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Pumps Centrifugal vs. Positive Displacement

( Edward P. Brunet, Jr., PE) 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 imparts velocity 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 diaphragm in 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 be covered in this course. %igure & below shows the ma"or pump categories and the types of pumps within each category.

!igure " #a$or Pump Categories 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 more space 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. .t is 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 /ptimum %low and #ressure Applications 0aximum %low $ate 3ow %low $ate 2apability 0aximum #ressure $e,uires $elief :alve !mooth or #ulsating %low :ariable or 2onstant %low !elf-priming !pace 2onsiderations 2osts

Centrifugal Pumps 0edium1+igh 2apacity, 3ow10edium #ressure &44,4445 6#0 'o 8,4445 #!. 'o !mooth :ariable 'o $e,uires 3ess !pace 3ower .nitial 3ower 0aintenance

&eciprocating Pumps 3ow 2apacity, +igh #ressure &4,4445 6#0 7es &44,4445 #!. 7es #ulsating 2onstant 7es $e,uires 0ore !pace +igher .nitial +igher 0aintenance

&otar' Pumps 3ow10edium 2apacity, 3ow10edium #ressure &4,4445 6#0 7es 9,4445 #!. 7es !mooth 2onstant 7es $e,uires 3ess !pace 3ower .nitial 3ower 0aintenance

%luid +andling

+igher #ower 3ower #ower 3ower #ower !uitable for a wide range!uitable for clean, clear,$e,uires clean, clear, nonincluding clean, clear, non-non-abrasive fluids. abrasive fluid due to close abrasive fluids to fluids with!pecially-fitted pumpstolerances abrasive, high-solid content. suitable for abrasive-slurry service. 'ot suitable for high viscosity fluids !uitable for high viscosity /ptimum performance fluids with high viscosity fluids 3ower tolerance for entrained gases +igher tolerance for+igher tolerance for entrained gases entrained gases

Ta%le " Comparison Ta%le

Capacit'

9 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 medium capacity applications. %or high capacity applications, axial-flow pumps are capable of delivering flow rates in excess of &44,444 gpm. 2entrifugal pumps are not stable at low flow rates, although there are special low-flow centrifugal pumps available that can deliver flow rates less than &4 gpm. +owever, for extreme low-flow applications <= & 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 &4,4445 gpm. .n theory, reciprocating pumps can be manufactured 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 <fractions of a gpm), making them particularly suitable for many chemical in"ection applications. Pressure 2entrifugal pumps and rotary pumps are best suited for low to medium pressure applications. $eciprocating pumps are usually specified for high pressure service, with capabilities exceeding &44,444 psi. 0ulti-stage centrifugal pumps can deliver at pressures of 8,4445 psi and may be the most economical choice at this pressure in high capacity applications. -ut, in most applications exceeding &,444 psig, reciprocating pumps are more suitable, particularly in low to medium capacity service. -oth reciprocating and rotary pumps will continually increase pressure when pumping against a closed discharge to the extent allowed by the driver>s horsepower. This can result in overpressure of the pump or piping components, so it is necessary to install a relief valve on the discharge of the pump capable of discharging the full capacity of the pump. A centrifugal pump>s pressure rise is limited to the shut-off pressure on the pump curve, which is always less than the design pressure of the pump <and the piping system if properly designed). A relief valve is only needed if no other measures are provided to detect low flow conditions and shut down the pump to prevent damage. The relief valve need only be si?ed to pass the minimum flow rate re,uired to maintain stable flow and prevent excessive temperature rise. (moot) or Pulsating !low 2entrifugal pumps and most rotary pumps provide smooth, non-pulsating flow, while reciprocating pumps produce a pulsating flow. A pulsating flow may re,uire special design considerations in the piping system. .f the pump is not located near the suction source, then acceleration head can contribute to low '#!+A problems, which may re,uire the installation of a suction stabili?er. A pulsation dampener may need to be installed in the discharge piping to reduce pressure surges resulting from the pulsating flow. *aria%le or Constant !low 2entrifugal pumps operate on a variable-flow, variable-head curve. As the discharge pressure decreases, the pump delivers a higher flow rate. At any given speed, reciprocating and rotary pumps operate at a constant flow rate regardless of the discharge pressure. There are specific applications that re,uire either constant flow or variable flow. 0etering pumps rely on a constant flow at varying pressures, which makes reciprocating pumps and rotary pumps suitable for this application. #iston pumps used for metering will often use an ad"ustable stroke length to allow the operator to vary the flow rate to meet the system re,uirements. 2entrifugal pumps are favored where process conditions often re,uire varying flow rates. %or example, a level control valve must throttle the flow rate from a vessel to maintain a constant level in the vessel. A centrifugal pump is well suited to handle this process condition, whereas a positive displacement pump would either re,uire a continuous recycle to suction or a variable speed driver to accommodate the variable flow.

D (elf+priming $eciprocating and rotary pumps are self-priming. This is an important consideration where a prime cannot be maintained on the pump. 2entrifugal pumps are not inherently self-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 applyA The reciprocating pump will generally 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 higher maintenance 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, there are 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-slurry service would probably have higher maintenance costs than a properly selected reciprocating pump. The close running clearances <particularly for high pressure service) re,uired in the rotary pump would likely result in premature wear and fre,uent maintenance. !luid ,andling 2entrifugal 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 high viscosities. 2entrifugal pumps are not normally specified for viscosities higher than about 9,444 !!B. 2entrifugal pumps are also not well suited to pumping entrained air. 0ost centrifugal pumps can handle up to about *C entrained gas and specially-designed pumps can handle up to about &4C. $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-slurry applications, provided that the pump is properly designed for the service. $eciprocating pumps maintain high efficiencies when pumping highly viscous fluids and can easily handle D4C 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 abrasive solids because of close clearances between rotating and static pump components.

(ummar'- The comparisons between different pump categories presented in this course are general.
The information is intended to familiari?e the student with some of the basic differences between centrifugal, reciprocating and rotary pumps. +owever, there are many different subcategories of pumps within these broad categories and there are many regions of overlap where multiple types of pumps in the same category and even in different categories would be suitable. !ince every pump application is uni,ue, each of the factors that influence the pump selection must be considered in detail.