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ChE 203

More on Pumps

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Contents

Basic Conversions Series and Parallel Configurations

Reciprocating Pumps

Rotary Pumps Rotary Lobe Pumps

Pump Performance

Centrifugal v/s PD pumps

Liquid Ring Vacuum Pumps

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Converting Head to Pressure

Convert ing head in m eter to pressure in bar

p = 0.0981 h (1)where

h = head (m)

p = pressure (bar)

Convert ing head in meter to pressu re in kg /cm2

p = 0.1 h (2)where

h = head (m)

p = pressure (kg/cm2)

Convert ing head in feet to pressure in ps i

p = 0.434 h (3) where

p = pressure (psi)

h = head (ft)

= specific gravity

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Converting Pressure to Head

Since pressure gauges often are calibrated in pressure - psi or bar, it

may be necessary with a conversion to head - feet or meter, commonused in pump curves.

Convert ing pressure in kg/cm2to head in meter

h = p 10 / (4)

where h = head (m)

p = pressure (kg/cm2

)

Convert ing pressure in bar to head in meter

h = p 10.197 / (5)

where h = head (m)

p = pressure (bar) Convert ing pressure in psi to h ead in feet

h = p 2.31 / (6)

where h = head (ft)

p = pressure (psi)

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Pumps in Serial - Heads Added

When pumps are arranged in series, their resulting pump

performance curve is obtained by adding heads at the same

flow rate.

For two identical pumps the head will be twice the head of a

single pump at the same flow rate.

Two Pumps in Series

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Pumps in Parallel - Flow Rate Added

When pumps are arranged in parallel, their resulting

performance curve is obtained by adding their flow rates at the

same head.

For two identical pumps the flow rate will be twice the flow

rate of a single pump at the same head

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Pump Types

Pumps

Positive Displacement Centrifugal

Reciprocating Rotary

Plunger/Piston

Diaphragm

Lobe/Gear

Vane

Gear

Screw

Radial Axial Mixed Flow

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Reciprocating Pump Types

Single Acting piston pump

discharge curve

Simplex Double Acting piston

pump discharge curve

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Reciprocating Pumps Flow

Characteristics

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Pulsation Dampeners

A pulsation damper absorbs only that portion of piston displacement

above mean flow, and then stores it momentarily before discharging it

during the portion of the cycle below mean flow (on the suction stroke).

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Why?

Reduce high pressure fluctuations in the piping system that caneither overpressure or fatigue components in the piping system

Attenuate the frequency of the pressure pulsations produced by thepump

Eliminate relief-valve chatter

Provide relatively steady flow if large variations cannot be tolerated

by the process Reduce acceleration head and friction losses to maximize NPSHAwhen installed as a suction stabilizer in the suction piping

Reduce pump brake horsepower

Minimize check valve wear

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Dampener Working Principle

Half way through the pistons forward travel (discharge stroke), fluid

velocity between the discharge check valve and the pulsationdamper begins to decay.

The corresponding drop in pressure causes the membrane inside thedamper to expand, since the internal gas pre-charge pressure is nowhigher than the line pressure.

The (stored) fluid now being displaced by the pulsation damper

maintains velocity downstream of the damper thereby reducing, if noteliminating, any downstream pulsations.

A pulsation damper removes pulses only from the line downstream ofthe dampernot upstream.

Discharge damper installations to be made as close to pumpdischarge nozzles as possible.

In an application of a damper for suction stabilization (reduction ofacceleration head losses) it is the velocity gradient between thesupply vessel and the suction nozzle that is minimized.

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Bladder Type Pulsation Dampener

Single liquid connection at one

end of the metal housing A gas-charging valve at the other

end with the bladder between.

The gas volume acts as a spring,compressing and expanding tomeet liquid system pressure

changes. Shown: charging valve, bladder in

the middle of the bottle, the anti-extrusion button, the port,Gas/liquid phases.

The charging must be made with atmospheric pressure at the liquid

port.

The pre-charge should be set between 50 percent and 70 percent of the

system pressure.

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Diaphragm Pumps

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Wilden T2 - 25 mm (1") Metal Pump FLOW RATE

Rubber/TPE 133 lpm(35 gpm) Teflon 95 lpm(25 gpm)

MAX PRESSURE 8.6 bar (125 psig)

MAX. SOLIDS PASSAGE

3.2 mm (1/8")

MAX. SUCTION LIFT (wet): Rubber/TPE 9.5 m (31.0')

Teflon 9.5 m (31.0')

(dry): Rubber/TPE 5.2 m (17.0')

Teflon 1.8 m (6.0')

SPECIFICATIONS Height: 279 mm (11.0")

Width: 268 mm (10.5")

Depth: 184 mm (7.3")

Air Inlet: 6 mm (1/4" FNPT)

Liquid Inlet: 25 mm (1")

Liquid Outlet: 19 mm (3/4")

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Flow Curve

To pump 394 lpm against a discharge pressure head of 1.4 bar requires 4.1 bar and 102

Nm3/hr air consumption. Dot on chart represents the plotted intersection and the circled

numbers are the air pressure and volume figures.

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Installation

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Discharge regulation

Limiting the volume and/or pressure of the air supply to the pump.

An air regulator is used to regulate air pressure. A needle valve isused to regulate volume.

Throttling the pump discharge by partially closing a valve in thedischarge line of the pump. This action increases friction loss whichreduces flow rate (control the pump from a remote location).

When the pump discharge pressure equals or exceeds the air supply

pressure, the pump will stop; no bypass or pressure relief valve isneeded, and pump damage will not occur.

The pump has reached a deadhead situation and can be restartedby reducing the fluid discharge pressure or increasing the air inletpressure.

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Surge Dampening

As the reciprocating pump begins its stroke, the liquid discharge pressureincreases and flexes the diaphragm inward, accumulating fluid in theliquid chamber.

When the pump redirects its motion, the liquid discharge pressuredecreases allowing the diaphragm to flex outward displacing the fluid intothe discharge line

A compressed air line attached to the air side of the air regulator bodysets and maintains pressure on the air side of the diaphragm.

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Efficiency of Reciprocating Pump

There are only two efficiency losses Volumetric and Mechanical.

Volumetric efficiency loss is induced by slippage through valves,

ratio of liquid chamber volume at end of stroke to plunger/piston

displacement volume, and liquid compressibility.

Mechanical efficiency loss occurs while overcoming mechanical

friction in bearing and speed reduction.

The overall efficiency of a reciprocating pump unit is generally above85 percent throughout its full operating range.

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Rotary Pumps

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Internal Gear Pumps Advantages Two moving parts

One stuffing box Positive suction, non-pulsating discharge

Ideal for high viscosity liquids

Constant and even discharge regardless of varying

pressure conditions

Low NPSH required

Easy to maintain

Disadvantages

Low speeds usually required

Medium pressure One bearing runs in pumped product

Overhung load on shaft bearing

Internal gear pumps are ideal for hig h-v iscos ity l iquid s, bu t they

are damaged w hen pump ing large sol ids .

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External Gear Pumps

Advantages

High speed

Medium pressure

No overhung bearing loads

Relatively quiet

Design lens itself to use of a wide variety of materials Disadvantages

Four bushings in liquid area

Four stuffing boxes

No solids allowed

External gear pum ps (shown is a

double pump) are typical ly used for

h igh -pressure appl icat ions such as

hydraul ics

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Lobe Pumps

Advantages

Pass medium solids

High acceptance

Little galling (frictional wear) possibility

Disadvantages

Timing gears More space required

May require factory service to repair

Two seals

Lobes in lobe pum ps do not m ake contact ,

because they are dr iven b y external tim ing g ears

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Sliding Vane Pumps

Advantages

Medium capacity Medium speed

Thin liquids

Sometimes preferred for solvents, LPG

Can run dry for short periods

Can have one seal or stuffing box Develops good vacuum

Disadvantages

Can have two stuffing boxes

Medium pressure

Complex housing

Not suitable for high viscosity

Not good with abrasives

Vane pumps have better dry pr iming

capabi l i ty than other pos it ive

displacement pumps.

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Rotary Pump Applications

Pump Selection Guide

Abrasives Thin Liquids Viscous Solids Dry PrimeDiff.

Pressure

Internal Gear G G E P A G

External Gear P G G P A E

Lobe G A E E A G

Vane P E A P G A

E = Excellent, G = Good, A = Average, P = Poor

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Rotary Lobe Pumps

Attributes include:

Gentle transfer of delicate

suspended solids.

Bi-directional operation.

Compact size with high

performance and lowenergy input.

Ability to pump shear

sensitive media.

Easy maintenance.

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EOU Dryer Feed Pump

FEED PUMPS 61P1, 62 P1, 63 P1

Make SRU2/018/HS withAINSI B16.5 Flange

Impeller Dia Trilobe

Motor Make Siemens

Motor rating 1.5kW/2 HP

Motor rpm 1420 rpm

Rated rpm 200 RPM

Fluid Sp Gr 1.3

Fluid temperature 30 C

Shanti Gear Box Type F20-YU and 2 HPND 90L

Coupling Guard SS 304

Plain Base Frame SS 304

H= Vertical

orientation

S= SS shaft

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Pump System Design

Confirm the Net Positive Suction Headrequirements of the pump (NPSHr) are met by thesystem, as this is crucial for ensuring the smoothoperation of the pump and preventing cavitation.

Avoid suction lifts and manifold/common suction

lines for two pumps running in parallel, as this maycause vibration or cavitation.

Protect the pump against blockage from hard solidobjects e.g. nuts, bolts etc.

Also protect the pump from accidental operation

against a closed valve by using one of the followingmethods: - relief valves, pressure switch, andcurrent limiting device.

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Slip

Slip is the fluid lost by leakage through the pump clearances.

The direction of slip will be from the high pressure to the low

pressure side of the pump i.e. from pump outlet to pump inlet.

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Factors Affecting Slip

The amount of slip is dependent upon

Pressure

Clearance

Viscosity.

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Pump Characteristic and System Curve

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Pump Performance Curve

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Net Positive Inlet Pressure (NPIP)

NPIPA = Pa +/- Pz - Pf - PvpPha

where NPIPA = net positive suction head available, kPa

Pa = absolute pressure working on the fluid in the suctionvessel or sump, kPa

Pvp = vapor pressure of the fluid being pumped at the

pumping temperature, kPa Pz = static height above (+) or below (-) centerline of the

cylinder, kPa

Pf = piping friction loss, m(based on equivalent length) kPa

Pha = acceleration head, m (based on actual length) kPa

NPIP Required (NPIPR) is a function of pump type, speed andviscosity of fluid pumped.

NPIPA must always be greater than NPIPR to preventcavitation.

NPIPR values published by manufacturers are expressed in

kPa units.

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NPIP- Rotary Pumps

Rotary pumps are often selected to move liquids with a low vapor

pressure point, or fluids with a lot of entrained bubbles. NPIP required (NPSH) is difficult to test and is established at the

point where

Cavitation noise is heard.

A 5% reduction in capacity at constant differential pressure and

speed A 5% reduction in power consumption at constant differential

pressure and speed.

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Flow Regulation

Discharge throttling

Not Possible Suction throttling

Not possible Recycle control

Efficient method of control for PD pumps. The flow rate constant, the power requirement is roughly

proportional to discharge pressure. The effect of recycle is to drop the discharge pressure, it results

in reductions in power requirement. Power wasted is in proportion to discharge pressure times

recycle flow. Speed Control Stroke Adjustment

C t if l V P iti

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Centrifugal Versus Positive

Displacement Effect of Viscosity

C i

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ComparisonParameter Centrifugal Pumps Reciprocating Pumps Rotary Pumps

Optimum Flowand PressureApplications

Medium/High Capacity,Low/Medium Pressure

Low Capacity,High Pressure

Low/Medium Capacity,Low/Medium Pressure

Maximum FlowRate

20000+ m3/h 2000+ m3/h 2000+ m3/h

Low Flow RateCapability

No Yes Yes

MaximumPressure

400+ kg/cm2 7000+ kg/cm2 250+ kg/cm2

Requires ReliefValve

No Yes Yes

Smooth orPulsating Flow

Smooth Pulsating Smooth

Variable orConstant Flow

Variable Constant Constant

Self-priming No Yes Yes

SpaceConsiderations

Requires Less Space Requires More Space Requires Less Space

Costs Lower Initial

Lower MaintenanceHigher Power

Higher Initial

Higher MaintenanceLower Power

Lower Initial

Lower MaintenanceLower Power

Fluid Handling Suitable for a wide rangeincluding clean, clear, non-abrasive fluids to fluids withabrasive, high-solid content.Not suitable for highviscosity fluidsLower tolerance forentrained gases

Suitable for clean, clear, non-abrasive fluids. Specially-fittedpumps suitable for abrasive-slurry service.Suitable for high viscosityfluidsHigher tolerance for entrainedgases

Requires clean, clear, non-abrasive fluid due to closetolerances

Optimum performance with highviscosity fluidsHigher tolerance for entrainedgases

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Liquid Ring Vacuum pumps

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Operating Principle

An impeller (A) is mounted eccentric toa round pump body (B) and enclosedbetween two endplates.

As the body is partially filled withservice liquid before operation, theimpeller rotation creates a liquid ring(C) concentric to the body by the actionof centrifugal force.

As the cavities pass the suction port,

the volume of liquid within themreduces, and this in-turn creates anegative pressure. This acts as thepumps suction, drawing more gas fromthe vessel to be evacuated.

As the impeller continues to rotate,these cavities become filled with theliquid ring, and the entrained gas is

compressed. As rotation continues past thedischarge port, all gas and someservice liquid is forced out of the pump.

The liquid ring can be regeneratedthrough the use of a separator (D).

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Design Data

Inlet pressure, usually expressed in mm Hga

Inlet temperature Mass flow rate, usually expressed in kg/hr and the molecular weight

of fluid components

Vapor pressure data for each fluid component

Seal fluid data, if other than water: specific gravity, specific

Heat, viscosity, thermal conductivity, molecular weight and vaporpressure data

Temperature of the seal fluid or cooling water

Discharge pressure, usually expressed in kg/cm2

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How To Avoid Cavitation

Use a colder seal fluid.

Use a seal fluid with a lower vapor pressure. The seal fluid shouldbe compatible with the process gas mixture and the materials ofconstruction.

Increase the inlet operating pressure beyond the range ofcavitation.

Check the loading and operating pressure. If the load is less than

design, a vacuum relief valve or air bleed valve can be used tointroduce additional load so the pump will be able to operate closerto the design pressure.

Install a booster upstream of the liquid ring vacuum pump tocompress and raise the absolute pressure of the vacuum pumpsuction.

An ejector works by using pressure energy of the motive toincrease the velocity of the entrained gas load. This produces ahigher absolute pressure at the outlet of the ejector.

Increase the throughput of seal fluid, if possible.