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

Summary• Pump Basics• Pump Selection• Common Pump Problems

– Imbalance– Misalignment– Suction Conditions– Nozzle Loads– Resonance

• Problem Solving Techniques

Centrifugal Pump Selection

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

• Rule #2: Require NPSHA greater than NPSHR plus 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

Pump Internal Hydraulic Design

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 = (Psuct – Pvap)/fluid density

– NPSHR: Suction head that causes 3% drop in TDH

43

*

HEAD

GPMRPMNs

43

*

RNPSH

GPMRPMNss

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

Pump Specific Speed Chart

Basic Pump Components

• Rotor– Shaft

– Impeller(s)

– Coupling(s)

• Casing– Diffusers / Volutes

– Stuffing Box

– Discharge Head (VTP’s)

– Bearing Housings

• Bearings• Seals

Basic Pump Designs

Single Stage End Suction Pump with Open Impeller

Horizontal Split Case Pump

Pacific RLIJ (Barrel Pump)

Pacific BFI (Barrel Pump)

Bingham MSD Pump

Vertical Turbine Pumps

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

Pump Performance

Typical Pump System Head Curve

Typical Pump System Head Curve

Pump Vibration vs. Flow Rate

Vibration Testing

Instrumentation Options

Data Processing:Converting from time domain to frequency domain with an FFT

Raw Time Signal

Result of FFT

Common Vibration Measurements

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

What Does Vibration Do?

• Bearing Failures

• Seal Failures

• Internal Wear (affects performance)

• Increases Power Consumption

Vibration Decreases Pump Reliability

And Increases Cost of Operation

Common Excitation Frequencies:Identifying the Source of the Problem

Balance and Alignment

Vibration Problem #1:1X Running Speed

Vibration Caused by an Oscillating Force - Imbalance

Balance: Single vs. Two Plane

Vibration Problem #2: 1X and 2X Running Speed

Angular Misalignment

Offset Misalignment

Checking Alignment – Reverse Dial Indicator Method

Dodd Bars for Continuous Monitoring of Alignment

(Thermal Effects)

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

Alig

nm

en

t A

cc

ura

cy

(m

ils/in

)

Unacceptable

Acceptable

Good

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

– Parallel: less than ½ 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 or delaminated foundation)

Pump / Driver Alignment Guidelines

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

Piping Design Issues

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 NPSHA vs. 3% Head drop NPSHR

– Operate Near Best Efficiency Point (BEP)

• Mechanical Considerations– Do Not Use Pump Nozzle as Pipe Anchor

– No Unrestrained Expansion Joints

Flow through Elbows(Courtesy Koch Engineering)

Static Piping Load Sources

• Unrestrained Expansion Joint (Like a Rocket Nozzle, F=P*A)

• ‘Bourdon Tube’ Straightening• Thermal Growth / Mismatch

Static Piping Loads are a Common Cause of Casing Deformation and Misalignment

Piping Loads(Misalignment due to Warped Casing)

Vibration Problem #3 High Vane Pass Frequency Vibration

Vane Pass Frequency Vibration

Key Internal Gaps

Vibration Problem #4 High Harmonics of Running Speed

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)

Typical Rotor Vibration Response vs. Speed

Typical Rotor Mode Shape

Avoiding Resonance:Campbell Diagram

Avoiding Resonance:Critical Speed Map

Improving Reliability by Avoiding Natural Frequencies and

Resonance

Natural Frequency

Approximating Natural Frequency

How Natural Frequencies Affect Vibration

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

Using Waterfall Plots to Identify Natural Frequencies

Natural Frequency

Approximate Identification of Natural Frequencies

Modal Test Results

Test Data

Analytical Prediction

Advanced Problem Solving

• Use test data to identify most likely source of problem

• 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

Conclusions• There’s more to pump and system vibrations

than 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