Introduction to Vibration and Related Test Methods for Troubleshooting

Transcription

1 Introduction to Vibration and Related Test Methods for Troubleshooting Eric J. Olson MSI Vice President of Business Development & Principal Engineer Juan D. Gamarra, P.E. Assistant Manager of Turbomachinery Testing & Senior Engineer 2017 Feedwater System Reliability Users Group (FSRUG) Austin, Texas January 15-19, 2017

2 Pump Vibration Troubleshooting The focus is on rotating machinery systems, especially pumps and turbines, but many aspects are also applicable to valves. Typical Vibration Problems Overview Vibration Measurement Units What is Resonance? Damaging Forces, Failure Modes, and Instrumentation Types Introduction to FFT Spectra Interpretation and Phase Angle The Impact of Off-Design and Transient Operation on Pump/System Vibration and Life Cavitation Detection and Damage Measurement Methods Introduction to Operating Deflection Shape (ODS) and Experimental Modal Analysis (EMA) testing methods Introductory Case History

3 Typical Rotating Machinery Vibration Problems 1. Imbalance at 1xN (40% Chance) 2. Misalignment at 2xN and 1xN (40% Chance) 3. Natural Frequency Resonance (10% Chance) 4. Everything Else (10% Chance) (Motor Electrical Problems, Pump or System Hydraulic Problems, Foundation Problems, etc.) *Problems 1 and 2 are the most common. But 3, when encountered, eats up maintenance budgets and can have a large impact on equipment/plant availability

4 Vibration Measure Numbers Displacement: How much the object is vibrating ( mils peak to peak ). Lower rpm and frequency. Velocity: How fast the object is vibrating ( inches per second peak ) Acceleration: Imagine being on the vibrating part - Rate of change of velocity over time (expressed in g peak). Higher frequency measurements.

5 Meaning of RMS vs. Peak Vibration

6 What Is Resonance? Resonance = A natural frequency being excited. A problem when there is not enough damping

7 Typical Failures in Centrifugal Pumps Aging Baseplate/Foundation Issues Such as Delamination

8 Sources of Damaging Forces in Centrifugal Pumps Pipe Strain

9 Damage Causing Forces Vibration Frequencies Fortunately (for troubleshooters), most damage causing forces have vibration and frequencies associated with them. More details about these frequencies and problems presented later.

10 Typical Vibration Measurement Hardware Types of accelerometers Single-axis, double-axis, and triaxis accelerometers. As small as 0.2 inches long (5.1 mm) and weighing oz (0.2 grams). Can vary in construction depending on the application, i.e. high temperature and submersible application

11 Typical Pressure Measurement Hardware Dynamic pressure transducers Threaded mounted (1/8 NPT) Flush mounted on the piping or through an isolated ball valve. Oscillating pressure range varies from 10 psi to 5000 psi Detection of pressure pulsations (i.e., vane pass) and acoustic natural frequencies from piping systems For pumps, piping, and valves [psi] Time(Signal 3) - Mark 1 Working : Linear Ave after HP and IP drum filling process before fire going on- line : Input : FFT Analyzer m 200m 300m 400m 500m 600m 700m 800m 900m [s] MSI can also use high frequency accelerometers mounted externally to a pipe, pump casing, valve body (non-intrusive) to measure pressure pulsations and to quantify cavitation damage.

12 Typical Strain Measurement Hardware Measure strain on casing nozzles, piping, and shafting in order to calculate stresses, and determine torsional oscillations in shafts. Can be applied to shafting to determine torque transmitted and with speed to determine horsepower required at any given operating condition. Torsional natural frequencies.

13 Pump Vibration Troubleshooting Introduction to FFT Spectra Interpretation and Phase Angle: Misalignment, imbalance, rubbing, looseness, vane pass, pump system resonance

14 Vibration Problem No. 1: 1x Running Speed

15 Vibration & Phase Angle vs. Frequency Natural Frequency The force leads the response in time. There is a phase lag between force and response.

16 Operating Deflection Shape (ODS) of Typical Misalignment Angular Misalignment Offset Misalignment

17 Vibration Problem No. 2: 1x & 2x Running Speed

18 Vibration Problem No. 3: High Vane Pass

19 Vibration Problem No. 4: High Harmonics of Run Speed

20 FFT scaling/format Feedwater pump with 7 vanes [in/s] m 20m 10m 5m 2m 1m 500u 200u 100u 50u 20u 10u 5u Autospectrum(Signal 3) - Mark 1 (Magnitude) \ FFT Analyzer 1x 2x Natural Frequency 3x 4x 5x 2u 1u [Hz] Amplitude on Y axis in log scale NF near running speed 6x 7x/ VPF [in/s] Autospectrum(Signal 3) - Mark 1 (Magnitude) \ FFT Analyzer [Hz] Amplitude on Y axis in linear scale. Both cases X axis out to about 2 times Vane Pass Frequency.

21 Pump Vibration Troubleshooting The Impact of Off-Design and Transient Operation on System Vibration and Life Recirculation, Stall, Rotating Stall, Hydraulic Forces, and Pressure Pulsations Cavitation Detection and Damage Measurement Methods Introduction to operating deflection shape (ODS) and experimental modal analysis testing methods Introductory case history

22 Vibration Problem No. 5: Sub-synchronous (below 1x rpm)

23 H-Q Pump Curve / System Curve

24 Flow Rate and the Angle-of-Attack Velocity triangle at or near design flow rate Velocity triangle at off-design. Common Vibrations Frequency Source 0.05X 0.35X Diffuser Stall 0.65X 0.95X Impeller Stall *Resonance problem IF the stall excites a natural frequency

25 Onset of Internal Recirculation Suction Recirculation Flow Discharge Recirculation Yellow: Discharge Recirculation Red: Suction Recirculation

26 Vibration (mils p-p) Vibration & Pulsation vs. Flowrate The flow rate where the impeller stalls (i.e., internal suction or discharge recirculation occurs depends on the specific pump design). For high energy pumps it may be very close to the best efficiency point (BEP). Example a large barrel pump initiated internal recirculation at 80% of BEP which would not have been to much of an issue except the 1 st rotordynamic critical speed was at 80% of running speed. Very exciting!

27 Piping Acoustic Natural Frequencies Pipe Nominal Diameter (in) Pipe Average D (in) Pipe Wall Thickness (in) Pipe ID a Schedule (in) (ft/sec)* * Value based on 4977 ft/sec speed of sound in water at 95 F Example: speed of sound in steam at 14.7 psia and 300 F is ~1654 ft/sec. Potential problem if fn A ½ or fn A ¼ = pump vane pass frequency Vane pass frequency = (rpm * the number of impeller vanes)

28 Potential for Cavitation from Pressure Depression at Impeller Eye P s Vapor Pressure Line Suction Suction Eye Discharge β 1 B P s Suction Eye Discharge

29 Scope Traces of Cavitation Event Cavitation Event: Microphone SPL 85dB Inboard Dynamic Pressure +/-150 psi pk Outboard Dynamic Pressure +/-150 psi pk Time (msec) => Is the cavitation event a nuisance or a problem (i.e., metal removal or performance impact)? Inboard Accel on Suction Wall +/- 300 G pk

30 Dynamic Pressure Transducer Reading During Cavitation Zoomed View Negative pressure amplitude clipped-off below vapor pressure indicates cavitation. Without cavitation the time waveform is symmetric above and below zero.

31 Determining Natural Frequencies Typical Structural Vibrations of Vertical Pumps Log 10 Displacement in Inches Frequency CPM

32 Determining Natural Frequencies Typical Lateral Rotordynamic Modes in Horizontal Pumps

33 Determining Natural Frequencies Typical Lateral Rotordynamic Mode Shape

34 Determining Natural Frequencies Approximation method using FFT plots No need to shut down the equipment Determine if resonance may be the problem root cause or eliminate resonance from list of possible root causes Typical FFT linear on x- and y-axis FFT linear on x- and but log scale on y-axis. Fluffs up noise floor revealing potential natural frequencies

35 Introduction to Experimental Modal Analysis (EMA) Also called impact or bump testing to determine rotordynamic and structural natural frequencies Best to perform on pumps while operating due to Lomakin Effect, water mass, bearings loaded up, etc.

36 Introduction to Experimental Modal Analysis (EMA) Short Force Application Excites Wide Frequency Band Principle of Time Averaging (cps)

37 Rotor Impact Testing Case History 14 Stage Axially Split Case Pump Pumps with a large distance between bearing centerlines depend on the support across the wear rings from the water flow through the running clearances for support (i.e., Lomakin Effect). The non-operating rotordynamic natural frequencies are very different from the operating natural frequencies.

38 Difference in NF between operating and non-operating Time Averaging Field Example on an operating pump: Watch the Harmonics Disappear In the End, Natural Frequencies Are Clear

39 Another Method for Finding Natural Frequency Resonances Waterfall Plot of Stacked FFT s from Test Must be able to start/shut down equipment Waterfall Plot of a Shutdown 1xN

40 Case History Nuclear Facility Barrel Type Charge Pump with High Vibration at the Outboard Bearing Housing

41 Background Induction motor driven high energy multistage (11 stage) barrel type charge pump through a gear increaser (1:2.69) Erratic shaft vibration amplitude and phase angle during normal monitoring Plant personnel were speculating a possible crack of the shaft due to electrical power loss Field testing rather than costly teardown inspection was taken to: a) characterize the problem b) recommend a fix

42 Field Vibration Testing Maximum vibration amplitude of 0.22 in/s (5.6 mm/s) peak horizontal direction at the OBB at 1x pump operating speed of 4812 rpm (80.2 Hz) Sister pump was only 0.05 in/s (1.3 mm/s) peak Amplitude (Y-axis) on a log scale so the noise floor is fluffed up and useful approximate natural frequency information is revealed.

43 Operating Deflection Shape (ODS) Testing Applies natural excitation vibration of the rotating equipment to various key points, as exaggerated motion (but to scale) Steady load operation Hundreds of vibration measurements with phase reference signal Data base of amplitude vs. frequency and phase angle 3-D CAD model assigning motion to each individual vibration data point Create animations of the equipment If a picture is worth 1,000 words, motion along with related piping and what is an animation worth when foundation trying to resolve a problem or explain it to management?

44 Field Vibration Testing EMA (impact) test while stationary - natural frequency at 93 Hz (+16% separation margin) TAP TM (impact) test while operating - natural frequency at 76.2 Hz (-5.0% separation margin) EMA FRF plot TAP TM FRF plot

45 Preliminary Conclusions Based on ODS and EMA test results, the source of the high vibration was due to the downward shift of the natural frequency of the pump close to the 1x rpm while it was operating The root cause of the shift in the static to the dynamic frequency was attributed to a soft foot condition The erratic vibration pattern could be attributed to the non-linear behavior of the soft foot contact MSI recommended re-torqueing the hold-down bolt. Customer wanted assurance it would work (hard to believe that it was only a soft foot causing the high bearing housing vibration problem). FEA analysis was used to verify this behavior and the fix

46 Preliminary Conclusions Pump natural frequency at 92.1 Hz with all bolted foot connections secure Pump natural frequency at 77.8 Hz with simulated soft foot on one outboard foot

47 Conclusions and Recommendations The ODS of the pump at 1x rpm indicated a loose connection or soft foot on the pump The stiffness of the pump support connection was varying from start to start causing the natural frequency to shift The variations in vibration amplitude and phase were a result of this natural frequency tuning in and out of the pump running speed This shift in natural frequency and its effects was confirmed with finite element analyses of the pump with and without the soft foot Ensure proper bolt torque values were obtained

48 ODS Animations Examples Premature seal failure due to lack of piping restraints. The excessive nozzle loading was deforming the pump casings. The excitation source is lobe passing frequency (gear pump).

49 ODS Animations Examples Premature bearing failure due to lack of piping restraints. The increased number of life cycles caused premature fretting on the pump bearings. The excitation source is vane passing frequency (5 vane impeller).

50 Questions? Eric J. Olson Vice President of Business Development & Principal Engineer Mechanical Solutions, Inc. ext. 125 Juan D. Gamarra Senior Staff Engineer Mechanical Solutions, Inc. ext Whippany, NJ

51 MSI Company History 1996 Company Founded As A New Jersey Corporation By Bill Marscher Primary Focus On Solving High Value Rotating Machinery Problems Via Specialized Testing And Analysis Relocated To Larger Facility In Whippany, NJ 2006 Opened Offices In MD, NH 2007 Initiated Fluids Engineering Division 2009 Purchased Assets Of Qinetiq NA/Foster Miller/MTI in Albany, NY Compact radial stream turbine generator and foil (air) bearing developer/supplier Opened Indianapolis, IN Office 2012 Opened Denver, CO Office Today: 40+ Personnel, $9M+/Yr Sales. 60% Commercial & 40% Government Geographical Coverage: 75% US, 25% International Customers are rotating machinery/valve users, OEMs, repair centers, and engineering consultants. MSI NJ HQ MSI Albany NY Labs Rotating Machinery/Valve CBM system Test Area In Albany NY Nuclear Pump, Turbine, and Valve Problem Solving

52 Company Description Small company with large company capabilities State of the art testing capabilities and analysis software Highly qualified staff with experience working for turbomachinery manufacturers and end users as well as third party auditors MSI has a proven track record in diagnosing and solving the most complicated turbomachinery related problems Turbomachinery focus Pre-construction design audits New machinery and retrofit design & analysis Troubleshooting

53 Fluid Dynamics Capabilities Flowpath Design & Analysis Steady state and transient CFD analysis Unsteady cavitation modeling NPSH Margin Analysis Cavitation surge Fluid Structural Interaction (FSI) Multi-component and stage CFD analysis Cycle Modeling Startup transient analysis Hydrostatic bearing CFD Typical Projects Gas turbines and steam turbines FCC axial and radial expander specialists LH 2, LOX, HC turbopumps including Ariane 5 LOX Starter Turbine design for F-22 Pumps & fans for commercial & DOD Process compressors and pumps 2-Stage vapor cycle compressor for F-22 Fighter

54 Structural Analysis Capabilities Finite Element Analysis Linear and Non-Linear Capability Stress, Fatigue, and Failure (LCF & HCF) Vibration and Natural Frequency (incl. Bladed CoMAT Disc) Preprocessor: TPA Geometry, Mesh, & Heat Boundary Transfer Conditions - Steady and Read Transient in as Initial Input Acoustics ASME Boiler & Pressure Vessel Code CAD Models & Mesh Sect. III or VIII Cracking, Fracture Mechanics Conditions 3-D Contact w/ Friction Plastic Flow & Creep Physical Boundary 2531 psi Rotordynamics, Bearings & Seals Rotor Stability Shaft Critical Speeds Rotor Forced Response Power Train Torsional Analysis Bearing & Seal Coefficients Squeeze Film Dampers Multi-rotor Systems with Flexible Supports Foil (air) bearing development, manufacturing, & tech transfer 1952 psi 1811 psi 1077 psi 12 DISTRIBUTION STATEMENT C. As described on Title Page. FLORIDA TURBINE T T E E C C H N H O N L O L G O I E G S, I I E N SC.

55 Test & Measurement Capabilities Turbomachinery performance Pressure, flow, efficiency, temperature, and cavitation, Blade Fatigue Detection (BFD) - Blade vibration and HCF Blade deflection & strain using a recently developed methodology Experimental modal analysis testing Impact hammer & shaker excitation Spin pit with mag excitation 300+ simultaneous channels of data collection (field or lab) Using a microwave sensor and the GSX software package, MSI s BFD product and test service is the modern non-contacting stress measurement system (NSMS) for determining turbomachinery bladed disk modes on the test stand or in the field.

56 Troubleshooting Combining specialized testing with experienced analysis to provide Solutions Advanced vibration analysis and diagnostics Operating deflection mode shape analysis Time-Averaged Pulse (TAP) testing during operation Design modifications to avoid resonance Blade Fatigue Detection and surge evaluation using MSI s BFD system. Machinery performance Pressure, flow, power & temperature Blade vibration, HCF, and LCF Using an MSI developed microwave sensor Experimental modal analysis Impact hammer excitation

57 Advanced Analytical & Test Tools $2M+ invested in design, analysis, and testing tools ANSYS Finite Element (FEA) Full Multiphysics Version ROMAC RotorLab Rotordynamics/ Bearing/ Seal Software Package Cfturbo, TurbAERO for turbomachinery flow path design and analysis CFX and STAR-CCM+ Computational Fluid Dynamics Software Packages Pro/Engineer Wildfire Solid Modeler, Siemens NX Version 8.5 (Unigraphics), & SolidWorks MSI s Sentry CBM system LabVIEW (Alliance Partner and 2 certified programmers on staff) NIST ACTIS Tribology Computer Program Set Multi-channel FFT and digital data recorders (over 300 channels simultaneously & data acquisition rates up to 50 khz) Sensors capable of measuring the behavior of the structure, rotor, and system: Dynamic pressure, SPL and SI acoustics, BFD for non-contacting bladed disk stress measurements, strain and torque measurement on rotating components via RF telemetry, Motor and generator voltage & current measurement, accelerometers, laser velocimeters, proximity probes ME-Scope Shock & Vibration Test Visualization Computer Program - Full Vibration & Acoustics Version Two high performance computing clusters totaling 276 cores with over 1.5 TB of 1333& 1866 MHz DDR3 memory and 10 & 56 Gbps interconnects.