pump vibration troubleshooting 05 2011

Piedmont ChapterTime msec => Microphone SPL 85dB Inboard Pressure +/-150 psi pk Outboard Pressure +/-150 psi pk Inboard Accel on Suction Wall +/- 300 G pk Cavitation Event v Scope Traces

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Piedmont Chapter Pump Vibration Troubleshooting

by William D Marscher, P.E.

President & Technical Director Mechanical Solutions, Inc

www.mechsol.com Whippany NJ

May 13, 2011

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Hub

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Discharge Nozzle Impeller

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Piedmont Chapter

Time (msec) =>

Microphone SPL 85dB

Inboard Pressure +/-150 psi pk

Outboard Pressure +/-150 psi pk

Inboard Accel on Suction Wall +/- 300 G pk

Cavitation Event

v

Scope Traces of Cavitation Event

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Axially Split Case Dbl Suction Pump

• 180 degree “drip

pocket” allows bearing

hsg and stuffing box to

flex downward due to

pressure pulsations,

e.g at 1x or vane pass

• Double suction

impeller typically

equalizes thrust

side-to-side, but at

off-design is prone to axial

shuttling

• Suction recirc & cav have been issues

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Boiler Feedwater Ring-Section Pumps

“Donut” Pump with

Balance Disk

• 180 Less robust than barrel

pump, but fewer Fn

problems

• Designs where thrust disk

totally replaces oil

lubricated thrust bearing

have encountered rub and

binding problems during

process upsets

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Hero or Enemy? The Thrust Balancing “Disk”

Can have rubs at start-up or during

severe process transients, may permit

“axial shutting”.

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Pacific BFI Barrel Pump

Barrel Pump with

Balance Drum and

Stiff Shaft

• 180 degree “drip

pocket” allows bearing

hsg Fn near vane pass

• Pedestals weaken with

age, structural modes

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Pacific RLIJ

Barrel Pump with

Balance Drum, and

Thin Shaft

• Rotor critical speeds

sensitive to Lomakin

Effect and therefore

wear ring & drum

clearances

drift into 1xRPM

resonance

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causing axial shuttling

drift into 1xRPM

resonance

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near vane pass

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interstage jet leakage

• Axial shutting at

off-design conditions

• High sensitivity of 1 st

bend mode to center

bushing clearance

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Pumps for Wastewater Applications

• Excitation of structural natural frequencies

– 1X, 2X, Vane Pass

• Packing vs mechanical seals

• Material selection

– ‘Wear’ parts – rings, etc.

– ‘Non-Wear’ parts – casings and impellers

• Vertical with intermediate shafting

• Submersible

– Wet pit – Dry pit

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• Bearings – low precision

– Affect on rotor behavior

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• Lineshaft is “violin string”

• Shaft enclosure tube Fn’s

• Disch nozzle loose joints

• Sump vortices or odd flows

• Column piping acoustics

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Vertical Pump Shaft Thrust Effects

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The “Concept” of Imbalance

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Vibration

Problem No 1: 1x Running

Speed

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What Is Resonance?

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Natural Frequency

Vibration & Phase vs Frequency

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Shaft Response to Imbalance

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Shaft Response to Imbalance

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Balance: Single vs Two Plane

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Piedmont Chapter Angular Misalignment

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Piedmont Chapter Offset Misalignment

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Vibration

Problem No 2: 1x & 2x

.Running Speed

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Observing Alignment Continuously with “Dodd Bars”

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Flow

Through Elbows

(Courtesy Cheng Fluid Systems, Inc.)

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Piedmont Chapter Stationary Piping Load Sources

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Piping Acoustic Natural Frequencies

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“Lomakin Effect”

in Centrifugal Pumps

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Concept of the “Wet Critical Speed

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Lomakin Effect

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Lomakin Result:

Critical Speeds Shift Up

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Some Key Issues in Lomakin Effect Strength:

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Wild Cards: Swirl & Grooving

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Some Typical Exciting Frequencies

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Rolling Element Bearing Parameters

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Rolling Element Bearing Frequencies

-+

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Lateral Excitation Forces (Sulzer)

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Vibration

Problem No 3: High Vane Pass

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Vibration

Problem No 4: High Harmonics

of Run Speed

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Marked-up Copy of Pump Curves

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Flow Rate and the “Angle-of-Attack”

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Vane Stalling at Low Flows

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Example of Stalled Blade

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Onset of Internal Recirculation

Discharge Recirculation

Suction

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Vibration & Pulsation vs Flowrate

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An Unexpected Hydraulic Problem:

Rotating Stall

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Four Stage High Speed Boiler Feed Pump

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Rotating Stall Pulsation Spectrum

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Stall Vibration vs Speed

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Vibration

Problem No 5:

Subsynchronous (below 1x)

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Rotor Dynamics

Critical Speeds & Mode Shapes

Forced Response

Stability

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Rotordynamics Is Best Evaluated by Computer

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Typical Rotor Vibr Response vs Speed Exhibits Several Natural Frequencies

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Avoiding Resonance w/ Campbell Diagram

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Rotordynamic Critical Speed Map

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Forward vs Backward

Precession

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Typical Torsional Critical Speeds

and Worst Case Excitations

x2 / POLES

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Typical Vibration Problems

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Instrumentation Options

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Converting Vibration vs Time to Vibr vs f

with “FFT”: Fast Fourier Transform

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Meaning of

RMS vs Peak Vibration

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Vibration Measure Numbers

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Vibration Measurement Format

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Housing vs Shaft Vibration:

Which Should Be Measured?

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Piedmont Chapter Natural Frequency

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Approximating Natural Frequency

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Vibration “Mode Shapes”

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Finding Nat Freq Resonances with a

“Waterfall” Plot of Stacked FFT’s from Test

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Approximate Identification

of Natural Frequencies

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A Modal Field Test In-Progress on

an Operating Pump

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Short Force Application Excites Wide Frequency Band

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Principle of Time Averaging

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Rotor Impact Testing

Case History

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14 Stage Axially Split Case Pump

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Time Averaging Field Example: Watch the Harmonics Disappear

In the End, Natural

Frequencies Are Clear

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Impact Test

Rotordynamic Result

Critical Speeds of

14 Stage Pump

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First Critical Speed, Dbl Suction SS Pump

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Sources of Damaging Forces

in Centrifugal Pumps

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Typical Failures in Centrifugal Pumps

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Stuffing Box & Bearing Skew

Case Histories

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Cartridge Pump

Seal Rub Example

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Vibration

Problem #6: Repeating

Impacts

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Double Suction Pump

“Breathing” Tilts Stuffing Boxes

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FEA Model of Twisting Pump Bearing Housing

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Vertical Pump Failure

Case Histories

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Typical Structural Vibrations of VTP’s

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Six Vertical Pumps on Platform, with Column Exposed to Air

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Multistage Pump Inboard Bearing Chronic Failure

Case History

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Pump with Inboard Bearing Failures

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Shaft Orbits of Problem Pump

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Vibration Spectra for Problem Pump

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Impact Test Results Showing Shaft

Critical Speed at 5280 rpm

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“What If” Analysis for Problem Pump

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Vibration Spectrum After Bearing Fix

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Use of Dynamic Absorbers

in Constant Speed Equipment

to Decrease Vibration Motion

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Dynamic Absorber Design- Vertical Pump/Motor

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Effect of Dynamic Absorber on Vibration

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Gear Box Noise & Wear Problem

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1.There’s More to Pump and

System Vibrations than You

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