PUMP TYPES, THE BASICS PRINCIPLES AND COMPONENTS
By: Cyrus 07 Sep. 2022
The nearly inexhaustible variety of flow phenomena – from the flow through blood vessels, the flow in centrifugal pumps to global weather events − is based on a few basic physical laws only. In this article these will be briefly reviewed and their general nature illuminated. Emphasis will be on the phenomena which are of special interest and significance to the pump engineer. Basic knowledge of the terms of fluid dynamics is taken for granted. There is a wealth of textbooks and handbooks on fluid dynamics, only a few can be quoted
Centrifugal pumps are turbomachines used for transporting liquids by raising a specified volume flow to a specified pressure level. The energy transfer in turbomachines is invariably based on hydrodynamic processes for which characteristically all pressure and energy differences are proportional to the square of the circumferential rotor speed. By contrast, positive displacement pumps (e.g. piston pumps) essentially deliver the same volume Vstroke at each stroke independently of flow velocity or rotor speed n. The flow rate then becomes Q = n×Vstroke; the pressure rise results solely from the imposed back pressure. A centrifugal pump according is essentially composed of a casing, a bearing housing, the pump shaft and an impeller. The liquid to be pumped flows through the suction nozzle to the impeller. The overhung impeller mounted on the shaft is driven via a coupling by a motor. The impeller transfers the energy necessary to transport the fluid and accelerates it in the circumferential direction. This causes the static pressure to increase in accordance with kinetics, because the fluid flow follows a curved path.The fluid exiting the impeller is decelerated in the volute and the following diffuser in order to utilize the greatest possible part of the kinetic energy at the impeller outlet for increasing the static pressure. The diffuser forms the discharge nozzle.
Single-stage volute pump with bearing frame, Sulzer PumpsA shaft seal, e.g. a stuffing box or a mechanical seal, prevents the liquid from escaping into the environment or the bearing housing. An inducer may be added at the impeller inlet for improving the suction performance. However, most applications do not use an inducer. Impeller and casing are separated by a narrow annular seal through which some leakage flows back from the impeller outlet to the inlet. A second annular seal on the rear shroud serves the purpose of counterbalancing the axial forces acting on the impeller front and rear shrouds. The leakage through this seal flows back into the suction chamber through “axial thrust balance holes” which are drilled into the rear shroud. The impeller can be described by the hub, the rear shroud, the blades transferring energy to the fluid and the front shroud. In some applications the front shroud is omitted. In this case the impeller is termed “semi-open”. The leading face of the blade of the rotating impeller experiences the highest pressure for a given radius. It is called pressure surface or pressure side. The opposite blade surface with the lower pressure accordingly is the suction surface or suction side
Meridional section and plan view of a radial impeller,
Radial impeller
Net positive suction head, NPSH
When the pressure in a liquid drops below the vapor pressure, a portion of the fluid will evaporate. Excess velocities due to the flow around the blade leading edge cause a local pressure drop, which may lead to such partial evaporation. This phenomenon is called “cavitation". Extensive cavitation can impair the performance or even interrupt the flow. Therefore the approach flow conditions at the suction nozzle are an important criterion for the layout and selection of a pump. The relevant parameter is the “net positive suction energy” NPSE or the “net positive suction head” NPSH. It is defined as the absolute suction head Hs, abs minus the vapor pressure expressed as head. We distinguish between the (usually measured) NPSH of the pump which is necessary in order to fully or partially suppress cavitation (“NPSH required” or NPSHR ) and the NPSH available in the plant (NPSHA ). Since the vapor pressure pv is given in the water/steam tables as an absolute pressure, absolute pressures must be inserted into Eqs. for calculating the NPSH.
Pump characteristics
When the flow rate of a pump varies, the head, the power consumption and the efficiency change too. Plotting these quantities against the flow rate we obtain the “pump characteristics”. At a certain flow rate the pump efficiency has a maximum value called the “best efficiency point” (BEP). The pump is designed for this BEP which is characterized by Qopt, Hopt, Popt and ηopt at a given speed.
Pump characteristics
Pump types
The following section presents some widely used pump types.
Single-stage, single-entry pumps with volute casings: Single-stage pumps with volute casings are common in many branches of industry. Water supply, sewage, chemical processing (including hydrocarbon processing) and power plants are important areas of application. They are also produced in great numbers as small pumps, e.g. for pumping cooling water in motor vehicles or as central heating circulation pumps in homes and buildings. Severe demands with respect to reliability and noise emission are made on these rather inconspicuous pumps which have impeller diameters typically below 100 mm.
Small inline pumps are often built as mono-block pumps, where the impeller is mounted on the end of the motor shaft. The motor and bearings are exposed to, and cooled by, the liquid pumped; no shaft seal is required. The stator windings of the motor are protected from the fluid by a liner; likewise the rotor windings are protected by a sleeve. Some liquid is passed through the gap between rotor and stator in order to remove the heat created by the motor losses and fluid friction. The bearings are lubricated by the liquid pumped. Leakage losses at the impeller inlet are reduced by a floating seal ring.
KSB Etaline circulating heat pump
Single-stage, double-entry pumps with volute casings:: Pumps with double-entry impellers require distinctly lower suction pressures (or NPSHA) than single-entry pumps for delivering a specified flow rate at a given speed s a double-entry vertical booster pump feeding a pump in a pipeline for the transport of drinking-water. The double volute used considerably reduces the hydraulic radial forces as compared to a single volute design. As the lower radial bearing is water lubricated, there is no need for a lower shaft seal. This bearing is cooled with water taken from the volute casing.
The upper radial bearing is a plain journal bearing while the axial forces are carried by a tilting-pad thrust bearing. Both are oil lubricated. As this is a low speed pump with a large shaft diameter.
Double-entry vertical pump KSB Omega
Multistage, single-entry pumps: If the specific speed for a required duty falls to a level where the efficiency would be too low and, consequently, the energy costs unacceptably high, multistage pumps are chosen (if the speed cannot be raised). Likewise, it is often necessary to limit the head per stage by using a multistage concept for reasons of mechanical design. Whereas all stages normally deliver the same volumetric flow, the heads of all the stages arranged in series add up to the total head. The maximum possible number of stages is essentially limited by vibration problems. One reason for this is that increasing the rotor length will cause the “critical speed” of the pump shaft to drop.
Multistage pump with suction impeller and balance KSB Multitec
Barrel-type multistage diffuser pumps are built for extreme pressures: Again the diffusers, with integral return vanes, are centered in stage casings which are bolted to each other. The result is an assembly unit (a “cartridge”) comprising the casing cover, the rotor, the bearing housings and the inlet casing insert. The cartridge can be removed from the barrel casing by disassembling the coupling and a small retention ring on the drive-end. The design shown is typical for boiler feed pumps in fossil-fueled power plants of several hundred MW. Similar pumps are used for water injection duties in oil fields for boosting the oil production. The inlet nozzle leads the water to an annular chamber from which it flows through a vane insert to the suction impeller. The radial vanes of the insert take out virtually all circumferential components of the inlet velocity and thus ensure a uniform flow to the impeller. The vane insert also provides a mechanically strong an
Barrel-type, high-pressure boiler feed pump, Sulzer Pumps
In multistage pumps the hydraulic axial forces of all stages add up to a considerable “axial thrust” on the rotor that cannot be carried by a reasonably sized thrust bearing. For this reason, multistage pumps usually have some device for axial thrust balancing. Another possibility to balance the axial thrust is the “back-to-back” arrangement of the impellers in two groups (also: “opposed-impeller design”). As an example. The casing is split axially, making it possible to remove the complete rotor after lifting off the upper half of the casing. The volutes are cast integrally into the lower and upper casing.
Axially split multi-stage pump Sulzer msd
The back-to-back arrangement requires a smaller thrust bearing than inline impellers. The leakage losses through the bushings in the center and at the inlet to the second stage group tend to be smaller than in inline pumps. By choosing back-to-back pumps, it is thus sometimes possible to improve the efficiency as compared to pumps with inline impellers. This is particularly true with low specific speeds. The center bushing is very favorable with respect to rotor dynamics since it provides additional support (stiffness and damping) in the middle of the bearing span.
The volutes discharge via diffusers into overflow channels (“short crossovers”) leading the fluid to the impeller of the subsequent stage. The suction and discharge nozzles are in the lower half of the casing so that it is possible to open the pump without disassembling the piping. The discharge nozzle is in the middle of the pump. A center bushing in the middle of the rotor controls the leakage from the second to the first group of stages. Another bushing is necessary at the inlet of the second stage group for balancing the axial thrust and relieving the pressure at the shaft seal. The castings fo
Multi stage Injection Pump. Sulzer Injection Pump
Semi-axial pumps: have optimum hydraulic behavior at specific speeds ranging from nq = 40 to 170. Vertical pumps of these specific speeds are used for transporting drinking- or cooling water and for irrigation or drainage. They are often installed in intake basins as “wet pit installations”. shows such a pump with a specific speed of nq = 150 and a suction bell for wet pit installation.
Seal-less pumps: Pumps without shaft seals (“seal-less pumps”) are used when hazardous or noxious substances must be prevented absolutely from leaking to the environment. With small pumps this very compact design significantly reduces installation costs, thus off-setting the higher costs of the canned motor. Using a magnetic coupling eliminates the risk of the motor windings coming into contact with the liquid in case of a defect in the liners which protect the motors. Permanent magnets, mounted on the shafts of the driver and the pump, transmit a rotating magnetic field (and therefore a torque) to the impeller via an air gap and a metallic liner.
Seal less Process Pump KSB Magnochem
Submersible pumps: Submerging the pump and motor into the liquid to be transported, reduces the complexity and cost of the pumping plant – in particular if long column pipes would be involved. One of the designs available uses oil-filled motors with a mechanical seal preventing oil leakage into the pumped fluid. Small sewage pumps are predominantly built as submersible pumps. These can be installed or withdrawn from the pit, from above the sewage level, by means of a chain without other manipulation for connecting the pump to the discharge pipe. The oil is cooled by a heat exchanger which can be distinguished in the figure just above the impeller rear shroud. The oil is circulated by an impeller arranged below the motors rotor. An external pipe leads the oil from the heat exchanger to the top of the motor. A special oil is used which does not harm the environment in case of a leakage. To avoid the oil circuit, other designs use water cooling of the motor casing; an air gap separates the rotor and stator windings of the motor in this design.
Submersible sewage pump, KSB Amacan
Comments
Post a Comment