Introduction
Centrifugal pump construction & component is intended to provide the components that are necessary to allow the impeller to do its job. The impeller is the primary working part of a centrifugal pump and is designed to rotate in the pump casing and add energy to the liquid. To accomplish this, a variety of impeller designs are used. They are usually supported by a shaft and bearings, while seals are used on the shaft to prevent leakage.
Objectives When you complete this module, Centrifugal Pump Construction & component, you will be able to: •Identify the major components of centrifugal pumps •Identify the sub-types of each component •Identify the mechanical similarity of different
centrifugal pump types •Explain the function of each component •Select the most appropriate sub-type component for
different applications.
Impellers The impeller is mounted on a shaft with bearings so that it can be rotated inside the ca casing. Note that where the rotating shaft enters the casing, it must be sealed against leakage of liquid.
This impeller has the inlet or suction on one side si de.. Hence ence,, it is si sin ngle gle suct suctio ion. n. It al also so has has plat plates es or shrouds on both sides of the vanes which enclose the impeller passages, and is called an enclosed impeller.
Many impellers are supplied with replaceable wearing rings which are fitted to the impeller. These rings are often made of wear resistant material to better withstand any rubbing contact with the casing.
The casing may also be fitted with a replaceable wearing ring. Care must be taken to assure that the two wearing surfaces can rub together without seizing.
What if the impeller has no shrouds? Then it must depend on the surrounding casing to confine the impeller passages, and is called an open impeller. impeller.
Another impeller design uses a shroud on on
A popular alternative to the single suction impeller is on which which is is mount mounted ed betwee between n beari bearings. ngs. The impell impeller er in in su pumps usually provides for inlet to both sides of the imp
Reverse Vane Impeller
Shafts
Pump shafts are designed to support the impeller on one end, overhung from the bearings or between the bearings. The overhung design allows straight liquid flow into the impeller, but results in greater radial load on the bearings
The overhung design eliminates one seal around the shaft and simplifies the construction of the casing. Some seal se alle less ss pump pump desi design gns s use use a stationary shaft or spindle on which the impeller and drive element rotate.
The between-bearing design divides the radial forces between the two bearings, resulting in lower bearing loads and allowing double suction impeller designs. However, two shaft seals are required in this design.
Multistage pumps typically have the impellers mounted betwee between n bear bearing ings. s. Howeve However, r, the the longer longer shaft shaft which which is necessary for holding more impellers becomes increasingly more flexible and vibration problems can be more prevalent.
Multistage well pumps, also called vertical turbine pumps, use long shafts, but sleeve bearings between stages support the shaft and impellers and minimize vibration problems. Shafts are made in about ten foot sections, which are connected with solid threaded couplings
Bearings: Rolling Element Bearings
Rolling element bearings are most commonly used to support centri centrifug fugal al pump pump shaft shafts. s. Single Single row or or double double row row ball ball bearin bearings gs are good for carrying radial and axial loads. Roller bearings are good for carrying high radial loads but not axial loads.
Bearings are limited to the load they can carry by the fatigue life of the the materi material al of of the rollin rolling g elemen elements ts and and racew raceways ays.. Higher Higher loads reduce life as shown in this graph. For pumps, bearings are usually selected so that the basic rating life L10 of the bearings is about two years of continuous operation. L10 is the expected life of 90% of the bearings.
Good bearing life depends on a clean operating environment and proper lubri lubrica cati tion on.. The The envi environ ronme ment nt is is usua usuall lly y kept clean by seals around the shaft which keep liquid and dirt out of the bearing housing.
Bearings can be lubricated with oil or grease. Oil is usually present in the bearing housing in the form of an oil bath. A constant constant level level oiler can be used used to maintain maintain the the level of of the oil oil bath at the center of the lowest ball in the bearing. Oil also can reach the bearings by using a flinger disc or slinger rings on the rotating shaft, which dip into the oil, tossing it onto the bearings.
Grease lubrication is usually provided by grease in a cavity in the bearing cove co ver. r. Grea Grease se is repl replen enis ishe hed d by a grease dispenser as shown or by a grease gun through a fitting on the bearing cover. Spent or excess grease is flushed through to the interior of the bearing housing. Care must be taken not to over lubr lubric icat ate e the the bear bearin ing. g. Grea Grease sedd-fo forrlife bearings are also used but may be more limited in their life.
Angular Contact Bearing
Double Angular Contact Bearing
Angular contact bearings are often used when higher thrust capability is required. Note in left figure that the ball is in contact with the outer raceway at an angle. angle. Double Double angula angularr conta contact ct bearin bearings gs show shown n in in right right figure are capable of carrying thrust in either direction.
Bearings: Journal Bearings
Journal bearings are used when higher speeds and high radial forces need to be carried. As the shaft rotates, a wedge of oil is carried into the space between the shaft and bearing which keeps the shaft from touching the bearing. A properly designed and lubricated journal bearing can operate indefinitely.
Journal bearings are usually accompanied by a tilting pad thrust bearing. The tilting pads in the bearing allow for a wedge of oil between the thrust collar and pads. Like the journal bearing, this design can also operate indefinitely.
When tilting pad thrust bearings are used, the pump design is usually limited to thrust capability in one direction only. Thrust capability in two directions requires bearings with opposing sets of tilting pads. With journal bearing designs, external lubrication systems are usually required.
Journal bearings are also used in canned motor and magnet drive pump designs as well as vertical turbine pumps. These bearings are lubricated by the product being pumped and may have grooves to aid lubrication. They may include one or more radial journal bearings as well as axial thrust bearings, both forward and reverse.
Shaft sleeves are used to provide harder and smoother surfaces under packing or journal bearings and to protect the shaft from wear. Sleeves are frequently made of hard metal or coated with hard metals or ceramics and ground to an extra smooth surface. They are usually considered replaceable parts.
Seals and Packing
In order to seal rotating shafts against leakage of the pumped liquid, liquid, soft soft pack packing ing is is often often used. used. Such Such packin packing g is usuall usually y made made of braided fibers impregnated with graphite or other lubricating materi material. al. The pack packing ing is reta retaine ined d by a gland gland whic which h can can be tightened to squeeze the packing close to the shaft. During operation, some leakage of liquid is necessary to keep packing from overheating.
Mechanical seals provide an alternate to packi packing ng.. Su Such ch sea seals ls inc inclu lude de a flat flat sta statio tiona nary ry fac face e and a flat rotating face held in close contact by sprin springs gs or or bell bellow ows. s. The The face face mat mater eria ials ls are are des desig igne ned d to resist wear and withstand many hours of oper operati ation on.. The The rota rotatin ting g face face is is often often carbo carbon n and and the the stationary face is hard metal or ceramic.
Casings
Pump casings collect the liquid from the pump impeller, convert the velocity energy to pressure energy, and guide the liquid to the pump discharge nozzle. This casing shows a dual (double) volute which helps balance the hydraulic forces.
Pipe flanges are used to connect to the pump system, although threaded pipe connections may be used on smaller pumps.
The radial split casing is often used and includes a radial mechanical joint which is sealed by a gasket or o-r o-rin ing g to fac facili ilita tate te ins instal tallat lation ion of the the imp impel eller ler.. A simple volute or spiral shape is used to collect the liquid from the impeller.
The casing can also be axially split to facilitate the impeller assembly. This design allows for the double suction impeller and between-bearings arrangement.
Casings: Diffuser Design
Multistage pumps require a diffuser design which collects liquid from the impeller and guides it back to the the next next stag stage e impe impelle ller. r. Su Such ch des desig igns ns may may hav have e two two diffusers or dual volutes, or multiple diffusers, as determined by the designer.
The diffusers or ring sections can be bolted together by tie rods and rely on gaskets or o-rings to seal against leakage between each diffuser.
Multistage diffusers may also be contained in a cylindr cylindrical ical barre barrell that contain contains s interstag interstage e liquid liquid and can withstand higher pressure than ring section designs. This configuration is common on electric utility boiler feed applications.
In a similar fashion, the diffusers can be contained in an axial split casing. This design often includes the diffusers in the casing casting.
Vertical turbine deep well pumps are similar in concept to ring section pumps, but usually use bolts between each ea ch st stag age. e. The The sta stage ge se sect ctio ions ns are are called bowls, bowls, and the assembly is called the bowl assembly. assembly.
The discharge flow from the bowl assembly is directed up the column pipe to the pump discharge head. The liquid being pumped often lubricates the shaft that drives the impellers. Bearings are often made of soft rubber material.
The deep well pump discharge head supports the t he pump driver. It also allows for sealing around the drive shaft, and directs the liquid flow to the discharge pipe connection.
Couplings
The configurations which have been described so far are freestanding pumps that are driven by separate motors, turbines or engines. The pump is connected to the driver by a flexible shaft coupli coupling. ng. Such Such couplin couplings gs may use use rubber rubber cushi cushions ons,, flexibl flexible e plates plates,, springs, or other devices such as gear teeth to provide a limited flexibility between the pump and driver shaft
To simplify design and avoid couplings and pump bearings, the impeller may be mounted directly on the driver shaft. This arrangement is called a close-coupled design. Motors for such pumps are usually made to a standard shaft and mounting face design.
A unique close-coupled design uses a motor which has the motor rotor and bearings immersed in the liquid being pumped. This configuration is called a canned motor pump which refers to the cyli cylindri ndrical cal can can that that separa separates tes the the motor motor rotor rotor and and stato stator. r. This This design avoids the need for shaft packing or mechanical seals.
Another Another sealless sealless design is is one which which uses permanen permanentt magnets magnets that transmit torque through a cylindrical containment shell or rear casing. Magnets are mounted on a rotating shaft and are aligned with with other other magn magnets ets insi inside de the the can. can. The roto rotorr will will turn turn in perf perfect ect unison with the rotating magnet assembly. Magnetic drive pumps are available in both close-coupled and separately-coupled designs.
Drivers
Drivers for centrifugal pumps are usually electrical induction motors produced with drip-proof or totally enclosed bodies. In some cases, steam turbines or internal combustion engines may be used.
Variable speed drive controllers are growing in popularity and provide increased system efficiency when used with appropriate motors.
Accurate shaft alignment between the pump and driver is critical and is maintained by mounting both on a structural baseplate baseplate usually usually made made of of steel steel or cast cast iron. It is also normal normal to bolt bolt the the baseplate baseplate to a foundati foundation on and further ensure rigidity by adding concrete grout material betwee between n the basep baseplat late e and the the foundat foundation ion..
Drivers: Design Configurations
With so many variations in centrifugal/vertical pump construction, pump designs have evolved into about twenty common design configurations. These have been categorized in a family tree shown here.
Drivers: Design Configurations The most common configuration is the overhung impeller design, which has two major branches — close-coupled and separately-coupled The close-coupled configuration is further divided into •end suction and •in-line. The separately-coupled configuration is divided into six configurations: •in-line, •frame-mounted, •centerline support, •ANSI B-73.1, •wet pit volute and •axial flow.
The second common configuration is impellerbetween-bearings, which is available as single stage and multistage. Single stage casings may be •axially split or radial ally ly spli split. t. •radi Multistage casings also can be •axially split or •radially split
Third is the vertical turbine type, which is also available as single and multistage. They are further categorized as •deep well turbine type, •barrel or can pump and •short set pumps.
Two special designs include the regenerative turbine and special effects designs.
Summary In this module you learned that the major components of centrifugal pumps include the following: •Impellers •Shafts •Bearings •Seals •Casings •Couplings •Drivers There are different types and applications for each.
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