Assignment 3- PD Pump

8/9/2019 Assignment 3- PD Pump

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Assignment 3 BIO 3073 Bioprocess Plant & Equipment

INTRODUCTION

Pump is a device used to move liquids orslurries. A pump moves liquids from

lower pressure to higher pressure, and overcomes this difference in pressure by addingenergy to the system (such as a water system). A gas pump is generally called a

compressor, except in very low pressure-rise applications, such as in heating, ventilating,

and air-conditioning, the equipment is known asfans orblowers.

Diagram of pump

Positive displacement pumps can be further classified as either rotary-type or

reciprocating-type.A positive displacement pump causes a liquid to move by trapping a

fixed amount of fluid and then forcing (displacing) that trapped volume into the discharge

pipe.

Example of Positive-displacement Pump

Positive-Displacement Pump - 1 -
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BASIC OPERATING PRINCIPLE

Positive-displacement Pump has an expanding cavity on the suction side of the

pump and a decreasing cavity on the discharge side. Liquid is allowed to flow into the pump as the cavity on the suction side expands and the liquid is forced out of the

discharge as the cavity collapses.

By definition, PD pumps displace a known quantity of liquid with each revolution

of the pumping elements (i.e., gears, rotors, screws, vanes). PD pumps displace liquid by

creating a space between the pumping elements and trapping liquid in the space. The

rotation of the pumping elements then reduces the size of the space and moves the liquid

out of the pump.

ADVAN

TAGES & DISADVANTAGES



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SELECTION OF POSITIVE DISPLACEMENT PUMP

Selection of a positive displacement (PD) rotary pump is not always an easy

choice. There are four common types of PD pumps available: internal gear, externalgear, timed lobe, and vane. Most PD pumps can be adapted to handle a wide range of

applications, but some types are better suited than others for a given set of circumstances.

The first consideration in any application is pumping conditions. Usually the need

for a PD pump is already determined, such as a requirement for a given amount of flow

regardless of differential pressure, viscosity too high for a centrifugal pump, need for

high differential pressure, or other factors.

Inlet conditions, required flow rate, differential pressure, temperature, particle

size in the liquid, abrasive characteristics, and corrosiveness of the liquid must be

determined before a pump selection is made.

A pump needs proper suction conditions to work well. PD pumps are self-

priming, and it is often assumed that suction conditions are not important. But they are.

Each PD pump has a minimum inlet pressure requirement to fill individual pump cavities.

If these cavities are not completely filled, total pump flow is diminished. Pumpmanufacturers supply information on minimum inlet conditions required. If high lift or

high vacuum inlet conditions exist, special attention must be paid to the suction side of

the pump.



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NTERNAL GEAR PUMPS

The crescent internal gear pump has an outer or

rotor gear that is generally used to drive the inner or

idler gear (Figure 1). The idler gear, which is smaller

than the rotor gear, rotates on a stationary pin and

operates inside the rotor gear. The gears create voids as

they come out of mesh and liquid flows into the pump.

As the gears come back into mesh, volumes are

reduced and liquid is forced out the discharge port.

Liquid can enter the expanding cavities through the rotor teeth or recessed areas on the

head, alongside the teeth. The crescent is integral with the pump head and prevents

liquids from flowing to the suction port from the discharge port.

The rotor gear is driven by a shaft supported by journal or antifriction bearings.

The idler gear contains a journal bearing rotating on a stationary pin in the pumped

liquid. Depending on shaft sealing arrangements, the rotor shaft support bearings may run

in pumped liquid. This is an important consideration when handling an abrasive liquid

and can wear out a support bearing.

The speed of internal gear pumps is considered relatively slow compared to

centrifugal types. Speeds up to 1,150 rpm are considered common, although some small

designs operate up to 3,450 rpm. Because of their ability to operate at low speeds,

internal gear pumps are well suited for high-viscosity applications and where suction

conditions call for a pump with minimal inlet pressure requirements.

Internal gear pumps are made to close tolerances and are damaged when pumping

large solids. These pumps can handle small suspended particulate in abrasive

applications, but gradually wear and lose performance. Some performance loss is restored

by adjusting the pump end clearance. End clearance is the closeness of the rotor gear to

the head of the pump.


Figure 1.Internal gear pumps are

ideal for high-viscosity liquids, but

they are damaged when pumping large

solids.


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EXTERNAL GEAR PUMPS

External gear pumps are similar inpumping action to internal gear pumps in that

two gears come into and out of mesh to produce

flow (Figure 2). However, the external gear

pump uses two identical gears rotating against

each other. Each gear is supported by a shaft

with bearings on both sides of each gear.

Typically, all four bearings operate in the

pumped liquid.

Because the gears are supported on both sides, external gear pumps are used for

high pressure applications such as hydraulics. Usually, small external gear pumps operate

at 1,750 or 3,450 rpm and larger versions operate at speeds up to 640 rpm.

The design of external gear pumps allows them to be made to closer tolerances

than internal gear pumps. The pump is not very forgiving of particulate in the pumped

liquid. Since there are clearances at both ends of the gears, there is no end clearance

adjustment for wear. When an external gear pump wears, it must be rebuilt or replaced.

External gear pumps handle viscous and watery-type liquids, but speed must be

properly set for thick liquids. Gear teeth come out of mesh a short time, and viscous

liquids need time to fill the spaces between gear teeth. As a result, pump speed must be

slowed down considerably when pumping viscous liquids.

The pump does not perform well under critical suction conditions. Volatile liquids

tend to vaporize locally as gear teeth spaces expand rapidly. When the viscosity of

pumped liquids rises, torque requirements also rise, and pump shaft strength may not be

adequate. Pump manufacturers supply torque limit information when it is a factor.

LOBE PUMPS


Figure 2.External gear pumps (shown

is a double pump) are typically used for high-

pressure applications such as hydraulics.


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Lobe pumps (Figure 3) are similar to

external gear pumps in operation, except the

pumping elements or lobes do not make

contact. Lobe contact is prevented by external

timing gears. Pump shaft support bearings are

located in the timing gear case. Since the

bearings are out of the pumped liquid, pressure

is limited by bearing location and shaft

deflection. There is not metal-to-metal contact

and wear in abrasive applications is minimal.

Use of multiple mechanical seals makes seal construction important.

Lobe pumps are frequently used in food applications, because they handle solids

without damaging the pump. Particle size pumped can be much larger in lobe pumps than

in other PD types. Since the lobes do not make contact, and clearances are not as close as

in other PD pumps, this design handles low viscosity liquids with diminished

performance. Loading characteristics are not as good as other designs, and suction ability

is low. High-viscosity liquids require considerably reduced speeds to achieve satisfactory

performance. Reductions of 25% of rated speed and lower are common with high-

viscosity liquids.

Lobe pumps are cleaned by circulating a fluid through them. Cleaning is

important when the product cannot remain in the pumps for sanitary reasons or when

products of different colors or properties are batched.


Figure 3.Lobes in lobe pumps do not

make contact, because they are driven by

external timing gears. This design handles low-

viscosity liquids.


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Flexible Member

This principle is similar to the Vane principle except the vanes flex rather than

slide. The fluid pumping and sealing action depends on the elasticity of the flexible

members. The flexible members may be a tube, a vane, or a liner. Figure 5 shows a

flexible vane pump.

Diagram of Flexible Member type

Circumferential Piston

Fluid is carried from inlet to outlet in spaces between piston surfaces. Rotors

must be timed by separate means, and each rotor may have one or more piston elements.

Diagram of Circumferential Piston type



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Single screw pumps

Single screw pumps are commonly called progressive cavity pumps. They have a

rotor with external threads and a stator with internal threads. The rotor threads are

eccentric to the axis of rotation.

Diagram of Single Screw type

Multiple screw pumps

Multiple screw pumps have multiple external screw threads. These pumps may be timed

or untimed

Diagram of Multiple Screw type



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VANE PUMPS

Sliding vane pumps (Figure 4) operate quite

differently from gear and lobe types. A rotor with

radial slots, is positioned off-center in a housing bore.

Vanes that fit closely in rotor slots slide in and out as

the rotor turns. Vane action is aided by centrifugal

force, hydraulic pressure, or pushrods. Pumping action

is caused by the expanding and contracting volumes

contained by the rotor, vanes, and housing.

Vanes are the main sealing element between the suction and discharge ports and

are usually made of a nonmetallic composite material. Rotor bushings run in the pumped

liquid or are isolated by seals.

Vane pumps usually operate at 1,000 rpm, but also run at 1,750 rpm. The pumps

work well with low-viscosity liquids that easily fill the cavities and provide good suction

characteristics. Speeds must be reduced dramatically for high-viscosity applications to

load the area underneath the vanes. These applications require stronger-than-normal vane

material.

Because there is no metal-to-metal contact, these pumps are frequently used with

low-viscosity nonlubricating liquids such as propane or solvent. This type of pump has

better dry priming capability than other PD pumps. Vane pumps can run dry, but are

subject to vane wear.

Abrasive applications require the proper selection of vane material and seals.

Vane pumps have fixed end clearances on both sides of the rotor and vanes similar to

external gear pumps. Once wear occurs, this clearance cannot be adjusted, but some

manufacturers supply replaceable or reversible end plates. Casing liners are a low-cost

way of restoring pump performance as wear occurs. Unlike lobe pumps, vane pumps

cannot handle solids.


Figure 4.Vane pumps have better dry

priming capability than other positive

displacement pumps.


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TROUBLESHOOTING

Pump Problems can be either caused by:

1. Mechanical Problem with the Pump

To determine if this noise is a mechanical problem with the pump drains it, close

both suction and discharge valves and run the pump briefly. If the noise continues

you have a mechanical problem. The mechanical noise can be caused by:

i. Debris in the Impeller

ii. Impeller Rubbing

iii. Impeller out of Balance

iv. Bent or Twisted shaft

v. Bad bearings

vi. Coupling Misalignment

vii. V-Belt sheave Misalignment

viii. Pipe Stress

In any case the problem can be corrected by taking it apart and simply fixing it by

either replacing the damaged parts or correcting the installation. Items Required:

a. Pump Installation Manual

b. Pump Operation Manual

c. Pump Maintenance Manual

d. Parts List



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2. Pump System Problem

However if the Noise goes away after draining the pump etc. then the noise is

caused by the Pumping System.Pump System Noise is usually caused by:

a. Cavitation

b. Vortexing

In order to troubleshoot a pump system problem the following tools are required:

i. Combination Vacuum/Pressure Gauge - To Check Pump Suction

Reading

ii. Pressure Gauge - To Check Pump Discharge Pressure Reading

iii. Amp Meter - To Check Horsepower Load

iv. Tachometer - To Check Pump Speed

v. Pump Performance Curve - To Check all Readings Against the

Expected Pump Performance



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MAINTENANCE

The maintenance for positive-displacement pump similar in many ways to that of

centrifugal pumps. Alignment is one common concern, since PD pumps are oftenpedestal-mounted and have close-tolerance components that wear or bind if misaligned.

Some key considerations are:

If the pump is belt driven, the belt tension is important. The rule of thumb

is that the belt should be able to be depressed in midlength. The faces of

both pulleys also need close alignment; a long straightedge can be used as

described for checking a coupling.

The length of the service life of PD pumps often depends on how clean the

pumped liquids are; therefore, filters and strainers should be scrupulously

maintained, and the air line feeding double diaphragm pumps monitored for

contamination.

Motors rarely need maintenance today. The ball bearings are often double

sealed for the life of the bearing; if they are not, care should be taken to avoid

the dangers of overgreasing.


Assignment 3- PD Pump

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