Micrometer: Probe [PDF]

Zitiervorschau

Micrometer It is on the SI unit of length equal to one million of a meter, or equivalently one thousand of a millimetre. It is also commonly known as a micron. It can be written in scientific notation as 1*10-6 m , measuring 1/10,00,000 mill-unit of length or displacement equal to 0.001 inches or 25.4 micrometres. Probe Especially a proximity probe transducer sometimes used to describe any transducers. Probe Gap The physical distance between the face of a proximity probe to tip and the observed surface the distance can be expressed in terms of displacement (mills, micrometre) or in terms of voltage (millivolts) standard polarities convention dictates that a decreasing gap result is increasingly less negative output signals. Proportional Values Direct vibration amplitude, probe gap, and other vibration amplitude and phase information proportional values are mentioned below. Vibration Probe Field Settings 1. Measure the vibration proximitor drive voltage between common an V/T. It should be -24 VDC 2. Measure the DC output voltage between common and output. It should be between -9.5 V and -10 VDC (as per company this voltage standard will variable) if the output voltage is not between -9.5 V and -10 VDC then the vibration probe will need to be set. 3. Remove the vibration probe cover and slacker off the probe lock nut with two spanners. One on the lock nut and other on the probe adjustment nut adjust the probe depth until the reading on the voltmeter is between -9.5 and -10 Volts. Note: If the reading is below -9.5 V then the probe must be slacked (anticlockwise). If the voltage is above -10 V then probe must be tightened (clockwise). When the correct voltage is obtained tightened the bollon lock nut. Refit vibration probe cover. 4.

If the probe adjustment does not affect the voltage reading try & different probe and if this does not work replace to vibration interface proximiter. 5. If the fault is still present, move the CO-AX cable from both the proximiter and the vibration probe-measure resistance of the CO-AX cable by placing a short circuit between the cable center conductor and the shield at line another end. 6. The resistance should be approximately 0.8 ohms per meter length

Objective: To check the and healthine probes ( make

characteristics ss of vibration : Bently Nevada )

Principle: Proximity transducers use Eddy Current principle to measure the distance between the probe Tip and the surface to be observed. The proximeter generate a low power radio frequency (RF) signal. This RF signal is connected to a coil of wire inside the probe tip by the extension cable. When No conductive material is within the range of RF signal which surrounds the probe tip, virtually all of the power released to the surrounding area is returned to the probe. When a conductive surface approaches the probe tip, the Rf signal sets up small eddy currents on the surface. These eddy currents creates a measurable power loss in the Rf signal. when vibration tip nearer the target the greater the power loss. The system uses this power loss to generate as output voltage. The output voltage of the proximate is linearly proportional to gap over a wide range.

Procedure: Follow the following steps 1) Physically check the vibration probe and extension cable for any damages, if it is please replace with new one. 2) Check resistance of vibration probe and continuity of extension cable it should be in between 5 To 9Ω and 5 to 20 Ω (resistance value varies from model to model) 3) Place the vibration probe on TK-3 spindle and adjust to the target plate surface and make sure the scale should be zero. 4) Connect multimeter on proximate common and Volt terminals. Apply input voltage. 5) Measure the output voltage (multimeter reading) and increase the target distance in the TK-3 calibrator and follow the same steps as per given table and note down the output voltages at different displacements (gaps). 6) After this make a graph which shows the relation between gap and voltage. 7) If graph is linear in between 10 mils to 80 mils then probe is in good condition.

Note: Change in gap is within 80 mils Linear Range which is in between 10 mils and 90 mils

Determine Lifting of Shaft using Proximity Sensor

Say, for example, a machine is installed with Bentley Nevada proximity transducer system [3300 XL 8 mm], as per the catalog of this proximity sensor, the sensitivity of the probe is 7.87 V/mm or 200 mV/mil (it is understood that proximity probe is installed within the distance specified in the catalog). The below graph is available in the catalog, which shows the sensor linear range. Suppose proximity probes are set at -10V output, it means that the gap between sensor and shaft is ~55 mils. This is reading prior to jacking. When the Jacking oil pump is started, and the shaft is lifted at that time, naturally, the gap between the probe and shaft will reduce. This is what we want to measure! Suppose after jacking, proximity reading went to -8 VDC. So 2VDC is reduced, which means that (7.87 VDC-> 1 mm then 2VDC->mm?) 0.25 mm (~10 mils) Gap has been reduced between probe and shaft with respect to earlier. This is the way that proximity probe can be used as distance measurement as the shaft is not rotating, it is only lifted. Now take the case 1 from the above proximity probe readings, Using Pythagoras formula, (2Z)2 = X2 + Y2… when we have X=Y then Z =  [1 / √2 ] * X X and Y can be known from Gap vs Voltage output. For above example, Z = [1 / √2 ] * 10 = 7.07 mils (0.18 mm) has been jacked up due to jacking oil. This can be rechecked with your field pointer dial reading. Once the proximity reading matches the field reading, this will reduce the time for the OEM engineer when he finds improper jacking. Although proximity sensors are used for various diagnostic analysis, for rotary equipment this proximity sensor reading helps calculating the actual jacking during initial installation and related maintenance & troubleshooting as this reading will serve as a base condition of shaft/rotor with respect to the casing. What is TSI? Turbine Supervisory Instrumentation system which supervises the turbine with the help of instruments Turbine Supervisory Instrumentation (TSI) Monitors some important parameters which directly or indirectly are linked & predict the healthiness of the turbine. Turbovisory Parameters

Vibration 1. 2.

Shaft Vibration Bearing Vibration

Turbine Expansion 1. 2. 3.

Absolute Expansion [HIP/LP] Differential Expansion [HIP/LP] Axial Shift

Speed Measurement 1. Key Phasor Also Read: Turbine Protective Devices

In order to achieve the purpose of the said measurement indicators, a tap with the world’s trusted manufacturer for vibration monitoring must be carried out, enter Bently Nevada. Tested by time and experience, the Bently Nevada 3500 rack processes the vibrations, axial positions and differential expansions.

Bently Nevada 3500 System configuration It communicates with the native Turbine Control system via Modbus TCP/IP, through its 3500/92 com gateway card (see Bently Nevada 3500 Rack in detail), in order to display analog figures in front of the Operator Work Stations (HMI).

Trip signals of Shaft Vibrations and Thrust positions are connected in a hardwire fashion from the 3500/32 relay modules to the Turbine Control system which will then demand to shut the solenoid trip block halting the flow of hydraulic oil to the governor and stop valves, closing them on fail-safe action. 1) 3500/15 Dual-Redundant Power Supply modules

Half-height module (120.7mm) each with the lower card acting as the primary source of power whiles the other upper card as the secondary (back-up) power. 2) 3500/22 Transient Data Interface (TDI)

Acts as the interface between the 3500 monitoring system and compatible software such as system condition monitoring and diagnostics TDI interfaces with the M series monitors connected in the rack to continuously collect steady-state and transient dynamic data. It also enables each monitor module in the rack backplane to be read and configured by the dedicated 3500 Rack System Configuration Software. 3) 3500/25 Key Phasor

Half height module which receives input signals from proximity probes or magnetic pickups and converts the signal to digital key phasor signals that indicate when the Key Phasor mark on the shaft is under the Key Phasor probe. A key phasor signal serves as a digital timing signal that is being utilized by monitor modules and external diagnostic equipment to measure vector parameters such as 1X amplitude and phase. 4) 3500/42M Proximitor / Seismic Monitor

A four channel monitor that accepts input from proximity and seismic transducers, it conditions each signal to generate various static vibrations and position measurements and compares the conditioned signals with user-programmable alarm and danger setpoints. Each channel can be configured to condition radial vibration, velocity and differential expansion among others. 5) 3500/45 Position Monitor : A four channel monitor almost similar with 42M in which it also

accepts inputs from proximity transducers as well as rotary position transducers and AC/DC Linear variable differential transformers (LVDTs).

Parameters considered as “proportional values” are conditioned from the inputs then compared to user-programmed alert and danger setpoints. 6) 3500/32M 4 Channel Relay

A four channel relay module that provides four relay hardwire outputs. Each relay used on the module can be programmed for an “Alarm Drive Logic” which is composed of AND and OR voting logic for alarm, danger and “Not-OK” statuses. The programming can be executed using the same 3500 Rack Configuration Software to meet the needs of the application. 7) 3500/92 Communication Gateway

Provides digital communication capabilities of all rack monitored values and statuses for integration with process control and automation systems, the Turbine Control system so to speak. The module can communicate via Ethernet TCP/IP and serial (RS232/422/485) protocols or on top of each other. Dual Over-speed Racks Apart from the Bently Nevada 3500 system, a TSI will not be complete without its speed supervising mechanism, enter JAQUET FT3100 rack which has the primary job of assuring that the rotor train’s speed is within limits and shall act when over-speed setpoints occur. Redundant sensing elements must be installed to make room for frequency control and governing calculations. It can be set-up in such a way that a dedicated set of sensors will be directly wired to the solenoid trip block while the other set takes care of the frequency conditioning function, while still wired to the same trip block. Turbine Speed As mentioned, turbine speed is picked up by probes which measures across a number of shaft gear teeth. A helpful formula when trying to simulate speed by means of a pulse generator will make the statement true.

For instance, take full speed no-load 3600 rpm measured across 132 teeth. The frequency you will be needing to inject is 7920 Hz. Take note that a certain terminal resistance is required in parallel to each speed acquisition channel. Usually a 1kΩ resistor will suffice. Shaft Position The thrust bearing is a fix point of the shaft itself relative to the casing. The thrust bearing can move axially within its clearances. The supervision of the axial shaft position allows to protect the turbine against shaft displacements caused by excessive thrust rates during transient conditions and disturbances.

Differential Expansion The axial clearances in blading and shaft seals are dimensioned so that during steady state and transient operation, no rubbing between rotating and stationary parts shall occur. Differential expansion must be supervised at locations where maximum values can occur during disturbances.

Shaft Vibration The duty of the shaft vibration measurements is to detect and monitor changes in the vibrational behavior of the turbine.

A pair of sensors are installed 45 degrees from the identified center line of the rotor shaft. This will constitute the left and right sectional vibration measurements.

The shaft vibration proximity sensor’s clearance must be adjusted according to a linear range of “gap voltage.” This value ranges from -11 to -9 VDC. Gap voltage (VDC) is a direct representation of distance between the probe’s tip and the shaft’s surface. This is the quiescent voltage that needs to be adjusted between the proximitor’s output voltage range limits. Any vibration of the shaft will cause the proximitor’s output voltage to vary in precise step: 200 mV/mil *1 mil = 1/1000 inch

(As to why it is negative DC voltage in the first place is an entire different story to tell. I might include the legend behind it as this article improves in the future.)

Proximity Transducer System provides an output voltage directly proportional to the distance between the probe tip and the observed conductive surface. It is capable of both static (position) and dynamic (vibration) measurements, and is primarily used for vibration and position measurement applications on fluid-film bearing machines, as well as Keyphasor and speed measurement applications.

 Proximity Transducer System

Proximity Transducer Systems provide an electrical signal that represents the distance between a conductive surface and the probe tip of the system.

The Proximitor contains electronics that provide two functions:

1. 2.

Generate a radio frequency (RF) signal using an oscillator circuit. Condition the RF signal to extract usable data using a demodulator circuit.

When conductive material is present in the RF field, Eddy Currents flow in the surface of that material. The penetration depth of the eddy currents depends on the material’s conductivity and permeability. 4140 steel penetration is around 0.003 inches (3 mils).

Once the probe is close enough to cause eddy currents to flow in a conductive material the RF signal is affected in two ways: 1. Amplitude is at a MINIMUM when distance (Gap) between probe and target material (Target) is at a MINIMUM. Maximum eddy current flow occurs. 2. Amplitude is at a MAXIMUM when distance (Gap) between probe and target material is at a MAXIMUM. Minimum eddy current flow occurs.

If the target is moving SLOWLY within the RF field, the signal amplitude INCREASES or DECREASES SLOWLY. If the target is moving RAPIDLY within the RF field, the signal amplitude INCREASES or DECREASES RAPIDLY. Oscillatory movement of the target causes the RF signal to modulate.

The demodulator circuit deals with slowly or rapidly changing signal amplitude in the same way. If the target is not oscillating, as might be the case with a thrust probe, the Proximitor output is a constant DC voltage, called the gap. If the target is oscillating (gap changing slowly or rapidly) the Proximitors output is a varying DC voltage (AC) shown above by a sine wave. If the probe is observing a vibration, the Proximitor will provide both a DC (gap) and an AC (vibration) component in the output signal. A typical system frequency response is from 0 Hz (DC) to 10 kHz. Newer transducer systems, such as the 3300XL proximity system have responses up to 12 kHz. Verification of Proximity Probes Probe response is verified by measuring and creating a calibration curve. Problems that can cause proximity probes to be out of tolerance: probe cable length power supply voltage crosstalk and sideview conditions target size and material Proximity Probe Used as a Keyphasor    

Proximity Probes Installation

Source: Bently Nevada A transducer that produces a voltage pulse for each turn of the shaft, called the Keyphasor. This keyphasor signal is used primarily to measure rotating shaft speed and serves as a reference for measuring vibration phase lag angle. It is an essential element in measuring rotor slow roll bow or runout information.The Keyphasor transducer is typically a proximity probe (recommended for permanent installations in which the probe observes a physical gap change event), an optical pickup (used for temporary installations in which the pickup observes a change in reflectivity event) or a magnetic pickup. Keyphasor is a Trademark owned by the Bentley Nevada Company. The system includes a proximity probe, extension cable, and proximitor sensor. Convention recommends that the prime keyphasor event is located on the driving unit. The keyphasor measurement required a coupling keyway or an elongated notch that can provide a once per turn event trigger for the signal pulse. Keyphasor

The keyphasor signal is a once per turn voltage pulse provided by a transducer, normally an eddy current proximity measurement. Also Read : Eddy Current Principle Keyphasor is an electric pulse, or trigger, which is derived from a point on a rotating shaft, it serves as a zero phase reference for determining where imbalance is on a rotor. The keyphasor in turbo machines are necessary to find phase angle of unbalance mass at the time of dynamic balancing.

You can monitor the machine health with vibration probes alone but if you want to analyze the cause of failure of the machine you need three-dimensional graph showing/analyzing the vibrations at any point of time when machine is running. If you don’t install the keyphasor you will come to know that there are vibrations in the machine but you will not point out at what direction, at what point, at what angle. The Keyphasor signal is used by monitoring, diagnostic, and management systems to generate filtered vibration. Amplitude, phase lag, speed and variety of other useful information. The phase is a critical part of this information. Without phase information, overall machine condition and machine faults would often be very difficult. The Keyphasor is used as a reference point of the shaft, 0 to 360 degrees. When the analysis is performed, the viewing of the 1X and 2X signals are important, X is defined as the running speed of the machine. The monitors read overall vibration signals at all frequencies, the Keyphasor is used to determine the speed. Let’s say a machine is running at 5,000 RPM, with a Key phasor, the instrument can filter the frequency to 5,000 rpm or 83.33 Hz. With this signal, one can view the shaft behavior or orbit inside the bearing. The phase is calculated as from the time the Keyphasor triggers to the first positive peak of the vibration signal. Keyphasor in turbomachines is necessary to find the phase angle of unbalance mass at the time of dynamic balancing. Balancing of the turbine rotor is essential. Manufacturers at their shop can balance the rotors with some other means even if keyphasor is not present. But when the rotor is to be balanced at the user’s site, keyphasor is required for balancing of Turbine rotor is essential. Advantages : The Keyphasor is signal is used to generate more than one third of the information regarding the condition of the machine.  Shaft crack detection  Rub Detection  Shaft Balancing  Shaft /Structural resonance detection. With the installation of the keyphasor, one can find out the direction the vibrations, at which point the vibrations are there, at which angle the shaft is vibrating.

Accelerometer Vibration Probe Principle In simple language we say that vibration is a movement or mechanical oscillation about an equilibrium position of a machine or component. Vibration is a by-product of an otherwise useful operation and is very difficult to avoid. Vibration unit is (m/s2) or units of gravitational constant “g,” where 1 g = 9.81 m/s2. Vibration measured two type as continuous measurement of critical machine and periodic measurement of machine after a selected interval. Why need to measure the vibration: Following are the reason to measure the vibration 1.

For checking the healthiness of critical machine (Like as compressor). By analysis the trend of vibration we can find the healthiness of critical machine. 2. Structural Analysis. Here we discussed about accelerometer type vibration measurement. Accelerometer type vibration Probe: Accelerometers type vibration probe works on piezoelectric effect. The acceleration of the gear box is transmitted to a seismic mass inside the accelerometer that generates a proportional force on the piezoelectric crystal. Then piezo electric crystal generates an electrical charge (mv) which is proportional to the applied force. This mv is transmitted to transmitter and then transmitter amplifies this mv and converts in  420 ma and send to the controller / display unit.

Also Read : Bently Nevada Vibration Probes Testing Point to choose best Accelerometer: 1. 2. 3.

On basis of mounting Range of measuring vibration Sensitivity: This is very important point to remember.Sensitivity is the relationship between the vibration and voltage produce by accelerometer at a reference Sensitivity defined mv/G. suppose a vibration probe which sensitivity is 5mv/G then its means at 1 gram forces its produce 5 mv.

Vibration Switch Working Principle A vibration switch is a device that recognizes the amplitude of the vibration to which it is exposed and provides some sort of response when this amplitude exceeds a predetermined threshold value. The switch response is typically an electrical contact closure or contact opening. The electrical contact may be either an electromechanical relay or solid-state device. Vibration Switch Working Principle Vibration Switch is working according to the pendulum switching principle.

A swinging permanent magnet holds a magnetic switch situated below in a particular position. Due to vibration, the magnetic field between Reed Switch and magnet changes. Thus the switch is actuated resulting in the stoppage of the equipment. The responding sensitivity of the system can be changed by adjusting the gap between switch and magnet. Thus vibrations with amplitude below a set value can be suppressed. Also Read : What is Keyphasor ? The free pendulum length and the responding frequency of the switch with a frequency slider can be adjusted exactly to the natural frequency of the equipment to be protected. Why use a vibration switch? Vibration switches are primarily used for protecting critical machinery from costly destructive failure by initiating an alarm or shutdown when excessive vibration of the machinery is detected. Conversely, a vibration switch can be utilized to warn when there is an absence of vibration, such as when a conveyor ceases to function due to a broken drive belt. Applications include all types of rotating or reciprocating machinery such as cooling tower fans, pumps, compressors etc. Checking the probe: 1.

Check the resistance of probe it should be range between kilo ohms to mega ohms or refer the vendor manual. 2. Tap on the vibration probe tip and check in indicator that reading is vary or not.

How Vibration sensors Work ? Sensors used to measure vibration come in three basic types: displacement, velocity, and acceleration. Displacement sensors measure changes in distance between a machine’s rotating element and its stationary housing (frame). Displacement sensors come in the form of a probe that threads into a hole drilled and tapped in the machine’s frame, just above the surface of a rotating shaft. Velocity and acceleration sensors, by contrast, measure the velocity or acceleration of whatever element the sensor is attached to, which is usually some external part of the machine frame. Vibration sensors

A design of displacement sensor manufactured by the Bently-Nevada corporation uses electromagnetic eddy current technology to sense the distance between the probe tip and the rotating machine shaft. The sensor itself is an encapsulated coil of wire, energized with high-frequency alternating current (AC). The magnetic field produced by the coil induces eddy currents in the metal shaft of the machine, as though the metal piece were a short-circuited secondary coil of a transformer (with the probe’s coil as the transformer primary winding). The closer the shaft moves toward the sensor tip, the tighter the magnetic coupling between the shaft and the sensor coil, and the stronger the eddy currents. The high-frequency oscillator circuit providing the sensor coil’s excitation signal becomes loaded by the induced eddy currents. Therefore, the oscillator’s load becomes a direct indication of how close the probe tip is to the metal shaft. This is not unlike the operation of a metal detector: measuring the proximity of a wire coil to any metal object by the degree of loading caused by eddy current induction. In the Bently-Nevada design, the oscillator circuit providing sensor coil excitation is called a proximitor.

The proximitor module is powered by an external DC power source, and drives the sensor coil through a coaxial cable. Proximity to the metal shaft is represented by a DC voltage output from the proximitor module, with 200 millivolts per mil (1 mil = 1 /1000 inch) of motion being the standard calibration.

Since the proximitor’s output voltage is a direct representation of distance between the probe’s tip and the shaft’s surface, a “quiet” signal (no vibration) will be a pure DC voltage. The probe is adjusted by a technician such that this quiescent voltage will lie between the proximitor’s output voltage range limits. Any vibration of the shaft will cause the proximitor’s output voltage to vary in precise step. A shaft vibration of 28.67 Hz, for instance, will cause the proximitor output signal to be a 28.67 Hz waveform superimposed on the DC “bias” voltage set by the initial probe/shaft gap. An oscilloscope connected to this output signal will show a direct representation of shaft vibration, as measured in the axis of the probe. In fact, any electronic test equipment capable of analyzing the voltage signal output by the proximitor may be used to analyze the machine’s vibration: oscilloscopes, spectrum analyzers, peakindicating voltmeters, RMS-indicating voltmeters, etc. Photographs of a Bently-Nevada displacement sensor (sensing axial vibration on a “ring” style air compressor) and two proximitor modules are shown here:

It is customary to arrange a set of three displacement probes at the end of a machine shaft to measure vibration: two radial probes and one axial (or thrust) probe. The purpose of this triaxial probe configuration is to measure shaft vibration (and/or shaft displacement) in all three dimensions:

It is also common to see one phase reference probe installed on the machine shaft, positioned in such a way that it detects the periodic passing of a keyway or other irregular feature on the shaft. The “keyphasor” signal will consist of one large pulse per revolution:

The purpose of a keyphasor signal is two-fold: to provide a reference point in the machine’s rotation to correlate other vibration signals against, and to provide a simple means of measuring shaft speed. The location in time of the pulse represents shaft position, while the frequency of that pulse signal represents shaft speed. For instance, if one of the radial displacement sensors indicates a high vibration at the same frequency as the shaft rotation (i.e. the shaft is bowed in one direction, like a banana spinning on its long axis), the phase shift between the vibration’s sinusoidal peak and the phase reference pulse will indicate to maintenance machinists where the machine is out of balance. This is not unlike automatic tire-balancing machines designed to measure imbalance in automobile tire and wheel assemblies: the machine must have some way of indicating to the human operator where a balancing weight should be placed, not just how far out of balance the tire is.

In the case of machine vibration monitoring equipment, the keyphasor signal and one of the axial displacement signals may be simultaneously plotted on a dual-trace oscilloscope for the purposes of determining the position of the imbalance on the machine shaft Vibration Severity The severity of machine vibration is standardized by the International Standards Organization (ISO) in the ISO 10816 publication. The standard describes acceptable vibration levels for four different classes of machines. This data, as laid out in the chart below, proves useful as a reference point when analyzing vibration measurements.

Guide to vibration severity per ISO 10816 Applications and Industries The table below describes the industries which commonly use vibration measurement, as well as the product traits for sensors designed for each. The unique characteristics of each industry determine ideal sensor characteristics; for example, the very slow rotation of turbines in the mining and wind power industries results in the use of a vibration sensor with a very low frequency response of around 1 Hz or less. In contrast, the oil and gas industry requires high frequency (greater than 10 Hz – 10 kHz) sensors to cope with the faster rotation of turbines and gears. Standards Standards related to vibration measurement are published and maintained by numerous standards bodies. Examples of these include: BS 7385 (Evaluation and measurement for vibration in buildings) SAA AS 2625.1 (Evaluation of machine vibration by measurements on non-rotating parts) ISO 10816 (Evaluation of mechanical vibration on non-rotating parts)

Basics of Vibration Measurement What is Vibration Vibration is static and dynamic imbalance of equipment. Vibration is the oscillation, or moving back and forth of an object. The word vibrations consciously or unconsciously use it as a measure of how well things are running. For vibration to get start it takes some effort, either external or internal to get vibration going, some input of energy through an applied force. Once we have put energy into the system to make it vibrate, how do we characterize the vibration? Amplitude and frequency are common characteristics. When we deal with several vibration phase also will becomes important. We can say that 3 physical characteristics control the vibration. 1. Mass 2. Stiffness (spring) 3. Damping Vibration measurement The principle characteristics of the vibration signal that we measure are 1. Amplitude 2. Frequency 3. Phase 4. Amplitude Amplitude Amplitude is a measure of how severe the vibration is and can be expressed in 3 different ways: Peak to peak, Zero to peak and RMS, depending on what signal we are measuring.

Vibration is measured either in terms of displacement, velocity or acceleration. Vibration displacement is always measured as Peak to Peak, a measure of the total excursion of the rotor or machine casing in MILS or MICROMETERS. Vibration velocity and Acceleration are measured as Zero to Peak or RMS. Units used are “inches per second” or “millimeters per second” for velocity or in terms of “G” or “meters per second per second” for acceleration. Frequency Frequency is a measure of how fast a body is vibrating and is used to identify the source of vibration. Normally Frequency is expressed in shaft rotative speed. If a vibration is at the same frequency as the shaft speed, this will be 1X or 1 time shaft speed. If it is twice it is 2X. Also the frequency may be expressed in cycles per second or Hertz, or in cycles per minute. The period of vibration is measured in seconds and the reciprocal calculated will give in Hertz.

Phase Phase is a simple timing relationship between 2 events which may be 2 vibration signals for Relative Phase measurements or a vibration signal and a keyphasor reference signal for Absolute measurements. Both these are important vibration signal properties.

To measure the relative phase between 2 vibration signals, both signals should be at the same frequency and should be in the same units ie. Both displacements, both velocity or both acceleration. Both signals may be taken as the reference and the relative phase is expressed as an angle between Oo and 180o leading or lagging. Shape or Form The shape or form can be viewed by using the oscilloscope. The shape can be viewed by combining the signals from the vertical and horizontal proximity transducers. For most machines this will be either circle for uniform mechanical impedance or an ellipse with low eccentricity where the mechanical impedance is not uniform in all directions. The shape can be a good indicator of non uniform mechanical impedance, preloads such as misalignment and rotor to stator rubbing. Reference frame for vibration measurement Each vibration transducer measures the vibration in a different way, either a relative measurement or an absolute measurement. Relative measurement The proximity transducer system measures the motion of the shaft relative to the transducer tip. As the transducer is located close to the bearing (less than 6”) the proximity probe can be considered to measure the motion of the shaft relative to the bearing. This gives an indication of the amount of available clearance taken up by the shaft motion. If the transducer mounting is in motion due to vibration, this will result in an output from the transducer which will appear as if the shaft is moving. If the shaft and the transducer mounting are moving together in phase, the resultant output from the probe will be zero as if there is no shaft vibration. Great care in mounting should be taken to ensure that this situation will not arise. Absolute measurement Absolute measurement or seismic measurement are made using either a velocity or acceleration transducer mounted on the bearing housing or machine casing. Absolute measurements are needed where casing or housing motion is significant. The velocity or acceleration transducer measures motion relative to free space, with the coil as reference for the velocity transducer and the mass as reference for the acceleration transducer.

Shaft absolute measurement is made by measuring the shaft relative displacement using a proximity probe and the bearing displacement using either a velocity probe or accelerometer. The velocity or acceleration measurement are integrated (or double integrated in the case of acceleromter) and then subtracted from the shaft relative displacement. Only in rare cases is the shaft absolute displacement required or machine measurement, shaft relative displacement usually provides sufficient information. Position measurement Axial Thrust position This is a measurement of the rotor within the thrust bearing clearance. The measurement is usually made using two proximity probe mounted in the thrust bearing observing the thrust collar.

If this is not practical, the probes may be mounted at some location close to the bearing observing the shaft end or a specially fitted collar. To ensure reliable measurements, axial thrust position should always be made using 2 transducers. The signal from the transducers are monitored using a dual voting thrust position monitor which looks at both the signals and compares them with the alarm levels. If either signal exceeds the first preset alarm value the alarm will be indicated and relay will change state. If the levels increase to the second preset level the monitor will indicate the alarm but unless both this signals exceeds this value the relay will not change its state. Radial Position Radial position of the shaft within the bearing clearance can be measured using the Dc signal installed from the proximity probe.

The DC signal is measured when the machine is at rest with the shaft sitting at the bottom of the bearing and again when the machine is running. With the shaft supported on its oil film, the change in DC voltage measured can be used to calculate the new position of the shaft center line. This can be a very important measurement to determine the condition of the shaft alignment and also to indicate any bearing wear which might be occurring. The signal needed to make these measurement are available at the front panel of the monitors. Differential measurement For large steam turbines with long shaft systems, an additional axial position measurement may be required to measure the position of the rotor at a location away from the machine thrust bearing. In all machines the thrust bearing is rigidly fixed to the machine foundation and the casings are free to move due to thermal expansion in an axial direction. For large machines the thermal expansion of the rotor will not be the same as the expansion of the casing. The differential expansion measurement is to measure this difference and ensure that the rotor does not touch the stationary parts. Shaft eccentricity This is the bow or bend in a machine shaft and is measured at very low shaft speed in the order of a few revolutions per minute. Ideally the proximity transducer is mounted some distance away from the bearing so that the maximum deflection will be detected when the machine is run at slow roll speed. The measurement made by the transducer is then not due to dynamic motion but is a purely measure of the shaft bow. Why do we need vibration monitoring? It is essential to monitor critical machines in the plants for increasing their efficiency and reliability. Hence real time vibration monitoring is the key to reduce frequent failures of machinery & keep high uptime. What causes vibrations? Unbalance of shaft, Bearing problem, cracking of the rings, Fluid coupling problem, Shaft misalignment, Oil whirl and other dynamic instabilities What is on line monitoring of vibrations? Time Based Maintenance System (TBM) is called preventive maintenance. One can extend the life of the machines by monitoring these online in a cost effective way. Vibration Monitoring and Analysis is the easiest way to keep machines healthy and efficient in the long run and increase the overall efficiency of the plant. It reduces the overall operating cost as well as the down time period. Vibration sensors are used to predict faults in a running machine without dismantling it and give a clear indication of the severity by showing the amplitude of vibration. Where do we need on-line vibration monitor? In Industries rotating machines are divided according to their criticality into three categories: 

First critical machine – Turbine and generator Secondary critical machines – ID fan, FD fan, PA fan, and boiler feed pump, cooling water pump, condensate extra pump, critical large HT motors of mills and other large motors.



Balance of plant machines – Coal handling plant crushers, cooling tower fans, raw water pumps and make up water pumps This explains where Vibration Monitoring is required and how critical is each machine if there is shut down. For taking the machines for maintenance we need to know the healthy state of the machine without dismantling it. This is possible only by online monitoring. What are the types of sensors for vibration monitoring? The three principal vibration sensor types are displacement, velocity, and accelerometer. The displacement transducer is an eddy current device, the velocity transducer is often a spring held magnet moving through a coil of wire or piezo velocity sensor, and the accelerometer is a piezoelectric device somewhat similar to ultrasonic transducers. What is the benefit of vibration monitoring? With vibration Monitoring system, we can prevent problems from arising and this saves a lot of time, money, and avoids frustration.   To increase equipment protection   To improve safety for personnel  To improve maintenance procedures  To detect problems early  To avoid catastrophic failures  To extend equipment life  To enhance operations What is the benefits of Vibration Analysis & diagnostic Software? Vibration analysis & diagnostic system that is applicable to a variety of rotating machinery, helps safe operation and to improve operational efficiency. It precisely keeps track of and quickly feeds back conditions of rotating machinery which are the key production assets of plants. It Helps customers improve productivity and reliability by optimizing plant operation. 

Detects abnormal symptoms from vibration characteristics or subtle changes in vibration. Reduces risks of unplanned production shutdown by taking proactive approach.  Advanced diagnostics realize assumption of causes and areas of anomalies and detailed analysis. Help users practice optimum, efficient maintenance. What is ‘vibration’? Vibration is defined as the motion of the equipment or its part to and from its rest (static) condition. Explain following with their sensitivity (output) and the measuring units: Radial: A vibration measurement across the radius of a rotating shaft. It is measured in terms of Micron. The sensitivity of a radial vibration pickup (eddy probe) is 200mv DC/mill (refer to the drawings for the exact parameters). Velocity: It is defined as the rate of change of distance traveled by the equipment. Velocity measurement is generally used for measuring the equipment body vibration. The sensitivity of a velocity pickup is 500mv DC/inch/sec (refer to the manufacturer drawings for the exact parameters).

Acceleration: It is defined as the rate of change of velocity. Acceleration measurement is generally used for measuring the equipment body vibration. The sensitivity of a velocity pickup is 100mv DC/inch/sec2 (refer to the manufacturer drawings for the exact parameters). What is the resistance of an ‘Eddy probe’? An eddy probe resistance should be between 5-8 ohms. Draw a sketch of an eddy probe calibration graph? Eddy probe calibration graph : Check Here What is gap voltage? Why and how is it set? Gap voltage is feed back voltage derived by setting the standard gap between the eddy probe tip and the rotating shaft. The gap voltage is the base DC voltage set to get the AC pulses peak per the radial vibration measured on the shaft. The gap voltage is set at –8.00V DC i.e. equal to a gap of 40 mill (between the probe tip and the measuring surface). or at -10.00V DC as per the system standard. Why an eddy probe and its probe driver is a matched pair? Eddy probe output is always measured after the probe driver. The characteristic of the eddy probe slightly differs from one to another. Always an eddy probe is installed on the shaft, after measuring and plotting an eddy probe calibration graph with its driver. The calibration is done by adjusting the sensitivity potentiometer on the eddy probe driver assigned for that particular eddy probe. What do the AC and DC signal represent in radial vibration, where ‘AC is super imposed on DC’? DC signal is the gap voltage set for –8.00V DC. DC signal is always present whether the unit is running or stopped. How much is the radial vibration, if the signal measured on a DVM is 135.00mv AC? The DVM measures the AC voltage in RMS (root mean square value). AC peak to peak =135.00*2 root 2  =  181.837mv Eddy probe sensitivity=200mv/mill or 200mv/25.4 Microns Hence, the vibration in Micron =(381.837*25.4)/200   =  48.49 Micron Why is vibration measurement very important on gas turbine and compressor? Gas turbines and compressors are high speed rotating equipment. On equipment when vibration exceeds the manufacturer limits can cause sever damage to both itself and to its associated components/parts. Hence it is important to measure and monitor vibration on the running equipment. Generally a high vibration pre-alarm and shutdown limits are set as per the manufactures recommendations.

What type of vibration measuring instrument is used on a gas turbine? Where are the vibrations measuring points? Since a gas turbine is hot engine, its bearing vibration measurement cannot be done through an eddy probe system. Generally a velocity or an acceleration pick-up is used for measuring and monitoring the vibration on the body of the gas turbine. The vibration pick-ups are generally installed on the turbine’s CT/GP and PT points. What is the type of vibration pick-up (contact or non-contact type) commonly installed on a gas turbine? On gas turbine vibration measurement, the ‘contact type’ peck-up such as a velocity or acceleration pick-up is used. What type of vibration measuring instrument is used on the compressors? Where is the vibration measuring points? On a gas compressor shaft/bearing, measurement is carried out using a eddy probe displacement system. On each gas compressor, on both forward and aft radial bearings (journal bearing) two eddy probes are installed in the ‘x’ and ‘y’ positions. What is the type of vibration pick-up (contact type or non-contact type) commonly installed on a gas compressor? A ‘non-contact type’- eddy probe is installed on gas compressors. What is ‘x or y function’ and, ‘x and y function’ selection on a radial vibration monitor? X or Y function: in this mode, the unit shut down when either ‘X’ or  ‘Y’ probe detects a high vibration exceeding the set point. X and Y function: in this mode, the unit shut down only when ‘X’ and ‘Y’ probes detects a high vibration exceeding the set point. What is a ‘Probe driver’? What does is it do? What is the other name for a probe driver? Probe driver is an amplifier installed in the field closer to the sensor. It transmits the field vibration signal to the remote control panel. The other name for the probe driver is ‘proximeter’. Radial, velocity and accelerometer probe drivers are of different types. What are the three wires used on a probe driver? The three wires terminated on the probe driver are: -24V DC (power supply), common and output signal. What is a ‘charge amplifier’? Where is it used? Charge Amplifier is a probe driver installed for the accelerometer pick-up.

What is an ‘axial displacement’? How is it measured? Axial displacement is the movement of the rotary shaft in the axial direction to and fro. Every centrifugal shaft is permitted to have a fixed axial movement designed by the manufacturer, this is to allow the compressor to take the load on the thrust collar. Axial displacement is measure using an eddy probe system. It is measured using the proportional change in the DC voltage (gap voltage) caused by the movement of the shaft from its center position to either side (+or-). What is the unit measurement of axial displacement? Axial displacement is measured in terms of “Microns” What is the term ‘float/end float’ in a compressor? It is the free axial movement of the compressor shaft. End float is the maximum possible movement of the shaft. It is a small amount approximately 250 Microns (refer to the manufacturer drawings for drawings for the exact parameters). What are the following assessments/recommendations of a vibration analysis? How are they corrected? Misalignment: Misalignment may cause two time RPM vibration. This is corrected by re-aligning the compressor shaft one to another. Unbalance: Unbalance in the compressor shaft may cause one time RPM vibration. This is corrected by balancing the compressor shaft on a balancing wheel. Overhauling: Multiple RPM vibration is symptom of damaged bearings. This is corrected by a total overhaul of the compressor or replacement of parts. (The above are only a simple clue and not a standard confirmed judgment) What precautions are to be taken when installing a vibration measuring instrument? Vibration measuring instrument such as a eddy probe, velocity and acceleration pick-ups, eddy probe drivers, charge amplifiers are sensitive instruments. The following care has to be taken while installing in the field. -Do not drop the instrument; this may result in the loss of their characteristic. -Fixing and tightening to be done as per the manufacturers recommended torque. -Proper care of the signal cable insulation. -Prevent any loose connections. …Etc. What are the reasons for a vibration ‘spike signal’? The following may cause a ‘spike signal’ in the vibration measurement.

-Loss of signal cable insulation. -Signal cable passing next to high voltage lines. -Improper earthling facility. -Grounding of the wires. -Loose mounting of the instrument field components…Etc. List the manufacturers names of vibration measuring instruments? The common vibration measuring instrument manufacturers are: Bently nevada Forbes Marshall Shinkawa Dymac Bell and Howell Which of the above manufacturer’s product is more reliable and why? With past experience, it is observed that the ‘Bently nevada’ vibration measuring system is more reliable due to its component layout on the control panel, accuracy and high gauge signal cable system in the field.