equally well when driven in either direction. Precautions should be taken to ensure that the shaft rotation is correct when special features, such as built-in relief valves or a bleed back of the shaft seal, are used.
There are also models that use multiple sets of gears on one shaft to produce more capacity. External-gear pumps are compact in size and can produce high pressures. They are well suited for highly viscous fluids. They are easily manufactured in a broad range of materials to ensure compatibility with the pumped fluids. Because of their close tolerances, they are limited to clean-fluid applications.
The internal-gear pump is similar in principle to the external gear except the drive shaft turns a ring gear with internal teeth. The external gear tooth (idler) rotates on an offset center and meshes with the drive gear through only a segmental arc of rotation. A fixed crescent-shaped filter occupies the space between internal- and external-gear-tooth tips opposite the mesh point. As the gear teeth disengage at the input port, fluid enters and is trapped in the tooth space of each gear and is carried to the discharge port. The meshing of the two gears and the elimination of the tooth space forces fluid from the pump.
Lateral gear pumps are used in low-head applications. They are limited to a maximum backpressure of 100 psi and require a pressure-relief valve on the discharge side. Because small clearances exist, they cannot handle liquids that contain solids. The manufacturer should always be consulted before any gear pump is used with fluid-handling solids.
Lobe pumps operate in the same manner as gear pumps except the rotating elements have two, three, or four lobes instead of gear teeth. Lobes cannot drive each other, so timing gears are used. The lobes never come into contact with each other so the pump can be allowed to run dry. Lobes are used where product integrity must be maintained and in applications in which liquids are shear sensitive. The large volume created between the casing and lobes allows many products to be pumped without damaging the product itself. A major advantage is that there is no metal-to-metal contact between the lobes, thus the possibility of traces of iron, steel, or other pump-construction materials ending up in the product because of wear is greatly reduced. On the other hand, they are more expensive than gear or vane pumps and are difficult to repair and maintain.
Screw pumps can be single-rotor (progressive cavity) or multiple-rotor (intermeshing) design. Screw pumps are relatively high-speed pumps but, because of the reversal of flow required to enter the suction passage, NPSH can often be a problem. Screw pumps are used for high-head applications; they are the most common rotary-pump type in use in producing operations.
In the single-screw design, the fluid is trapped between the treads of a rotating screw and the treads of the internal stationary element. These pumps are used for viscous liquids and liquids with high solids content. They can produce significant suction lift and relatively high pressures. They can handle fluids ranging from clean water to sludges without changing clearances or components. On the other hand, they are expensive, bulky, and difficult to maintain, and replacement parts are expensive.
In the multiple-screw design, the fluid flows between a central drive screw and one or more idler screws in a close-fitting housing. In two-screw pumps, both shafts are driven with timing gears. In three-screw pumps, the screw treads are cut so one screw can drive the other two. The rotation of the screws produces a vacuum at the inlet, moves the fluid through the pump, and delivers the fluid to the discharge. In small sizes, they are used to supply lubricating oil to engines and industrial machinery. In intermediate sizes, they are used in office buildings as a source of hydraulic energy to operate elevators. In large sizes, they are used to load and unload barges and tankers.
Reciprocating pumps move liquid by means of a constant back-and-forth motion of a piston, plunger, or diaphragm within a fixed volume or cylinder. Reciprocating pumps can handle viscous and abrasive fluids. They are low-speed machines when compared with centrifugal and rotary pumps. They offer higher efficiencies, generally 85 to 94%, thus they require less horsepower. Reciprocating pumps are best suited for high-pressure and low-volume applications. They frequently require pulsation dampeners because of the pulsating nature of the flow. They have higher installed costs (usually offset by higher efficiencies) and higher maintenance costs than centrifugal or rotary pumps.
Plunger and piston pumps
In plunger pumps, a plunger moves through a stationary packed seal and is pushed into and withdrawn from a liquid cavity. In piston pumps, a piston moving back and forth within a liquid cavity pushes the fluid from the cylinder. Movement of either the plunger or piston creates an alternating increase and decrease of flow. As the plunger or piston moves backward, the available volume in the cylinder increases and a suction valve opens to allow the liquid to enter the cylinder through a one-way suction valve. As the plunger or piston moves forward, the volume available in the cylinder decreases, the pressure of the liquid increases, and the liquid is forced out through a one-way discharge valve.
Efficiencies remain high regardless of head or speed (tend to decrease slightly with increasing speed). Because reciprocating pumps run at lower speeds than centrifugal or rotary pumps, they are better suited for handling viscous liquids. They are capable of producing high pressures and large capacities and are self-priming. On the other hand, they require more maintenance because of the large number of moving parts. They are heavier in weight and require more floor space than centrifugal or rotary pumps. In addition, they are poor at handling liquids containing solids that tend to erode valves and seats. Plunger and piston pumps require larger NPSHs because of pulsating flow and pressure drop through the valves. As a result of pulsating flow, they require special attention to suction- and discharge-piping design to avoid both acoustical and mechanical vibrations.
Fig. 3 shows a typical fluid (gas-, air-, or liquid-powered) diaphragm pump. Its principle of operation is similar to plunger and piston pumps except that, instead of a plunger or piston, there is a flexible pulsating diaphragm that displaces the liquid. Varying power-fluid pressure on one side of the diaphragm causes the diaphragm to deflect alternatively drawing liquid into the pump-side chamber or discharging the liquid from the pump-side chamber. Diaphragm pumps are capable of pumping liquids that are viscous, erosive, corrosive, or that contain large amounts of solids. In addition, diaphragm pumps are self-priming, can run periodically without liquids, and are inexpensive to repair because they have no stuffing box and have few moving parts.
Diaphragm pumps are limited to small flow rates (90 gal/min), moderate discharge pressures, and moderate temperatures. They require frequent maintenance and exhibit fatigue failure with time. Leaks can cause a hazard by mixing power fluid with the process fluid. Gas-/air-powered diaphragm pumps are commonly used as sump pumps.
It is possible to use a diaphragm to power a plunger or piston pump. This type of pump is often used for chemical injection because it is well suited for low volume and large-head applications, and the speed can be controlled by a throttling valve on the power fluid.
Reciprocating pump performance considerations
Reciprocating pumps are constant-volume pumps. Variations in discharge pressures do not affect flow rate. Because these pumps continue to deliver the same capacity, any attempt to throttle the discharge flow may overpressure the pump casing and/or the discharge piping. Thus, no reciprocating pump should ever be started or operated with the discharge block valve closed. Flow is regulated by speed.
The capacity of a reciprocating pump is the cylinder displacement less slip. For a single-acting cylinder, cylinder displacement can be determined from
For double-acting cylinders, the cylinder displacement can be determined by
s = cylinder displacement
A = plunger or piston area
a = piston-rod cross-sectional area
LS = stroke length
N = speed
m = number of pistons or plungers.
Slip is the loss of capacity as a percentage of the cylinder displacement because of volumetric efficiency, stuffing-box losses, and valve losses. Volumetric efficiency (not to be confused with mechanical efficiency) is normally 95 to 97%. Efficiency is also reduced when pumping a light hydrocarbon that has some degree of compressibility.
The pump capacity can be determined from
q = pump capacity.
Speed is the primary factor that determines both the capacity of a reciprocating pump and its maintenance costs. Running at high speeds shortens packing life and increases acceleration and deceleration forces on all moving components. Operating below the maximum “rated” speed may be advantageous when the pump is operated unattended, when there are no spares and no standby, when there is a high penalty for down time, when unit maintenance is poor, when long life is desired, and when the NPSH margin is low. Operating at the maximum rated speeds requires:
- Clean, cool fluids
- Excellent piping layout with rigidly fixed piping
- Good NPSH margin
- Solid foundation
- Well-designed suction and discharge pulsation dampeners
- Good maintenance
Whenever it becomes necessary to operate above the maximum rated speeds, very close attention should be given to all design, operation, and maintenance details.
If positive-displacement pumps are properly installed and operated, satisfactory performance can be realized for a long time. These pumps are manufactured in a variety of designs for many different services. Each manufacturer’s instructions should be followed carefully for specific machines or application equipment. The following discussion relates to general installation guidelines for positive-displacement reciprocating pumps.
Foundations and alignment
Most pump foundations are constructed of reinforced concrete. The pump and driver are bolted to a cast iron or steel base plate, which is secured to the concrete foundation with anchor bolts. Small pumps need a foundation large enough to accommodate the base-plate assembly. Large pumps require a foundation that is three to four times the weight of the pump and driver.
Anchor bolt sleeve installation
Each anchor bolt is fitted with a washer and passed through a pipe sleeve that has a diameter three to four times greater than the bolt. The bolt-sleeve unit is set into the concrete at the predetermined base-plate hole positions. The flexibility in the sleeve washer unit allows minor adjustments to be made in the bolt position before final tightening even after the concrete foundation has set.
Metal shim adjustments
Metal shims are used to position the pump on the foundation. Adjustments are made until the pump shaft and flanges are completely level. Alignment between the pump and driver is then adjusted before connecting the pump to the suction and discharge lines. The latter should have been aligned during the initial positioning of the base plate.
Because of pipe strain, the entire pump assembly should be rechecked for alignment once the piping has been securely bolted. If the drive alignment has not been changed by bolting the piping, the space between the base plate and concrete foundations is filled with grouting. Grouting should be sufficiently fluid to fill all the available space under the base plate.
Operating temperature considerations
It is essential that the alignment between the piping, pump, and driver not change. Ideally, alignments should be made at the operating temperature after initial cold alignment of the pumping system, thus eliminating any alignment changes because of thermal expansion.
Next to the selection of operating speeds, proper piping design is the most important consideration in pump-installation design. Poor piping is often the result of inattention to details, which can lead to more than average down time, higher maintenance costs, and loss of operating-personnel confidence.
Suction piping should be direct, free of bends, as short as possible, and at least one nominal pipe size larger than the pump-suction connection. Directional piping changes should be made with long-radius elbows. A full opening block valve should be installed in the suction piping. The suction vessel should have sufficient retention time for the evolution of free gas and should be equipped with a vortex breaker at the discharge nozzle. The suction and bypass lines should enter the vessel below the minimum liquid level.
Suction piping should be large enough so that the velocity limits are not exceeded. Eccentric reducers with the flat side up should be used instead of concentric reducers. Suction piping should include a suction strainer and a pulsation dampener. Suction strainers should not be installed unless regular maintenance can be assured. A fluid-starved condition resulting from a plugged strainer can cause more damage to the pump than solids ingestion.
The discharge piping should be direct, free of excessive bends, and at least one nominal pipe size larger than the pump-discharge connection. Directional piping changes should be made with long-radius elbows. Concentric reducers may be used, but they should be placed as near to the pump as practical. To facilitate priming and starting, a bypass (recycle) line with check valve and block valve should be installed to the suction source. If a pulsation dampener is not included in the initial installation, a flanged connection should be provided if pulsation attenuation may be required. A relief valve should be installed upstream of the discharge block valve, in case overpressurization in the discharge piping occurs.
Flow from a reciprocating pump is not uniform. The oscillating motion of the plungers creates disturbances (pulsations) that travel at the speed of sound from the pump cylinder to the piping system. Pulsations are a function of the pump’s piston/plunger velocity, internal valves, and operating speed. Pulsations cause the pressure level of the system to fluctuate with respect to time.
Suction pulsations can cause the pressure level to drop instantaneously below the fluid vapor pressure, which results in cavitation. Caviation can cause failure of pump parts such as:
Caviation can also cause high piping vibrations that result in the failure of:
- Gauge lines
Normal pipe clamps and supports may not be effective in controlling these vibrations.
Pulsations can be amplified by the acoustical resonances of the piping system, which results in pump fluid-end failures and piping failures because of the shaking caused by pressure pulsation. For simple piping layouts and low-to-moderate pump speeds, pulsation dampeners are used to attenuate the effects of pulsating flows. Pulsation dampeners are normally installed on both the suction and discharge. Dampeners can be liquid-filled; gas-cushioned, or tuned acoustical filters. For complicated and multiple-pump piping and high pump speeds, acoustical filters are used.
The design of a pulsation-dampening system is beyond the scope of this chapter. Special expertise is needed for analyzing and controlling pulsations in multipump installations.
|A||=||plunger or piston area|
|a||=||piston-rod cross-sectional area|
|m||=||number of pistons or plungers|
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