Vibration Fundamentals: Machine-Train Monitoring Parameters PDF

# Chapter 11 - Machine-Train Monitoring Parameters This chapter covers normal failure modes, monitoring techniques to prevent premature failures, and the measurement points for monitoring machine-train components. Understanding the specific location and orientation of each measurement point is vita...

# Chapter 11 - Machine-Train Monitoring Parameters This chapter covers normal failure modes, monitoring techniques to prevent premature failures, and the measurement points for monitoring machine-train components. Understanding the specific location and orientation of each measurement point is vital for diagnosing incipient problems. ## Frequency-Domain Analysis The fast Fourier transform (FFT) signature acquired at each measurement point represents the machine-train component's motion at that specific location. It is crucial to know the location and orientation for accurate identification of problems. In simple terms, the FFT signature is a snapshot of mechanical motion at a specific point and time. ## Vibration Monitoring Process The vibration-monitoring process entails collecting, temporarily storing, and downloading data to a more powerful computer for lasting storage and analysis. Data collection relies on microprocessor-based vibration analyzers. It is crucial to establish a database with specific parameters before using the analyzers. **Narrowband** refers to a specific frequency window monitored due to known machine components or characteristics in that range. ## Measurement Point Orientation Each measurement point's orientation is significant during database setup and analysis. The optimal orientation for each machine-train component varies. For instance, a helical gear set creates specific force vectors during normal operation. As the gear set degrades, these force vectors transmit maximum vibration components. For machines with radials, the orientation should be aligned with the plane that maximizes vibration amplitude. ## Machine-Train Setup Each machine-train should be set up on a "common-shaft" with the outboard driver bearing as the first data point. Measurement points are numbered consecutively, starting with the outboard driver bearing and ending with the outboard bearing of the final driven component. This standardized numbering has benefits: - Immediate identification of the data point's location during analysis. - Grouping data points by "common shaft" allows for evaluation of parameters affecting each component. ## Drivers All machines need some form of motive power, a driver. This section focuses on the common drivers: electric motors and steam turbines. ### Electric Motors Electric motors utilize microprocessor-based vibration monitoring systems more than any other driver. Essential parameters to monitor for evaluating operating condition include: - Bearing frequencies - Imbalance - Line frequency - Loose rotor bars - Running speed - Slip frequency - V-belt intermediate drives **Bearing Frequencies**: Electric motors use either sleeve or rolling-element bearings. A narrowband window should be established to monitor normal rotational and defect frequencies associated with the specific bearing type. **Imbalance**: Instability or imbalance is caused by various forcing functions. Narrowbands monitoring the fundamental and harmonics of running speed help identify mechanical imbalance. Additionally, line frequency provides indications of instability. Modulations or harmonics of line frequency indicate the motor's inability to find magnetic center. Variations in line frequency also increase the amplitude of harmonics of running speed. Additionally, axial movement and the third harmonic of running speed indicate instability or imbalance. **Line Frequency**: The frequency of the alternating current supplied to the motor. Monitoring the fundamental and first three harmonics is crucial. **Loose Rotor Bars**: Loose rotor bars are problematic. Two methods can be used to identify them: - **High-frequency vibration components**: These frequencies are above the normal maximum frequency. A high-pass filter can monitor rotor bar condition. - **Slip frequency**: The passing frequency resulting from loose rotor bars energizes modulations. This method is preferred because these frequency components are within the normal bandwidth for analysis. **Running Speed**: Electric motors can be classified as variable-speed machines. A narrowband window should monitor the true running speed. **Slip Frequency**: The difference between synchronous speed and the motor's actual speed. A narrowband filter should be established for accurate identification. **V-Belt Intermediate Drives**: Electric motors using V-belt intermediate drives exhibit the same general failure modes. Monitoring unique V-belt frequencies is essential to determine improper belt tension or misalignment. Premature wear on the bearings due to V-belt drives is common. The primary data-measurement point on the inboard bearing housing should be located in the plane opposing the induced side load, with the secondary point at 90 degrees. The outboard primary data-measurement point should be in a plane opposite the inboard bearing with the secondary at 90 degrees. ### Steam Turbines Steam turbines vary in size. Common monitored parameters include: - Bearings: Turbine rotors utilize both rolling-element and Babbitt bearings. Narrowbands monitor the normal rotational frequencies and failure modes for each turbine. - Blade Pass: Turbine rotors consist of vanes or blades mounted on individual wheels. Each wheel unit, a stage of compression, has its blade-pass frequency. Narrowbands monitor this frequency. Flexing or loss of blades or wheels is detected by these narrowbands. - Mode Shape (Shaft Deflection): Most turbines feature long bearing spans and highly flexible shafts. These factors, coupled with variations in process flow, make them susceptible to shaft deflection. Monitoring the harmonics of shaft speed is essential. - Speed: Steam turbines operate near or above their critical speeds. A narrowband window should track the critical speeds. ## Intermediate Drives Intermediate drives transmit power from a primary driver to a driven unit. These include: - Chains: The meshing of sprocket teeth and chain links creates vibrations similar to a gear set. Slack in the chain amplifies tooth-mesh energy. Monitoring running speed, tooth-mesh, and chain speed is essential. - Couplings: Couplings cannot be monitored directly, but they generate forcing functions. Each coupling should be evaluated for the specific mechanical forces and failure modes. - Gearboxes: Used to change speed or rotation direction. Monitoring parameters include bearings, gear-mesh frequencies, and running speeds. ## Driven Components This chapter cannot comprehensively cover all driven component combinations. However, the guidelines provided here can be used for various machine-trains and process systems. ### Compressors Compressors fall into two categories: Centrifugal and Positive Displacement. #### Centrifugal - **In-Line:** Functions similarly to centrifugal pumps. They require the same monitoring and evaluation methods. - **Bullgear:** A multistage unit utilizing a large helical gear and pinion gears driving impellers. #### Positive Displacement - **Reciprocating:** Reciprocating compressors use one or more cylinders with pistons driven by a crankshaft. - **Screw:** Screw compressors utilize interlocked rotors and operate as positive-displacement devices. They are sensitive to instability. ### Fans - **Centerline:** The rotating element is located at the midpoint between two bearings. - **Cantilever:** The rotating element is located outboard of two bearings. ### Generators Generators utilize Babbitt bearings that rely on lubricating films to prevent wear. They are particularly sensitive to wear during startup and shutdown. Monitoring subharmonic frequencies in the bearing is crucial. ### Process Rolls - **Bearings:** Nonuniform loading and misalignment create uneven bearing load zones. The ball-pass outer-race frequency should be monitored. - **Load Distribution:** Rolls should be uniformly loaded. - **Misalignment:** Misalignment causes issues with the bearing and creates a signature similar to classical parallel misalignment. ### Pumps - **Centrifugal:** End-suction and horizontal split-case are two common types of pumps. - **Positive Displacement:** Common types include piston-type pumps and rotating element pumps. # Chapter 12 - Database Development Valid data is essential for vibration monitoring and analysis. Without accurate and complete data taken in the appropriate frequency range, it is impossible to interpret the vibration data from a machine-train. This is especially true for microprocessor/computer-based systems which rely on a database. ## Database Development Steps: 1. **Collection of machine and process data** 2. **Database setup**: This includes machine and process specifications, analysis parameters, data filters, alert/alarm limits, and other parameters related to the data acquisition process. ## Machine and Process Data Collection Accurate and complete database development includes: - **Equipment Information Sheets**: These sheets document the operating condition of each machine-train, including specific data about the type of operation and information on all components. - **Process Information Sheets**: These sheets describe each process being monitored. <start_of_image> schematic: - image: [Image unavailable for display] - description: A schematic of a typical double-pivot universal joint.