Optimum overhaul of pumps 2014

How to reduce the energy used by your pumps

Ray Beebe Speaker, trainer, author, including:

Predicting maintenance of pump using condition

monitoring(Elsevier, 2004)

World’s most common machine (after motors)

Use 25% of world’s total motor-driven electricity,

….or about 6.5% of global electricity production!!

Pumps

….for pumps:Bearing degradation - Oil sampling & analysis, vibration analysisCasing wear? - NDTMisalignment? - VibrationInternal wear, impeller and seals? - Performance analysis

Vibra tion ana lysis (ro ta ting m ach ines)

Perform ance ana lysis

Ana lysis o f wear partic les and contam inants

V isua l inspection /N D T

E lectrica l p lan t tests

C ondition m onito ring techn iques

Optimise energy usage – good for business AND greenhouse effect.

Optimise energy usage – good for business AND greenhouse effect.

Choose the best mix of techniques to detect and monitor the modes of degradation you expect.

Choose the best mix of techniques to detect and monitor the modes of degradation you expect.

Pump internal wear

• Erosion of

impeller

• Erosion at sealing/ wearing rings

Increased clearance allows recirculation

Pump internal wear

Sealing rings

Ring section diffuser pump

Internal leakage

Pump internal wear

Head

H

Flow Q

Internal leakagerecirculation

H-Q with wear

Pump internal wear

0

5

10

15

20

0 500 1000 1500

Days in service

% R

ed

uct

ion

in h

ead

Head reduction @ datum flow shows cooling water pump degradation (230kW)

Increasing internal leakage reduces Head at chosen

datum flow

Close to linear for 4500kW pump, too

y = -0.155x2 + 0.4907x - 0.1388

-12

-10

-8

-6

-4

-2

0

2

0 1 2 3 4 5 6 7 8 9 10

% r

ed

uc

tio

n in

He

ad

@ d

atu

m

flo

w

Time: years since overhaul

Boiler Feed Pump wear trend

Effect of increased internal wear in centrifugal pumps relates to Specific Speed:

Using data at Best Efficiency Point:

N = Rotation speed, r/min

Q = flow per impeller eye, m³/h

H = head per stage, m

(Number resulting is close to that you get if US units are used) 75.0H

QNN s

0

2

4

6

8

10

12

14

16

18

20

0 1000 2000 3000 4000 5000

Specific Speed (US units)

% Increase in power

Clearances worn to 2X design

Clearances worn to 1.5X design

1 Head-Flow method for CM

At around normal duty point is enough.

Checks condition of pump AND its system.

Repeatable pressure and flow measurement needed, and speed for variable speed pumps.

• Plant DCS etc may work for monitoring: e.g. boiler feed pump

operating H-Q point

DCS use off-line: historian

Boiler Feed Pump

Pressure measurement

Pressure measurement: quick connect couplings for non-hazardous liquids

Flow measurement:

orifice plate

Repeatability can be OK even if very short straight upstream pipe length!

Annubar™ and similar

Expedient flow measurement: ultrasonic flowmeter

(Several types)

[Note: pipe bore diameter must be known. If test flow seems unusual, check pipe wall thickness for presence of buildup in bore]

Flow measurement: tank in system: measure level change with time.

2 Shut-off Head

Simple test

Not always allowable: high energy pumps can explode if dead-headed too long

Note that pumps with a rising head-flow curve shape can give a greater shutoff head when worn!

Waste water pump, 19kW, Specific Speed 930

3 Measurement of thrust balance leakoff flow Annular clearance

wears: thrust balance flow increases,

……therefore likely that clearances up at the impellers, too.

• Thrust balance flow line is small diameter; low cost permanent flow monitor possible.

• High temp ultrasonic flow sensors available

This corresponds to 250kW wasted !

PLUS any impeller sealing leakage!

Boiler feed pump, variable speed (flow is proportional to speed, therefore was corrected to datum speed).

4 Thermometric method

Assumes inefficiency shows as increase in liquid temperature through pump

Well established in UK etc. water industry Special tapping points, 2D from suction,

discharge flanges (temp, pressure) Power measured: motor efficiency found Flow can be calculated Proprietary systems available

Thermodynamic process: use liquid properties (water/steam: www.pepse.com)

Entropy

Enthalpy

3 2: P2 T2

1: P1 T1

or use Whillier equation: for water up to 54 degC

(Units: degC, kPa, K)

]4160)2(003.01[

100

HeadTotal

riseTemptempInlet

Yatesmeter, also Robertson’s kit…

Precision power meter

Notebook takes data, calculates flow, efficiency

Pressure, temperature transducers at suction and discharge, away from pump flanges

Thermometric tests on boiler feed pump with pipe surface temperature

• Usable results, BUTmust allow time for outlet

metal temp to stabilise.

Optimum time for overhaul - on energy saving basis (1)

1 Pump wear causes drop in plant production

2 Pump duty is intermittent to meet demand

• Overhaul readily justified

• Wear means extra service time and extra energy

Optimum time for overhaul - on energy saving basis (2)

3 Pump wear does not affect plant production, at least initially.

Constant speed, output controlled by throttling – monitor control valve position

4 Pump wear does not affect plant production, at least initially.

Output controlled by varying speed –monitor pump speed

• Same basic method applies...

An example:

Overhaul would cost $50 000.

Cost of power 10c/kWh.

Pump runs for 27% of time on average

Test at 24 months since last overhaul

Motor efficiency is 90%, so the extra power consumed by motor/pump combined (WORN) is:

2300 – 2150 = 150kW ÷ motor efficiency

= 167kW

Calculate the current extra cost of electricity: (720h is average month):

167 × 0.10 × 0.27 × 720

kW $ % h

= $3240/month

Calculate the average cost rate of deterioration:

$ 3240 ÷ 24 = $ 135 /month/month.

Can now find the optimum time for overhaul:

= 27.2 months

C

OT

2

Total cost curve often fairly flat

around the optimum

Variable speed pump (1)

Variable speed pump (2)• Same method as before used, but with speed change.

Here, 31% increase in power to maintain constant system flow, as speed increases from 1490 to 1660 r/min

The method does not apply to all pumps…..

Small pumps may cost more to test than overhaul, and energy costs may be just too small to justify work

Pumps of Specific Speed above about 2000 (r/min, m3/h, m or US units) have a flat or declining Power-Flow curve, and increased leakage does not use more power

Is the pump always at fault?

Maybe the system has changed?

Note that a lower system resistance is also possible

Is the rotation correct?

(DC motor drive)

Condition monitoring is much more than vibration analysis

Performance analysis adds the energy-saving dimension - USE IT !

Conclusion

Happy Monitoring !

[email protected]

[Co-ordinator for 16 years of Monash University’s postgrad programs in maintenance and

reliability engineering: off campus learning (open to all: conditions

apply).

From Jan 2014, programs owned and run by Federation University

Australia]