Introduction
The transfer of liquids against gravity existed from time immemorial. A pump is one suchdevice that expends energy to raise, transport, or compress liquids. The earliest known pump devices go back a few thousand years. One such early pump device was called ‘Noria’, similar to the Persian and the Roman water wheels. Noria was used for irrigating fields.
Figure 1.1. Noria water wheel (From the Ripley’s believe it not)
The ancient Egyptians invented water wheels with buckets mounted on them to transfer water for irrigation. More than 2000 years ago, a Greek inventor, Ctesibius, made a similar type of pump for pumping water (Figure 1.2).
Figure 1.2 Model of a piston pump made by CtesbiusDuring the same period, Archimedes, a Greek mathematician, invented what is now known as the ‘Archimedes’ screw’ – a pump designed like a screw rotating within a cylinder (Figure 1.3). The spiraled tube was set at an incline and was hand operated. This type of pump was used to drain and irrigate the Nile valley. In 4th century Rome, Archimedes’ screw was used for the Roman water supply systems, highly advanced for that time. The Romans also used screw pumps for irrigation and drainage work. Screw pumps can also be traced to the ore mines of Spain. These early units were all driven by either man or animal power.
Figure 1.3 Archimedes’ screw pumpThe mining operations of the Middle Ages led to the development of the suction (piston) pump, types of which are described by Georgius Agricola in De re metallica (1556). Force pumps, utilizing a piston-and-cylinder combination, were used in Greece to raise water from wells (Figure 1.4).
Figure 1.4 Reciprocating hand pump in suction stroke
1.1 Applications
Times have changed, but pumps still operate on the same fundamental principle – expend
energy to raise, transport, or compress liquids. Over time, the application of pumps in the agricultural domain has expanded to cover other domains as well. The following are a few main domains that use pumps extensively :
Water supply: To supply water to inhabited areas.
Drainage: To control the level of water in a protected area.
Sewage: To collect and treat sewage.
Irrigation: To make dry lands agriculturally productive.
Chemical industry: To transport fluids to and from various sites in the chemical plant.
Petroleum industry: Used in every phase of petroleum production, transportation., and refinery.
Pharmaceutical and medical field: To transfer of chemicals in drug manufacture; pump fluids in and out of the body.
Steel mills: To transport cooling water.
Construction: Bypass pumping, well-point dewatering, remediation, and general site pumping applications.
Mining: Heavy-duty construction, wash water, dust control fines and tailings pumping, site dewatering, groundwater control, and water runoff.
Pumps are also used for diverse applications like in transfer of potatoes, to peel the skin of hazelnuts in chocolate manufacture, and to cut metal sheets in areas that are too hazardous to allow cutting by a gas flame torch. The artificial heart is also a mechanical pump. The smallest pump ever made is no bigger than the tip of a finger. It moves between 10 and 30 nl of liquid in one cycle (10- to 30-thousandths of a drop of water). It was not found to have any practical use so maybe it was created just for the records.
1.2 Pump types
Pumps can be classified on various bases. For example, a typical classification of rotating shaft (kinetic) pumps is given in Appendix. Pumps based on their principle of operation are primarily classified into:
Positive displacement pumps (reciprocating, rotary pumps)
Roto-dynamic pumps (centrifugal pumps)
Others.
1.2.1 Positive displacement pumps
Positive displacement pumps, which lift a given volume for each cycle of operation, can be divided into two main classes, reciprocating and rotary. Reciprocating pumps include piston, plunger, and diaphragm types. The rotary pumps include gear, lobe, screw, vane, regenerative (peripheral), and progressive cavity pumps.
1.2.2 Roto-dynamic pumps
Roto-dynamic pumps raise the pressure of the liquid by first imparting velocity energy to it and then converting this to pressure energy. These are also called centrifugal pumps. Centrifugal pumps include radial, axial, and mixed flow units.
A radial flow pump is commonly referred to as a straight centrifugal pump; the most common type is the volute pump. Fluid enters the pump through the eye of impeller, which rotates at high speed. The fluid is accelerated radially outward from the pump casing. A partial vacuum is created that continuously draws more fluid into the pump if properly primed.
In the axial flow centrifugal pumps, the rotor is a propeller. Fluid flows parallel to the axis of the shaft. The mixed flow, the direction of liquid from the impeller acts as an in-between that of the radial and axial flow pumps.
1.2.3 Other types
The other types include electromagnetic pumps, jet pumps, gas lift pumps, and hydraulic ram pumps.
1.3 Reciprocating pumps
Reciprocating pumps are positive displacement pumps and are based on the principle of the 2000-year-old pump made by the Greek inventor, Ctesibius.
1.3.1 Plunger pumps
Plunger pumps comprise of a cylinder with a reciprocating plunger in it (Figure 1.5). The head of the cylinder houses the suction and the discharge valves.
In the suction stroke, as the plunger retracts, the suction valve opens causing suction of the liquid within the cylinder. In the forward stroke, the plunger then pushes the liquid out into the discharge header. The pressure built in the cylinder is marginally over the pressure in the discharge. The gland packings help to contain the pressurized fluid within the cylinder.
The plungers are operated using the slider-crank mechanism. Usually, two or three cylinders
are placed alongside and their plungers reciprocate from the same crankshaft. These are called as duplex or triplex plunger pumps.
Figure 1.5 Plunger pump
1.3.2 Diaphragm pumpsDiaphragm pumps are inherently plunger pumps. The plunger, however, pressurizes the hydraulic oil and this pressurized oil is used to flex the diaphragm and cause the pumping of the process liquid. Diaphragm pumps are primarily used when the liquids to be pumped are hazardous or toxic. Thus, these pumps are often provided with diaphragm rupture indicators.
Diaphragm pumps that are designed to pump hazardous fluids usually have a doublediaphragm which is separated by a thin film of water (for example, see Figure 1.6). A pressure sensor senses the pressure of this water. In a normal condition, the pressure on the process and oil sides of the diaphragms is always the same and the pressure between the diaphragms is zero.
Figure 1.6 Double diaphragm pumps (Lewa pumps)
However, no sooner does one of them ruptures than the pressure sensor records a maximum of process discharge pressure. The rising of this pressure is an indicator of the diaphragm rupture (Figure 1.7). Even with the rupture of just one diaphragm, the process liquid does not come into contact with the atmosphere.
Figure 1.7 Diaphragm pump
1.4 Rotary pumps
1.4.1 Gear pump
Gear pumps are of two types:
1. External gear pump
2. Internal gear pump.
External gear pump
In external gear pumps, two identical gears rotate against each other. The motor provides the drive for one gear. This gear in turn drives the other gear. A separate shaft supports each gear, which contains bearings on both of its sides .
Figure 1.8 External gear pump
As the gears come out of the mesh, they create expanding volume on the inlet side of the pump. Liquid flows into the cavity and is trapped by the gear teeth while they rotate. Liquid travels around the interior of the casing in the pockets between the teeth and the casing. The fine side clearances between the gear and the casing allow recirculation of the liquid between the gears.
Finally, the meshing of the gears forces liquid through the outlet port under pressure. As the gears are supported on both sides, the noise levels of these pumps are lower and are typically used for high-pressure applications such as the hydraulic applications.
Internal gear pump
Internal gear pumps have only two moving parts (Figure 1.9). They can operate in either direction, which allows for maximum utility with a variety of application requirements. In these pumps, liquid enters the suction port between the large exterior gears, rotor, and the smaller interior gear teeth, idler. The arrows indicate the direction of the pump and the liquid.
Liquid travels through the pump between the teeth of the ‘gear-within-a-gear’ principle.
The crescent shape divides the liquid and acts as a seal between the suction and the discharge ports. The pump head is now nearly flooded as it forces the liquid out of the discharge port. Rotor and idler teeth mesh completely to form a seal equidistant from the discharge and suction ports. This seal forces the liquid out of the discharge port.
Figure 1.9 Internal gear pump
The internal gear pumps are capable of handling liquid from very low to very high viscosities. In addition to superior high-viscosity handling capabilities, internal gear pumps offer a smooth, nonpulsating flow. Internal gear pumps are self-priming and can run dry.
1.4.2 Lobe pump
The operation of the lobe pumps is similar to the operation of the external gear pumps (Figure 1.10). Here, each of the lobes is driven by external timing gears. As a result, the lobes do not make contact. Pump shaft support bearings are located in the gearbox, and since the bearings are not within the pumped liquid, pressure is limited by the location of the bearing and shaft deflection.
Figure 1.10 Lobe pump
As the lobes come out of mesh, they create expanding volume on the inlet side of the pump. The liquid then flows into the cavity and is trapped by the lobes as they rotate. The liquid travels around the interior of the casing in the pockets between the lobes and the casing and it does not pass between the lobes.
Finally, the meshing of the lobes forces the liquid through the outlet port under pressure. Lobe pumps are frequently used in food applications because they can handle solids without damaging the product. The particle size pumped can be much larger in lobe pumps than in any other of the PD types.
1.4.3 Vane pump
A vane pump too traps the liquid by forming a compartment comprising of vanes and the casing (Figure 1.11). As the rotor turns, the trapped liquid is traversed from the suction port to the discharge port. A slotted rotor or impeller is eccentrically supported in a cycloidal cam. The rotor is located close to the wall of the cam so a crescent-shaped cavity is formed. The rotor is sealed in the cam by two side plates. Vanes or blades fit within the slots of the impeller.
As the impeller rotates and fluid enters the pump, centrifugal force, hydraulic pressure, and/or pushrods push the vanes to the walls of the housing. The tight seal among the vanes, rotor, cam, and side plate is the key to the good suction characteristics common to the Vane pumping principle.
The housing and cam force fluid into the pumping chamber through the holes in the cam. Fluid enters the pockets created by the vanes, rotor, cam, and side plate. As the impeller continues around, the vanes sweep the fluid to the opposite side of the crescent where it is squeezed through the discharge holes of the cam as the vane approaches the point of the crescent. Fluid then exits the discharge port.Vane pumps are ideally suited for low-viscosity, nonlubricating liquids.
Figure 1.11 Vane pump
1.4.4 Peripheral pump
As shown in Figure 1.12, the impeller has a large number of small radial vanes on both of its sides. The impeller runs in a concentric circular casing. Interaction between the casing and the vanes creates a vortex in the spaces between the vanes and the casing, and the mechanical energy is transmitted to the pumped liquid.
Figure 1.12 Peripheral pump impeller
Peripheral pumps are relatively inefficient and have poor self-priming capability. They can handle large amounts of entrained gas. They are suitable to low flow and highpressure applications with clean liquids.
1.4.5 Screw pump
In addition to the previously described pumps based on the Archimedes’ screw, there are
pumps fitted with two or three spindles crews housed in a casing. Three-spindle screw pumps, as shown in Figure 1.13, are ideally suited for a variety of marine and offshore applications such as fuel-injection, oil burners, boosting, hydraulics, fuel, lubrication, circulating, feed, and many more. The pumps deliver pulsation free flow and operate with low noise levels. These pumps are self-priming with good efficiency. These pumps are also ideal for highly viscous liquids.
Figure 1.13 Three-spindle screw pump – Alweiller pumps
1.5 Centrifugal pumps
The centrifugal pumps are by far the most commonly used of the pump types. Among all the installed pumps in a typical petroleum plant, almost 80–90% are centrifugal pumps. Centrifugal pumps are widely used because of their design simplicity, high efficiency, wide range of capacity, head, smooth flow rate, and ease of operation and maintenance.
The ‘modern’ era pumps began during the late 17th and early 18th centuries AD. British engineer Thomas Savery, French physicist Denis Papin, and British blacksmith and inventor Thomas Newcomen contributed to the development of a water pump that used steam to power the pump’s piston. The steam-powered water pump’s first wide use was in pumping water out of mines.
However, the origin of the centrifugal impeller is attributed to the French physicist and inventor Denis Papin in 1689. Papin's contribution lies in his understanding of the concept of creating a forced vortex within a circular or spiral casing by means of vanes. The pump made by him had straight vanes.
Following Papin’s theory, Combs presented a paper in 1838 on curved vanes and the effect of curvature, which subsequently proved to be an important factor in the development of the centrifugal impeller. In 1839, W.H. Andrews introduced the proper volute casing and in 1846, he used a fully shrouded impeller.
In addition, in 1846, W.H. Johnson constructed the first three-stage centrifugal pump, and in 1849, James S. Gwynne constructed a multistage centrifugal pump and began the first systematic examination of these pumps.
Around the same time, British inventor, John Appold conducted an exhaustive series of
empirically directed experiments to determine the best shape of the impeller, which culminated in his discovery that efficiency depends on blade curvature. Appold’s pump of 1851 with curved blades showed an efficiency of 68%, thus improving pump efficiency three-fold.
The subsequent development of centrifugal pumps was very rapid due to its relatively inexpensive manufacturing and its ability to handle voluminous amounts of fluid. However, it has to be noted that the popularity of the centrifugal pumps has been made possible by major developments in the fields of electric motors, steam turbines, and internal combustion (IC) engines. Prior to this, the positive displacement type pumps were more widely used.
The centrifugal pump has a simple construction, essentially comprising a volute (1) and an impeller (2) (refer to Figure 1.14). The impeller is mounted on a shaft (5), which is supported by bearings (7) assembled in a bearing housing (6). A drive coupling is mounted on the free end of the shaft.
Figure 1.14 Centrifugal pump – basic construction
The prime mover, which is usually an electrical motor, steam turbine, or an IC engine, transmits the torque through the coupling. As the impeller rotates, accelerates, and displaces the fluid within itself, more fluid is drawn into the impeller to take its place; if the pump is properly primed. The impeller thus, impacts kinetic or velocity energy to the fluid through mechanical action. This velocity energy is then converted to pressure energy by the volute. The pressure of the fluid formed in the casing has to be contained and this is achieved by an appropriate sealing arrangement (4). The seals are installed in the seal housing (3).
The normal operating speed of pumps is 1500 rpm (1800 rpm) and 3000 rpm (3600 rpm). However, there are certain designs of pumps that can operate at speeds in the range of 5000–25 000 rpm.
1.5.1 Types of centrifugal pumps
Centrifugal pumps can be categorized in various ways. Some of the main types are on the following basis:
Orientation of the pump shaft axis
This refers to the plane on which the shaft axis of the pump is placed. It is either horizontal or vertical as shown in Figure 1.15.
Figure 1.15 Vertical pump and horizontal pump
Number of stages
This refers to the number of sets of impellers and diffusers in a pump. A set forms a stage
and it is usually single, dual, or multiple (more than two) stages (Figure 1.16).
Figure 1.16 Multistage pump
Suction flange orientation
This is based on the orientation of the pump suction flange. This orientation could be
horizontal (also known as End) or vertical (also known as Top) (Figure 1.17).
Figure 1.17 Multistage pump with end suction
Casing split
This classification is based on the casing split. It is either Radial (perpendicular to shaft
axis) or Axial (plane of the shaft axis) (Figure 1.18).
Figure 1.18 Axial split casing
Bearing support
This is judged based on the location of the bearings supporting the rotor. If the rotor is supported in the form of a cantilever (Figure 1.19), it is called as an Overhung type of pump. When the impellers on the rotor are supported with bearings on either side, the pump is called as an in-between bearings pump.
Figure 1.19 Models of pump supports
Shaft connection
The closed coupled pumps are characterized by the absence of a coupling between the motor and the pump. The motor shaft has an extended length and the impeller is mounted on one end (Figure 1.20). The vertical monobloc pumps have the suction and discharge flanges along one axis
and can be mounted between pipelines. They are also termed as ‘in-line pumps’.
Figure 1.20 Closed coupled monobloc pumps with end suction
Sealless pumps
Pumps are used to build the pressure in a liquid and if necessary to contain it within the casing. At the interface of the rotating shaft and the pump casing, mechanical seals are installed to do the job of product containment. However, seals are prone to leakages and this maybe unacceptable in certain critical applications. To address this issue, sealless pumps have been designed and manufactured.
These are of two types – canned and magnetic drive pumps:
1. Canned pumps: In the construction of this second type of sealless pump, the rotor comprises of an impeller, shaft, and the rotor of the motor. These are housed within the pump casing and a containment shell (Figure 1.21). The hazardous or the toxic liquid is confined within this shell and casing. The rotating flux generated by the stator passes through the containment shell and drives the rotor and the impeller.
Figure 1.21 Canned pump
2. Magnetic drive pumps: In magnetic drive pumps, the rotor comprises of an impeller, shaft, and driven magnets. These housed within the pump casing and the containment shell ensures that the usually hazardous/toxic liquid is contained within a metal shell (Figure 1.21). The driven magnets take their drive from the rotating drive magnets, which are assembled on a different shaft that is coupled to the prime mover.
Figure 1.21 Magnetic drive pump
1.5.2 Pump standards
In order to bring about uniformity and minimum standards of design and dimensional specifications for centrifugal pumps, a number of centrifugal pump standards have been developed. These include the API (American Petroleum Institute), ISO (International Standards Organization), ANSI (American National Standards Institute), DIN (German), NFPA (Nation Fire Protection Agency), and AS-NZ (Australia–New Zealand). Some of the famous standards, which are used in the development and manufacture of centrifugal pumps are API 610, ISO 5199, 2858, ANSI B73.1, DIN 24256, NFPA-21.
Figure 1.22 Pump built to API 610 standard
In addition to the above, there are many National Standards. Some of these are:
• France: NF E 44.121
• United Kingdom: BS 5257
• German: DIN 24256
• Australia & New Zealand: AS 2417-2001, grades 1 and 2.
Usually, the service criticality or application of the pump forms the deciding factor for a choice of standard. A critical refinery pump handling hazardous hydrocarbons would be in all probability built as per standard API 610 (Figure 1.22).
However, ordinary applications do not require the entire API-specified features and so the premium that comes with an API pump is not justified. Such pumps can be purchased built to lesser demanding standards like the ANSI B73.1. One big advantage of ANSI pumps is the outline dimensional interchangeability of same size pumps regardless of brand or manufacturer, something that is not available in the API pumps. In a similar way, pumps meant for firewater applications are usually built to the design specifications laid out in NFPA-21.
There are some standards like the ISO 2858, which are primarily meant as dimensional standards. This does not provide any requirement for the pump’s construction. The standard from ISO that addresses the design aspects of pumps is ISO 5199. For a good comparative study of the API, ANSI, and ISO standards, it is recommended to read the technical paper called, ‘ISO-5199 Standard Addresses Today’s Reliability Requirements For Chemical Process Pumps’, by Pierre H. Fabeck, Product Manager, Durco Europe, Brussels, Belgium and R. Barry Erickson, Manager of Engineering, The Duriron Company, Incorporated, Dayton, Ohio. This paper was presented at the
7th (1990) Pumps Symposium at the Texas A&M University.
1.5.3 Pump applications
The classification of pumps in the above sections is based on the construction of the pump and its components. However, on the basis of the applications for which they are designed, pumps tend to be built differently. Some of the applications where typical pumps can be found are:
• Petroleum and chemical process pumps
• Electric, nuclear power pumps
• Waste/wastewater, cooling tower pumps
• Pulp and paper
• Slurry
• Pipeline, water-flood (injection) pumps
• High-speed pumps.
As this needs an introduction to the components/construction of the pump, these are covered in detail in subsequent topics.
