Unit 2 Pump Classification

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UNITS IN THIS COURSE

UNIT 1 PUMP TERMINOLOGY.

UNIT 2 PUMP CLASSIFICATION.

UNIT 3 RECIPROCATING PUMPS.

UNIT 4 ROTARY PUMPS.

UNIT 5 CENTRIFUGAL PUMPS.

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TABLE OF CONTENTS

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2.0 COURSE OBJECTIVES 3

2.1 WHY PUMPS ARE USED 4

2.2 PUMP CLASSIFICATION 4

2.3 RECIPROCATING PUMPS 5

2.3.1 Piston Pumps 6

2.3.2 Plunger Pumps 7

2.3.3 Diaphragm Pumps 7

2.4 ROTARY PUMPS 8

2.4.1 Gear Pumps 8

2.4.2 Vane Pumps 9

2.4.3 Lobe Pumps 9

2.5 CENTRIFUGAL PUMPS 10

2.5.1 Volute Pumps 10

2.5.2 Diffuser Pumps 11

2.5.3 Turbine Pumps 12

2.5.4 Propeller Pumps 13

2.6 IMPELLERS 14

2.7 MULTI-STAGING 15

2.8 INSTALLATION 15

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2.0 COURSE OBJECTIVES

This course describes the different types of pumps commonly found in processplants. The course has five Units. These Units describe the basic terminology ofpumps and pumping, the types of reciprocating, rotary and centrifugal pumps,start up of centrifugal pumps, pump problems, and pump control systems. Whenhe has finished the course, the student will be able to:

Explain the basic terminology of pumps.

identify different kinds of pumps and explain their functions.

Identify the differences between the main types of reciprocating pumps and

explain the end uses of these pumps in process plants.

Explain the function of pulsation dampeners.

Identify the differences between the main types of rotary pumps and explain

the end uses of rotary pumps in process plants.

Explain how centrifugal pumps increase pressure.

Explain how pump shafts are sealed and how axial and radial thrust are

balanced in single stage centrifugal pumps.

Explain the reasons for, and the advantages of, multi-stage centrifugal

pumps.

Explain how axial thrust is balanced in a multi-stage centrifugal pump.

Explain how cavitation and vapour lock occur in a centrifugal pump and

explain how these problems are overcome.

Explain why centrifugal pumps are connected in parallel or in series.

Explain the pre-start checks and start-up procedure for centrifugal pumps

with electric motor and steam turbine prime movers.

Explain the procedure for centrifugal pump changeover.

Use pump readings to identify pump problems.

Identify and explain pump control systems.

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2.1 WHY PUMPS ARE USED

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Pumps are machines which move liquid from one place to another.

Pumps work by adding energy to the liquid they are pumping. The liquid beingpumped leaves the discharge of the pump with more energy in it than when itentered the pump. The added energy is in the form of pressure, so there is anincrease in pressure across the pump.

The pump must add enough pressure energy to the liquid to maintain the correctflow rate through the process.

Figure 2-11 A Simple Pumping System

Figure 2-1 shows a simple pumping system which draws liquid from a tank andpumps it to a process vessel operating at 100 psig. The discharge pressure of thepump must be sufficient to overcome:

The pressure in the process vessel.

The discharge head (the difference in level between the location of the pump

and the location of the process vessel).

The friction losses in the pipeline.

2.2 PUMP CLASSIFICATION

Pump classification describes:

How the pump is built.

How the pump moves the liquids through the process.

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There are two main classifications (groups) of pumps:

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Positive Displacement (PD) pumps, which are best for high pressures.

Centrifugal pumps, which are best for high volumes.

Positive displacement pumps work by trapping a set volume of liquid in a cylinder.Then the liquid is forced out of the pump discharge at higher pressure by themovement of a piston in the cylinder.

Centrifugal pumps work by using a centrifugal force to increase the velocity of theliquid. Then the design of the pump casing changes the velocity energy to pressureenergy. The liquid is discharged at a higher pressure.

Each of the groups above includes a number of different types of pumps which arebuilt for:

Handling special fluids.

Giving different discharge pressures.

Giving different discharge volumes.

Handling different liquid temperatures.

The two main types of positive displacement pumps are:

Reciprocating pumps which operate with a reciprocating or straight line motion.

Rotary pumps which operate with a rotating or circular motion.

2.3 RECIPROCATING PUMPS

There are three main types of reciprocating pumps:

piston pumps,

plunger pumps,

diaphragm pumps.

Some reciprocating pumps have only one cylinder. Other reciprocating pumps havemore than one cylinder working together in parallel. Special words are used todescribe these pumps:

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Simplex -one cylinder.

Duplex - two cylinders. A duplex pump is good for large volume.

Triplex three cylinders. A triplex pump is good for even larger volume.

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The movement of the piston, plunger or diaphragm from one end of the cylinder tothe other is called one pump stroke.

Pumps are designed to discharge liquids in two methods:

Single acting which means that the liquid is discharged at only one end of thecylinder. Therefore, it takes two strokes of the piston to discharge the liquid.

Double acting which means that the liquid is discharged at both ends of thecylinder. Therefore, liquid is discharged on each stroke of the piston.

Figure 2-2 Piston Pumps

2.3.1 Piston Pumps

In a piston pump the piston forces the liquid out of the cylinder. A single actingpump uses only one side of the piston to force out the liquid. A double acting pumpuses both sides of the piston to force out the liquid.

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2.3.2 Plunger Pumps

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Figure 2-3 Plunger Pump

In a plunger pump the plunger forces the liquid out of the cylinder. Plunger pumpsare only single acting pumps.

2.3.3 Diaphragm Pumps

Figure 2-4 Diaphragm Pump

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In a diaphragm pump the movement of the diaphragm forces the liquid out of thecylinder. Most diaphragm pumps are single acting, but they can be double acting.

Diaphragm pumps are good for pumping corrosive or abrasive liquids.

2.4 ROTARY PUMPS

Rotary pumps are classed as positive displacement pumps. The three mostcommon types are:

Gear pumps.

Vane pumps.

Lobe pumps.

2.4.1 Gear Pumps

In a gear pump the gears do the pumping. Gear pumps may have external orinternal gears. In an external gear pump the two gear wheels are meshed togetherside by side. In an internal gear pump one gear wheel is fitted inside the other.

Figure 2-5 Gear Pumps


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2.4.2 Vane Pumps


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Figure 2-6 Vane Pumps

Vane pumps may be sliding vane pumps or swinging vane pumps. This describesthe way the vanes are attached to the rotor of the pump. In a sliding vane pump thevanes move in and out of the rotor with the shape of the pump casing. In a swinging

vane pump the vanes are attached to the rotor by a hinge. By moving on the hingethe vanes are able to rotate with the shape of the casing.

2.4.3 Lobe Pumps

Figure 2-7 Lobe Pumps

Lobe pumps have two specially shaped rotors. The rotors mesh together to movethe liquid through the pump. The rotors of a lobe pump may have two or threelobes.


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2.5 CENTRIFUGAL PUMPS

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he four main types of centrifugal pumps are:

Volute pumps.

Diffuser pumps

Turbine pumps.

Propeller pumps.

2.5.1 Volute Pumps

Figure 2-8 Volute Pump

Volute pumps are the most common type of centrifugal pump to be found in aprocess plant. They usually operate without problems and do not require muchmaintenance.


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A volute pump works like this:

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2.5.3 Turbine Pumps

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Figure 2-10 Turbine Pump

Turbine pumps are small. They are best for low capacity, high pressure service ofclean liquids. Because the clearances inside the pump are very close, rapid 'Weartakes place if the pumped liquid contains abrasives.

A turbine pump works like this:

The impeller vanes travel in a channel in the pump casing.

The liquid being pumped is directed to the outside edge of the impeller.

The impeller makes the liquid travel in almost a complete circle.

As the liquid travels it receives a series of 'pushes' from the impeller vanes.

Each push increases the pressure of the liquid.


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2.5.4 Propeller Pumps

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Figure 2-11 Propeller Pump

Propeller pumps are small. They are used for low pressure, high capacity service.

The pump works in a similar way to the blades of an electric fan or the propellers ofan aircraft. The shape of the impellers increases the pressure of the pumped liquid.


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2.6 IMPELLERS

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In a volute pump the device which rotates inside the pump casing to give velocityenergy to the liquid is called an impeller.


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Figure 2-12 Types of Impellers for Volute Pumps

Open impellers are used to pump liquids that contain small amounts of solids thatmight block the pump or become trapped between the impeller and the casing

Semi-open and semi-closed impellers are used to pump liquids that contain manysolid materials.

Enclosed impellers are used for liquids which contain only small amounts of solidmaterials. This type of impeller is the most common and is very efficient.


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2.7 MULTI-STAGING

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Centrifugal pump impellers only give a small amount of pressure energy to theliquid.

To increase the final pump discharge pressure the impellers are connected togetherin series on one shaft. This is called multi-staging.

If each impeller gives 10 barg of pressure to the liquid and there are 6 impellers inseries, the final pump discharge pressure will be 60 barg.

Each impeller is called a stage, so the pump described above is a 6 stage pump.Pumps may have many stages from 1 to 60 depending on the processrequirements.

2.8 INSTALLATION

Centrifugal pumps can be installed horizontally or vertically depending on theprocess requirements.

Unit 2 Pump Classification

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