Pumps Types and Classifications

Egyptian Electricity Holding Company

Cairo Electricity Production Company

Code : G101 - 1 - Mechanical

Pumps Types And Classifications




Chapter 1

Content

- Purpose . ...................................................... 1

- Centrifugal Pump Classification .................. 3

- Mechanical Features Of Centrifugal Pump .. 10

- Positive Displacement Pumps ..................... 23

- Gear And Van Pump .................................... 23

- Cam And Piston , And Screw Pump ............. 26

- Operating Characteristics Of Rotary Pump .. 28

- Mechanical Features Of Rotary Pump .......... 29




I . Purpose

Just as the human heart is an important part of the circulatory system; the pump can be considered the heart of a power plant. there are many types of pumps used in a power plant . we will consider the major types . these can be broken into two categories :

a) Centrifugal pumps . b) Positive displacement pumps .

Basically, a pump is a mechanical device used to force liquids, or gases, from one location to another. gases and liquids are called fluids - a fluid being any substance that flows .

Each of the above categories of pumps has its own characteristics , but each possesses features that have a common terminology with one another .

II. Centrifugal Pump Theory

A. Simple Centrifugal Pumps

In a centrifugal pump, the liquid is forced by either atmospheric , or another source of pressure, into a set of rotating vanes that discharge the liquid at a higher pressure and a higher velocity .

Whenever a centrifugal pump starts up, it must have water in it. if there is no water in it , no centrifugal force can be generated, and nothing will be pumped .

Single volute , twin volute, and diffuser type pumps are the simplest kinds of pumps . there are many variations in design, but the principles involved are the same. there must be an enclosed




space in which an impeller rotates ; this impeller imparts a high velocity, followed by some type of divergent stationary area, which converts the velocity to pressure .

1. Single Volute Pump

most of the velocity energy is converted into pressure energy by means of a volute, or by a set of stationary diffuser vanes surrounding the peripheral edge of the impeller . pumps with volute casings are called volute pumps; those with diffusion vanes are called diffuser pumps .

Figure II-1 shows a volute casing pump and the flow path of the liquid through the pump .

Figure II-1 Volute Casing Pump




Let us consider Figure II-1 :

1) Liquid (water) enters the suction at the center of the impeller .

2) The impeller is spinning rapidly, so the water starts spinning, and centrifugal force moves it out .

3) As the water moves out, it spins in a larger circle . as it is enclosed by the impeller, it must spin at the same rpm, but, because the circle is larger at its periphery, the water is moving faster. as a result, the water has more kinetic energy. the kinetic energy generated comes from the action of the impeller, which pushes the water around and therefore provides the force for it to move faster .

4) When the water enters the volute casing, it is moving very fast .

5) The shape of the casing changes the kinetic energy of the water to pressure head .

6) When the water leaves the pump through the discharge its speed is slower, but it has enough pressure to push it through the remainder of the system .

As the impeller throws out liquid by centrifugal force, more liquid rushes into the center or low pressure area. this is the area to which a suction pipe is connected. this process continues and a steady discharge, characteristic of centrifugal pumps , is formed .

2. Twin Volute Pump

Figure II-2 illustrates another variation of the volute-type pump, the twin volute pump .




Figure II-2 Twin Volute Pump

to balance radial thrust, the single volute can be replaced by two half-capacity volutes , each taking their discharge from 180 degrees of the impeller. the twin volute pumps are used particularly for large capacity at a high head .

3. Diffuser Type Pump figure II-3 shows how the diffuser type pump is built internally .




Figure II-3 Diffuser Type Pump

In this pump, the diffusion vanes change the direction of flow and convert the velocity energy imparted to the liquid by the impeller into pressure energy. these pumps can be found in many high pressure units .

B. Other Centrifugal Pumps

1. Mixed Flow-type Mixed flow centrifugal pumps (figure II-4) are ideal for low head, large capacity applications. the design of the pump is such that the head is developed partly by the lift of the vanes on the liquid . This type of pump has a single inlet impeller with the flow entering axially and discharging in an axial and radial direction. pumps of this type usually have a specific speed from 4,200 to 9,000 rpm.




Figure II-4 Mixed Flow Pump

2. Axial Flow-Type The axial flow type pump (figure II-5), often called the propeller pump, has flow parallel to the axis of rotation or strictly axial. it develops most of its head by the propelling, or lifting action of the vanes on the liquid . it has a single inlet impeller with the flow entering axially and discharging nearly axially. pumps of this type have a specific speed above 9,000 rpm .

Figure II-5 Axial Flow Pump




3. Turbine Or Regenerative Type In the turbine or regenerative type pump (figure 11-6), the impeller acts on the liquid on its periphery for almost a full revolution, with the liquid being circulated in and out of the impeller vanes so the true path of the liquid is a circular spiral .

Figure II-6 Turbine Or Regenerative Type Pump

This type of pump fills a need between other centrifugal and usual rotary designs for low to medium capacity , high head for clear liquids .

This pump can be used on a liquid-vapor combination. care must be given in selecting this type of pump. the running clearances are important, and corrosive or erosive grit will affect them. the life of this type of pump is not as long as that of other centrifugal pumps .




C. Classification Of Centrifugal Pumps Figure II-7 Shows Classification Of Centrifugal Pumps .

Figure II-7 Classification Of Centrifugal Pumps

Figure II-8 shows profiles of several pump impeller designs ranging from the low-specific-speed radial flow on the left to a high-specific speed axial impeller flow design on the right. these profiles are placed according to where each design fits on the specific-speed scale.




values of specific speeds

profiles of several pump impeller designs ranging from the low-

specific-speed radial flow on the left to a high-specific-speed axial

impeller flow design on the right, placed according to where each

design fits on the specific-speed scale .

Figure II-8 Comparison Of Pump Profiles

D. Multi-Stage Centrifugal Pump Theory Based on the basic pump theory presented earlier, if we are to have a large amount of pressure with a centrifugal pump we must impart very high velocities to the liquids being pumped .




This can be attained in two ways: First : build a pump whose impeller will be very large in

diameter. Second: provide the pump with a very high rotational speed.

The first method is unsuitable, because the pump would be too large, and the second is unsuitable because of cavitation .

To overcome this problem and still meet both of the design requirements, two pumps are used. one pump, designed to work with a small net positive suction head (NPSH), discharges into a system at 100 psig, or more, to feed the suction of a larger pump, which boosts the pressure up to around 1,000 psig. but this arrangement is not very convenient because it means that there must be two or more independent pumps in series in order to meet the higher pressure requirements. this involves more costs, space, spare parts, and maintenance to keep all of these superfluous units operational. to overcome this, pumps have been designed with a series of impellers keyed to one shaft. these impellers are called "stages" . the stages act like pumps operating in series to raise the pressure gradually. this design eliminates the need for greater speed and minimizes the diametral size of the impeller .

figure II-9 shows a cutaway section of a multi-stage high pressure pump.




Figure II-9 High-Pressure Multiple-Stage Pump

E. NPSH And Cavitation Two terms have been used 80 far in this lesson , and both affect the operation of a centrifugal pump enormously. these terms are net positive suction head and cavitation. let us discuss these at this point.

Net positive suction head (NPSH) is a term used to describe the prevention of water from changing into a gas at the impeller because of the presence of a liquid at or below its vapor pressure. the absolute pressure (including velocity head, v2/2g) of the fluid at the inlet must exceed the vapor pressure by an amount to overcome (1) any entrance or frictional losses between the point of entry into the pump and the impeller, and (2) the shock losses occurring at the impeller inlet .




Unless NPSH available at least equals NPSH required at any condition of operation, some of the fluid will vaporize in the pump inlet and bubbles of vapor will be carried into the impeller. these bubbles will collapse violently at some point downstream of the pump inlet (usually some point within the impeller) and produce very sharp , crackling noises, frequently accompanied by physical damage of adjacent metal surfaces. this phenomenon is known as cavitation and is generally highly undesirable .

Exercise

On the following curve what is the NPSH , efficiency, head capacity for the following flow rates: 4,000 gpm, 5,000 gpm, 7,000 gpm, and 7,750 gpm? compare your answers with the those on the next page.




At 4,000 gpm head 238 ft Efficiency = 78%

At 5,000 gpm head = 220 ft Efficiency = 85%

At 7,000 gpm head = 178 ft Efficiency = 83%

At 7,750 gpm head = 132 ft

Efficiency = 81%

F. Impeller/Casing Design And Function

1. Single-Suction Impeller In this type of impeller, the liquid pumped enters from one side. this type of impeller is easy to manufacture. because of the unbalanced axial thrust, a larger thrust bearing is needed. single suction impellers are almost always used for pumps handling liquids containing suspended matter, such as sewage.

2. Double-Suction Impeller

The double-suction impeller is in effect two single-suction impellers cast back-to-back and is in axial hydraulic balance; therefore, an oversized thrust bearing is unnecessary. the greater suction area permits the pump to operate with less net absolute suction head for a given capacity (see figure II-10).




3. Open , Semi-Open , And Closed Impeller An open impeller (figure II-11) has vanes only, while a semi-open impeller (figure II-12) has a shroud on one side. the closed impeller (figure II-13), used almost universally in centrifugal pumps handling clear liquids, has shrouds on each side that totally enclose the waterways from the suction eye. water slippage does not occur with this type, but a running joint is needed between the impeller and the casing to separate the pump discharge and suction chambers .

Figure II-11 Open Impeller




Figure II-12 Semi-Open Impeller

Figure II-13 Closed Impeller




4. Axial-Flow Impeller In an axial flow type of impeller, flow is parallel to the axis of rotation . The axis of rotation can be horizontal, vertical, or inclined.

5. Casings

Solid or split casings refer to the design. practically speaking, a solid casing cannot be used and is radially split. however, if a casing is made of two or more parts fastened together, it is split. if split, they are split axially, radially, or diagonally . Casings are also classified by the location of the suction and discharge nozzles .

6. Method Of Drive

A centrifugal pump may be classified as to method of drive : a) direct-connected (coupled or close-coupled) b) geared c) chain or belt driven




7. Type Of Service centrifugal pumps are termed boiler feed, general purpose, sump, deepwell, refinery, condensate, vacuum, process, sewage, trash, circulating, self priming, sanitary, bait, booster, paper stock, chemical, fire, jet, sand, slurry, ash, glass, stoneware, submersible tail water, etc.

8. Special Features Each type has specific features of design, construction, and materials and thus can be classified for particular service.

G. Mechanical Features Of Centrifugal Pumps To understand the operation of centrifugal pumps, a few mechanical features should be mentioned.

1. Wearing Rings Earlier in the discussion of types of impellers used, mention was made of the closed type. in this type there is a running joint formed by a portion of the impeller and a portion of the casing separating the chambers forming the suction and discharge waterways. leakage of liquid through this running clearance joint will cause wear on the adjacent surfaces. to prevent costly wear of casing and impeller, wearing rings are installed. usually they are removable and can be replaced. the following diagrams, figure II-14, show different types of rings used .




Figure II-14 Wearing Rings

The first shows this joint between casing and impeller. the other five show ring designs used. form varies with pressure, service, etc .

2. Shaft And Interstage Sleeves Sleeves protect the pump shafts against corrosion, erosion, and wear affecting its strength (see figure II-15). many forms are used on large pumps, but on some small pumps the sleeve is often left off to cut hydraulic losses. Because of these losses, small pumps have shafts made of corrosion-and wear-resisting stainless steel .

Figure II-15 Sleeves




3. Lantern Rings And Stuffing Boxes Lantern rings (Figure II-16) are used to prevent air leakage into the pump when running with a suction lift and to distribute the sealing liquid uniformly around shaft-sleeve surface. Stuffing boxes (Figure II-17) are also used to prevent leakage. They hold leakage out of the casing when pressure is above atmospheric and stop air leaking into casing when pressure is below atmospheric. Some stuffing boxes have lantern rings, others are solid-packed and do not .

Figure II-16 Lantern Ring




Figure II-17 Stuffing Box

III. POSITIVE DISPLACEMENT PUMPS

Introduction To Positive Displacement Most rotary pumps are positive displacement units - that is, a given quantity of liquid is discharged each revolution of the shaft. The pumps consist of a fixed casing containing gears, vanes, pistons, cams, segments , screws, etc., operating with minimum clearance. Instead of "throwing" the liquid as a centrifugal does, rotaries trap it, push it around the closed casing and discharge it in a steady flow without large pressure pulsations .




Rotary pumps are generally divided into two classes: 1) gear and vane types; 2) cam-and-piston and screw types.

A. Gear And Vane Pump Rotary gear pumps (Figure III-1) have two or more gears in a casing. During rotation, the liquid fills the spaces between the gear teeth on the suction side. It is then carried around and squeezed out as they mesh on the other side of the pump. Sliding-vane rotary pumps (Figure III-2) are built in many ways. In the one Shown, vanes move in and out of the off-center rotor .

When the rotor is turned counterclockwise, liquid flows into the cavity formed by the rotor bottom and casing wall. As the rotor turns, the next vane traps liquid in a cavity. The liquid is finally discharged at the other end of the pump. The vanes are held against the inner wall of the casing by centrifugal force.

Figure III-1 Gear And Vane Pump




Figure III-2 Sliding Vane Rotary Pump

The gear and vane class of rotary pumps also includes the lobe or shuttle-block designs .

The lobular pump (Figure III-3) resembles the gear type in action. The two or more rotors have two, three, four, or more lobes on each rotor, synchronized by external gears .

Figure III-3 Lobular Pump




The shuttle-block pump has a cylindrical rotor turning in a concentric casing . A shuttle-block (Figure III-4) and piston reciprocated by an eccentrically located idler pin is located in the rotor producing suction and discharge .

Figure III.4 Shuttle Block Pump

B. CAM—AND-Piston And Screw Type Pumps Cam-and-piston pumps (Figure III-5) consist of an eccentric with a slotted arm at its top. Shaft rotation causes eccentric to trap liquid in the basing, discharging it through the slot to outlet .

Screw pumps (Figure III-6) draw and trap the liquid in the pockets formed by the threads. It is moved along to the discharge point much like a nut on a thread. They may have one, two, or three screws .




Figure III-5 CAM And Piston Pumps

Three-Screw

Figure III-6 Screw Pumps

The important factor in rotary pump designs of today is the small clearance used. These pumps are often thought of as viscous-liquid pumps, but they are not confined to this service. They will handle any fluid free of hard solids.




C. Operating Characteristics Of Rotary Pumps The displacement of a rotary pump varies directly as speed, except as capacity may be affected by viscosity and other factors (see Figure III-7) . Slip or loss in capacity through clearances between the casing and rotating element assuming constant viscosity, increases as pressure increases. Power input increases with liquid viscosity, efficiency decreases.

FIGURE III-7




D. Mechanical Features Of Rotary Pumps Adjustable gland-packed stuffing boxes are most common on rotary pumps. They are supplied with lantern gland seal rings for packing lubrication or pressure breakdown when required. Spring-loaded stuffing boxes and mechanical seals are also used . Materials in the rotating elements must not have a higher coefficient of expansion than the casing material .

Plastic and rubber parts are seldom used in rotary pump construction .

Theoretically, a rotary pump, being a positive displacement pump, will continue to discharge against any pressure . To protect pump and system from a possible rupture, relief valves are used. Some pumps have a variable-capacity device which reduces the pump’s displacement as pressure increases .

The discharge valve should be open while starting .

Reducing the speed of the pump or opening bypass line is the best way to reduce pump delivery .

E. Service Of Rotary Pumps Rotary pumps are used to pump all liquids that do not contain solid matter which can abrade or clog and also liquids that do not corrode . Pressure limitations are usually maintained below 1,000 psi. They carry liquids varying in viscosity from gasoline and water to oils, paints, varnishes, asphalts, and greases.

Pumps Types and Classifications

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