basics of vibration measurement - inst tools

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.

The force we apply to vibrate directly affects the vibration. The more force we apply, the greater the vibration amplitude. But what acts to limit the vibration? As we make stiffer, like a spring, the amplitude of vibration decreases.

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 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 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.

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.

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 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.

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.

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.

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.

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.

At our plant we have a 72OD x 1/2thick carbon steel pipe fitted with a 20 long stainless steel bellows expansion joint that has a 69.5OD x 3/8thick x 15 long carbon steel cantilevered liner that is welded to the inside surface of the 72OD pipe at one end. Inside the expansion joint is steam at partial vacuum and it is vibrating. We would like to measure the current orbit of the liner and try reducing vibration by installing jacking screws around the perimeter to dampen vibration. Do you have a proximity sensor that could detect the motion of the liner?

vibration sensor: working, types and applications

At present in the industry like research and development, the ability of monitoring, measuring as well as analyzing the vibration is very important. Unfortunately, the suitable techniques for making a measurement system for vibration with precise & repeatable are not always clear to researchers with the shades of test tools & analysis of vibration. There are some challenges related while measuring the vibration which includes a selection of suitable component, the configuration of the system, signal conditioning, analysis of waveform and setup. This article discusses what is a vibration sensor, working principle, types, and applications

The vibration sensor is also called a piezoelectric sensor. These sensors are flexible devices which are used for measuring various processes. This sensor uses the piezoelectric effects while measuring the changes within acceleration, pressure, temperature, force otherwise strain by changing to an electrical charge. This sensor is also used for deciding fragrances within the air by immediately measuring capacitance as well as quality.

The sensitivity of these sensors normally ranges from 10 mV/g to 100 mV/g, and there are lower and higher sensitivities are also accessible. The sensitivity of the sensor can be selected based on the application. So it is essential to know the levels of vibration amplitude range to which the sensor will be exposed throughout measurements.

Thus, this is all about vibration sensor. From the above information, finally, we can conclude that vibration is a difficult measurement which includes different parameters. Based on the goals of vibration measurement, the measurement technologies have benefits and drawbacks. These sensors are mainly used for measuring, analyzing, displaying, proximity, acceleration, displacement, etc. Here is a question for you, what are the advantages of vibration sensor?

coriolis mass flow meter working principle | helloinstru flow

The operation of a Coriolis flow meter is based on the mechanics of motion. The Coriolis force happens when a mass moves in a rotating inertial frame. The rotation is created by vibrating two opposing tubes on the flow meter. When a fluid flows through the opposed vibrating tubes, the tubes twist due to the Coriolis force. The twisting alternates with the vibration and creates two phase-shifted sinusoidal wave forms on coils mounted to the tubes. The amount of shift is proportional to the mass flow rate. In addition, the frequency of vibration is proportional to the fluid density. The signal shift and the frequency of vibration can be precisely measured, which makes the Coriolis meter one of the most accurate types of flow meters.

As this principle measures mass flow independent of what is within the tube, it can be directly applied to any fluid flowing through it LIQUID or GAS whereas thermal mass flow meters are dependent of the physical properties of the fluid. Furthermore, in parallel with the phase shift in frequency between inlet and outlet, it is also possible to measure the actual change in natural frequency. This change in frequency is in direct proportion to the density of the fluid and a further signal output can be derived. Having measured both the mass flow rate and the density it is possible to derive the volume flow rate.

transients induced either by switching of heavy electrical loads or lightning. Compatible surge protectors recommended by OEM for signal and power supply, with necessary mounting accessories shall be included in the offer. The surge protector shall be designed and tested as per the requirements of BS EN 62305 /IEEEC62.41 /IEC

rate.The signal shall be linearly proportional to the volume / Mass flow rate which shall be read by the batch controller. Number of pulses generated for a unit Volume/Mass transferred, shall be user configurable.

W: Witness pointimplies that Purchaser intends to witness the designated inspection feature. If Purchaser decides not towitness the relevant feature, production can proceed without permission of Purchaser. (Notification reqd)

S: Witness,but spot check basis. Initial operation will be witness point and subsequent operation will be witnessed at discretion of Purchaserconsidering the results of previous inspection. (Notification not reqd : Randam Inspection)

4) Monitoring/Observation ofMass Flow MeterVendor shop daily routine works (i.e.Storage of materials, Adherence to approved procedures,Testing tool calibration check, workmanship,cleanliness and etc.) shall be done by Purchaser during inspection visit.

5) 100% Mass Flow Meterinspection which is covered by the combination of witness / spot check / record review inspection & tests listed above and vendors originalQC activity shall be confirmed by Purchasers inspector prior to shipment.

measurement principle - laser doppler vibrometry

Light reflected from a moving object is subject to a change in frequency proportional to the objects velocity (Doppler effect). Measuring this frequency shift with an interferometer allows the precise determination of the vibrational motion of the object.

If waves are emitted from an approaching (or receding) source, successive wave crests reach the detector in smaller (larger) time intervals than they had been emitted. This phenomenon is observed as a shift in frequency is known as Doppler effect. An acoustic example is the apparent change in tone pitch of a siren of an ambulance passing by a pedestrian. The measured frequency shift fc of a laser with wavelength c is for practical vibration applications to high accuracy proportional to the velocity v,

The detected frequency shift is used to derive the velocity v(t) of the surface from which the laser light is reflected, as well as the displacement d(t) and acceleration a(t). In the case of a harmonic vibration with frequency f and displacement d(t) = D sin(2 f t), the amplitudes of displacement, velocity, and acceleration related through

The changes in frequency are by means of a Mach-Zehnder-interferometer converted into a time-series in intensity, whose frequency domain is accessible to further electronic processing. Within the interferometer the laser beam is split into a reference beam and a measurement beam. The light reflected from the probe is brought to interference with the reference beam. The intensity recorded in a photodetector contains, apart from the intensities of the reference beam Ic, and the reflected light Iv, a contribution which depends on the difference in optical path z,

The variation in intensity is independent on whether the object is approaching or moving away from the vibrometer. This ambiguity is removed by heterodyning.When the frequency of the reference beam is shifted by a fixed amount fb , the interference of both beams for a non-moving probe results in a harmonic intensity variation with frequency fb . This carrier signal cos (2 fb) is modulated by the motion at the measurement object. Depending on its direction of motion the frequency of the intensity is shifted towards larger or smaller frequencies.

The information on the motion of the object measured is obtained via demodulation from the intensities. After conversion into a digital signal, a signal processor determines displacement, velocity and acceleration of the measurement object in real time. Demodulation (often also referred to as decoding) is done either for displacement, velocity or acceleration.

vibrating tube densitometers working principle - instrumentationtools

One of the many parameters that must be accurately measured for product quality control, custody transfer, process control, or liquid interface detection purposes is liquid density. Often density measurement is combined with flow measurement to determine the mass flow rate of a liquid in a pipeline. What follows is a discussion of the principle of operation of vibrating tube densitometers.

There are different types of densitometers in usetoday. Some of the various operational principles forthese devices are: vibration, buoyancy, nuclear, andacoustic. Each operation principle has advantagesand disadvantages.

The simplest vibrating system consists of a spring andmass that are mechanically connected. If the mass isdisplaced and released, the system will vibrate at a knownfrequency defined by the equation,

If the spring constant or the masschanges, the frequency of vibrationor natural frequency will change.This concept can be related to avibrating tube densitometer. Thespring constant (k) can be related to the stiffness of thetubing.

A vibrating tube densitometer is basically a spring-masssystem in which the frequency of vibration of the tubingis measured and related to the fluid density. The tubeassembly is supported at each end and is mechanicallyexcited or displaced using electromechanical devices,so that the assembly will vibrate at the natural frequency.

As previously discussed, the frequency of vibration of thetube assembly will vary as the density of the fluid in thetubing changes. The tube assembly must have appropriatemechanical properties to resist corrosive attack from thefluid, be able to contain the pipeline pressure, and haveproper vibration characteristics.

The actual arrangementof the tubes will vary with manufacturer; parallel tubes,U-tube, and in-line tubing are the most common. The tubematerial is usually Ni-Span C, stainless steel or Hastelloy,although other materials have been used.

An efficient densitometer installation design will ensurethat representative liquid pipeline sample is located insidethe tube(s) at all times. The following equation is used torelate the frequencyfluid density.

For increased accuracy, compensation for the effects ofchanging temperature and pressure on the tubing must beperformed. To relate to the spring-mass example, changesin temperature and pressure cause small changes to thestiffness of the spring, causing variation in the frequencyof vibration.

During field calibration (or proving) ofthe densitometer, a density correction factor (DCF) isused to adjust the indicated or observed density of thedensitometer to the actual density of the liquid. If thedensity correction factor varies beyond manufacturerrecommendations, or the accuracy varies with the fluiddensity, the densitometer must be examined for possibledefects and recertified.

The signal output from a vibrating densitometer isa square wave signal with a period or frequencyequalling the vibration frequency of the tube assembly.From this signal, the frequency must be measured andthe corrections for the effects of fluid temperature andpressure must be applied. The final determination is thedensity of the fluid.

A flow computer is used to performthese calculations. In many cases, the flow computer isalso measuring the pipeline fluid flow rate using a turbineflow meter, an orifice meter, or other flow measurementdevice.

Using the accumulated data, the fluid mass flowrate can be calculated. Depending on the application,the flow computer output values may be transferredusing normal 4-20 mA signals, or conventional digitalcommunication techniques such as RS-232C.

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