Pumping System Vibration Understanding the Fundamentals Eric J. Olson Mechanical Solutions, Inc. (MSI) ejo@mechsol.com
Agenda 1. Vibration testing and analysis fundamentals 2. Structural and acoustic resonance 3. Reducing the risk of installing vibration problems
Why is vibration important? 1. Useful for monitoring machine/system condition in terms of remaining useful life. Continuous or periodic monitoring. 2. Specialized vibration test and analysis techniques used to determine a problem root cause. 3. Computational vibration analysis methods can be used to: a. Design and analyze a problem solution before it is implemented. b. Analysis and modification of a system prior to equipment installation and commissioning (Reference ANSI/ Hydraulic Institute 9.6.8).
Why is vibration important?
Definitions Vibration: The oscillation of an object about it s position of rest * Cycle: Movement of a mass through all it s positions back to it s point of rest Period: The time it takes for one cycle to complete Frequency: Number of cycles in a given amount of time, (e.g., 1 minute) CPM: Number of cycles in one minute CPS: Number of cycles in one second (Hz) * Forced vibration - Force causing vibration Free vibration occurs without force like when a string is plucked. Natural frequency and resonance discussed later in the presentation
Vibration Measurement Numbers Displacement: How much the object is vibrating ( mils peak to peak ). Typically used for lower rpm (<600rpm) and frequency. Velocity: How fast the object is vibrating ( inches per second peak ) Typically used for CPM range of 600 to 60,000 Acceleration: Imagine being on the vibrating part - Rate of change of velocity over time (expressed in g peak). Higher frequency measurements >60,000 CPM. Take-away: When talking about vibration you might assume velocity (ips) units but units should be noted in any report or communication not assumed.
Vibration Output - Domain 1. Overall or unfiltered with no reference to frequency: a single value usually in three directions for each measurement point. Used for specifications, post-installation acceptance test criteria, and first-pass monitoring. 2. Time Wave Form Amplitude versus frequency output from the analyzer. 3. Filtered or FFT Spectra Amplitude versus frequency. Fast Fourier Transform (FFT) A computerized technique to calculate the frequency components of a time waveform from the digitized voltage measurements. The result is a display of Amplitude vs. Frequency (Spectra).
Vibration Output - Domain Time wave form Time domain FFT to get The FFT is essentially viewing the time wave from the side. FFT spectra Frequency domain Most used
Meaning of Root Mean Square (RMS) vs. Peak Vibration
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 0.007 oz (0.2 grams). Can vary in construction depending on the application, i.e. high temperature and submersible application
Typical Vibration Measurement Hardware Proximity Probes Usually threaded mounted Typical Gap Voltage: 10 volts (50 mils) Typical calibration: 200 mv/mil Has poor sensitivity against non-magnetic steels
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 40 30 20 10 0-10 -20-30 -40 0 100m 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.
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/ resonance.
Vibration Analysis Diagnostics Typical Failures in Centrifugal Pumps
Sources of Damaging Forces in Centrifugal Pumps Pipe Strain
Damage Causing Forces Vibration Frequencies Fortunately (for troubleshooters), most damage causing forces have vibration and frequencies associated with them. One example provided (next slide). More details about these frequencies and problems are topics for a longer seminar.
Vibration Problem No. 1: 1x Running Speed
Typical Pump and Pump System 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. often also resonance related) *Problems 1 and 2 are the most common. But when 3 is encountered, it eats up maintenance budgets and can have a large impact on equipment/plant availability
What Is Resonance? Resonance = A natural frequency being excited. A problem when there is not enough damping Resonance Take-aways: 1. Vibration amplified by 5, 10, 15 times (example, 0.10 ips rms becomes 1.0 ips rms) 2. Specialized troubleshooting and analysis required to fix. 3. Avoidable by specifying pre-construction analysis (ANSI/HI 9.6.8).
Determining Natural Frequencies Typical Structural Vibrations of Vertical Pumps Log 10 Displacement in Inches Frequency CPM
Determining Natural Frequencies Typical Lateral Rotordynamic Modes in Horizontal Pumps
Introduction to Experimental Modal Analysis (EMA) Testing 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.
Typical Non-Clog Pump Resonance Problem Experimental Modal Analysis (i.e. impact) test performed Pump running Confirmed natural frequency near vane pass (resonance) Right - Mode shape animation @ 38 Hz
Operating Deflection Shape (ODS) Testing Right - ODS test animation Exaggerated scaled animation of actual pump system motion and deflection A solution can often be implemented based on TAP and ODS results. In this case a solution was designed ODS data used to calibrate a pump system finite element analysis model.
Piping Acoustic Natural Frequencies
Adjusted Speed of Sound for Different Water Piping Sizes Where: aadjusted - the adjusted speed of sound a - the speed of sound for the fluid D - the average pipe diameter Ks - the bulk modulus for the fluid t - the pipe wall thickness E - the elastic modulus for the pipe material Pipe Nominal Diameter (in) Pipe Average D (in) Pipe Wall Thickness (in) Pipe ID a Schedule (in) (ft/sec)* 24 30 23.4 0.562 22.9 4167 24 20 23.6 0.375 23.3 3887 20 30 19.5 0.5 19.0 4207 20 20 19.6 0.375 19.3 4020 16 30 15.6 0.375 15.3 4167 14 30 13.6 0.375 13.3 4247 6.0 30 5.7 0.277 5.4 4513 * Value based on 4977 ft/sec speed of sound in water at 95 F
Piping System Resonance Motion Amplified Envision Motion Video Amplified Video
Pump Operation for Good Vibrations Rule # 1: Match Design Point to System Head & Flow Requirements Rule # 2: For Pumps, Require NPSHA Above NPSHR, with Margin Rule # 3: Use a Long Straight Piping Run to the Inlet Rule # 4: Careful When & How You Throttle Rule # 5: Avoid H-Q Slopes Being Similar, Machine vs. System Rule # 6: Minimize Nozzle Loads & Use Expansion Joint Tie Rods Rule # 7: Avoid Structural Natural Frequencies & Rotor Criticals Rule # 8: Minimize Load Cycling, if Practical Rule # 9: Select Materials Based on Corrosion, Galling, Fatigue & Erosion Resistance Rule # 10: You Get What You Spec & Pay For
Reduce the Risk of Installing Vibration Problems Use for new pump/driver installations or modifications Possibly specify a pre-manufacturing: Dynamics (vibration) analysis; Torsional rotordynamics, lateral rotordynamics, and System structural analysis Structural analysis must include the foundation/support system and nearby piping (not pump/motor alone) Correct potential issues before equipment delivery and installation Specify a post acceptance vibration test Applicable to pumps, blowers, and digesters/mixers
Analysis for risk-reduction How to specify a pump system dynamics (vibration) analysis Hydraulic Institute/ANSI Guideline 9.6.8 provides: 1. A Risk Uncertainty Numbering (RUN) method Owner, AE, manufacturer provide input to determine if and what level of analysis is required. 2. Sample specifications provided for three levels of analysis Why specify a pre-construction analysis? 1. Helps avoid commissioning or life-limiting vibration problems 2. Owner, AE, and manufacturer collectively pursue mutual risk reduction. 3. System analysis and post-installation acceptance testing can be performed by an independent specialist company.
Questions? Eric J. Olson Vice President of & Principal Engineer Mechanical Solutions, Inc. ejo@mechsol.com ext. 125 973-326-9920 www.mechsol.com Whippany, NJ
Learner Outcomes 1. The difference between vibration measurement numbers: A) Displacement, B) velocity, and C) acceleration. 2. The different types of vibration data output: A) overall or unfiltered, B) Time Wave Form; and C) Filtered or FFT Spectra. 3. Understand natural frequency, resonance, and be aware of more specialized test methods for addressing resonance problems. 4. Be able to access a published method for reducing the risk of installing vibration problems during a new plant construction or plant modification.