Ansys traning document. Fatigue analysis in offshore engineering by ansys 14.5Full description
Ansys traning document. Fatigue analysis in offshore engineering by ansys 14.5Full description
Fatigue analysisFull description
PSV Connection in Caesar IIDescripción completa
TutorialFull description
Descripción: Caesar II mainly used for pipe stress analysis and water hammer is one of the frequent cases to be analysed as per this document
Training
good one
Fatigue
Fatigue analysis in SACSFull description
Fatigue analysis in SACS
CAESAR II adalah program computer untuk perhitungan Stress Analysis yang mampu mengakomodasi kebutuhan perhitungan Stress Analysis
Descripción: Tutorial caesar II Traducido
Dynaflow Lectu Dynaflow Lectures res – Recip Reciprocat rocating ing compr compressor essors s Acoustics and Mechanical Response Rotterdam, December 10th 2009
Compr ompre essor pipin piping g vibration ana nalysis lysis Two parts: pa rts: 1. Acoustical/pulsation Acoustical/pulsation study 2. Mechanical response analysis
•Labor intensive modeling •Large number of load cases.
EXAMPLE
Compr ompre essor pipin piping g vibration ana nalysis lysis Two parts: pa rts: 1. Acoustical/pulsation Acoustical/pulsation study 2. Mechanical response analysis
•Labor intensive modeling •Large number of load cases.
EXAMPLE
Sequ que enc nce e of depe depend nde enc nce e
A c o u s t i c s i s ab Ac abo o u t p r o p ag agat atii o n o f pressure puls pulsa ations in i n piping syst ems Sourc ource e of Pressur Pressure e pulsations:
Recipr Re cipr ocating compr essor ssors s and and pumps
Pressure Pressu re waves waves are propaga pro pagated ted thru th ru the piping sys te tem. m.
Pressure waves are reflected (partly) and tra tr ansm nsmitt itt ed (part (partly) ly) at at geometrical discontinuities
Pressure Pressu re pulsations puls ations gene generate rate unbalanced unbalanced force forc es tha th at are the source of pi ping vibration
Sustaine ustained d vibration v ibration may result in i n fatigue failures
Ag A g en end da
Elements of Acoustics
Aspects
of Mechanical Response Respon se
Examples of Mechanical Response
Recip ciproc roca atin ting g compressors com pressors and pum pumps ps inhe in here rentl ntly y produce prod uce pulsations in the suction and discharge pipin piping g
Double actin acting g cylind cy linde er: Pisto iston n di splace splacement ment opens and clos es suction sucti on and discharge di scharge valves valves
Actual Piston movement (not purely sinusoidal) due to finite rod length
Valve openings result in a “ Sawtooth” type of gas flow Due the sequence of piston movement and valve opening and closing
The shape of the sawtooth is determined by the rotational speed of the compressor, the geometry of the cylinder and the pressure ratio.
Flow time history for a single acting cylinder With ideal instantaneous reacting valves
Resulting Flow Frequency Spectrum (discrete) for single acting cylinder
Double acting cylinder (slightly unsymmetrical) Head end ≠ cranck end because of the piston rod volume
Flow Frequency Spectrum (discrete) for double acting unsymmetrical cylinder
Uneven frequency components finite but small due to imperfect symmetry
Flow pulsations result in pressure pulsations
Pressure pulsations pr opagate thru th e piping sy stem at the speed of sound
Speed of sound depends on:
Gas composition
Gas Temperature
Gas Density
Pressure/Flow pulsations reflect at geometrical di scont inuities
Wave length of propagating wave depending on s peed of sound and pulsation frequency λ
c =
f
Wave reflection and wave interaction result s in system acoustic al natural fr equencies. e.g. for wave length/frequency th at match a geometrical length scale standi ng waves may occur
Presence of Acoust ical natural frequencies may result i n Acoustical resonance
System wil l show an acoustic al response to an acoust ical excitation
Example of acoustical natural frequency result
Limited accuracy of acoustical model
Accuracy of prediction of acoustical natural frequencies relatively large
Error margin relatively small: +/- 5%
Errors controlled by limited number of parameters:
Geometry
Speed of sound
Compressor RPM
Guidelines for acoustical pulsation levels according API618
Guidelines for acceptable pulsation levels. Acceptable levels are related to (inversely proportional to) frequency, pipe diameter and (proportional to) average pressure level
Introduction of damping (orifice plates at location of max oscillating flow)
Introduction of additional volumes with or without internals (creating filters)
Increasing size of bottles (“windkessel” function).
Pulsation Bottles are a way to reduce pulsations Bottles serve two effects: (1) Surge volume and (2) Filter function 1. SURGE VOLUME
2. FILTER FUNCTION
Pulsation reduction is proportional to surge volume size
Maximum filter function for pulsations with a wave length that matches the bottle length
Minimum filter function (attenuation) for pulsation with a half wave length that matches the bottle length
Pulsation Bottles located near the compressor
EXAMPLE
COMPRESSORS Inlet scrubbers
Two bottles per compressor
Multiple pistons per compressor
Guidelines for Pulsation Bottle sizing 1. SINGLE CYLINDER BOTTLE
2. MULTICYLINDER BOTTLE
Acoustical filters
Volumes connected by choke tubes Filter frequency f h:
Filter fr equency r esponse
Agenda
Elements of Acoustics
Mechanical Response
Example of Mechanical Response
Mechanical response calculation fifth edition of API 618
Guidelines for pulsation levels.
If pulsation levels exceed guidelines system may be qualified by means of mechanical response analysis.
Vibration control by mechanical means is a possible option
Large uncertainty margin in mechanics during design (minimum 10-20%)
Acoustic is more accurate (typically +/- 5%) Preference for reduction of pulsations and thereby shaking forces by means of acoustical measures e.g. filtering (e.g. Helmholtz resonator)
Accuracy of prediction of mechanical natural frequencies Error margin: 10-20% or many time even larger
Modeling of Boundary conditions
Modeling of rack structures
Support clearance
Support lift off (thermal), support settling
Support stiffness i.e. stiffness of clamps and restraints
Influence of friction
Nonlinear supports (supports with gaps or single acting supports)
Uncertainties in masses
Differences between “as built” and “design”
Interaction between parallel pipes in pipe racks
Stiffness of concrete sleepers and pedestals
Many vibration problems related to attached components
Examples:
Valve Actuators
Small bore branch connections
Instrument connections
Level indicators
Stairs & Ladders
Mechanical properties and pulsations
Rule of thumb: minimum mechanical natural frequency 20% above second compressor harmonic. Question: is this feasible???
Mechanical properties and pulsations (2)
Mechanical resonance difficult t o avoi d due to uncertainty in mechanical nat. freq..
Variable speed compressor makes separation virtually impossible.
At resonance condition amplitude limited by damping only (typical damping factors of 2%-3% of critical)
High sti ffness results i n lower amplitudes.
Application of filters in combination with high mechanical natural frequencies looks ideal
Agenda
Acoustics
Mechanical Response
Example of Mechanical Response analysis in design
Example: Mechanical Response of NAM Oude Pekela Compressor plant Air cooler A-174
EXAMPLE
Acoustical results of suction piping
EXAMPLE
Focal area
Unbalanced shaking forces in [kN peak to peak] per pipe section and per compressor harmonic
Additional discharge volumes to reduce pulsation levels in remaining piping
Discharge Line
Aircooler E-174 nozzles
Suction Line
EXAMPLE
Additional discharge volumes
EXAMPLE
Harmonic frequency assessment in CAESAR II Sweep from 4 -56 Hz with 1 Hz steps
EXAMPLE
Harmonic forces are inserted in the model Conservative Shaking force set taken from acoustic pulsation report
EXAMPLE
Maximum dynamic stress amplitude calculation Max amplitude 6 MPa
EXAMPLE
At a stress amplitude level of 6 MPa the number of cycles is > 1011
EXAMPLE
Carbon Steel Fatigue Curve in the high cycle range
6 MPa
Agenda
Acoustics
Mechanical Response
Example of Mechanical Response analysis “as built”
Issue: Unacceptably high vibration level in compressor suction piping
EXAMPLE
In 5 steps to solution
1. Vibration Measurements: identification of main contributions in frequency domain 2. Acoustical Resonance: verification of acoustical natural frequencies 3. Mechanical Resonance: verification of mechanical natural frequencies 4. Identification of source of vibration problem 5. Modification proposal
Compressor plant
Structure and support details around the compressor (I)
Structure and support details around the compressor (II)
Details of the compressor location
EXAMPLE
Step 1. Vibration Measurements
120.00
33 Hz 66 Hz
49 Hz
100.00
16 Hz
99 Hz 83 Hz
80.00
) B d ( e d u 60.00 t i l p m A
40.00
20.00
0.00 0.0
10.0
20.0
30.0
40.0
50.0
Frequenc y (Hz)
60.0
70.0
80.0
90.0
100.
Intermediate conclusion from step 1
Vibrations are at compressor harmonics
Vibrations must be result of
1
Acoustical resonance
or 2
Mechanical resonance
or 3
High pulsation forces without resonance (compressor bottle sizing problem)
Purple vertical lines represent pi pe system natural fr equencies
10.00
0.00 00
10 0
20 0
30 0
40 0
50 0
60 0
70 0
80 0
90 0
100 0
Conclusion from step 3 & Identification of cause of vibration problem
Apparently there is mechanical resonance at 33 Hz and 66 Hz and near mechanical resonance at 83 Hz
No mechanical resonance condition at the first compressor harmonic (16.5 Hz.) and at 49 Hz. and 99 Hz
The high vibration levels 33 Hz, 66 Hz and 83 Hz are of mechanical nature
The high vibration level at 16.5 Hz most probably is an acoustical resonance problem
The high vibration level at 49 Hz and 99 Hz. must be the result of High pulsation forces without resonance (compressor bottle sizing problem)
Step 4. Identification of cause of vibration problem
EXAMPLE
The high vibration level at 16.5 Hz most probably is an acoustical resonance problem. Apparently there is mechanical resonance at 33 Hz and 66 Hz and near mechanical resonance at 83 Hz.
The high vibration levels 33 Hz, 66 Hz and 83 Hz are of mechanical nature
No mechanical resonance condition at the first compressor harmonic (16.5 Hz.) and at 49 Hz. and 99 Hz.
The high vibration level at 49 Hz and 99 Hz. must be the result of: High pulsation forces without resonance (compressor bottle sizing problem)
Examination of mechanical behavior
EXAMPLE
Example of 66 Hz. mode shape
Large amplitude movement in suction manifold
Step 5. Modifications
EXAMPLE
1. The high vibration levels 33 Hz, 66 Hz and 83 Hz are of mechanical nature and need a mechanical solution Better supporting Improved support stiffness 2. The high vibration level at 16.5 Hz is due to acoustical resonance and needs an acoustical solution, I.e. different bottles and/or orifice plates to introduce more damping 3. The high vibration level at 49 Hz and 99 Hz. are the result of high pulsation forces without resonance and must be resolved by compressor bottle (re)sizing.
Modified structure implemented and connected to attached piping AS BUILT SITUATION
EXAMPLE
IMPROVED AND IMPLEMENTED SITUATION
Conclusion from example
Compressor vibration problems many cases are of a mixed nature
Part is mechanical
Part is acoustical
Each category requires a different approach and result in different solutions
Not all vibration problems can be solved by mechanical measures.
