Pumps Principle , Operation and Maintenance
2.0
ROTODYNAMIC PUMPS: Rotary pumps operate on the principle that a rotating vane, screw, or gear traps the liquid in the suction side of the pump casing and forces it to the discharge side of the casing. These pumps are essentially selfpriming due to their capability of removing air from suction lines and producing a high suction lift. In pumps designed for systems requiring high suction lift and self-priming features, it is essential that all clearances between rotating parts, and between rotating and stationary parts, be kept to a minimum in order to reduce slippage. Slippage is leakage of fluid from the discharge of the pump back to its suction. Due to the close clearances in rotary pumps, it is necessary to operate these pumps at relatively low speed in order to secure reliable operation and maintain pump capacity over an extended period of time. Otherwise, the erosive action due to the high velocities of the liquid passing through the narrow clearance spaces would soon cause excessive wear and increased clearances, resulting in slippage. There are many types of positive displacement rotary pumps, and they are normally grouped into three basic categories that include gear pumps, screw pumps, and moving vane pumps.
Centrifugal: The description centrifugal pump is applied generally to all types of pumps with an impeller having fixed blades housed in a suitable shaped casing so that when the impeller rotates momentum is applied to liquid in the pump casing transporting it from the inlet to the outlet side. To achieve this it follows that the pump casing has to be full of fluid (i.e. a basic pump of this type is not self-priming). Centrifugal pumps basically consist of a stationary pump casing and an impeller mounted on a rotating shaft. The pump casing provides a pressure boundary for the pump and contains channels to properly direct the suction and discharge flow. The pump casing has suction and discharge penetrations for the main flow path of the pump and normally has small drain and vent fittings to remove gases trapped in the pump casing or to drain the pump casing for maintenance. (Figure 2.1) is a simplified diagram of a typical centrifugal pump that shows the relative locations of the pump suction, impeller, volute, and discharge. The pump casing guides the liquid from the suction connection to the center, or eye, of the impeller. The vanes of the rotating impeller impart a radial and rotary motion to the liquid, forcing it to the outer periphery of the pump casing where it is collected in the outer part of the pump casing called the volute. The volute is a region that expands in cross-sectional area as it wraps around the pump casing. The purpose of the volute is to collect the liquid discharged from the periphery of the impeller at high velocity and gradually cause a
Prepared by: NAK
10
Pumps Principle , Operation and Maintenance
reduction in fluid velocity by increasing the flow area. This converts the velocity head to static pressure. The fluid is then discharged from the pump through the discharge connection.
Figure 2.1: Centrifugal pump.
Centrifugal pumps can also be constructed in a manner that results in two distinct volutes, each receiving the liquid that is discharged from a 180o region of the impeller at any given time. Pumps of this type are called double volute pumps (they may also be referred to a split volute pumps). In some applications the double volute minimizes radial forces imparted to the shaft and bearings due to imbalances in the pressure around the impeller. A comparison of single and double volute centrifugal pumps is shown on (Figure 2.2)
Figure 2.2: Single and double volutes.
Prepared by: NAK
11
Pumps Principle , Operation and Maintenance
Diffuser: Some centrifugal pumps contain diffusers. A diffuser is a set of stationary vanes that surround the impeller (Figure 2.3). The purpose of the diffuser is to increase the efficiency of the centrifugal pump by allowing a more gradual expansion and less turbulent area for the liquid to reduce in velocity. The diffuser vanes are designed in a manner that the liquid exiting the impeller will encounter an ever-increasing flow area as it passes through the diffuser. This increase in flow area causes a reduction in flow velocity, converting kinetic energy into flow pressure.
Figure 2.3: Centrifugal pump diffuser.
Impeller classification: Impellers of pumps are classified based on the number of points that the liquid can enter the impeller and also on the amount of webbing between the impeller blades. Impellers can be either single suction or double-suction. A single-suction impeller allows liquid to enter the center of the blades from only one direction. A double-suction impeller allows liquid to enter the center of the impeller blades from both sides simultaneously. (Figure 2.4) shows simplified diagrams of single and double-suction impellers.
Prepared by: NAK
12
Pumps Principle , Operation and Maintenance
Figure 2.4: Single-suction and double-suction impellers.
Impellers can be open, semi-open, or enclosed. The open impeller consists only of blades attached to a hub. The semi-open impeller is constructed with a circular plate (the web) attached to one side of the blades. The enclosed impeller has circular plates attached to both sides of the blades. Enclosed impellers are also referred to as shrouded impellers. (Figure 2.5) illustrates examples of open, semi-open, and enclosed impellers.
Figure 2.5: Open, semi-open and enclosed impellers.
The impeller sometimes contains balancing holes that connect the space around the hub to the suction side of the impeller. The balancing holes have a total cross-sectional area that is considerably greater than the cross-sectional area of the annular space between the wearing ring and the hub. The result is suction pressure on both sides of the impeller hub, which maintains a hydraulic balance of axial thrust.
Prepared by: NAK
13
Pumps Principle , Operation and Maintenance
Centrifugal pump classification by flow: Centrifugal pumps can be classified based on the manner in which fluid flows through the pump. The manner in which fluid flows through the pump is determined by the design of the pump casing and the impeller. The three types of flow through a centrifugal pump are radial flow, axial flow, and mixed flow. Radial centrifugal flow pumps: In a radial flow pump, the liquid enters at the center of the impeller and is directed out along the impeller blades in a direction at right angles to the pump shaft. The impeller of a typical radial flow pump and the flow through a radial flow pump are shown in (Figure 2.6)
Figure 2.6: Radial flow centrifugal pump.
Axial centrifugal flow pumps: In an axial flow pump, the impeller pushes the liquid in a direction parallel to the pump shaft. Axial flow pumps are sometimes called propeller pumps because they operate essentially the same as the propeller of a boat. The impeller of a typical axial flow pump and the flow through a radial flow pump are shown in (Figure 2.7)
Figure 2.7: Axial flow centrifugal pump.
Prepared by: NAK
14
Pumps Principle , Operation and Maintenance
Mixed centrifugal flow pumps: Mixed flow pumps borrow characteristics from both radial flow and axial flow pumps. As liquid flows through the impeller of a mixed flow pump, the impeller blades push the liquid out away from the pump shaft and to the pump suction at an angle greater than 90o. The impeller of a typical mixed flow pump and the flow through a mixed flow pump are shown in (Figure 2.8)
Figure 2.8: Mixed flow centrifugal pump.
Multi-Stage Centrifugal Pumps: A centrifugal pump with a single impeller that can develop a differential pressure of more than 150 psi between the suction and the discharge is difficult and costly to design and construct. A more economical approach to developing high pressures with a single centrifugal pump is to include multiple impellers on a common shaft within the same pump casing. Internal channels in the pump casing route the discharge of one impeller to the suction of another impeller. (Figure 2.9) shows a diagram of the arrangement of the impellers of a four-stage pump. The water enters the pump from the top left and passes through each of the four impellers in series, going from left to right. The water goes from the volute surrounding the discharge of one impeller to the suction of the next impeller. A pump stage is defined as that portion of a centrifugal pump consisting of one impeller and its associated components. Most centrifugal pumps are single-stage pumps, containing only one impeller. A pump containing seven impellers within a single casing would be referred to as a seven-stage pump or, or generally, as a multi-stage pump.
Prepared by: NAK
15
Pumps Principle , Operation and Maintenance
Figure 2.9: Multi-stage centrifugal pump.
Centrifugal pump components: Centrifugal pumps vary in design and construction from simple pumps with relatively few parts to extremely complicated pumps with hundreds of individual parts. Some of the most common components found in centrifugal pumps are wearing rings, stuffing boxes, packing, and lantern rings. These components are shown in (Figure 2.10) and described on the following pages. Wearing rings: Centrifugal pumps contain rotating impellers within stationary pump casings. To allow the impeller to rotate freely within the pump casing, a small clearance is designed to be maintained between the impeller and the pump casing. To maximize the efficiency of a centrifugal pump, it is necessary to minimize the amount of liquid leaking through this clearance from the high pressure or discharge side of the pump back to the low pressure or suction side.
Prepared by: NAK
16
Pumps Principle , Operation and Maintenance
Figure 2.10: Centrifugal pump components.
Some wear or erosion will occur at the point where the impeller and the pump casing nearly come into contact. This wear is due to the erosion caused by liquid leaking through this tight clearance and other causes. As wear occurs, the clearances become larger and the rate of leakage increases. Eventually, the leakage could become unacceptably large and maintenance would be required on the pump. To minimize the cost of pump maintenance, many centrifugal pumps are designed with wearing rings. Wearing rings are replaceable rings that are attached to the impeller and/or the pump casing to allow a small running clearance between the impeller and the pump casing without causing wear of the actual impeller or pump casing material. These wearing rings are designed to be replaced periodically during the life of a pump and prevent the more costly replacement of the impeller or the casing. Stuffing Box In almost all centrifugal pumps, the rotating shaft that drives the impeller penetrates the pressure boundary of the pump casing. It is important that the pump is designed properly to control the amount of liquid that leaks along the shaft at the point that the shaft penetrates the pump casing. There are many different methods of sealing the Prepared by: NAK
17
Pumps Principle , Operation and Maintenance
shaft penetration of the pump casing. Factors considered when choosing a method include the pressure and temperature of the fluid being pumped, the size of the pump, and the chemical and physical characteristics of the fluid being pumped. One of the simplest types of shaft seal is the stuffing box. The stuffing box is a cylindrical space in the pump casing surrounding the shaft. Rings of packing material are placed in this space. Packing is material in the form of rings or strands that is placed in the stuffing box to form a seal to control the rate of leakage along the shaft. The packing rings are held in place by a gland. The gland is, in turn, held in place by studs with adjusting nuts. As the adjusting nuts are tightened, they move the gland in and compress the packing. This axial compression causes the packing to expand radially, forming a tight seal between the rotating shaft and the inside wall of the stuffing box. The high speed rotation of the shaft generates a significant amount of heat as it rubs against the packing rings. If no lubrication and cooling are provided to the packing, the temperature of the packing increases to the point where damage occurs to the packing, the pump shaft, and possibly nearby pump bearings. Stuffing boxes are normally designed to allow a small amount of controlled leakage along the shaft to provide lubrication and cooling to the packing. The leakage rate can be adjusted by tightening and loosening the packing gland. Mechanical Seals: In some situations, packing material is not adequate for sealing the shaft. One common alternative method for sealing the shaft is with mechanical seals. Mechanical seals consist of two basic parts, a rotating element attached to the pump shaft and a stationary element attached to the pump casing. Each of these elements has a highly polished sealing surface. The polished faces of the rotating and stationary elements come into contact with each other to form a seal that prevents leakage along the shaft.
Prepared by: NAK
18
Pumps Principle , Operation and Maintenance
Summery: Centrifugal pumps summary: The impeller contains rotating vanes that impart a radial and rotary motion to the liquid. The volute collects the liquid discharged from the impeller at high velocity and gradually causes a reduction in fluid velocity by increasing the flow area, converting the velocity head to a static head. A diffuser increases the efficiency of a centrifugal pump by allowing a more gradual expansion and less turbulent area for the liquid to slow as the flow area expands. Packing material provides a seal in the area where the pump shaft penetrates the pump casing. Wearing rings are replaceable rings that are attached to the impeller and/or the pump casing to allow a small running clearance between the impeller and pump casing without causing wear of the actual impeller or pump casing material. The lantern ring is inserted between rings of packing in the stuffing box to receive relatively cool, clean liquid and distribute the liquid uniformly around the shaft to provide lubrication and cooling to the packing.
Prepared by: NAK
19
Pumps Principle , Operation and Maintenance
Axial and Mixed flow: Axial and Mixed Flow Pumps are the normal choice for high-volume, low-pressure pumping duties and particularly for large-scale primary water supplies, flood control, irrigation and drainage. Such types are available in a wide range of sizes and capacities. Typical bowl sizes range from 0.3-2.1 m (12-48 in). capacities range from about 9000 lit/min (2000 gal/min) to over 9000000 lit/min (200000 gal/min). Heads generated may range from about 1.5 m (5 ft) to 22 m (75 ft). It is a general characteristic of such pumps that the power input curve is much flatter than that of a centrifugal pump, thus input power demand does not vary very much with the working point. Hydraulic performance, however, differs appreciably. Axial flow pumps generate head pressure through an axial motion developed by a combination of propeller and internal vane design. The combination induces fluids to travel strictly along the axial of the pump drive shaft. The mixed flow pump employs a chamber /impeller /vane design to impart both an axial and radial motion to fluids traveling downstream. This powerful motion is created by lifting the fluids with an impeller, while simultaneously forcing fluids out against the bowl like impeller chamber. The outward movement known as an upper diffuser section. By combining axial and centrifugal motion, the mixed flow pump generates greater head pressure to downstream fluids than is possible with a single motion axial pump.
Q
Q
Q
Q
Hd
η,HP,
Hd
η,HP,
Hd
η,HP,
Hd
η,HP,
Hd
η,HP,
η,HP,
Hd
These differences are noted in the generated comparative H-Q curve shown in Figure (2.12).
Q
Q
Figure 2.12: Typical impeller shapes and effect on performance for various speeds.
Prepared by: NAK
20
Pumps Principle , Operation and Maintenance
In the case of a mixed flow pump, the H-Q curve tends to be steep, with the point of maximum efficiency displaced towards maximum capacity. The H-Q curve of an axial flow pump has substantial falling characteristics although the actual head obtainable is much lower. Efficiency is higher over a greater (percentage) range of head than for a centrifugal pump. General form: Axial and mixed flow pumps are commonly mounted vertically, directly over a sump, (Figure 2.13). This is most often the case when the source of water is a river or reservoir. Fluids enter through a flared suction bell that has been reinforced by heavy vanes. The vanes, in addition to lending support to the suction bell, act to direct the liquid flow parallel to the drive shaft as it travels upstream. Once past the suction bell, fluids encounter either an axial propeller or a mixed flow impeller. While differing in design, both are always dynamically balanced, onepiece castings, mounted on a stainless steel pump shaft. Driver: The drive shaft may be encased in a tube or open, with supports properly spaced for intermediate shaft bearings. Positive drive is provided by a heavy-duty drive key and thrust collar. A variety of drivers may be used, however, electric motors and right angle gears are most common. For the purposes of this manual, these types of drivers can be grouped into two categories: 1. Hollow shaft drivers - the pump shaft extends through a tube in the center of the rotor and is connected to the driver by a clutch assembly at the top of the driver. 2. Solid shaft drivers - the rotor shaft is solid and projects below the driver-mounting base. This type driver requires an adjustable coupling between the pump and driver. Column assembly: The column assembly is of two basic types, either of which may be used on close coupled units: 1. Open line shaft construction utilizes the liquid being pumped to lubricate the line shaft bearings. 2. Enclosed line shaft construction has an enclosing pipe around the line shaft and utilizes oil, grease or injected liquid to lubricate the line shaft bearings.
Prepared by: NAK
21
Pumps Principle , Operation and Maintenance
The column assembly will consist of column pipe, which connects the bowl assembly to the discharge head and carries the pumped liquid to the discharge head, the shaft, which connects the bowl assembly to the discharge head, the head shaft, which connects the line shaft to the driver. Column pipe may be either threaded or flanged and may contain bearings if required for the particular unit. Note: Some units will not require a column assembly, having the bowl assembly connected directly to the discharge head. Elbows commonly terminate in either an ANSI standard flange or a plain end to accommodate a dresser type coupling. Depending upon the installation, engineering considerations will dictate whether the discharge elbow is positioned above or below the main flow. Discharge head assembly: The discharge head supports the driver and bowl assembly as well as supplying a discharge connection (the underground discharge connection will be located on one of the column pipe sections below the motor stand). A shaft sealing arrangement is located in the discharge head to seal the shaft where it leaves the liquid chamber. The shaft seal will usually be either a packing box or mechanical seal assembly. Bowl assemblies: The bowl assembly consists of impellers rigidly mounted on the bowl shaft, which rotate and impart energy to the fluid. The bowls (or diffusers) contain the fluid at increased pressure and direct it vertically to the next stage and eventually to the column pipe. The suction bell or case directs the fluid into the first stage impeller. Bearings are located in the suction bell, discharge case, and between each impeller. The rotating element is mounted in an individual housing, which is usually replaceable and situated just above the section bell, close to the pump inlet. The housing may be bronze, stainless steel or any other material similar to the impeller. After emerging from the housing, fluids travel past an upper diffuser chamber and out through the guide case. The guide case, consisting of a column pipe and discharge elbow is of welded steel plate or cast iron. Sections are flanged or bolted together. A registered fit is used to maintain proper alignment on all mated parts. Pullout options: Pullout options, which make for easier maintenance and inspection, are a common feature of these pumps, the best designs permit removal of the entire bowl assembly (including all rotating parts, diffuser, impeller housing and suction bell) through the outer shell, without disturbing either the discharge or floor plate connections.
Prepared by: NAK
22
Pumps Principle , Operation and Maintenance
Driver
Head shaft
Discharge elbow Seal
Column assembly Pump shaft
Impeller
Bowls
Suction bell
Figure 2.13 Mixed flow pump with open impeller.
Prepared by: NAK
23
Pumps Principle , Operation and Maintenance
Figure 2.14: Vertical turbine pumps
Channel impeller pumps: A channel impeller pump employs an impeller with turbine-type blades mounted on the periphery running in an annular channel (or channels) surrounding the periphery of the wheel. In practice, only two main subtypes emerge, known generally as the peripheral pump and side channel pump respectively. The main difference is in the form and positioning of the channels. The peripheral pump has a double-sided channel in which the liquid circulates, this channel being located partly in the cylindrical part of the casing and partly in the side plates. The side-channel pump has two channels cut in Prepared by: NAK
24
Pumps Principle , Operation and Maintenance
the side plates only, adjacent to each side of the impeller blades. Characteristics are quite different, eg the side-channel pump is selfpriming and can develop a much higher head, although its efficiency is lower. Hence, the two are distinct types (Figure 2.15). The peripheral pump is also known as a regenerative pump. Side-channel HEAD,BHP & Efficiency
HEAD,BHP & Efficiency
Peripheral Efficiency
H-Q
BHP
H-Q
BHP Efficiency
100%
100% Capacity
Capacity
Figure 2.15: Characteristic curves for peripheral and side-channel pumps.
Regenerative pumps have a substantially straight and sleep H-Q curve (Figure 2.16). Pressure developed tends to rise sharply with decreasing capacity making this type unsuitable for discharge regulation by throttling. Power output falls with increasing capacity.
HEAD
Centrifugal
Axial flow Regenerati
Axial flow
Capacity Figure 2.16: H-Q curves for some rotodynamic pumps.
Prepared by: NAK
25
Pumps Principle , Operation and Maintenance
Turbine pumps (Regenerative): Turbine pumps obtain their name from the many vanes machined into the periphery of the rotating impeller. Heads over 900 feet are readily developed in a two-stage pump. The impeller, which has very tight axial clearance and uses pump channel rings, displays minimal recirculation losses. The channel rings provide a circular channel around the blades of the impeller from the inlet to the outlet. Liquid entering the channel from the inlet is picked up immediately by the vanes on both sides of the impeller and pumped through the channel by the shearing action. The process is repeated over and over with each pass imparting more energy until the liquid is discharged (Figure 2.17).
Figure 2.17: Turbine pump
Special pumps: Viscous Drag or Disk Pump: The viscous drag pump operation utilizes two principles of fluid mechanics: boundary layer and viscous drag. These phenomena occur simultaneously whenever a surface is moved through a liquid. Boundary layer phenomenon occurs in the disk pump when liquid molecules lock onto the surface roughness of the disk rotor. A dynamic force field is developed. This force field produces a strong radially accelerating friction force gradient within and between the
Prepared by: NAK
26
Pumps Principle , Operation and Maintenance
molecules of the fluid and the disks, thereby creating a boundary layer effect. The resulting frictional resistance force field between the interacting elements and the natural inclination of a fluid to resist separation of its own continuum creates the adhesion phenomenon known as viscous drag. These effects acting together are the motivators in transferring the necessary tangential and centrifugal forces to propel the liquid with increasing momentum towards the discharge outlet located at the periphery of the disks (Figure 2.18).
Figure 2.18: Viscous drag pump.
• Advantages of using a viscous drag pump: 1. Minimal wear with abrasive materials. 2. Gentle handling of delicate liquids. 3. Ability to easily handle highly viscous liquids. 4. Freedom from vapor lock. Screw centrifugal pump: This pump incorporates a large-diameter screw instead of the more common radial impeller that is found in centrifugal pumps (Figure 2.19). Thick sludge and large particle solids can be moved because of the low Net Positive Suction Head (NPSH) requirements, which result from the utilization of the inducer-like impeller. Because the pumped material enters at a low entrance angle, a low shear, low turbulence condition exists, which results in very gentle handling of the liquid. The gentle handling makes it possible to
Prepared by: NAK
27
Pumps Principle , Operation and Maintenance
pump slurries of fruits and vegetables without undue breakup of constituents.
FIGURE 2.19: Screw centrifugal pump.
The pump can also be operated in the reverse direction. This characteristic is advantageous for clearing clogged suction lines. Rotating case pump: • The basic concept of this pump is unique (Figure 2.20), liquid enters the intake manifold and passes into a rotating case where centrifugal force accelerates it. A stationary pickup tube situated on the inner edge of the case, where pressure and velocity are the greatest, converts the centrifugal energy into a steady pulsation-free high pressure stream. • The following characteristics attest to the simplicity of the pump: - Only one rotating part (the casing) - The seal is exposed only to suction pressure - No seal is required at the high pressure discharge • The pump, turning at speeds from 1,325 to 4,500 rpm will generate heads approximately four times that of a single stage centrifugal pump operating at a similar speed. Single stage heads up to 3,000 feet are readily attainable even in sizes up to 200 gpm.
Prepared by: NAK
28
Pumps Principle , Operation and Maintenance
Figure 2.20: Rotating case pump.
Vortex pump: A vortex pump comprises a standard concentric casing with an axial suction intake and a tangential discharge nozzle (Figure 2.21). The straight radial-bladed impeller is axially recessed in the casing. The recess can range from 50% to 100% where the impeller is completely out of the flow stream. The rotating impeller creates a vortex field in the casing that motivates the liquid from the centrally located suction to the tangentially located discharge. Because the pumped liquid does not have to flow through any vane passages, solid content size is limited only by the suction and discharge diameters. A vortex pump can handle much larger percentages of air and entrained gases than a standard centrifugal pump because pumping action is by induced vortex rather than by impeller vanes. Advantage of a vortex pump: 1. Can handle high solid-content liquids, entrained gas liquids, and stringy sewage while requiring a relatively low NPSH. Disadvantage of a vortex pump: 1. Comparatively low efficiency of 35% to 55%.
Prepared by: NAK
29
Pumps Principle , Operation and Maintenance
FIGURE 2.21: Vortex pump.
Prepared by: NAK
30