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Addressing Pump Reliability

Problems

Matthew A. Gaydon

May 9, 2006

Mechanical Solutions, Inc.

11 Apollo Drive

Whippany NJ 07981

973-326-9920

www.mechsol.com

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Summary

Pump Basics Pump Selection

Common Pump Problems

Imbalance Misalignment

Suction Conditions

Nozzle Loads

Resonance

Problem Solving Techniques

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Centrifugal Pump Selection

Rule #1: Match Pump BEP to System Head & Flow

Rule #2: Require NPSHAgreater than NPSHRplus margin

Rule #3: Use a Long Straight Piping Run to the Suction Nozzle

Rule #4: Thou Shalt Not Dead-Head

Rule #5: Avoid Flat or Positive-Slope H-Q Characteristics Rule #6: Minimize Nozzle Loads & Use Expansion Joint Tie Bars

Rule #7: Avoid Structural Natural Frequencies and Rotor Critical Speeds

Rule #8: Minimize Load Cycling

Rule #9: Select Materials Based on Corrosion, Galling, Fatigue, Erosion

Resistance Rule #10: You Get What You Specify & Pay For

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Pump Internal Hydraulic Design

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Pump Design Fundamentals

The Fan Laws

Specific Speed (Ns)

Describes Impeller Design

Suction Specific Speed (Nss)

Describes Suction Performance

Cavitation Potential NPSHA: net positive suction head

available at the centerline of the

impeller NPSHA = (PsuctPvap)/fluid density

NPSHR: Suction head that causes 3%

drop in TDH

43

*

HEAD

GPMRPMNs

43

*

RNPSH

GPMRPMNss

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Pump Characteristics

Pumps follow the fan laws or affinity laws

2

1

2

1

N

N

Q

Q

2

2

1

2

1

N

N

H

H

3

2

1

2

1

N

N

HP

HP

Flow

Power

Head

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Pump Specific Speed Chart

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Basic Pump Components

Rotor

Shaft

Impeller(s)

Coupling(s)

Casing

Diffusers / Volutes Stuffing Box

Discharge Head (VTPs)

Bearing Housings

Bearings

Seals

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Basic Pump Designs

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Single Stage End Suction

Pump with Open Impeller

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Horizontal Split Case Pump

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Pacific RLIJ (Barrel Pump)

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Pacific BFI (Barrel Pump)

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Bingham MSD Pump

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Vertical Turbine Pumps

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Pump Selection

A properly selected pump will operate at or near its

Best Efficiency Point (BEP)

Pumps operating in parallel will operate at the same

head point on their curves

Two identical pumps operating in parallel at different

speeds will not operate properly

A pump will operate where its performance

characteristic matches the system resistance

characteristic

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Pump Performance

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Typical Pump System Head

Curve

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Typical Pump System Head

Curve

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Pump Vibration vs. Flow Rate

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Vibration Testing

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Instrumentation Options

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Data Processing:Converting from time domain to

frequency domain with an FFT

Raw Time Signal

Result of FFT

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Common Vibration Measurements

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Typical Pump Vibration Issues

Imbalance at 1X RPM (40%)

Misalignment at 2X and 1X RPM (40%)

Natural Frequency Resonance (10%) Everything Else (10%)

Excessive Vane Pass Forces

Hydraulic Forces, Including Rotating Stall Motor Electrical Problems

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What Does Vibration Do?

Bearing Failures

Seal Failures

Internal Wear (affects performance) Increases Power Consumption

Vibration Decreases Pump Reliability

And Increases Cost of Operation

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Common Excitation Frequencies:

Identifying the Source of the Problem

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Balance and Alignment

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Vibration Problem #1:

1X Running Speed

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Vibration Caused by an

Oscillating Force - Imbalance

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Balance: Single vs. Two Plane

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Vibration Problem #2: 1X and

2X Running Speed

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Angular Misalignment

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Offset Misalignment

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Checking AlignmentReverse Dial

Indicator Method

D dd B f C ti

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Dodd Bars for Continuous

Monitoring of Alignment

(Thermal Effects)

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Typical Alignment Limits

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 2 4 6 8 10 12 14 16 18 20

RPM X 1000

AlignmentAccuracy(mils/in)

Unacceptable

Acceptable

Good

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General Guideline for Acceptable Misalignment Offset: less than 2 mils * (3600/RPM)

Parallel: less than mil * (3600/RPM) per axial inch

Remember: Alignment when machine is cold and non-

pressurized will be different than when machine is hot

and pressurized. Machines may have cold offsets for

best COS alignment, and may need compromise

alignment for variable COS

Beware of soft foot (e.g. teetering pump ordelaminated foundation)

Pump / Driver Alignment

Guidelines

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Modern Laser Alignment

Same Principle as Dial Indicator

Eliminates

sagging indicator brackets

sticking / jumping dial indicators

low resolution / round-off error

reading errors: sign error, parallax error, etc.

looseness in mechanical linkages offset error due to tilted dial indicator

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Piping Design Issues

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Suction Piping and Inlet Design

Hydraulic Considerations

Long Straight Pipe Leading into Pump Suction Minimize bends or elbows close to the pump inlet

Minimize restrictions before inlet

Ample NPSHAvs. 3% Head drop NPSHR

Operate Near Best Efficiency Point (BEP)

Mechanical Considerations

Do Not Use Pump Nozzle as Pipe Anchor

No Unrestrained Expansion Joints

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Flow through Elbows(Courtesy Koch Engineering)

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Static Piping Load Sources

Unrestrained Expansion Joint (Like aRocket Nozzle, F=P*A)

Bourdon Tube Straightening

Thermal Growth / Mismatch

Static Piping Loads are a Common Cause ofCasing Deformation and Misalignment

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Piping Loads(Misalignment due to Warped Casing)

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Vibration Problem #3 High Vane Pass

Frequency Vibration

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Vane Pass Frequency Vibration

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Key Internal Gaps

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Vibration Problem #4 High Harmonics

of Running Speed

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Vibration Problem #5 Excitation of a

Natural Frequency (Rotor or Structure)

All structures have natural

frequencies

Natural frequencies are harmful if

they can become excited

Common excitation frequencies are:

1X rotational speed

2X rotational speed

NX rotational speed (where N =

number of impeller blades)

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Typical Rotor Vibration Response vs.

Speed

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Typical Rotor Mode Shape

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Avoiding Resonance:

Campbell Diagram

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Avoiding Resonance:

Critical Speed Map

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Improving Reliability by Avoiding

Natural Frequencies andResonance

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Natural Frequency

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Approximating Natural Frequency

H N t l F i Aff t

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How Natural Frequencies Affect

Vibration

Vib ti I t R

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Vibration Impact Response

(Bump Testing)

Concept: Provide artificial

stimulation to a machinery

system to identify rotor or

structural natural

frequencies

Practice: Measure vibration

response for a known input

excitation

U i W t f ll Pl t t Id tif

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Using Waterfall Plots to Identify

Natural Frequencies

Natural Frequency

A i t Id tifi ti f N t l

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Approximate Identification of Natural

Frequencies

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Modal Test Results

Test Data

Analytical Prediction

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Advanced Problem Solving

Use test data to identify most likely source ofproblem

Model pump to analytically approximate installation

Adjust analytical model to match site measurements Use calibrated model to test possible fixes

Accurate model allows us to avoid eliminating one

problem, but causing a new one

Testing + Analysis = SOLUTION

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Conclusions

Theres more to pump and system vibrationsthan you might expect

Keys to success: knowledge, experience, and

the right tools

Good rules-of-thumb exist

Good condition-based methods and

instrumentation are getting better

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