2 Pump types and performance data
2.3 Pump types and their applications
2.3.3 Pump types
Centrifugal pumps are used in so many different domains and applications that it would by impractical to enumerate them all. Table 2.4 gives examples of some applications of eminent economic significance; some particular requirements are listed too.
In many systems the pump is only one component in a complex process and its failure means serious economic losses – think for example of the boiler feed- pumps in a large power plant. Therefore the demand for maximum operating reli- ability is a characteristic feature of many pump specifications. This is true not only for special pumps but also for mass-produced items like water cooling pumps for automobiles or central heating circulation pumps.
2.3 Pump types and their applications 53
Fig. 2.4. Circulating pump for central heating installations, WILO AG
tected 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. The pump is mounted between flanges in the pipeline; no additional support is required. Electronic controls are integrated into the motor block.
Not only radial but also semi-axial impellers up to about nq =140 are used in pumps with volute casings. Small pumps with low specific speeds can be built also with an annular casing instead of a volute. For pumping liquids with a high content of solids or gas there are a range of special pump designs with various types of closed or open impellers to choose from. Figure 7.17 shows some special impeller designs and Fig. 7.21 shows the section of a sewage pump.
When pumping high-temperature fluids, cooling must be provided for the shaft seal (see the cooling box and cooling water connections in Fig. 2.1). Moreover, the bearing housing and the pump casing should be supported at the level of the shaft centerline in order to minimize any negative effects resulting from thermal deformation. The objective is to avoid a driver misalignment and a distortion of shaft and casing resulting in a possible seizure of the impeller ring on the station- ary wear ring.
The range of materials used, depending on the medium to be pumped and the application, comprises cast iron, carbon steel, bronzes and all kinds of stainless steel as well as plastics, glass and ceramics.
Single-stage, double-entry pumps with volute casings, Fig. 2.5: 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, Chap. 6.2.4. Also the efficiencies of double-entry pumps are often superior to those obtained with single entry pumps (at nq < 40). With a double-entry design the hydraulic forces acting axially on the impeller are to a great extent balanced for symmetry reasons (Chap. 9.2). Thus a smaller axial bearing is sufficient.
Figure 2.5 shows 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 (Chap. 9.3). 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.
Fig. 2.5. Double-entry vertical pump, Sulzer Pumps, n = 596 rpm, Qopt = 2.4 m3/s, Hopt = 48 m, Popt = 1250 kW, nq = 36
2.3 Pump types and their applications 55
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, a stuffing box is a good solution for the shaft seal.
Single-stage, double-entry pumps are designed for specific speeds between ap- proximately nq = 10 to 100. Mostly they are built as horizontal units with the suc- tion and discharge nozzles in line. Thus the pump can be installed in a straight pipeline without elbows or additional fittings. For large-bore piping this is an im- portant cost advantage if the installation concept is compatible with this arrange- ment. For cold liquids (nonetheless up to 130 °C) double-entry pumps are mostly built as “axial-split” designs. In this type, a flange splits the pump casing through the rotor axis and allows the removal of the complete rotor assembly after lifting off the upper half of the casing. Discharge and suction nozzles are below the split flange and do not hinder the opening of the pump. For high temperatures, high pressures and for chemical processes, single-stage, double-entry pumps are also designed with barrel casings and one or two casing covers which support the bear- ings and house the shaft seals. The design concept is similar to Fig. 2.11.
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 vi- bration problems. One reason for this is that increasing the rotor length will cause the “critical speed” of the pump shaft to drop. Another reason is that the forces which excite vibrations grow with the square of the circumferential speed u2 of the impellers: the greater u2, the higher is the danger of forced as well as self- excited vibrations. As a result, the number of allowable stages decreases with growing circumferential speed.
Multistage pumps are built in Europe as a rule with diffusers, whereas in North America double volutes are often used for many multistage applications. Fig- ure 2.6 shows a 6-stage segmental pump (also called “ring section pump”) with diffusers. Usually the fluid enters through an inlet casing with a radially oriented suction nozzle. However, there is also a pump concept with an axial suction noz- zle and, consequently, a product-lubricated bearing on the suction side. The pump shown in Fig. 2.6 has product-lubricated bearings. This feature favors a compact design resulting in reduced rotor length, space, weight and costs, [2.2].
The impeller of the first stage can be designed with a greater inlet diameter than the impellers of the following (“series”) stages in order to reduce the NPSH required, Fig. 2.6. The exit casing, which may be in the form of an annulus or a volute, leads the fluid from the last stage into the discharge nozzle.
The stage casings are arranged between the inlet and the exit casings, which are braced by strong tie bolts. Each stage casing comprises an impeller and a diffuser which is centered in the pressure-bearing casing element. Return vanes guide the
Fig. 2.6. Multistage segmental pump with suction impeller and balance disk, Sulzer Pumps fluid from the diffuser outlet to the impeller inlet of the subsequent stage. The bearing housings are bolted via flanges to the inlet and exit casings.
Barrel-type multistage diffuser pumps are built for extreme pressures, Fig. 2.7.
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.
Fig. 2.7. Barrel-type, high-pressure boiler feedpump, Sulzer Pumps
n = 5800 rpm, H = 3900 m, P = 34’000 kW. The cartridge assembly is shaded.
2.3 Pump types and their applications 57
The cartridge can be removed from the barrel casing by disassembling the cou- pling and a small retention ring on the drive-end. The design shown in Fig. 2.7 is typical for boiler feedpumps in fossil-fueled power plants of several hundred MW. The pump shown is of the same type as described in [2.3]. 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 vaned 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 vaned insert also provides a mechanically strong and symmetric connection between the stage casings and the drive end cover.
Multistage pumps may be equipped with an “intermediate take-off” withdraw- ing a fraction of the flow rate1. An example is an intermediate take-off on a boiler feedpump for controlling the steam temperature upstream of the re-heater by wa- ter injection. In this case, the flow rate through all stages is not the same; this fact must be taken into account in the pump layout. Some boiler feedpumps are equipped with a “kicker stage”, which consists of a small additional impeller mounted behind the last stage. It is designed for a fraction of the discharge flow rate with the purpose of controlling the steam temperature upstream or down- stream of the super-heater of a fossil-fired boiler.
In multistage pumps the hydraulic axial forces of all stages add up to a consid- erable “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 ax- ial thrust balancing, Chap. 9.2. The pump in Fig. 2.6 is designed with a balance disk, whereas the barrel-type pump in Fig. 2.7 has a balance piston.
Another possibility to balance the axial thrust is the “back-to-back” arrange- ment of the impellers in two groups (also: “opposed-impeller design”). As an ex- ample Fig. 2.8 shows a 12-stage pump with double volutes. 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
Fig. 2.8. Multistage, axial-split pipeline pump with double volutes. Sulzer Pumps
1 In Fig. 2.7 the intermediate take-off is from the second stage, but it can be arranged at any other stage as well.
halves, as are the long crossovers leading from the first to the second stage group.
The volutes discharge via diffusers into overflow channels (“short crossovers”) leading the fluid to the impeller of the subsequent stage. The suction and dis- charge 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 for the casing in Fig. 2.8 are quite complicated and a hindrance to standardization.
The back-to-back arrangement requires a smaller thrust bearing (right-side in Figs. 2.8 and 2.9) than inline impellers as per Fig. 2.7. 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 some- times possible to improve the efficiency as compared to pumps with inline impel- lers. 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 (Chap. 10).
Figure 2.9 shows a back-to-back arrangement of a 12-stage diffuser pump for seawater injection. The performance data are given in the figure caption, note the head of 5300 m. Similar to Fig. 2.7 it is a cartridge design. Between the two stage groups there is a centerpiece with crossover channels. One set of channels, shown below the center bush, leads the water from the first stage group to the second.
Another set of channels (shown above the center bush) directs the fluid from the second stage group to an annular chamber and the discharge nozzle. The inlet nozzle (left-side in Fig. 2.9), directs the water into an annulus from where it flows through a vaned insert to the suction impeller (as in Fig. 2.7).
Multistage pumps of the types shown in Figs. 2.6 to 2.9 can also be built with a double-entry impeller as shown in Fig. 2.10.
Fig. 2.9 High-pressure sea water injection pump. Sulzer Pumps
Rated conditions: n = 6000 rpm, Q = 400 m3/h, H = 5300 m, P = 7700 kW, nq = 21
2.3 Pump types and their applications 59
Fig. 2.10 High-pressure back-to-back barrel pump with a double-entry suction impeller.
Sulzer Pumps (discharge nozzle not in drawing plane)
This arrangement can be advantageous if a direct drive (e.g. 3600 rpm) is to be used, since it allows a higher speed (smaller pump) for a specified available NPSHA. The double-entry suction impeller discharges into a diffuser and subse- quently into an annular casing. Several channels lead the fluid from there to the second stage. Similar to Fig. 2.9, the stages are arranged back-to-back in a car- tridge which is mounted from the non-drive end “NDE” into the barrel (from the left in Fig. 2.10). The heavy cover on the NDE is a “twist-lock” design, which avoids the large bolting required for a conventional design as in Fig. 2.9. The back-to-back design is used for two-stage pumps too, Fig. 9.16.
Figure 2.11 shows a “radial split” process pump with a double-entry suction stage and a single-entry second stage. Both impellers can be dismantled through
Fig. 2.11 Two-stage process pump with double-entry suction impeller, Sulzer Pumps
casing covers arranged on either side of the pump. The double volutes and cross- overs are cast into the casing.
Multistage, double-entry pumps: 2- or 3-stage pumps may also be built as dou- ble-entry designs. Figure 2.12 shows a two-stage water transport pump with this concept. The fluid is distributed by a forked pipe to both suction nozzles and from there via the inlet casings to both the single-entry suction impellers (which operate in parallel). The water then flows through the diffusers and return channels to the second, double-entry stage. This delivers via a double volute into the discharge nozzle which is arranged inline with the entry to the forked pipe. The double- entry design reduces the NPSHA requirements (as compared to the single-entry pump) making it an economical solution with regard to investment costs. More- over, it achieves a better efficiency (and therefore lower energy costs) because no axial thrust balance device is needed. The volumetric and mechanical efficiencies are accordingly higher, since there is no leakage through a balance device and the axial bearing is smaller. The slightly more expensive design of a multistage, dou- ble-entry pump thus pays off quickly in large pumping plants due to reduced civil engineering and energy cost and also due to very good reliability (i.e. lower main- tenance cost).
Fig. 2.12. Two-stage, double-entry water transport pump, Sulzer Pumps n = 1490 rpm, Qopt = 1.7 m3/s, Hopt = 465 m, Popt = 8600 kW, nq = 23
Semi-axial pumps: The semi-axial concept helps to reduce the outside diameter of the diffuser in comparison to the volute casing of a radial design. That is why vertical bore-hole pumps for water supply which require, for economic reasons, the smallest possible diameter are mostly multistage, semi-axial pumps. For low flow rates in this kind of application, even pumps with specific speeds as low as approximately nq = 20 are designed with semi-axial diffusers. The impellers hardly vary from those employed in a radial design. Vertical, multistage process or condensate pumps with low specific speeds are often built as semi-axial can- type pumps. Figure 2.13 shows such a design with suction and discharge nozzles
2.3 Pump types and their applications 61
arranged inline. The diffusers are cast into the stage casings which are bolted to- gether to form the pump unit. The fluid exits from the last stage into a column pipe. The pump assembly is fitted into a tank (a “can”) from where the fluid enters the suction impeller via a suction bell. Axial thrust reduction is accomplished by balance holes and an annular seal on the impeller rear shroud, similar to Fig. 2.1.
A design similar to Fig. 2.13 uses volutes with 3 or 4 channels instead of vaned diffusers.
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 trans- porting drinking- or cooling water and for irrigation or drainage. They are often installed in intake basins as “wet pit installations”. Figure 2.14a shows such a pump with a specific speed of nq = 150 and a suction bell for wet pit installation.
a)
b)
Fig. 2.13. Multistage, semi-axial process can- type pump, Sulzer Pumps
Fig. 2.14. Single-stage vertical pump for wet pit installation:
a semi-axial pump, b axial pump, Sulzer Pumps
The diffuser discharges into a column pipe in which the shaft is supported by product-lubricated bearings. For low heads (and consequently high specific speeds) special attention has to be paid to good hydraulic design of the discharge bend, since it causes pressure losses of several per cent of the head and thus has an impact on the energy costs.
Axial pumps: For specific speeds above nq > 170 axial or (“propeller”) pumps are used, Fig. 2.14b. The flow through impeller and diffuser is purely in axial direc- tion. The mechanical design is very similar to that of semi-axial pumps.
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. The technology of seal-less pumps is extensively discussed in [B.14].
One way to eliminate the need for shaft seals is the “canned” or “wet motor”
design as shown in Fig. 2.4. With small pumps this very compact design signifi- cantly 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 mo- tor. Permanent magnets, mounted on the shafts of the driver and the pump, trans- mit a rotating magnetic field (and therefore a torque) to the impeller via an air gap and a metallic liner, Fig. 2.15.
Fig. 2.15. Seal-less process pump driven by a magnetic coupling, Sulzer Pumps
2.3 Pump types and their applications 63
Submersible pumps: Submerging the pump and motor into the liquid to be trans- ported, 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 in- stalled 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.
Figure 2.16 shows a submersible sewage pump with an oil-filled motor. 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 mo- tor 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 cas- ing; an air gap separates the rotor and stator windings of the motor in this design.
Fig. 2.16. Submersible sewage pump,
Wilo-Emu SE Fig. 2.17. Submersible vortex pump
for sewage applications, Wilo-Emu SE