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Pumps Centrifugal vs. Positive Displacement

Two Categories Kinetic (Centrifugal) and Positive Displacement

There are two main categories of pumps - kinetic and positive displacement. Almost all

pumps fall into one of these two categories. The main difference between kinetic and

positive displacement pumps lies in the method of fluid transfer. A kinetic pump impartsvelocity energy to the fluid, which is converted to pressure energy upon exiting the pump

casing. A positive displacement pump moves a fixed volume of fluid within the pump

casing by applying a force to moveable boundaries containing the fluid volume.

Kinetic pumps can be further divided into two categories of pumps centrifugal and

special effect. pecial effect pumps include !et pumps, reversible centrifugal, gas lift,

electromagnetic and hydraulic ram. pecial effect pumps are not commonly used relative

to centrifugal pumps, so they will not be covered in this course.

"ositive displacement pumps are also divided into two ma!or pump categories reciprocating and rotary. #eciprocating pumps transfer a volume of fluid by a crankshaft,

eccentric cam or an alternating fluid pressure acting on a piston, plunger or a diaphragmin a reciprocating motion. #otary pumps operate by transferring a volume of fluid in

cavities located between rotating and stationary components inside the pump casing. The

relative features of reciprocating and rotary pumps, as well as centrifugal pumps, will becovered in this course.

$igure % below shows the ma!or pump categories and the types of pumps within each

category.

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Figure 1 Maor Pump Categories

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Comparison Ta!le Centrifugal vs. Positive Displacement Pumps

Table % below outlines some of the main differences between centrifugal pumps,reciprocating pumps and rotary pumps. &ote that 'centrifugal(, 'reciprocating( and

'rotary( pumps are all relatively broad categories. The table below provides a

comparison of features between these pump categories that generally holds true.

)owever, there are exceptions. $or example, reciprocating pumps generally re*uire morespace than centrifugal pumps for a given flow rate. +ut, there may be specific

applications where a positive displacement pump re*uires less space relative to a

centrifugal pump. Also, note that Table % lists typical maximum flow rates and heads. tis possible to build special pumps outside the upper bounds of the pressures and flow

rates listed, but such pumps would be prohibitively expensive for most applications.

Parameter Centrifugal Pumps "eciprocating Pumps "otar# Pumps

ptimum $low and

"ressure Applications

edium/)igh 0apacity,

1ow/edium "ressure

1ow 0apacity,

)igh "ressure

1ow/edium 0apacity,

1ow/edium "ressure

aximum $low #ate %22,2223 4" %2,2223 4" %2,2223 4"

1ow $low #ate

0apability

&o 5es 5es

aximum "ressure 6,2223 " %22,2223 " 7,2223 "#e*uires #elief 8alve &o 5es 5es

mooth or "ulsating

$low

mooth "ulsating mooth

8ariable or 0onstant

$low

8ariable 0onstant 0onstant

elf-priming &o 5es 5es

pace 0onsiderations #e*uires 1ess pace #e*uires ore pace #e*uires 1ess pace

0osts 1ower nitial

1ower aintenance

)igher "ower

)igher nitial

)igher aintenance

1ower "ower

1ower nitial

1ower aintenance

1ower "ower

$luid )andling uitable for a wide range

including clean, clear, non-

abrasive fluids to fluids withabrasive, high-solid content.

&ot suitable for high

viscosity fluids

uitable for clean, clear,

non-abrasive fluids.

pecially-fitted pumpssuitable for abrasive-slurry

service.

uitable for high viscosity

#e*uires clean, clear, non-

abrasive fluid due to close

tolerances

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1ower tolerance for

entrained gases

fluids

)igher tolerance forentrained gases

ptimum performance

with high viscosity fluids

)igher tolerance forentrained gases

Ta!le 1 Comparison Ta!le

Capacit#

The wide variety of centrifugal pumps manufactured offer a relatively large range of

available capacities. #adial-flow and mixed flow pumps are used for low to mediumcapacity applications. $or high capacity applications, axial-flow pumps are capable of

delivering flow rates in excess of %22,222 gpm. 0entrifugal pumps are not stable at low

flow rates, although there are special low-flow centrifugal pumps available that candeliver flow rates less than %2 gpm. )owever, for extreme low-flow applications 9: %

gpm;, positive displacement pumps are a better selection.

#eciprocating and rotary pumps are capable of capacities ranging from low to medium,with flow rates peaking at %2,2223 gpm. n theory, reciprocating pumps can bemanufactured to deliver more capacity, but they become prohibitively large and

expensive at high flow rates. +oth reciprocating and rotary pumps are capable of

delivering product at extremely low flow rates 9fractions of a gpm;, making themparticularly suitable for many chemical in!ection applications.

Pressure

0entrifugal pumps and rotary pumps are best suited for low to medium pressure

applications. #eciprocating pumps are usually specified for high pressure service, withcapabilities exceeding %22,222 psi. ulti-stage centrifugal pumps can deliver at

pressures of 6,2223 psi and may be the most economical choice at this pressure in high

capacity applications. +ut, in most applications exceeding %,222 psig, reciprocatingpumps are more suitable, particularly in low to medium capacity service. +oth

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reciprocating and rotary pumps will continually increase pressure when pumping against

a closed discharge to the extent allowed by the driver

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$elf'priming

#eciprocating and rotary pumps are self-priming. This is an important consideration

where a prime cannot be maintained on the pump. 0entrifugal pumps are not inherentlyself-priming, although some manufacturers do specially design self-priming units.

>xternal priming sources, such as an eductor or vacuum pump can also be employed.

Costs and $pace Considerations

n an overlap region where the conditions are suitable to use a centrifugal, reciprocating

or a rotary pump, the following rules generally apply? The reciprocating pump willgenerally have higher initial capital costs and will re*uire more space relative to the

centrifugal pump or the rotary pump. The reciprocating pump will generally have highermaintenance costs relative to the centrifugal pump or the rotary pump. The centrifugal

pump will generally have higher annual power consumption costs relative to the

reciprocating pump or the rotary pump because of lower efficiencies. f course, thereare many exceptions. These are !ust general guidelines. A pump that is selected for an

application outside of its optimum operating parameters will almost certainly not follow

these rules. $or example, a rotary pump operating in a high pressure, abrasive-slurryservice would probably have higher maintenance costs than a properly selected

reciprocating pump. The close running clearances 9particularly for high pressure service;

re*uired in the rotary pump would likely result in premature wear and fre*uentmaintenance.

Fluid andling

0entrifugal pumps are suitable for transferring a variety of fluids ranging from clean,

clear non-abrasive fluids to abrasive-slurries. )owever, a centrifugal pump is not the best

choice for pumping highly viscous fluids due to dramatic drops in efficiency at highviscosities. 0entrifugal pumps are not normally specified for viscosities higher than

about 7,222 @. 0entrifugal pumps are also not well suited to pumping entrained air.ost centrifugal pumps can handle up to about B entrained gas and specially-designed

pumps can handle up to about %2B.

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#eciprocating pumps are well suited for transferring clear, non-abrasive fluids, as well as

abrasive slurries. n fact, the relatively low velocities of moving parts within a

reciprocating pump make it particularly resistant to erosion in abrasive-slurryapplications, provided that the pump is properly designed for the service. #eciprocating

pumps maintain high efficiencies when pumping highly viscous fluids and can easily

handle C2B and higher volumes of entrained gas.

#otary pumps can also handle high viscosity fluids and high volumes of entrained gas. n

fact, many rotary pumps operate at their best efficiency at higher viscosities. )owever,

rotary pumps are not well suited for pumping corrosive fluids or fluids with abrasivesolids because of close clearances between rotating and static pump components.

$ummar#

The comparisons between different pump categories in this course are general. The

information is intended to familiari=e the student with the basic differences between

centrifugal, reciprocating and rotary pumps. )owever, every pump application is

uni*ue. "rior to selecting a pump for a particular application, further study is re*uired onthe factors that go into selecting a pump. The student is also encouraged to learn more

about the many types of pumps contained within the broad categories of centrifugal,

reciprocating and rotary pumps.