Thursday, 29 November 2012

ONLINE & OFFLINE VIBRATION SOLUTIONS


ONLINE & OFFLINE VIBRATION SOLUTIONS

Vibration monitoring is just like checking the pulse of the patient. Now two methods can be adopted based on the condition of the patient. One can be a periodic scanning of the pulse & other can be continuous pulse monitoring in ICU. Similarly for the machines which lie in less critical zone, periodic vibration scanning system is advisable. Machine which are critical with respect of safety, environment & directly affect the production, online vibration monitoring systems are advocated. Following is the broad criterion for implementation of different technologies-

1.    Online System: This system holds well for critical category of assets which have immediate impact on safety, environment & production for example turbines, compressors, critical pumps & blowers etc.

2.    Offline System: Less critical or semi critical assets are good targets for portable/offline technology. On these assets periodic scanning is good enough to judge the present condition.

ONLINE VIBRATION MONITORING: These are continuous vibration monitoring systems employed on critical rotating assets like turbines, critical compressors, blowers, pumps etc. Sensor & system selection depends on many factors like machine type, type of bearings, machine speed, machine components, machine elements etc. These types of systems act as protection safeguard against excessive vibration & also do the condition monitoring job if the analysis software & module has been included in the system. As far as turbines are concerned where bearing of interest is fluid film (journal), Bently Nevada System is one of the best systems that I have worked on. Bently system is really good in rotor dynamics due to its long experience in this field. On rolling element bearings SKF has deep understanding and does a really good diagnostic on them. CSI is also good solution in this area.

PORTABLE VIBRATION MONITORING SOLUTION: As already discussed portable or vibration scanning solution is good for less critical assets. These are the assets which can be periodically scanned or can be scanned when there is some unusual indication regarding performance of the machine. Normally amplitude monitoring & FFT analysis is done on these kind of assets. Other techniques like phase analysis & orbit analysis may also be employed where we have facility to tap the signal through BNC port. Again SKF & CSI are very good in this area. Bently Nevada Snapshot & ADRE are also useful tools. Recently some new players in the condition monitoring field like Fluke have also introduced some exciting solution in the offline vibration segment. They have introduced two models that I have used till now. One of them is FFT based vibration analyser with auto fault diagnostic & auto advisory feature. This is a good tool for the shop floor engineers / technicians who have limited knowledge of vibration. The system is easy to use & interpret the faults.

Disclaimer: The discussed solutions are based on my own experience & are my personal views.

Monday, 19 November 2012

Vibration Monitoring & Analysis- Sensor selection Criterion


Sensor selection criterion:

Sensor selection is one of the most important things in vibration monitoring / Analysis system. Basically it’s the quality of vibration data input that decides the accuracy & relevance of the measurement. No matter how advanced electronics & complex analysis algorithm is used, the sensor being the first element in the line of measurement, has high importance in determining the quality of measurement. The mantra is gold in gold out, garbage in garbage out. Various factors and operating conditions like ambient temperature, magnetic field interferences, g range, frequency range, electromagnetic compatibility etc. decide the ruggedness of sensors in the field. Other machine parameters like type of bearings (Rolling element or journal) & machine speed decide the choice of measurement units & kind of sensor to be put in the field. Once the proper sensor selection has been done, the proper installation & sensor orientation becomes very important.
DISPLACEMENT SENSORS: These are also known as proximity sensors or eddy current sensors & are used to measure relative shaft vibration, shaft position and clearance. These probes are put on sleeve or oil film bearings (Journal bearings). These sensors are best suitable to measure low frequency and low amplitude displacements. If there is not any viability of mounting proximity probes on the machine, the accelerometer with double integrator circuit is also used for the displacement measurement. Normally in turbines & large compressors we find these probes.

VELOCITY SENSORS: Unlike proximity probes, velocity sensors are contact type sensors. Earlier electromagnetic sensors were used but due to mounting constraints & change in sensitivity issue with respect to time, now much rugged & sturdy piezoelectric sensors are used. These are accelerometers which are integrated once to get the velocity output. These sensors are used for low to medium frequency measurements (approx. up to 5000Hz). Majority of the machines lie in this frequency range (low to medium RPM) for vibration monitoring & balancing operations.

ACCELEROMETERS: Accelerometer is made of piezoelectric wafers & produces emf when there is force applied to it. These are the most preferred sensors for measuring vibration & have a very wide frequency range (almost from DC to 20 KHz). They are very useful for high speed machines & rolling element bearings. These are rugged devices & can sustain hash ambient conditions like corrosive environment & extremely high temperature (e.g. gas turbines). The sensor & associated electronics (charge amplifier) can be separated for high temperature applications. They have very good signal to noise ratio.   

 One should ask the questions like application frequency, type of bearing, environmental conditions, machine type, sensor size & mounting constraints before selecting suitable sensor.

Sunday, 18 November 2012

Vibration Monotoring & Analysis- An important condition monitoring tool


Different technologies can be applied for the maintenance of different category of asset. We know that (Moubray’s PF chart) vibration is one of the earliest indicators of start of machine failure. In this section we will cover the vibration monitoring & analysis technology. Again human analogy becomes relevant to the machine also. Machines are like human body. The human body has complex balance system to maintain the internal environment of the body to a predetermined level. This is known as homeostasis, for example it maintains the body temperature around 98.6° Fahrenheit. Whenever there is some problem or a breakdown in this regulatory system, this control mechanism is disturbed. This is reflected in terms of rise in temperature, shivering etc. Similarly, if there is some mechanical / electrical problem or a process disturbance, the machine also expresses the problem in form of different variables like rise in temperature, vibration, change in electrical parameters etc. Since we know that vibration is one of the earliest indicators of the machine fault, it becomes important for us to use the available technologies for early detection of machine faults.

Vibration: Everything in the nature, either static or dynamic vibrates. So, question is what makes all these things to vibrate? The answer lies in the basic fundamentals of Physics. Atoms are the basic building blocks of the matter and they have inherent property of vibration. Vibration is the harmonic motion of a machine or machine part in either side of its neutral or stationary position & the response of a system to some internal or external excitation / force applied to it. So, the question is why different machines vibrate differently? The answer lies in the machine design & fundamental conditions like difference in mass, stiffness & damping. Even the same model & same design machine behave differently under similar conditions. God has created human beings & all are different from each other. Even two identical twins have some difference & they behave differently. Similarly no two machines can be same, even if all the design parameters are same & this follows law of nature. This distinction makes the machine diagnostic discipline not only a pure science but also an art which can be developed by the detailed study of the discipline & wealth of experience on different set of machines.

Units of Measurement: Vibration can be measured into three units-

 
1.    Displacement: The total distance travelled by the vibrating part, from one extreme limit to the other. This can be explained by simple spring mass balance where displacement is equal to the peak & bottom position travelled by the mass. The unit of displacement is microns (Pk-Pk).

2.    Velocity: Velocity is the speed at which displacement occurs. Since the seed is changing constantly, peak or RMS velocity are usually selected & the unit is mm/sec (Pk / RMS) & ips (inches per second)

3.     Acceleration: Acceleration is the rate of change of velocity.  At the extreme limit of travel of the vibrating part, acceleration is maximum or peak & most popular unit of acceleration is - g’s (peak).

 Relation between Displacement, Velocity & Acceleration

Integration of Acceleration =   Velocity, Integration of Velocity = Displacement