VIBRATIONS AND FAILURE INVESTIGATIONS OF ROTATING MACHINERY By Dr Bhaskar Sarma Sampoorna Rotor Dynamics consultancy Introduction Infrastructural Developments are vital for the growth of the country. This involves establishing petrochemical plants, power plants for generation of power in small medium and large segments. The downtime of any PowerPoint or rotating equipment is quite huge and to avoid such downtimes apart from predictive maintains, Quick failure investigation plays a major role. Clear understanding of Condition monitoring coupled with Rotor dynamic analysis helps in solving critical problems. This paper describes the procedures and various factors need to be considered for vibrations and failure investigations. Design Auditing Before looking at Fault diagnosis, it is required to carryout rotor dynamics auditing. The information like critical speeds , mode shapes, Unbalance response , parametric studies, torsional critical speeds torsional mode shapes are to be collected or to be generated using advanced software tools. For very critical jobs it is advisable to carryout Time dependent transient whirl studies. The above figure shows the critical speed map which was developed by Lin and prohl in 1951 which is a valuable tool to be adopted for solving various vibration problem. The rotor mode shapes as shown in figure will help in identifying balancing planes and bearing locations to control vibrations.
Transcript
VIBRATIONS AND FAILURE INVESTIGATIONS OF ROTATING MACHINERY
ByDr Bhaskar Sarma
Sampoorna Rotor Dynamics consultancy
Introduction
Infrastructural Developments are vital for the growth of the country. This involves establishing petrochemical plants, power plants for generation of power in small medium and large segments. The downtime of any PowerPoint or rotating equipment is quite huge and to avoid such downtimes apart from predictive maintains, Quick failure investigation plays a major role. Clear understanding of Condition monitoring coupled with Rotor dynamic analysis helps in solving critical problems. This paper describes the procedures and various factors need to be considered for vibrations and failure investigations.
Design Auditing
Before looking at Fault diagnosis, it is required to carryout rotor dynamics auditing. The information like critical speeds , mode shapes, Unbalance response , parametric studies, torsional critical speeds torsional mode shapes are to be collected or to be generated using advanced software tools. For very critical jobs it is advisable to carryout Time dependent transient whirl studies.
The above figure shows the critical speed map which was developed by Lin and prohl in 1951 which is a valuable tool to be adopted for solving various vibration problem. The rotor mode shapes as shown in figure will help in identifying balancing planes and bearing locations to control vibrations.
To resolve some problems we need to estimate the unbalances existing on the rotor. Before attempting filed balancing , it is advisable to study the most sensitive planes and unbalance levels to reduce the time for balancing at site. Such vibration response is shown as below.
Though the designs are made to be stable, yet we will come across, instability problems. The half frequency whirls normally appear on machines even for well designed machines. To understand such problems , it is advisable to generate damped critical speeds , mode shapes and stability of the system by simulating some parameters. Model of such charts are shown as below.
Another important tool is Engine order interference diagram. This is a valuable piece of information which will identify possible major and minor resonances due to several harmonic forces. This diagram is also named after its inventor Campbell.
Apart from these, the rotor system will have several components like Turbine, gearbox, compressor, generator etc connected with couplings. Due to torque pulses, the system will vibrate torsion ally and may lead to gearbox and coupling failure or some times the failure is catastrophic. The figure below shows the model of the system , torsional mode shapes and responses.
Vibration Measurements & Diagnosis tools
Most primitive and useful first hand information obtained is vibration velocity and displacement with contact probes. The overall values will indicate whether the system is safe or having abnormality. Several charts and experiences are available. The vibration limit values depending on construction features and operational environment and life expectancy of the machine. The allowable vibrations vary from 2 mm/sec to 24 mm/sec based on the above factors.
The next level of vibration analysis is vibration spectrums either with contact or noncontact measurements. This provide indications like several harmonic excitations, assembly problems, improper surface contacts, alignment problems. Correlation with theritical cambel digarm with Spectrum will provide excellent tool for diagnosis. A typical Spectrum is shown below.
Torsion Mode Shape Plot
-500
0
500
1000
1500
2000
0 2000 4000 6000 8000 10000 12000
Axial Location, mm
1398.7 cpm
4518.4 cpm
7145.7 cpm
XlTorsion template in SI UnitsUse this to start your ow n torsional model f ile w ith SI units (mm,N,kg)
Next is coast down plot. This may called as run or rundown plot or bode plot. The plot is phase, amplitude with respect to speed on horizontal axis. The free run shows, rubbing, critical speeds looseness of the system shifting of critical speeds etc. Figure below shows such a plot.
Failure studies
Several types of failures are possible in rotating machinery. This section describes some of the recent studies made.
1. Axial compressor during its first run got severally damaged. Figure shows some rubbing marks occurred during its MRT test and the bode plot confirmed the marks with spikes. Since the values are low it was neglected.
2. If the torsional natural frequencies are close to the grid frequencies and wrong synchronization occurs, the coupling which is sensitive to that frequency can create a failure as shown below.
3. Most of the blowers, Fans and boiler feed pumps and compressors are VFD driven system to save energy. The systems will have several electrical harmonics. If the torsional Natural frequency is close to one of the harmonic, based on the intensity of the harmonic, if the system is not tuned properly it may experience catastrophe.
4. The failure due to short circuit conditions is catastrophic if the torsional calculations are not made and couplings and gear boxes are not selected from dynamics point of view, devastation like below may occur.
5. Thermal imbalance in the system due to material defect, frequent startup and shut down can shift critical speeds either higher side or lower side. The shift can cause bearing rubbing and may bend. Such system is very difficult to handle during coast up and load. A very slow roll with prolonged time to take the speed step by step by carefully controlling the vibrations is the only answer. Over a period of time due to heat soaking, the vibrations my come down to normal level.
6. Temporary thermal bend is possible if the rotor is subjected to repeated starts and shutdowns in very less intervals of time. The variations in mass density across the shaft can give rise to variations in mass and asymmetric stiffness causes a parametric instability.
7. The combined systems like gear box with generator or Turbine with gearbox , some times due to rigid coupling or behavior of coupling due to simulated rigidity due to erection or site conditions, may change loading condition and cause frequent bearing failures.
8. Assembly looseness of the bearing housing and higher overhung of couplings can give rise to very high vibrations and noise levels. The FFT collected from such case was already shown in FFT.Assembly looseness some times occur due to sudden load through off either due to grid failures or process requirements. In such cases, the foundation bolts, sole plate or coupling bolts may become loose.
9. Chillers in compressor train can also create instability and high axial forces leading to bearing failures. The symptoms are as shown.
Axial vibrations at E &F location when dehydrator off line
Vibration problems in rotating machinery are quite complex and are unique. With the available design soft wares and vibration measuring equipment, it is possible to identify the problems and provide solutions.
