SelectionofPumpsBasedonEfficiencyLoadingforEvaluationofBids

7/28/2019 SelectionofPumpsBasedonEfficiencyLoadingforEvaluationofBids

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Selection of Pumps Based on Efficiency Loadingfor Evaluation of Bids

Introduction:- Designing of pumps for water supply schemes are usually based on total pumping quantity & totalhead as per system head curve. Accordingly, procurement of pumps is being carried out, in various configurationsof working & standby arrangement for discharge & DPH as computed and evaluating Best Efficiency Point (BEP) ofvarious offers & loading thereon, through tendering process, asking the bidders to provide pumps with fairlyconstant efficiency curve, within the head range of minimum & maximum levels of sump and HSR.

Do the pumps so selected operate at DPH and give optimum performance?Do we verify the fairly constant efficiency characteristics c laimed by bidder?

The exercise in the subject matter is to develop a model procedure, which validates and insure accurately; theoperating system it represents and simple enough as well as practicable for implementation. The pumps all over

the world consume about 50 % of electrical energy generated, and it is very much essential to have uniform normfor proper selection of pumps with optimum performance.

Facts:- This assumption clearly indicates the Duty Point Head for pump so chosen is for parallel operation ofpumps (No of working in the design data).

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The actual operating head varies between head range due to one or all of the following parameters:-

1) Partial pumping (less than total no of working pumps as per design)

2) Level variations of sump and HSR/GSR/MBR/BPT

3) Variations in power supply frequency.

In the tender conditions, the efficiency variation within this head range is not clearly defined quantitatively, such as

percent of efficiency at BEP; rather it is mentioned as fairly constant.

Accordingly we rely on the graphical representation of characteristic curves only. The actual operating point ofpumps fluctuates within the head range; hence the loading of efficiency and acceptance of pumps for economicperformance, on the presumption of operation at DPH & BEP seldom fails, considering the actual operation.

The discharge & head at which pumps will operate will be decided by the system resistance curves. There will bedifferent system resistance curves for different levels since static head will vary. If we superimpose pumpperformance curves of these system resistance curves, then the operating range becomes clear. If we considerminimum, normal & maximum levels then intersection points of pump performance curves with maximum &minimum water level decide the operating range (for single as well as parallel operation). Now the levelsconsidered for this exercise, are reasonable approximation of actual pattern over the years.

Field Conditions:- For Raw Water Pumps, weir level vary as per discharge from upstream dam/reservoir, it isobserved that 10 % of the time the levels are low, 70 % of the time the levels are normal and 20 % of the time thelevels are maximum over the year period

For Pure Water Pumps, the sump water level varies hourly and it is observed that out of 24 hours; 5 hours levelsare lowest, 5 hours levels are highest and about 14 hours levels are normal. Same is the case forHSR/GSR/MBR/BPT.

Solution:- The most important point is that the correct assessment of bids for proper selection of pumps as peractual field condition by incorporating efficiencies in this operating zone and verification of optimum performance ofpumps over the entire head range.

The practical limitations from manufacturers point of view, for fairly constant performance of pumps within theentire head range are also need to be considered, due to manufacturing constraints, such as the impellers are cast& due to minor variations in the suction angle, discharge angle & the vane configurations, the efficiencies may varyfor each cast impeller. Hence, by making some adjustments, to maintain Q, H at BEP, they offer guaranteedperformance at Duty Point only. Also the actual performance may vary than the quoted values at high head & lowhead.

Now the relevant IS also stipulates guarantee for duty point only, with or without tolerance. In my opinion, the timehas come to adopt strict control over specifications/ranges/tolerances as a buyer, in view of scarcity of electricityand implementation of energy conservation.

One more point relating to the subject issue, generally over sighted by presuming that, for a given duty point all the

bidders (assuming various make of pumps) will have offered pumps having specific speed almost identical or in thenearby range and in consideration to this, the efficiency variation between high head, low head & duty point will bealmost similar and hence the current practice of bid evaluation by using only duty point efficiency will be trueindication of actual power consumption. It is to note here that without the standardize procedure to verify thecommitted figures of Q, H & for entire operating range quantitatively, one may quote for higher duty pointperformance by offering the pump with higher specific speed at par the cost by other, who may offer slightly lowerduty point performance but fairly constant performance in entire head range.


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The practical solution for loading of efficiency for optimum performance & competitive comparison and toaccommodate the commitment of fairly constant efficiency characteristic by bidders to achieve the proper selectionof pumps, I would like to suggest the loading procedure as follows:-

Effective Efficiency Calculation:-

A) Raw Water Pumps:-

For raw water pumps, we should propose loading of efficiency by evaluating effective efficiency for completehead range as 20% of lowest head efficiency plus 70% of BEP condition plus 10% of highest head efficiency.

effective=(20%lowhead+70%DPH +10%highhead)

B) Pure Water Pumps:-

For pure water pumps, we should propose loading of efficiency by evaluating effective efficiency for completehead range as 20% of lowest head efficiency plus 60% of BEP condition plus 20% of highest head efficiency.

effective=(20%lowhead+60%DPH +20%highhead)

The efficiency of the motor remains fairly constant in above loading conditions. The sample calculations by takingfigures from the quoted values by bidders with present loading & proposed loading is tabulated below.

Location Raw Water Pumps

Efficiency

Quoted

Low head DPH High head

Loading 20% 70% 10%

EffectiveEfficiency

of pump

MotorEfficiency

OverallEfficiency

Bidder 1 0.79 0.872 0.81 0.8494 0.92 0.7814

Bidder 2 0.842 0.863 0.823 0.8548 0.925 0.7907

Bidder 3 0.835 0.86 0.825 0.8515 0.92 0.7834

Overall Efficiency by Current Method:-

Bidder 1:- 0.872 X 0.92 =0.8022

Bidder 2:- 0.863 X 0.925 =0.7983

Bidder 3:- 0.86 X 0.92 =0.7912

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Location Pure Water Pump

EfficiencyQuoted

Low head DPH High head

Loading 20% 60% 20%

EffectiveEfficiencyof pump

MotorEfficiency

OverallEfficiency

Bidder 1 0.79 0.872 0.81 0.8432 0.92 0.7757

Bidder 2 0.842 0.863 0.823 0.8508 0.925 0.7870

Bidder 3 0.835 0.86 0.825 0.8480 0.92 0.7802

Overall Efficiency by Current Method:-

Bidder 1:- 0.872 X 0.92 =0.8022

Bidder 2:- 0.863 X 0.925 =0.7983

Bidder 3:- 0.86 X 0.92 =0.7912

Considering the existing system of loading we may consider the pump from bidder 1 as highest efficiency; howeveras per modified approach to suit actual operating condition the pumps from bidder 2 will give optimumperformance. The financial loading & present worth may be calculated based on effective efficiency so arrived.

I have one more suggestion that there should be bonus for higher efficiency at site & penalty for lower efficiency atsite, also sustenance of efficiency over prolonged period must be considered, rather initial efficiency. Otherwise,

first few months, Pumps operate at good efficiency but later the efficiency drop is very fast. This should nothappen. Worldwide there is trend of sustained efficiency. This may result into lower initial efficiency but forprolonged period efficiency will not deteriorate.

Dipak D. DahakeAssistant Engineer (E&M)

MIDC, E&M Division, MahadEmail: [email protected]
mailto:[email protected]:[email protected]

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SelectionofPumpsBasedonEfficiencyLoadingforEvaluationofBids