Answer: Foundation of Machine Protection

• Relative Vibration (Shaft Vibration):
  1. Measurement: Measures the displacement of the rotor shaft relative to the bearing housing. This requires non-contact proximity probes (Eddy Current Sensors).
  2. Purpose: Essential for monitoring the shaft's stability within its clearance and detecting common shaft-related faults like imbalance, misalignment, and oil whirl/whip.
  3. Application: Critical for large, high-speed fluid-film bearing machines (e.g., Turbines, Centrifugal Compressors) where bearing housing movement is minimal compared to shaft movement. It is the primary measurement for machine protection.
• Absolute Vibration (Casing/Bearing Housing Vibration):
  1. Measurement: Measures the absolute motion (acceleration or velocity) of the bearing housing or machine casing relative to a fixed reference point (like the earth). This uses seismic sensors (Accelerometers or Velocity Transducers).
  2. Purpose: Detects faults that create high energy transfer to the casing, such as bearing defects (rolling element bearings), gear mesh problems, or structural looseness.
  3. Application: Preferred for machines with rolling element bearings or structural issues, and is also used on large machines to monitor structural integrity when shaft movement is transmitted to the housing.

Answer: Eddy Current Sensing

• Working Principle: The probe operates on the Eddy Current principle. A high-frequency AC current is applied to the probe tip, generating a magnetic field. When the probe tip is brought close to a conductive target (the shaft), the magnetic field induces eddy currents on the target's surface.
• Signal Generation: These induced eddy currents generate their own magnetic field, which opposes the probe's field. This opposition causes an energy loss, resulting in a change in the probe's internal impedance.
• Measurement Output: The system (Proximitor) converts this impedance change into a proportional DC voltage. The **DC voltage is inversely proportional to the distance (gap)** between the probe tip and the shaft. Changes in this gap due to vibration are measured as AC voltage signals superimposed on the DC voltage (gap voltage).
• Main Components:
  1. Probe Tip: Contains the coil and is mounted near the shaft.
  2. Extension Cable: A coaxial cable connecting the probe to the Proximitor.
  3. Proximitor (Driver): An electronic circuit that supplies the high-frequency signal, processes the impedance change, and outputs the DC gap voltage and AC vibration signal.

Answer: Integrated Protection System

• Primary Function: The 3500 System is a highly reliable, rack-based **machinery protection system** designed to continuously monitor critical parameters (vibration, position, speed) of rotating equipment. Its main role is to provide alarms and, most critically, automatically trip the machine (shutdown) if monitored parameters exceed hazardous levels, preventing catastrophic failure.
• Core Components:
  • Rack & Power Supplies: The physical enclosure with redundant (A/B) power for high availability.
  • Rack Interface Module (RIM): Provides communication with external systems (DCS/PLC) and manages the system configuration.
  • Monitor Modules (e.g., 3500/42M, 3500/50): The intelligent cards that receive raw transducer signals, process them, compare them against setpoints, and generate alarms/trips. Each module handles a specific type of measurement (Vibration, Thrust, Temperature, Overspeed).
  • Relay Modules (3500/32 or 3500/34): Receive trip signals from the monitor modules and execute the physical action of opening or closing contacts to interface with the machine's control system (for safe shutdown).

Answer: Protecting the Aerodynamic Limit

• Surge: Surge is the unstable flow condition where flow reversal occurs inside the compressor, leading to rapid pressure/flow oscillations and potential mechanical damage. Anti-Surge Control is vital.
• Key Measurements for the Controller:
  1. Inlet Volumetric Flow (Q): Typically measured using a Differential Pressure (DP) cell across an orifice plate or a Venturi meter on the compressor suction line. This defines the operating point on the compressor map.
  2. Suction/Discharge Pressures (P_1, P_2): Required to calculate the compression ratio and head, accurately locating the operating point relative to the surge line. High-accuracy pressure transmitters are used.
  3. Suction Temperature (T_1): Required for compensating the flow and pressure measurements to determine the actual *volumetric* flow and thermodynamic conditions.
  4. Speed (N): Measured by a proximity probe (Keyphasor) or magnetic pickup, crucial because the surge line location depends heavily on compressor speed.
• Final Control Element: The **Anti-Surge Valve (ASV)**, which must be a high-performance, fast-opening control valve, often equipped with a large-volume booster and smart positioner for rapid response to imminent surge conditions.

Answer: Rotational Reference

• Function: The Keyphasor is a proximity probe (similar to a vibration probe) focused on a specific point (keyway or notch) on the shaft. It provides a **once-per-revolution reference signal**.
• Output Signal: It generates a single **TTL (Transistor-Transistor Logic) pulse** or a sharp voltage spike every time the keyway/notch passes the probe tip.
• Importance in Monitoring:
  1. Rotor Speed: The frequency of the pulse train directly gives the rotational speed (RPM) of the machine.
  2. Phase/Vector Analysis: It serves as a timing reference point. All vibration data (amplitude and phase angle) is synchronized with the Keyphasor pulse. This allows dynamic analysis (e.g., Bode, Polar plots) to determine the location of the heavy spot on the rotor relative to the shaft reference.

Answer: Axial Position Monitoring

• Thrust Bearing Function: Absorbs residual axial forces (thrust) generated by the compressor or turbine stages, preventing the rotor from moving too far in the axial direction.
• Monitoring Method: Monitored using two proximity probes placed axially to measure the rotor's Axial Position (Shaft Position). This typically uses a **2-out-of-2** voting system for reliability.
• Alarm/Trip Philosophy:
  1. Alert (Warning): Typically set when the shaft moves approximately **half the available total clearance**, indicating early wear of the active thrust pad.
  2. Danger (Trip): Set when the shaft moves a set distance further, indicating imminent failure (wiping) of the thrust pad and potential rotor-stator contact. Axial position trips are often considered the most critical and fastest-acting trips on a rotating machine.
• Criticality: If axial movement is unrestricted, the rotor can crash into stationary parts (impellers against diaphragms, turbine blades against casings), leading to instantaneous catastrophic failure of the machine.

Answer: Differential Expansion (Diff Exp)

• Concept: Differential Expansion (Diff Exp) measures the **difference in axial thermal growth** between the rotor (which heats up faster) and the turbine casing (which heats up slower) during startup and shutdown.
• Instrumentation: Typically a long-range **proximity probe system** (e.g., Bently Nevada 3300 XL) is used to monitor the axial movement of the casing relative to the rotor at a specific point.
• Significance:
  1. Rotor Lagging/Leading: Ensures that the rotor doesn't grow too far relative to the casing, which could cause internal rubs at critical seals (like labyrinth seals).
  2. Controlled Startup: High Diff Exp alarms or trips slow down the heating rate of the turbine, controlling the differential growth and preventing damage during thermal transients.

Answer: RTDs and Thermocouples

• Primary Sensors:
  1. Resistance Temperature Detectors (RTDs - typically Pt100): Preferred for measuring Lube Oil/Bearing metal temperatures due to their high **accuracy and stability** over narrow temperature ranges.
  2. Thermocouples (Type K or J): Used where **higher temperatures** are involved (e.g., turbine exhaust gas) or when a faster response is needed, but they are generally less accurate than RTDs for bearing metal.
• Typical Setpoints (Oil Film/Bearing Metal):
  • Alert (Warning): Around 90 °C (194 °F). Indicates high friction or oil film breakdown risk.
  • Danger (Trip): Around 100 °C to 110 °C (212 °F to 230 °F). Indicates immediate risk of bearing wipe (melting of babbitt layer) and requires an immediate shutdown.

Answer: Dealing with Shaft Imperfections

• Mechanical Runout: Physical imperfections on the shaft surface (scratches, ovality, eccentricity) that are not true vibration but are picked up by the proximity probe as a 1 (once-per-revolution)** signal.
• Electrical Runout: Localized variations in the magnetic properties or conductivity of the shaft material, which also cause a false 1 signal from the Eddy Current probe.
• Compensation Method (Vector Compensation):
  1. Data Acquisition: The 1 vibration data (amplitude and phase) is measured at a **slow roll speed** (below 500 RPM), where actual dynamic vibration is negligible. This measurement is primarily the combined runout vector.
  2. Vector Subtraction: The measured runout vector is then **subtracted vectorially** (using phase information) from the total measured 1 vibration amplitude at operating speed.
  3. Result: This leaves only the true dynamic 1 vibration component caused by imbalance, thermal bow, or other operational forces.

Answer: Preventing Catastrophic Failure

• Overspeed: An uncontrolled increase in the rotational speed of a machine (especially turbines) that can lead to centrifugal forces exceeding the yield strength of the materials, causing the wheel to burst.
• Instrumentation: Typically involves **three independent speed sensors** (proximity probes or magnetic pickups) focused on a gear or speed wheel. The signal is fed into a dedicated Overspeed Detection System (e.g., Bently Nevada 3500/50).
• Triple Redundancy (2-out-of-3 Voting):
  1. Safety: The system is configured to initiate a trip only if *at least two* of the three independent speed channels indicate that the overspeed setpoint has been exceeded.
  2. Availability: This configuration prevents a single false signal (e.g., a faulty sensor or loose wiring) from causing an unwarranted shutdown, maximizing machine availability.
  3. Failure Detection: It allows one sensor to fail safely without compromising protection (if one fails high) or availability (if one fails low). The single failed sensor can be identified and replaced while the machine is running.

Answer: Static Rotor Position Check

• Gap Voltage Significance: The gap voltage (DC output) represents the **static distance** between the probe tip and the shaft surface. This distance must be set to the middle of the probe's linear range, known as the **Preload**.
• Checking Procedure:
  1. Measurement Point: The DC voltage is typically measured at the **monitor module test point** or directly at the Proximitor output (before the monitor).
  2. Target Value: For a standard 8 mm probe (e.g., 200 mV/mil), the typical linear range is -2 VDC to -18 VDC. The desired preload (mid-range) is usually set around -10 VDC (or a gap of 100 mils or 2.54 mm).
• Purpose: A correct preload ensures that:
  • The shaft's dynamic movement (vibration) will be accurately captured without exceeding the linear range (avoiding clipping).
  • The shaft's static position is centered within the bearing clearance.

Answer: Units and Frequency Response

• Accelerometer (Measures Acceleration):
  1. Unit: Measures acceleration in **g's** (multiples of gravity).
  2. Preference: Ideal for **high-frequency** faults (above 1,000 Hz), such as rolling element bearing defects, gear mesh problems, or blade passing frequency, where the energy is concentrated.
  3. Principle: Uses the piezoelectric effect; the output voltage is proportional to the force (acceleration) exerted on a seismic mass.
• Velocity Transducer (Measures Velocity):
  1. Unit: Measures velocity in **in/sec** or **mm/s**.
  2. Preference: Ideal for **medium-frequency** faults, typically on casing measurements for machines with rolling element bearings or intermediate-speed gearboxes.
  3. Integration: An accelerometer can be electronically integrated once to get velocity, or twice to get displacement. Modern systems often use accelerometers and perform integration digitally.

Answer: Speed and Load Control

• Primary Control: The governor system is the core speed and load control mechanism. It precisely modulates the **steam (or fuel) flow** to the turbine by manipulating the control valves.
• Key Functions:
  1. Speed Regulation: Maintains the turbine speed at a precise setpoint (isochronous control) or allows speed to drop slightly with increasing load (droop control, common in power generation).
  2. Load Control: For turbines driving compressors or generators, the governor adjusts speed/steam flow to meet the process load demand (e.g., maintaining constant discharge pressure on a compressor).
  3. Startup/Shutdown: Safely controls the ramp rate of the machine during startup and coast-down phases, minimizing thermal stress and avoiding prolonged operation near critical speeds.

Answer: Lubrication vs. Containment

• Lube Oil Pressure Monitoring:
  1. Purpose: Ensures an adequate oil film for cooling and friction reduction in the journal and thrust bearings.
  2. Trip Logic: A **low lube oil pressure** trip is among the most immediate and critical shutdowns, as bearing failure can occur within seconds of oil loss.
• Seal Oil Pressure Monitoring (For hazardous gas compressors):
  1. Purpose: Provides a liquid barrier to prevent the process gas (which may be hazardous) from escaping to the atmosphere or mixing with the main lube oil system.
  2. Control Requirement: Seal oil pressure must be maintained at a **differential pressure** slightly higher (e.g., 5 to 10 psi) than the process gas pressure being sealed. This requires a dedicated DP transmitter and controller.
  3. Trip Logic: **Low differential pressure** (Seal Oil pressure <= Process Gas pressure) triggers an alarm or shutdown to prevent gas leakage.

Answer: Raw Vibration Signal

• Definition: The Time Waveform (TWF) is the **raw, unprocessed voltage signal** from the vibration transducer, plotted as amplitude versus time. It captures the full fidelity of the vibration, including all frequency components and phase information.
• Diagnostic Utility (Superior to FFT):
  1. Impact and Rub Events: TWF clearly shows sharp peaks or sudden amplitude spikes, which are characteristic signatures of **rotor-stator rubs**, blade strikes, or looseness (impacting the casing). FFT can sometimes miss these transient features.
  2. Non-Synchronous Events: It helps identify and visualize complex, non-periodic behavior like **oil whirl/whip** and random turbulent flow noise, which appear as characteristic patterns.
  3. Signal Clipping: It is the definitive tool to check if a vibration signal is being "clipped" or saturated, which indicates that the proximity probe is operating outside its linear range.

Answer: Tracking Synchronous Vibration

• Definition: A Bode Plot consists of two graphs plotted against rotational speed: **Amplitude** (of 1 vibration) and **Phase Angle** (of 1 vibration).
• Diagnostic Use during Run-up:
  1. Critical Speed Identification: The plot precisely locates the **Critical Speed (N_cr)**, where the amplitude curve shows a distinct peak and the phase curve shifts rapidly by approximately 90° to 180°.
  2. Rotor Balance Condition: The phase angle at the critical speed indicates the location of the **heavy spot** (imbalance). Analyzing the phase change across the N_cr is critical for determining where to place balance weights.
  3. Resonance Assessment: Allows engineers to verify that the machine's primary operating speed is safely separated from the N_cr (API 670 specifies a separation margin of at least 10%).

Answer: Rotor Resonance

• Definition: Critical speed (N_cr) is the rotational speed at which the operating frequency of the machine aligns with the **natural frequency** of the rotor-bearing system. At this speed, the imbalance forces are highly amplified, leading to large, often dangerous, vibration amplitudes.
• Monitoring Importance:
  1. Transient Alarms: Dedicated transient trip multipliers are often set up in monitoring systems (like BN 3500) to temporarily allow higher vibration levels while the machine rapidly passes through the N_cr during startup/shutdown.
  2. Separation Margin: Continuous monitoring confirms that the machine is not running near N_cr. A shifting N_cr (due to stiffness change or foundation looseness) is an indication of a major mechanical fault.
  3. Safe Operation: Prolonged operation at a critical speed is prohibited due to the high stress levels and is a key machine protection rule.

Answer: Actuation Fluid Health

• Pressure Monitoring (Most Critical):
  1. High-Pressure Trip Oil: High-accuracy pressure switches and transmitters on the trip header (trip oil pressure). A drop in this pressure indicates a trip has been initiated and triggers the main stop valve to close.
  2. Main Hydraulic Pressure: Transmitters monitor the main pump discharge pressure, ensuring enough force is available to operate the steam control valves and the stop valve. Low pressure alarms/trips.
• Quality and Level Monitoring:
  • Reservoir Level: Level switches (LSH/LSL) and transmitters (LT) to monitor minimum oil inventory (low level is a trip).
  • Temperature: RTDs or thermocouples monitor reservoir temperature and oil cooler outlet temperature for control and protection.
  • Filtration DP: Differential Pressure across filters/strainers indicates blockage and potential for contaminated oil, which can stick valves.

Answer: Shaft Centerline Movement

• Configuration: Two non-contact proximity probes are mounted orthogonally ($90^\circ$ apart, typically designated 'X' and 'Y') near each journal bearing. Both measure the shaft's displacement relative to the bearing housing.
• Orbit Plot Creation:
  1. Data Acquisition: The two sinusoidal displacement signals are simultaneously acquired and plotted against each other (X vs. Y).
  2. Visualization: The resulting plot is the **shaft orbit**, which visually represents the physical path the shaft centerline takes inside the bearing clearance during rotation.
• Diagnostic Use:
  • The orbit shape (round, elliptical, lopsided) helps diagnose specific faults like **imbalance, misalignment,** and **oil whirl/whip**.
  • The orbit direction (forward/reverse precession) is a key indicator of internal rub or fluid dynamic instability.

Answer: Safety and Intervention Time

• Primary Goal (Intervention): The Alert setpoint is a **warning** that a deterioration in machine health has occurred. The 2 times scaling provides maintenance and operations personnel with an adequate, quantifiable window of time to investigate the cause of the vibration and plan corrective action *before* a catastrophic failure occurs.
• Safety and Reliability:
  1. Proportional Risk: Vibration energy is often proportional to the square of the amplitude. Doubling the amplitude represents a significantly higher risk, justifying the trip action.
  2. Avoiding Spurious Trips: Setting the trip level too close to the alert level could result in a **spurious trip** (unnecessary shutdown) due to minor process fluctuations, reducing plant availability. The 2 times margin provides robustness.

Answer: Combustion Verification and Safety

• Function: Flame detectors (often using UV/IR technology) are mounted on the combustion casing to continuously monitor the intensity and presence of the flame within the combustion chamber.
• Safety Role (Flame-Out Protection):
  1. Ignition Verification: Confirms the presence of a stable flame during the ignition sequence, allowing the fuel valves to remain open.
  2. Preventing Explosion: If the flame is lost (a "flame-out") while fuel is still being supplied, the unburnt fuel would accumulate in the hot exhaust section. The flame detector must immediately signal the system to close the fuel valves to prevent a **secondary ignition/explosion**.
  3. Signal Output: The detector typically provides a binary (ON/OFF) signal to the turbine control system (TCS) indicating flame presence.

Answer: Emissions and Efficiency Control

• Oxygen (O2) Analysis:
  1. Sensor Type: Typically an **in-situ Zirconia Oxide analyzer**. It measures the residual O2 in the flue gas.
  2. Criticality: O2 is the primary indicator of **combustion efficiency** and **air-to-fuel ratio**. Maintaining optimal O2 ensures complete combustion while minimizing excess air (which wastes energy).
• Carbon Monoxide (CO) Analysis:
  1. Sensor Type: Often a **Non-Dispersive Infrared (NDIR)** or electrochemical analyzer.
  2. Criticality: O2 is a toxic gas and a byproduct of **incomplete combustion**. Monitoring CO is essential for meeting environmental emissions limits and can indicate burner or fuel control problems.

Answer: Protection vs. Accelerometer Systems

• API 670 (Machinery Protection Systems):
  1. Focus: Sets the minimum requirements for **protection systems** on critical rotating machinery (turbines, large compressors).
  2. Scope: Covers monitoring of **shaft parameters** (relative vibration, axial position, speed) and critical bearing temperatures (RTDs/T/Cs). It mandates specific requirements for redundancy and trip functionality.
• API 678 (Accelerometer-Based Vibration Monitoring Systems):
  1. Focus: Specifies requirements for vibration monitoring using **accelerometers** on **less critical** equipment or machines with rolling element bearings.
  2. Scope: Focuses on casing-based vibration (absolute vibration) and is more oriented toward condition monitoring and diagnostics rather than primary machine protection for fluid-film bearing machines.

Answer: Binary vs. Diagnostic

• Vibration Switch:
  1. Operation: Typically a **seismic** device that measures overall casing velocity or acceleration. When vibration exceeds a single, preset threshold, it triggers a simple **latching relay** (ON/OFF) signal.
  2. Application: Used for **non-critical** or "balance-of-plant" equipment (e.g., small pumps, fans) where detailed diagnostics are not necessary or cost-justified. It provides no trend data or frequency analysis.
• Continuous Monitoring System (CMS):
  1. Operation: Measures raw signals (displacement, velocity, or acceleration), performs complex signal processing (**FFT, phase analysis**), tracks multiple setpoints (Alert/Danger), and provides continuous data trending.
  2. Application: **Mandatory for critical, high-speed machinery** (turbines, large compressors) where real-time diagnostics and automatic shutdown protection are essential for safety and uptime.

Answer: Eliminating Noise (EMI/RFI)

• Problem: Vibration sensors output highly sensitive, low-voltage signals (often in the millivolt range). These signals are susceptible to Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI) from nearby motors, power cables, and process noise.
• Shielding (Noise Prevention):
  1. Mechanism: Coaxial cables use a mesh shield surrounding the center conductor. This shield acts as a **Faraday cage** to intercept external electrical noise.
  2. Requirement: The shield must be connected to earth ground at **one point only** (typically at the monitor end) to prevent ground loops.
• Grounding (Potential Equalization):
  • Ensures that all components (machine casing, Proximitor housing, monitor rack) are at the same electrical potential, preventing common-mode noise and ground loops which can inject false vibration signals.

Answer: 3D Frequency Analysis

• Definition: A Waterfall Plot is a series of **FFT (Frequency Spectrum) plots** stacked together to show how the vibration characteristics change over time or, more commonly, over a change in rotational speed (e.g., during a startup or coast-down).
• Plot Axes:
  • **X-Axis:** Frequency (CPM or Hz)
  • **Y-Axis (Z-Axis in 3D):** Amplitude (mils,mm/s, or g's)
  • **Z-Axis (Vertical Stacking):** Time or Speed (RPM)
• Diagnostic Utility:
  1. Identifying Natural Frequencies: Tracks spectral peaks that remain fixed in frequency but change in amplitude with speed, indicating a **structural resonance**.
  2. Tracking Synchronous/Non-Synchronous Events: Clearly separates faults related to speed (e.g., 1 times imbalance) from faults not related to speed (e.g., oil whirl at 0.4 times).

Answer: Protecting Bearings from EDM

• Purpose of Grounding Brush: To provide a low-resistance path to earth ground for any stray electrical currents generated on the shaft (often due to static electricity, magnetic fields, or VFDs).
• Risk of Failure: If the brush fails or contact resistance is too high, the current will seek the next easiest path to ground, which is often through the **bearing oil film**. This results in **Electrical Discharge Machining (EDM)**, causing pitting, frosting, and premature failure of the bearing metal.
• Instrumentation/Monitoring:
  1. Shaft Voltage Probe: A proximity probe (or specialized sensor) is often used to continuously measure the **DC voltage potential** between the shaft and the machine casing (ground).
  2. Alarm Condition: If the measured voltage exceeds a set limit (e.g., 10 VDC), it indicates that the grounding brush is ineffective, triggering an alarm for immediate attention.

Answer: Steam Cycle Integration

• Standard Protection: BFP-Ts, being critical, require full API $670$ protection: **Relative Vibration, Axial Position, Dual Overspeed**, and Bearing Temperature monitoring.
• Specific Steam Path Monitoring:
  1. Steam Chest Pressure: Pressure transmitter and control valve positioner are required for the governor to manage speed and load by controlling the high-pressure steam admission.
  2. Exhaust Casing Temperature: Monitored closely by thermocouples. Extremely high temperatures can indicate insufficient cooling, low steam flow (windage), or broken seals. Low temperatures can indicate water carry-over.
  3. Water Induction Protection: High-level switches/probes in the exhaust casing or low-point drains detect condensate buildup (water slugs), which can severely damage blades and require a trip.

Answer: Automatic vs. Absolute Shutdown

• Trip-Multiply (Vibration Protection):
  1. Function: A temporary, automatic increase in the vibration trip setpoint (e.g., 1.5 times or 2 times the normal Danger level), typically engaged only during the time required to pass through a Critical Speed during startup/shutdown.
  2. Control: Software-controlled and temporary, intended only to maintain protection integrity during expected high-vibration periods.
• Emergency Trip (E-Trip / Emergency Shutdown):
  1. Function: An immediate, absolute shutdown command that often bypasses the primary protection system and directly actuates the main stop valve (via a dedicated solenoid or hydraulic circuit).
  2. Trigger: Activated manually (E-Stop push button) or by external severe hazards (e.g., high-high gas detection, fire detection). It is the final safety layer.

Answer: Misalignment and Looseness

• Detection: 2 times vibration is detected as a distinct peak on the **FFT Spectrum** at a frequency exactly twice the 1 times (running speed) frequency. This fault component is typically higher than the 1 times component when the fault is severe.
• Common Causes:
  1. Misalignment (Angular and Parallel): The most common cause. The shaft deflects twice per revolution due to coupling stresses, generating a strong 2 times component. Angular misalignment often causes strong axial 2 times vibration.
  2. Mechanical Looseness: Excessive clearance in bearing pedestals, foundation, or even internal bearing clearance. The rotor "hits" or contacts the limiting surface twice per revolution as it attempts to travel its elliptical orbit.
  3. Bent Shaft: If the shaft has a complex or severe bow, it can generate higher-order harmonics, including $2 \times$.

Answer: Relative Shaft vs. Absolute Casing

• Proximity Probe (Relative, Shaft):
  • Measures **displacement (mils)** of the rotor shaft relative to the bearing housing.
  • Essential for **critical machines with fluid-film bearings** (turbines, compressors) to monitor shaft stability and internal clearance.
• Velocity Sensor (Absolute, Casing):
  • Measures **velocity (in/s)** of the bearing housing or casing relative to the ground.
  • Essential for machines with **rolling-element bearings** or for monitoring structural defects, as the energy transfer is higher at the casing.
• Combined Use: On large machines, both may be used: proximity probes for shaft protection, and velocity sensors/accelerometers for structural integrity and rolling element health (if present).

Answer: Verifying Linear Range and Scale Factor

• Equipment Required: A specialized **micrometer fixture** with a known shaft target material, a DC voltmeter, and a measuring tool (e.g., feeler gauge).
• Calibration Steps:
  1. Setup: The probe and its Proximitor (driver) are connected to the test bench and power supply.
  2. Gap Adjustment: The micrometer is used to mechanically adjust the distance (gap) between the probe tip and the target plate across the probe's specified linear range (e.g., 10 to 90 mils).
  3. Scale Factor Check: At several known gap points, the output DC voltage is recorded. The difference in voltage for a known change in gap distance is calculated to verify the probe's **Scale Factor** (e.g., 200 mV/mil).
  4. Linearity: The process verifies that the voltage output is linear across the entire required operating range, confirming the system's accuracy.

Answer: Preventing Lube Oil Overheating

• Purpose: The cooling water system removes heat from the lube oil via a heat exchanger (cooler). Loss of cooling water flow leads to rapidly increasing lube oil temperature, which can damage the babbitt bearings and break down the oil's properties.
• Instrumentation Used:
  1. Flow Switches/Transmitters (FS/FT): Often installed on the cooling water outlet header. Low flow alarms trigger an investigation. If flow is completely lost (e.g., due to valve closure or pump failure), a trip may be initiated depending on machine criticality and run-down time.
  2. Temperature Control: A Temperature Control Valve (TCV) regulates the cooling water flow based on the Lube Oil temperature setpoint, ensuring optimal oil temperature is maintained.

Answer: Stiffness Anisotropy and Sub-Synchronous Peaks

• Mechanism: A crack introduces non-uniform stiffness (anisotropy) into the rotor. The rotor's dynamic behavior changes depending on whether the crack is open or closed, which happens twice per revolution.
• Vibration Signatures (Critical):
  1. High 2 times Vibration: The most common initial indicator is a sudden, high peak at **2 times running speed** in the spectrum, often accompanied by a rapid change in the 1 times phase angle.
  2. Sub-Synchronous Component: The appearance of a vibration component at **0.5 times running speed** (or fractional multiples) can be a strong indicator of a severe crack.
  3. Monitoring 1 times Vector Trend: The 1 times vibration amplitude and phase angle trend over time often exhibit strange, non-linear behavior, especially in response to load or thermal changes.

Answer: Fault Frequency Correlation

• Displacement (mils or mu m):
  • Measures physical distance moved.
  • Best for **low-frequency** faults: **Imbalance** and **Rotor Bow**. (Monitored via Proximity Probes).
• Velocity (in/sec or mm/s):
  • Measures the speed of movement (proportional to energy).
  • Best for **mid-frequency** faults: **General looseness** and **Misalignment**. (Monitored via Velocity Transducers).
• Acceleration (g's):
  • Measures the rate of speed change.
  • Best for **high-frequency** faults: **Rolling element bearing defects** (impacts), **Gear mesh frequency**, and **Cavitation**. (Monitored via Accelerometers).

Answer: Channel Malfunction

• Definition: 'Not OK' is a system health check status that indicates a malfunction in a specific monitoring channel (sensor, cable, or monitor module). It means the channel can no longer be trusted to provide accurate data or protection.
• Common Causes:
  1. Sensor/Cable Fault: Open circuit, short circuit, or excessive noise on the transducer wiring.
  2. Probe Gap Out of Range: The DC gap voltage is outside the safe operating limits (e.g., shaft has moved too close or too far).
  3. Module Error: An internal electronic failure within the monitor module itself.
• Fail-Safe Action: When a channel goes 'Not OK', the associated monitor module will typically **bypass its trip logic** for that channel to prevent a spurious trip (fail safe). An alarm is always generated immediately, requiring operator attention.

Answer: Max Temperature Protection

• Instrumentation: Multiple RTDs (often three) are embedded in the babbitt layer of the bearing metal, typically positioned in the active (load zone) and inactive thrust faces, or spread radially in a journal bearing.
• Hot Spot Detection: A failure may occur as a highly localized "hot spot."
  1. Max Select Logic: For protection purposes, the monitoring system is almost always configured to initiate an alarm or trip based on the **maximum reading** from all RTDs in that bearing. This ensures the fastest response to the hottest point.
  2. Averaging (Diagnostics): The average temperature may be calculated and trended for long-term health monitoring, but it is not typically used for the primary trip function, as it would dilute the high temperature of a dangerous local hot spot.

Answer: Aerodynamic Efficiency Deterioration

• Mechanism of Fouling: Fouling (deposits on impellers, blades, or diffusers) changes the aerodynamic shape, increasing roughness and reducing the volumetric flow capacity and efficiency of the compressor.
• Key Instrumentation and Metrics:
  1. Pressure and Temperature Transmitters (P_1, P_2, T_1 and T_2): These are used to calculate the actual **Isentropic Efficiency** and **Head** developed by the compressor. Fouling causes a measurable drop in efficiency and head for a given mass flow.
  2. Flow Measurement (FT): Required to determine the throughput. Fouling causes the required **power consumption** (monitored by generator/motor instrumentation) to increase for the same output flow, or flow to drop for the same power.
  3. Inlet DP: Differential pressure transmitters across the suction filter indicate blockage, which is related to the source of fouling.

Answer: Signal Isolation (High Impedance)

• Function: A buffer amplifier (or unity gain amplifier) has a very **high input impedance** and a very **low output impedance**. Its primary job is to **isolate** the protection monitoring circuit from any external diagnostic or data acquisition systems.
• Importance:
  1. Preventing Loading: Ensures that connecting an external device (e.g., a portable analyzer or DCS analog input) does not "load down" the sensitive vibration signal, which could degrade the signal quality or compromise the integrity of the primary protection trip circuit.
  2. Signal Integrity: It copies the signal faithfully, providing a robust, buffered output for multiple users without impacting the signal being processed by the primary monitor.

Answer: LVDT Measurement

• Purpose: To ensure that the turbine casing expands symmetrically and within design limits during heating (startup) or cooling (shutdown), preventing excessive stress or misalignment with connected equipment (e.g., a generator).
• Instrumentation: **Linear Variable Differential Transformers (LVDTs)** are the preferred sensors for this measurement.
• LVDT Mechanism:
  1. Installation: The LVDT body is mounted on the foundation (fixed reference), and its core is attached to the turbine casing.
  2. Measurement: As the casing expands (moves), the LVDT core moves, producing an electrical signal (AC or DC voltage) proportional to the absolute linear displacement.

Answer: Contacting Displacement

• Principle: A Shaft Rider Probe is a **contacting sensor** that uses a stylus or wheel pressed against the shaft surface (or sometimes a fixed collar). The mechanical movement of the stylus is converted into an electrical signal, typically measuring shaft displacement.
• Application and Limitation:
  1. Use Case: They are mainly found on older, less critical, or lower-speed machines, and sometimes used temporarily for initial run-out studies.
  2. Limitation: Due to the mechanical contact, they introduce friction, wear, and potential signal noise, and are not compliant with API 670 requirements for high-speed, fluid-film bearing machinery, which mandates non-contact proximity probes.

Answer: Governor Feedback and Safety

• Purpose: The position of the governor control valves determines the amount of steam (or fuel) entering the turbine, which directly controls the speed and load. Monitoring the position is crucial for feedback control and safety.
• Instrumentation:
  1. LVDTs (Linear Variable Differential Transformers): The preferred sensor due to high accuracy, repeatability, and non-contact operation with respect to the valve stem.
  2. Potentiometers: May be used in older or less critical systems, providing an electrical signal proportional to the valve stem's mechanical position.
• Control Function: The position signal acts as the **primary feedback** to the turbine governor (TCS). The governor uses this signal to verify that the valve has moved to the commanded position, closing the control loop for speed and load regulation.

Answer: Timing the Vibration Peak

• Definition: The 1 times Phase Angle is the angular measure (in degrees,0 to 360°) of the **time difference** between the once-per-revolution reference signal (Keyphasor pulse) and the peak of the 1 time vibration signal.
• Derivation Process:
  1. Reference Signal: The **Keyphasor probe** generates a voltage pulse aligned with a specific shaft feature (keyway or notch).
  2. Vibration Signal: The **Radial Proximity Probes** measure the shaft's 1 time displacement.
  3. Measurement: The monitoring system measures the time (or fraction of a revolution) between the Keyphasor pulse and the next positive-going peak of the 1 times vibration wave. This time is converted to an angle: 360° corresponds to one full revolution.
• Diagnostic Use: The phase angle pinpoints the exact **location of the high spot** (heavy spot) on the rotor, which is essential for trim balancing.

Answer: Preventing Water Induction

• Water Induction Risk: Condensate (water) can build up in low points of the turbine casing during startup or low load. If a slug of water enters the steam path, it can cause severe damage to the rotating blades due to water induction.
• Instrumentation Loop:
  1. Level Measurement: **Level switches (LSL)** or **Level Transmitters (LT)** are installed in the critical low points of the casing (e.g., steam chest or exhaust drain pots).
  2. Control Element: The signal controls a **pneumatic drain valve (LCV)** or a solenoid valve on the drain line.
  3. Safety Trip: A separate high-level switch (**LSHH**) provides a safety trip to the turbine's main stop valve if water reaches a dangerous level, protecting against catastrophic water induction.

Answer: Shaft Centerline Tracking

• Shaft Centerline Monitoring (SCM): The DC gap voltage, when trended over time and compared between the X and Y probes, gives the precise **static position** of the shaft center within the bearing clearance.
• Diagnostic Indications:
  1. Bearing Wear: A slow shift in the mean gap voltage can indicate gradual wear or degradation of the babbitt bearing (the shaft drops).
  2. Foundation/Casing Shift: A sudden, parallel shift in the X and Y gap voltages can indicate thermal growth issues or an external movement of the machine casing relative to the probes.
  3. Integrity Check: The DC gap confirms the monitoring channel is "OK." If the gap voltage moves out of the linear range, the channel is declared 'Not OK' to prevent protection errors.

Answer: High-Frequency Pressure Pulsation

• Purpose: To detect high-frequency pressure oscillations (pulsations) within the combustion chamber or gas path. These dynamics can indicate combustion instability, which leads to high-cycle fatigue damage of hot gas path components.
• Instrumentation:
  1. Piezoelectric Pressure Transducers: Specialized, high-frequency pressure transducers (often water-cooled) are installed directly on the combustion cans or transition pieces. These sensors are capable of measuring rapid pressure changes (up to 5 kHz or more).
  2. Monitoring System: The signals are processed by high-speed data acquisition systems (often integrated into the Turbine Control System or a dedicated module) that perform real-time Fast Fourier Transforms (FFT).
  3. Alarm Criteria: Alarms/Trips are based on spectral amplitude peaks at specific, known combustion resonant frequencies.

Answer: Signal Conditioning for Diagnostics

• Filters in Monitoring: Filters allow the analyst to isolate specific frequency ranges to focus on certain fault types, improving the signal-to-noise ratio for that specific analysis.
• Filter Types:
  1. Low-Pass Filter: Allows frequencies *below* a cutoff point to pass. Used to remove high-frequency noise from accelerometers when focusing on low-frequency components like 1 times or 2 times
  2. High-Pass Filter: Allows frequencies *above* a cutoff point to pass. Used to remove low-frequency content (like 1 times or electrical runout) to focus on high-frequency, impacting events like bearing faults.
  3. Band-Pass Filter: Allows frequencies *within* a specific narrow range to pass. Used to isolate a single frequency (e.g., 1 times or a gear mesh frequency) for precise amplitude and phase tracking.

Answer: Shaft Orbit Visualization

• Lissajous Figure (General): In a general sense, a Lissajous figure is the pattern created when two sinusoidal inputs are plotted orthogonally.
• Vibration Context (Shaft Orbit): In rotating machinery, the term refers to the trace created by plotting the two orthogonal proximity probe signals (X-probe vs. Y-probe). This trace is specifically called the **Shaft Orbit Plot**.
• Significance:
  1. Dynamic Motion: The orbit visualizes the actual path the shaft centerline takes during one or more revolutions.
  2. Fault Diagnosis: The shape of the orbit is crucial for diagnosing faults: an elliptical orbit indicates misalignment, a figure-eight or highly distorted orbit indicates looseness or a rub, and a small, round orbit indicates a healthy shaft.

Answer: Bode Plot Characteristics

• Test Procedure: The machine is subjected to a controlled, slow run-up or coast-down, and the 1 times amplitude and phase angle data are recorded continuously (Bode Plot).
• Identification Criteria (Simultaneous Events):
  1. Amplitude Peak: The rotational speed where the 1 times vibration amplitude curve reaches its **first major peak** (resonance).
  2. Phase Shift: Coincident with the amplitude peak, the 1 times phase angle curve exhibits a rapid, continuous shift of approximately **90° to 180°**.
  3. Rotor Lag: As the machine passes through N_cr, the rotor moves from vibrating approximately 0° to the heavy spot (below N_cr) to vibrating approximately 180° opposite the heavy spot (above N_cr).