Twin Screw vs Centrifugal & Reciprocating Article

7/29/2019 Twin Screw vs Centrifugal & Reciprocating Article

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Viscosity Vs. Efficiency

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Efficien

cy-% 2-Screw Pump

Centrifugal Pump

Typical Centrifugal Pump Curve

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Twin-Screw Pumps vs. Centrifugal and Reciprocating Pumps

In this article the Twin-Screw pump will be explored and compared to centrifugal andreciprocating pumps. Primary areas of explanation will include development of energy,

limitation of energy generation, effect of viscosity and flow characteristics.

The Centrifugal PumpThe centrifugal pump is the most widely used pump in industry, is well understood byusers and accepted for most all applications. The pump is typically low in price, and

available in a wide selection of sizes andconfigurations. Centrifugal pumps generateenergy by accelerating the pumped fluid to high

speed through the spinning impeller andconverting the velocity energy to pressure

energy. As a result, the pumps operate at highspeeds which causes high fluid shear rates. Dueto the generated head being a function of the

energy imparted to the fluid by the spinningimpeller, centrifugal pumps produce dynamic

head, which is related to pressure as a function Figure 1 Centrifugal Pump Headof the fluids specific gravity. This dynamic head can be expressed as the force exerted onthe bottom of a column of fluid in terms of meters of fluid. The head produced isa

combination of impeller tip speed and pump design and is generally not a function of thepumped fluid, especially with thin fluids. For

instance, if a pump produces a head of 100meters, it will produce 100 meters of headregardless of the fluid pumped as illustrated in

Figure 1. This is a general statement and it mustbe noted that fluid viscosity and the presence of

solids will affect the total dynamic head.

As the flow rate increases within the centrifugal

pump impeller and case, there are additionalenergy losses such as eddy currents and friction

which decrease the produced head as theflow Figure 2 Centrifugal Pump Curverate through the pump increases. This results inthe familiar shape of the typical centrifugal

pump head/capacity curve as shown in Figure 2.

Due to the high fluid shear rates, centrifugalpumps can become extremely inefficient whenpumping fluids of higher viscosities when

compared to Twin-Screw pumps. Figure 3illustrates a typical effect of viscosity on

efficiency of a centrifugal pump vs. a Twin-Screw pump. In an application where acentrifugal pump requires 1000 Kw, a Figure 3 Typical Efficiency Curves


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Power Difference Vs. Cost

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$0.02 to $0.20 per Kw/Hr

$0.02

$0.04

$0.06$0.10$0.20

8000 Hours Operation Per Year

Twin-Screw pump may only require 670 Kw. Based on the power difference energy

curves in Figure 4 the centrifugal pump would require an additional annual energy costof $100,000 for 8000hour/year at $0.04/Kw-Hr.

Single stage overhung centrifugal pumps in low pressure applications are typically lower

in price than Twin-Screw pumps. At pressures above approximately 10-14 Bar, thepumps must be manufactured in multiple stages which begins to rapidly approach theprice of positive displacement pumps. For

that reason, single stage centrifugal pumpsare typically used at pressures below 14BAR in low viscosity fluid applications.

The reduction in efficiency at even modestviscosities of 80-100 cSt requires

alternative pumping technology beconsidered for any application with higherfluid viscosities regardless of the pressure.

Centrifugal pumps are not self priming and

if gas is present in the fluid the pump may Figure 4 Power Difference Energy Costfill with air/gas and quit pumping (vapor lock).

Centrifugal pumps have relatively high net positive suction head requirements as the flowincreases and the pump approaches the best efficiency point. Net positive suction head

requirements range from the capability to handle vacuums to requiring positive pressuresand are a function of overall pump design and size.

Common centrifugal pump materials of construction include steel or cast iron bearingframes, steel, stainless steel, iron or Ni-Resist wetted parts depending on the application.

In general most centrifugal pumps use antifriction bearings. Both high axial and radialloads may be present and must be considered when making final bearing type and size

selections. Typical antifriction bearing design lives are 20,000 to 100,000 L10 hours withlubrication by either grease or oil splash.

Advantages of centrifugal pumps include wide acceptance for low viscosity applications,ability to handle various inlet conditions, operation at induction motor speeds, high

temperature capability and being well understood by the users. The pump can handle awide range of non-lubricating, lubricating, abrasive and corrosive fluids. At low

viscosities the pump exhibits high mechanical efficiency. Installation and maintenancecost are typically low. Disadvantages include low efficiency at high viscosities, high fluidshear rates, limited pressure capability and vapor locking at moderate gas content or

when suction pipe leaks are present.


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Typical Reciprocating Pump Curve

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The Reciprocating Pump

The reciprocating pump is a constant-speed, constant-torque pump with a constant outputand are available in piston and plunger versions. Piston pumps are typically limited to

flows of 100-120 M3/Hr and pressures of 70 Bar or less with plunger pumps covering the70 Bar to 2070 Bar range. This article is restricted to the piston pump since it more

suitably covers the same hydraulic range as the Twin-Screw pump.

A series of pistons are driven through an eccentric crankshaft with output determined by

overall piston size, piston speed and number of pistons. Pump speeds are typically low inthe several hundred RPM range. The pump generates energy by pushing a slug of fluidinto the pump discharge piping system at whatever pressure is required to perform that

action. As the piston retracts in the bore,the pump chamber volume increases

which creates a low pressure region andopens the inlet valve and allows fluid toenter the chamber. At the end of the

suction stroke, the low pressure regionceases to exist and the inlet valve closes.

The piston then begins the dischargestroke which reduces the dischargechamber volume and forces the fluid to

exit through the outlet valve and into thepiping system.

Figure 5 Piston Pump Cross Section

The ability to generate pressure is only a function of the structural capacity of the pump

and driver power. If system pressures become too high, catastrophic failure of the pump,piping or drive components can occur. For that reason, all piston pumps should have a

relief valve installed in the discharge piping to prevent over pressurization.

Volumetric efficiency is a function of

valve slippage, packing leakage and (insome instances) fluid compressibility.

Slip flow occurs within the pump as aresult of back flow through the valves asthey close. Viscosity, pump speed and

pressure can also have an effect. Typicalslipped flow ranges from 2 to 10% of total

flow. Overall mechanical efficiencies arerelatively high in the 80 to 90% rangedependent on design and pressure

differential. The resulting speed capacitycurve is essentially a fixed flow

at a fixed speed as illustrated in Figure 6. Figure 6 Typical Piston Pump Curve


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Due to the start/stop type operation of the piston movement, there is extensive variations

in the flow rate which results in pressure pulsations. This variation in flow can be reducedby increasing the number of pistons. For that reason, piston pumps are typically

manufactured in single chamber and multiple parallel chambers of 2 through 7 chambersor more . A common arrangement found in the oil fields is the 3 chamber or triplex

pump. Flow variations for a 3 chamber pump are shown in Figure 7 while Table 1 showsthe effects of pistons number vs. flow variations. These flow variations can result inpressure pulsations so severe that pulsation dampeners (accumulators) are required to

smooth the pulses to a level atwhich other system componentssuch as, piping, valves and

structural components are notadversely effected. Fatigue

failure of these criticalcomponents is a major concernwhen subjected to the cyclic

loading of the pressurepulsations.

Figure 7 Triplex Pump Flow Pulsations

Net positive suction

head requirements aregenerally similar to

other pumps but anacceleration head factormust be included to

insure the fluid iscapable of filling the

bore as the pistonretracts or pump knocking will result. In general, positive inlet pressures are required.

Common materials of construction include steel frames, steel, iron or bronze pistons andNi-Resist cylinder liners. Pistons and liners may also employ various coatings such as

chrome or ceramics to improve abrasion resistance.

Piston pumps are designed in both sleeve and antifriction bearing configurations. Sleeve

bearings are considered to have infinite life when properly installed and lubricated.However, sleeve bearings are designed to operate within a limited speed range and

operation outside of this range (either higher or lower in rpm) can result in lost of thelubrication film and bearing failure. In order to start a pump with sleeve bearings, thepump must be first brought up to speed through a bypass line under no pressure load.

Antifriction bearing pumps can be started without the need for a bypass line with typicalbearing design lives of 30,000 to 50,000 L10 hours. Antifriction bearing lubrication is

typically by splash if sufficient pump rpm is present. At low speed a force feedlubrication system may be required to insure proper bearing lubrication.


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Advantages of the piston pump include being relatively well understood by the users,

high mechanical efficiency and large acceptance by the oil industry. Disadvantagesinclude pulsation levels, resulting vibration problems and low operating speed which

requires gear box or V-Belt drives. Disadvantages may also include low flow capabilitydepending on requirements. The installed cost can be high due to the need for multiple

pumps, speed reducers and accumulators. Repair cost can be high and the pump maywear excessively when exposed to non-lubricating fluids (water) and abrasive slurriessuch as sandy crude oil. Leakage of pumpage past the packing rings on the piston rod

also pose environmental concerns.

The Twin-Screw Pump

The Twin-Screw Pump, also called the Two-Screw pump, is a double ended positivedisplacement rotary pump. Both product lubricated and oil lubricated gear and bearings

versions are available as illustrated in Figure 8. Some manufactures lubricate the non-gear end bearings using grease. The pump incorporates either a one-piece integral bodywith the screw bores machined as part of the body or an external case with the screw

bores machined in a replaceable liner. Normal pumping action splits the incomingsuction flow at the inlet flange into two equal portions which are directed to the inlet end

of the screws. Transfer chambers form as the meshed screw set rotates which conveysthe pumped fluid axially to the discharge chamber in the center of the pump. Specialhigh viscosity pump designs exist with the fluid entering the center of the screw set and

discharging at the screw ends.

Product Lubricated Gears/Bearings External Lubricated Gears/BearingWith a Single Mechanical Seal With Mechanical Seals or Packing

Figure 8 Two-Screw Pump Configurations


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In doubled ended screw pumps, the axial hydraulic forces cancel out. If used, helical

timing gears generate axial shaft forces which must be carried by the axial locator (ball orspherical roller) bearings. Hydraulic forces within the meshed screw set generate radial

forces which bend the screws toward the bore. These forces are a function of pressure,screw pitch, shaft strength, bearing span and to a minimum degree the screw profile.

Shaft bending direction is predictableand direction depends on theorientation of the bores as shown in

Figure 9. The amount of bending is adirect function of pressure, screw pitchand shaft material modulus of

elasticity, shaft diameter tothe 4th

power and bearing span to the 3rd

power. The most important of these isthe bearing span and for this reason,manufacturers offer short andlong

bearing bracket pump versions asshown in Figure 10. The Long-Bearing Figure 9 Shaft Bending Direction

Pumps have longer bearing brackets which allows operation at higher pumpagetemperature and installation of exotic sealing arrangements such as double mechanicalseals or packing. In Short-Bearing pumps the bearing brackets are shorter which results

in the bearing being held close to the pump body to minimize shaft bending. Short-Bearing pumps are typically used for lower temperature applications that do not require

exotic mechanical seals.

Long-Bearing Version Short-Bearing Version

Figure 10 Long and Short Bearing Designs


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To maximize pressure capability prior to the screws contacting the bore, manufacturers

increase the distance between the screws and bore by machining extra metal (scalloping)out of one side of the bores or offseting the screws to one side of the bores to give extra

room for the screws tobend. Scalloped and

offset bore and screwarrangements areshown in Figure 11.

To increase shaftoverall strength andminimize bending,

manufacturers havebegan to change from

shell or pinned screws Scalloped Bores Offset Screwsto integral (one-piece) Figure 11 Obtaining Screw Bending Clearancescrews. The integral screw is stronger as multiple components are not involved with

tolerances and fits and the overall shaft diameters at the seal areas can be maximized asillustrated in Figure 12.

The advantages of the shelldesign including the ability

to mix and match materialsis minimal due to increases

in material technologies.The shell screws alsoproved next to impossible to

repair in the field due todifficulties in controlling

runout and thread startlocations. Therefore,virtually all repaired pinned

screws are treated asintegral screws during the

repair process or werereturned to the OEM tohave new pinned screws Figure 12 Pinned Vs. Integral Shafts

made and installed on the shaft.

The Twin-Screw pump typically operate at induction motor speeds. The pump generatesenergy by trapping a pocket of fluid and transferring it from the suction chamber to thedischarge chamber. As the fluid pocket traverses down the bore, the pressure increases

linearly until discharge pressure is obtained in the discharge chamber. Due to internalclearances within the pump, some flow slips back from the higher pressure discharge

chamber to the suction chamber. This slipped flow is a function of internal pumpclearances, differential pressure, number of screw turns, viscosity and pump speed whichresults in a straight line performance curve as illustrated in Figure 13. The ability to


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Typical Twin-Screw Pump Curve

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43 cSt 2200 cSt

generate pressure is a function of

volumetric efficiency which should notdrop below 50% and the structural

capacity of the pump. As with thereciprocating pumps, a relief valve

should be installed in the dischargepiping to prevent over pressurization.Typical maximum flows for Twin-

Screw pumps is as high 3400 M3/Hrwith pressures to 100 Bar.

NPSH requirements are a function ofpump design, pump size, speed,

screw pitch and viscosity. NPSHrequirements are typicallysignificantly Figure 13 Typical Twin-Screw Pump Curveless than centrifugal pumps and may be one of the primary reasons for selection of this

pumping technology.

Pump efficiencies are typically high but decrease as viscosities increase due to increasedfrictional horsepower. However, efficiencies do not decrease as rapidly as centrifugalpumps as illustrated in Figure 3.

By design, there is no metal-to-metal contact inside the pump. However, the screws may

be designed to run in boundary layer lubrication conditions against the bores as describedelsewhere in this article. Torque is transferred from the drive shaft to the driven shaftthrough a timing gear set located either in the product or an isolated cooling and

lubricating chamber with the bearings. These gears synchronize the rotation of thepumping screws and hold the meshing clearance so no internal metallic contact occurs

between the screws. The externally housed gears and bearings typically require noadditional systems for cooling and lubricating. In some cases, high horsepower pumpsmay require auxiliary bearing and gear oil cooling systems.

There are two different fundamental design variations of the Twin-Screw pump which

can be categorized based on the bore length. For purposes of labeling, the authorcategorizes the pumps into either the Short-Bore Design or Long-Bore Design. Details ofthe Short-Bore and Long Bore Designs are outlined below.

Short-Bore Design While each manufacturer has modifications to the basic design,

Table 2 and Figure 14 detail the basic Short-Bore Design configuration.

Table 2 Short-Bore Design Configuration

Locator Bearings 2 Ball Anti-Friction Integral Body or Replaceable Liner

Radial Bearings 2 Roller Anti-Friction Spur, Helical or Herringbone Gears

Short Bore Length (3/4 to 1 times Bore Dia) Circular or Scalloped Bores

Light To Non-Contacting Screw/Bore Screws Centered In Bore Diameter

Corrosion and Abrasive Resistant Screw Coatings


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Due to the short bores, Short-Bore Design pumps are typically designed as either non-

contacting between the screws and bores or light contacting. As pressures increase thereis the possibility the

screws may run inboundary layer lubrication

conditions against thebore. Screw and borecoatings or treatments are

typically used only toimprove corrosion orabrasion resistance.

Advantages of the Short-Bore Design include

shorter bores which reducecost and the ability to mixand match screw and liner

materials to eliminate thenecessity of screw and/or

bore coatings. Figure 14 Typical Short-Bore/Short-Bearing Design

Long-Bore Design Configuration - While each manufacturer has modifications to the

basic design, Table 3 and Figure 15 detail the basic Long-Bore Design.

Table 3 Long-Bore Design Configuration

Locator Bearings 1 or 2 Spherical Rolleror Ball Anti-Friction

Integral One-Piece Body

Radial Bearings - 4 or 5 Heavy Duty Roller

Anti-Friction

Spur or Herringbone Gears

Long Bore Length (1-1/2 times Bore Dia) Circular Bores

Non-Contacting to Heavy ContactingScrew/Bore

Screws Offset To One Side Of Body Bore

Extensive Use Of Screw Coatings andTreatments

Extensive Use Of Body Bore Coatings andTreatments

Figure 15 Typical Long-Bore/Long-Bearing Design


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Typical System Requirement Curve

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System 2

Due to the long bores and bearing span, the shafts may bend more in the Long-Bore

Design as compared to the Short-Bore Design for equivalent size pumps under similarconditions. This may require the screws to run against the body bores in boundary layer

lubrication conditions and necessitate extensive usage of screw O.D. coatings and bodycoatings/treatments. Limitations to the screw/body bore loading is based on

manufacturers priority empirical data for materials of construction and past experience.Advantages of the Long-Bore Design include longer bores which allow longer screwpitches and therefore greater flow capacity in a smaller diameter screw. At shorter leads

more screw turns are present which results in lower pressure differentials per turn whichreduces slipped flow and wear rates. Some applications such as high pressure thin fluids(i.e. naphtha), require the Long-Bore Design to allow sufficient screw turns to minimize

slipped flow and allow acceptable volumetric efficiency. Bearings are also typicallyheavier duty when compared to the Short-Bore Design pump.

Table 3 Twin-Screw Pump Advantages

Dry Running (for limited time periods) Low Noise and Fluid Pulsations

Self Priming, Capable of StrippingServices and Slugging Flows Can Handle High Gas Content Fluids

Lubricating, Non-Lubricating, Corrosiveand Abrasive Fluids

Screw Pitch Changes Allow Different FlowRates

Abrasive and Non-Abrasive Slurries No Metal-To-Metal Contact

Operate At Induction Motor Speeds Flow Varies With Speed

Low NPSH Requirements Multiple Sealing Arrangements

High Temperature Capability Slow Pressure Buildup

High Flow and Pressure Capability Internal and External Bearing Arrangements

High Efficiency (Lower Electrical Cost) Horizontal and Vertical Pump Arrangements

Low Fluid Shear Rates Reverse Operation Possible

Multi-Phase Capability Low Starting Torque

Disadvantages of the Twin-Screw pumps may include cost, a relatively complicateddesign, use of 4 mechanical seals for external bearing designs and not being understoodby most users.

System Requirements

Hydraulic systems, piping, valves,filters, etc. require energy to move fluidthrough the system. This is a

combination of velocity energy to

accelerate the fluid, friction andelevation change. As the flow increases,the energy requirements increase asshown in Figure 16. Note that the curve

starts at 50 meters of liquid which in thisspecific example is the elevation change

in fluid level.Figure 16 System Head Requirement Curve


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Typical System Requirement Curve

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System 2System 1

Centrifugal

Piston2-Screw

Overlaying the different pump curves onto the system curves show how each type ofpumping technology reacts as the system changes. This is illustrated in Figure 17 in

which thecentrifugal pumps flow varies 20% dependent on the system requirementswhile the Twin-Screw varies less than 5% and the Piston pumps flow remains

approximately the same. Based on a viscosity of 330 cSt the centrifugal pump is themost inefficient while the piston pump is the most efficient. Assuming an energy cost of$0.04 per KW-Hr and an 8000 Hour year, the centrifugal pump would have an additional

energy cost of approximately $56,400over the Twin-Screw pump. Thepiston pump is the most efficient but

approximately 6 pumps in paralleloperation would be required to produce

the 600 M3/Hr flow rate.

Table 4 below outlines the

performance of the selected pumpsbased on the projected performance

data assuming an 8000 hour year withan energy cost of $0.04/Kw-Hr.

Figure 17 Pump System Performance

Table 4 Typical Performance Overview At 330 cSt

System 1 System 2

Pump TypeQuantityPumps

FlowM3/Hr

PowerKw

EnergyCost

FlowM3/Hr

PowerKw

EnergyCost

Centrifugal 1 600 485 $155,200 485 492 $157,440Piston 6 600 255 $86,100 600 346 $110,720

Twin-Screw 1 600 309 $98,800 575 379 $121,280

Note: Energy cost demand charge is not included in the above cost projections.

References:

Pump Handbook, Igor J. Karassik, MaGraw-Hill Book Company, New York, 1976,Section 3.1 Power PumpsCameron Hydraulic Data, 16th Edition, C.R. Westaway and A.W. Loomis, Ingersoll-

Rand, Woodcliff Lake, NJ, 1979, Section 1 Hydraulics


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About The Author:

Stephen Smith has over 28 years of pump experience as both a manufacturer and user.He is a graduate of Columbus State University and Wingate University and a registered

professional engineer in the state of North Carolina. Mr. Smith is currently the ScrewPump Specialist for Flowserve Corporation. Previous positive displacement positions

included Chief Engineer-Twin Screw Products for Imo Pump and Warren Pumps andEngineering Manager at Brunswick Defense for a Reverse Osmosis Water PurificationUnit which used a variety of pumping technologies including reciprocating and

centrifugal pumps. Prior to that, Mr. Smith was V.P. Engineering for Pekor Pumps, amanufacturer of centrifugal slurry and dredge pumps.

Twin Screw vs Centrifugal & Reciprocating Article

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