EC Pumps-R

7/30/2019 EC Pumps-R

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ENERGYCONSERVATIONI

N PUMPS

Presented By

M.V.PandeDy.Director

NPTI, Nagpur


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INTRODUCTIONPumps provide the motive force to move liquidsagainst the resistance of a piping system andagainst changes in elevation.

Pumping systems account for nearly 20% of theworlds electrical energy demand.Furthermore, they range between 25-50% of theenergy usage in certain industrial plant

operationsA centrifugal pump is one of the simplest piecesof equipment in any process plant.


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WATER PUMPS IN POWER STATION


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BOILER FEED WATER PUMPBoiler feed pump is the major power consumer

among all power consuming equipment in thepower plant. BFP may constitute about 25% of thetotal auxiliary power consumption.

BFP Main Parameters for 210 MW unit

Model SpeedRPM

Disch. headmWC

CapacityTPH

Motor Power kW

200 KHI4320

1834.6 430 4000

FK 6D 30 5050 2104 398 3500


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BFP LAYOUT

BFP LAYOUT


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Centrifugal Pumps

The purpose of acentrifugal pump is tofirst convert energy ofa prime mover, such asa electric motor orturbine, into velocity orkinetic energy,

Then this isconverted further intopressure energy of afluid that is being

pumped .


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Pumping System Characteristics

Static HeadStatic Head vs. Flow

HEAD LOSSTo transfer a liquid from a source to a required destination, or to circulate liquid around a system,

pressure is needed to make the liquid flow at the required rate. This must overcome the head losses in the system. Losses are of two types, static and friction head.

STATIC HEAD Static head is simply the difference in height of the supply and destination reservoirs

and is independent of flow.


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Dynamic (Friction) Head Friction head is the head required to overcome the resistance to flow in thepipe and fittings.

It is dependent upon the size, condition and type of pipe, number and type ofpipe fittings, flow rate, and nature of the liquid.

The friction losses are proportional to the square of the flow rate.A closed loop circulating system without a surface open to atmosphericpressure, would exhibit only friction losses and would have a system frictionhead loss vs. flow curve.

Friction Head vs. Flow


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System With High Static Head


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System With High Static Head


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Pump CurveThe performance of a pump can beexpressed graphically as head against flow

rate.

The centrifugal pump has a curve where

the head falls gradually with increasing flow.


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Pump Operating Point

The operating point of the pump isdetermined by the intersection of thesystem curve and the pump curve

A pump application might need tocover several duty points, of whichthe largest flow and/or head willdetermine the rated duty for the pump.

For a centrifugal pump, an increasingsystem resistance will reduce the flow,but the maximum head is limited


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Pump Suction Performance (NPSH)

Cavitation may occur when the local static pressure in a fluid reach a levelbelow the vapor pressure of the liquid at the actual temperature. This mayhappen when the fluid accelerates in a control valve or around a pumpimpeller.The vaporization itself does not cause the damage. Instead the damagehappens when the vapor almost immediately collapses after evaporation

as velocity is decreased and pressure increased.There are three undesirable effects of cavitation:1. The collapsing cavitation bubbles can erode the vane surface, especially

when pumping water-based liquids.2. Noise and vibration are increased, with possible shortened seal and

bearing life.

3. The cavity areas will initially partially choke the impeller passages andreduce the pump performance. In extreme cases, total loss of pumpdeveloped head occurs.

The value, by which the pressure in the pump suction exceeds the liquid vapour pressure, is expressed as a head of liquid and referred to as net positive suction head available, NPSHA.


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Pump Performance Curve


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Hydraulic Power, Pump Shaft Powerand Electrical Input Power

Pump output/Hydraulic power is the liquid horsepower deliveredby the pump

Hydraulic power P h = Q (m3 /s) x Total head, h d - h s (m) x (kg/m

3) x g (m 2 /s)1000

Where hd

- discharge head, hs suction head, - density of the fluid, g

acceleration due to gravity

Pump shaft input (Ps) is the actual power delivered to the pumpshaft

Pump shaft power Ps = Hydraulic power, P

hpump efficiency, Pump

Electrical input power = Pump shaft power P

Motor


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Typical Pump Characteristic Curves This figure illustrates a typical vendor-supplied head-flow curve for acentrifugal pump. Pump head-flow curves are typically given for clearwater. The choice of pump for a given application depends largely on how thepump head-flow characteristics match the requirement of the systemdownstream of the pump.


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Affinity LawsFor a centrifugal pump, there is a relationship between the impellerperipheral velocity and the generated head. Peripheral velocity is directlyrelated to shaft rotational speed, for a fixed impeller diameter.Subsequently varying the rotational speed has a direct effect on theperformance of the pump.All the parameters like head, efficiency, power etc. will change if the speed

is varied.Therefore, it is important to have an appreciation of how these parametersvary in order to safely control a pump at different speeds.The equations that relate roto-dynamic pump performance parameters of

flow, head and power absorbed, to speed are known as the affinity laws.They say that flow rate is proportional to the rotating speed; head is

proportional to the square of the rotating speed; and that power isproportional to the cube of the rotating speed.

It is relevant to note that flow control by speed regulation is always moreefficient than by a control valve.


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Flow vs Speed

If the speed of the impeller is increasedfrom N 1 to N 2 rpm, the flow rate willincrease from Q 1 to Q 2 as per the givenformula:

Flow: Q1 / Q2 = N1 / N2

Example: 100 / Q 2 = 1480/740

Q 2 = 50 m3 /hr

The affinity law for acentrifugal pump withthe impeller diameterheld constant and thespeed changed:


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Head Vs speedThe head developed(H) will be

proportional to the square of thequantity discharged, so that

Head:

H1/H2 = (N1 2) / (N2 2)Example: 100 /H 2 = (1480 2) / (740 2)

H 2 = 25 m


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Power Vs Speed

The power consumed(W) will bethe product of H and Q, and,therefore

Power(kW): kW1 / kW2 = (N1 3) / (N2 3)

Example: 50 /kW 2 = (14803

) / (7403

)kW 2 = 6.25 As can be seen from the affinity laws, reducing the speed by half of thecentrifugal pump, will reduce the power consumption by 8 times. This forms the basis for energy conservation in centrifugal pumps withvarying flow requirements .


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Energy Efficiency

Opportunities


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Avoiding over sizing of pump

ENERGY CONSERVATION OPPORTUNITIES

Head

Head

Partiallyclosed valve

Const. Speed

A

B

C

Meters

Pump Efficiency 77%

82%

Pump Curve at

Full open valveSystem Curves

Operating Points

500300

50 m

70 m

Static

42 m

Flow (m 3/hr)

Oversize Pump

Required Pump


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1.Avoiding Over sizing of Pump- contd.

Efficiency Curves

28.6 kW

14.8 kW


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If we select impeller E, then the pump efficiency is 60%

Hydraulic Power = Q (m 3 /s) x Total head, h d - h s (m) x (kg/m 3) x g (m 2 /s)1000

= (68/3600) x 47 x 1000 x 9.811000

= 8.7 kW

Shaft Power - 8.7 / 0.60 = 14.5 kW Motor Power - 14.8 / 0.9 = 16.1 kW

(considering a motor efficiency of 90%)


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If we select A, then the pumpefficiency is 50%

Hydraulic Power = Q (m 3 /s) x Total head, h d - h s (m) x (kg/m 3) x g (m 2 /s)1000

(68/3600) x 76 x 1000 x 9.811000

= 14 kW

Shaft Power - 14 / 0.50 = 28 kWMotor Power - 28 / 0.9 = 31 kW (considering a

motor efficiency of 90%)


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1.Avoiding Over sizing of Pump- contd.

As shown in the drawing, we should be using impeller "E" todo this, but we have an oversized pump so we are using thelarger impeller "A" with the pump discharge valve throttledback to 68 cubic meters per hour, giving us an actual head of

76 meters. Hence, additional power drawn by A over E is 31 16.1 = 14.9 kW. Extra energy used - 8760 hrs/yr x 14.9 = 1,30,524 kw.

= Rs. 5,22,096/annumIn this example, the extra cost of the electricity is more than the costof purchasing a new pump.


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2.Using variable speed drives (VSD) forvariable flow

Most existing systems that require flow control make use of bypass lines, throttlingvalves, or pump speed adjustments. As mentioned earlier, the most efficient of theseis pump speed control. When a pumps speed is reduced, less energy is imparted tothe fluid and less energy needs to be throttled or bypassed.There are two primary methods of reducing pump speed : Multiple speed pumpmethod & variable speed drive. We are looking closer at the variable speed drive, alsocalled VSD.

VSDs allow pump speed adjustments over a continuous range and avoid the need tojump from speed to speed as with multiple-speed pumps.The major advantages of VSD application in addition to energy saving are :First, improved process control. By matching pump output flow or pressure directlyto the process requirements, small variations can be corrected more rapidly by a VSDthan by other control forms, which improves process performance.Secondly, improved system reliability: Any reduction in speed achieved by using a

VSD has major benefits in reducing pump wear, particularly in bearings and seals.Furthermore, by using reliability indices, the additional time periods betweenmaintenance or breakdowns can be accurately computed.Third, reduction of capital and maintenance costs: Offset the capital cost of a VSD byeliminating control valves, by-pass lines, and conventional starters.Fourth, soft starter capability: VFDs allow the motor the motor to have a lower startupcurrent.


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Effect of speed variation

P fl l b i


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Pump flow control by varyingspeed: Pure friction head

Reducing speed inthe friction loss

system moves theintersection point onthe system curvealong a line of

constant efficiencyThe affinity laws areobeyed

P fl l b i


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Pump flow control by varyingspeed: Static + friction head

Operating point for the pumpmoves relative to the lines of constant pump efficiency whenthe speed is changed

The reduction in flow is no longerproportional to speedA small turn down in speed couldgive a big reduction in flow rateand pump efficiencyAt the lowest speed illustrated,(1184 rpm), the pump does notgenerate sufficient head to pumpany liquid into the system


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3.Reducing impeller diameter

Changing the impeller diameter gives a proportional change inperipheral velocityDiameter changes are generally limited to reducing the diameterto about 75% of the maximum, i.e. a head reduction to about 50%Beyond this, efficiency and NPSH are badly affectedHowever speed change can be used over a wider range withoutseriously reducing efficiencyFor example reducing the speed by 50% typically results in areduction of efficiency by 1 or 2 percentage points.

It should be noted that if the change in diameter is more thanabout 5%, the accuracy of the squared and cubic relationships canfall off and for precise calculations, the pump manufacturersperformance curves should be referred to

Th ffi i l f if l i h h


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Flow: Q1 / Q2 = D1 / D2 Example: 100 / Q2 = 8/6 Q2 = 75 m 3 /hr Head: H1/H2 = (D1) x (D1) / (D2) x (D2) Example: 100 /H2 = 8 x 8 / 6 x 6 H2 = 56.25 m

Power (KW):kW1 / kW2 = (D1) x (D1) x (D1) / (D2) x (D2) x (D2)Example: 5/kW2 = 8 x 8 x 8 / 6 x 6 x 6 kW2 = 2.1 kW

The affinity law for a centrifugal pump with thespeed held constant and the impeller diameter

changed


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Impeller Diameter Reduction on CentrifugalPump Performance


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4. Impeller Trimming For this option, the following should be considered:

This option cannot be used where varying flow patterns exist.The impeller should not be trimmed more than 25% of the original

impeller size, otherwise it leads to vibration due to cavitation andtherefore decrease the pump efficiency.

The balance of the pump has to been maintained, i.e. the impellertrimming should be the same on all sides.

Changing the impeller itself is a better option than trimming the

impeller, but is also more expensive and sometimes the smaller impelleris too small .

f ll l


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5. Use of Pumps in parallel tomeet varying demand

Particularly for systems wherestatic head is a high proportion of the total, another energy efficient

method of flow control is to installtwo or more pumps to operate inparallel.

Variation of flow rate is achievedby switching on and off additional

pumps to meet demand.The combined pump curve isobtained by adding the flow ratesat a specific head


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Energy Conservation Opportunities inPumping Systems- Summary

Understand system characteristicsAvoid over sizing of pumps

Consider impeller trimming and other easy -to implementalternatives

Consider variable speed drives wherever possibleOperate pumps near best efficiency point.Modify pumping system/pumps losses to minimize throttling.Stop running multiple pumps - add an auto-start for an on-linespare or add a booster pump in the problem area.Conduct water balance to minimise water consumptionReplace old pumps by energy efficient pumps


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