Variable Speed Pumps Am

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VARIABLE SPEED PUMPS APPLICATION IN WATER SYPPLY SYSTEMSAjjampur Ramadas (advisor- power), GMR

Synopsis:It is very uncommon that the pumps, usually operating at constant speed, areoperated at their duty point, i.e. at best efficiency point, because of the variationsof the system demand and thus utilizing excess amount of energy. As the latter isnot in the control of the designer, one of the optimum ways to run the pumps atthe best efficiency point or closer to it is to go in for variable speed pumps. Thedetails of the same are explained in the paper.

1.Introduction:In a number of installations, large capacity pumps are normally envisaged withalmost 100 % standby where availability and reliability are very important, such aspublic water distribution system, large capacity power plants, etc. The pumpsmay be of horizontal type or vertical type driven by constant speed inductionmotors. The capacity of each pump will be usually selected in such a way as tohave more no. of pumps from the point of availability as well operation flexibility.As such, the no of pumps could be two (50 %) or three (33%) or four (25 %) oreven five ( 20 %) or even more also. Correspondingly the required no ofstandby pumps are also selected. It is usual practice to bring in or cut off thenumber of pumps in operation depending upon the water demand in the system,thus straining the system. Further, since these pumps are usually provided withbutterfly valves on the discharge side, these valves are used to throttle, thusintroducing additional resistance (and thus wasting electrical energy ) to keep thepumps operating in its safe operating zone. Is there any solution to all these ? YES. That is variable speed pumps.

2. Discussion:a) There are basically two types of water supply system:

i)Storage type


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ii)Direct feed typeb) In the storage type, pumps supply the water to an overhead tank situated

at a suitable elevated spot or storage reservoir located at ground level (maybe partly protruding above ground). The pumps are started and stoppedaccording to the levels in the tank/reservoir. In this arrangement, thepumps are operating reasonably at their duty point, but they are subjectedto frequent starts and stops, as per the levels in the tank/reservoir.

c) In the other type, i.e. direct feed type, the pumps feed directly to thedistribution system and their operation is governed by the pressure in thepump discharge pipes. When there is a heavy demand, the pressure inthe system drops. Drop in pressure will be sensed and energises thepreselected pump/s to start. When there is no demand and pumpscontinue pumping the water, the pressure builds up in the pipe system.Once again sensed by the pressure switches, the operating pumps will bestopped.

d) In both these types of systems, the pumps have to be frequently startedand stopped or sometimes throttled (with the help of discharge valves ororifice pates) to meet the demand. However, both the systems, ifoperated with constant speed pumps (which is the normal practice), wouldhave the following drawbacks:i)For satisfactory performance, it is preferable to normally restrict the

pump operation within +/- 15 % of its design duty point and alsolimit the number of start and stop operations to the minimum.However, this calls for increased number of pumps resulting inrelatively high initial investment besides the high operating andmaintenance costs.

ii)The frequent starts and stops of the pumps induce considerablestresses in the various components of the system due to pressurefluctuations as a result of transient condition (This calls for accuratedesign of pressure surge system to be incorporated in the pumpdischarge side).


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iii)

iv)

Excessive power consumption as the pumps will be operated underthrottled condition or inefficient condition (i.e. operating away fromtheir design duty point).While starting and stopping of the pumps, associated pipe fittingslike check valves, butterfly valves, etc. are subjected to wear andtear, as they are also operated with every start and stop of the pump.

3. Pump parameters:One of the most important factors, that ensures the satisfactory operation andlong life of a pump set, is that it should always be operated at its best efficiencypoint (b.e.p.). Quite often, this is not achieved due to various reasons such as

i) Selection of pump will be such that operating condition is not at itsb.e.p. This could be due to the decision governed by severalconsiderations such as less (attractive) initial cost, to meet urgentemergency situation buying from off shelf itself.Pump parameters decided conservatively like adding margin todischarge capacity and calculating the friction loss based on the old pipebasis, (while actually the system being new) and further padding up thisby adding certain margin for unknown contingencies.

The result will be that the pump (duty conditions) parameters will bedifferent from the actual system requirements. Due to this, in practiceone has to introduce additional resistance in the system either bythrottling the valve or introducing orifice plate in the system for the safeoperation of the pump and to meet the system demand, resulting inunnecessary excessive power consumption.

4. Throttling OperationThe essential function of a valve or an orifice is to dissipate the energyrepresented by the difference between the pressure head of the pump and therequired head of the system at the reduced flow which is the actual requirementof the system. Further, at the design duty condition, the pump operates at or

ii)


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near its peak efficiency, while at other points, the efficiency falls appreciably, asexplained in the figure 1.From the figure 1, it is observed that system resistance curve intersects the pumpcharacteristic curve at point A, indicating the pump will be pumping flow Q1,

which is higher than required Q (duty point). In order to obtain designed Q,it isnecessary to introduce the additional resistance by throttling the valve orintroducing orifice plate. Thus unnecessarily, the excess amount of power (i.e.Q x HT/ efficiency of pump) is utilised. Further, we are subjecting the valve or orifice to wear and tear thereby more maintenance.

5. Variable Speed Pumpa) The above problem could be overcome by adopting variable speed pumps.

In a variable speed pump, basic parameters like capacity (Q), head (H),andpower (P) are directly proportional to the speed (N) of the pump, as below:

Q directly proportional to N,H proportional to square of N, i.e. N2P proportional to cube of N, i.e. N3

Without throttling of the valves or introducing any orifice plate, thereduced outputs are obtained by reducing the pump speed. If the systemresistance does not have any static head component, instead only frictioncomponent, the variable speed pump becomes an ideal choice to obtainreduced outputs, as the operating point moves along the best efficiencypoint (as the specific speed remains reasonably constant ) as indicated infigure 2. However, otherwise (i.e. system resistance with static headcomponent), the operating point, with reduced speed, will be near the bestefficiency of the pump.

b) A typical comparison of electrical input for the two types of volume controlis shown in figure 3. It can be seen that over the entire normal operatingrange, the variable speed system required significantly lower power thanthat of the constant speed drive pump.


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c) Certain important criteria, while selecting variable speed pump in a system,are :i)If the system resistance consists of mainly static height component

and negligible friction component, (like pumping directly to anoverhead tank located close to the pumping plant), then it becomesnecessary to procure a pump having steep slope H-Q characteristiccurve. This enables the system to have a wide range of speedreduction and corresponding flow variation.

ii)The variable speed application offers maximum advantage if thepump mainly has to overcome friction losses and relatively smallstatic elevation.

iii)Due to possible occurrence of partial thrusts at reduced flows, thethrust bearing of such variable speed pumps will have to be designedwith a large service factor.

iv)The preferred range of speed variation normally 40 to 100 %(Sometimes it is possible to go beyond 100 %, provided there is amargin in the rated capacity of the motor.)

d) The construction features of a variable speed pump will be same as that ofa constant speed pump. While the constant speed pumps are driven byconventional squired cage induction motors, the variable speed pumps aredriven either by slip ring type or wound rotor type motors. The choice ofcontrols normally adopted for variable speed pumps could be of solid state(a.c. or d.c.) or electro mechanical (eddy current type or wound rotor type).It is possible to have speed variation by using mechanical systems likerubber belt type or metal chain (using different pulley diameter) orhydraulic couplings.

6. ConclusionAlthough variable speed drive offers advantages such as improved flow controland economy in operation, it calls for higher initial investment (sometimestwice or even more than that of conventional sets), adequate facilities and of


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course competent personnel for maintenance. As such, the importance ofgiving due consideration to selection of the type of drive for any specificapplication cannot be over emphasized.

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The figures are enclosed below.


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