Skip to main content

Tagged With "Criticality"

Blog Post

VFD Security Risk

Reliability ·
Vendors include this setting intentionally in the device’s registry so operators will always know the top speed-limit for motors. But the devices reveal this critical speed to anyone who queries them. They use the Modbus protocol to communicate, and a hacker can send a simple query to the drive’s control board to obtain this critical information then use it against the device.
Topic

2x Turning speed critical in paper machine rolls

stevenlee ·
Is there anyone else out there who knows about this phenomenon. Balance tech or anyone else. I can clearly see it in the balancer but I do not know how the rolls actually react in the machine. Does the felt actually dampen the critical or not. My theory is that the energy is carried out to the bearings, anyone know for sure. thanks Steve
Topic

Critical Speed

ALI HAIDERR ·
Dear Experts How many critcal speeds of this rotor Rotor speed 3000 RPM,
Topic

Critical speed range

Rohit02 ·
Dear experts, We are having a turbine rotor which during High Speed balancing shows a critical speed range from 1500 RPM to 4000 RPM(actual critical speed 3200. I have read that critical band is +/-20%. But considering the above data it is about 50%. We do not have earlier records as the rotor is more than 20 years old and gone under several repairs. I just want to know is it possible that during time course the band has increased. If yes then what can be the reasons Secondly can multiple...
Reply

Re: High speed balance question

William_C._Foiles ·
Some shafts running at that speed or even higher operate as stiff shafts. As such, a low speed balance has proven effective. The specifics may need review for a better answer.
Reply

Re: High speed balance question

electricpete ·
1. when do we need high speed balance exactly in industry? I've heard a thumbrule: when machine operates more than 75% of its first flexible-rotor critical speed. 2. is it possible a balanced rotor in 40000 rpm (above second critical speed), show unbalance in 3000 rpm balance machine? I say yes, it is certainly possible. Vastly different modeshapes may apply at these two speeds. For example you might have unbalance present at the node of the dominant 40000rpm modeshape that don't show up at...
Reply

Re: High speed balance question

William_C._Foiles ·
Two instances where we see rotors in this speed range are turboexpanders and integrally geared compressors. Turboexpanders are low speed balanced. I don't remember one high speed balanced. Integrally geared compressors have a 'major' overhung mass on one or two ends making the efficacy of a two plane balance likely. Other high speed rotors may or may not be a suitable. Low speed balancing doesn't imply that only the final assembly has been balanced by low speed methods; often or usually...
Reply

Re: Critical Speed

John from PA ·
I'm guessing this is a multiple choice question off some examination so it probably is best to share the possible answers. This isn't the best data for determination of critical speeds. Since the plot lacks header information, we don't know much about the data, even the transducer type or mounting angle. We also know very little information about the machine. We see three peaks, and well into perhaps a 4th. Except for some large nuclear 50 cycle generators, it might be a bit unusual to see...
Reply

Re: Critical Speed

Shurafa ·
I agree with John. To answer your question with confidence, I would look for information, other locations and plot types such as polar. Furthermore, looking at the vibration alone could be misleading if you want to know the system characteristics without knowing the forces. Many interpretations of Bode plot are based on the assumption that the exciting force is at 1X which is not always the case. Regards- Ali M. Al-Shurafa
Reply

Re: Critical Speed

Walt Strong ·
The OP states "Critical Speed" which is sometimes referred to rotor critical speed or balance resonance when the rotor has distinctive deflection. I have only seen one rotor balance resonance below operating speed for large 2-pole motors. Therefore, I would expect that one or more (at least 2) of the 3 peaks to be natural frequencies of the structure and not the motor rotor. Structural vibration test by impulse-response or operating deflection shape (ODS) could confirm this hypothesis. The...
Reply

Re: Critical Speed

aziz58 ·
For determine critical frequency recommended to get coast down bode plot becouse to eliminate effect of transient force or internal forces after the rotor stable at nominal speed.
Reply

Re: Critical Speed

John from PA ·
Walt, point well taken on motors but the OP does not refer to a motor, he refers simply to a rotor. I have frequently worked on large nuclear TG sets, both 60 Hz and 50 Hz and 1000 Mw, where the generator, which is physically massive may pass through multiple flexural modes.
Reply

Re: Critical Speed

Walt Strong ·
John, I read OP fast and assumed it was a motor rotor. I will wait for the details of the actual machine and measurement details. Walt
Reply

Re: Critical Speed

ALI HAIDERR ·
Thanks all It is Generator rotor directly coupled with frame 9 Gas turbine. It is coast down data taken by proximity probe of system 1. Again thanks for your support.
Reply

Re: Critical Speed

John from PA ·
As best I recall, the frame 9 generator probes are 45° left and 45° right. It is entirely possible you have a split critical. You might try virtual probe rotation as a tool to help identify the dynamic behavior.
Reply

Re: Critical Speed

aziz58 ·
Dear john Can you please explain more about virtual probe and split critical.may i face this state at frame 5 and 6 Thanks
Reply

Re: Critical Speed

John from PA ·
Quoting from the book Rotordynamics by Agnes Muszynska "Virtual Probe Rotation - A mathematical transform of vector data from an existing XY pair of vibration transducers to produce vectors that would be measured by XY probes mounted at some other, arbitrary, orientation." The technique requires special software to be accomplished. GE/BN diagnostic software has the ability to show data from the virtual probes but I do not know what else in the marketplace may have the capability. As far as a...
Reply

Re: Critical Speed

William_C._Foiles ·
A couple of points (leave out the o and that would be a couple of pints), there is more than one type of frame 9; this will make a difference. Rigidly coupled machinery trains often have noticeable train modes, not simple rotor modes. Gas turbines often have the casing and support system involved in the modes. Sometimes it helps to use both shaft relative and casing measurements if you want to better understand the dynamics. Generators may have pedestal resonances. It can help to look x, y,...
Reply

Re: Critical Speed

Rotating Guy ·
Dear Ali, With regards to your given plot and in the absence of other plot like polar, orientation and etc of probe, i assume that the critical speed is around 2000 rpm, the hump around 700 & 1300 rpm is like more on damping due to horizontal stiffness. but then, you can contact your OEM or in manual for the critical speed, even in nameplate this will be seen. regards,
Reply

Re: 2x Turning speed critical in paper machine rolls

John from PA ·
Review the thread at https://www.maintenance.org/topic/half-critical . Hopefully the links mentioned still work.
Reply

Re: 2x Turning speed critical in paper machine rolls

John from PA ·
In the thread I referenced above my first link doesn’t work anymore but takes you to a site in of a Swedish instrument vendor, perhaps OLI. Maybe a better link OLI to the old material?
Reply

Re: 2x Turning speed critical in paper machine rolls

jvoitl ·
This link worked for me John.
Reply

Re: 2x Turning speed critical in paper machine rolls

John from PA ·
Yes, but it the first link (from me) within that thread that takes you to a Swedish site. The one at 3/3/11 7:50 PM.
Reply

Re: 2x Turning speed critical in paper machine rolls

stevenlee ·
I have seen these posts before but I have never heard from anyone if the 2 x can be seen in the paper machine I know that system Dynamics play a part and I would like to know how much damping the felt provides thanks
Reply

Re: Critical speed range

Kishore kumar ·
I am not Turbine expert but Yes , i have observed in some cases .Any major changes can impact on natural frequency of a Rotor or a Structure.
Reply

Re: Critical speed range

John from PA ·
How are you measuring the vibration that leads you to this conclusion, casing or proximity probes?
Reply

Re: Critical speed range

Rohit02 ·
Casing vibration is measured in routine monitoring however during high speed balancing proximity probes were installed to measure shaft vibration. During high speed balancing this range is observed. In routine monitoring transient data is not collected.
Reply

Re: Critical speed range

John from PA ·
Can you add some data to this thread, preferably compensated polar and bode plots?
Reply

Re: Critical speed range

Rohit02 ·
Sorry..I have no plots but data only. I just wanted to know is this a normal phenomenon or something to look into
Reply

Re: Critical speed range

Vibration_guy ·
What happened to vibration above 4000 RPM? And Where you were performing the High Speed Balancing Procedure?
Reply

Re: Critical speed range

Walt Strong ·
I would consider two possible issues: 1) Damping has increased which tends to broaden a resonance peak. If this were the case, then amplification factor is less and peak vibration amplitude would be lower. 2) More than one vibration mode is present within the speed range. This is more likely with casing measurements. If the speed runup is too fast, then vibration response can be smeared and not show 2 or more individual peaks. Typical coast-down vibration response can be different from...
Reply

Re: Critical speed range

john borycki ·
Not possible to have a critical speed for that length of speed. What is turnine actual operating speed? Are you sure the shaft mode does not change during this speed change? Are you sure this is the 1st critical or 2nd critical l? Does your phase change 90 degrees or more during this speed change? How many mils are you talking about during range or at peek? Many factors involved in evaluation and high speed balancing
Reply

Re: Critical speed range

John from PA ·
I think we are all guessing here without seeing some data.
Reply

Re: Critical speed range

William_C._Foiles ·
John is correct; all is speculation at present. If critical just means elevated vibration, this could be anything.
Reply

Re: Critical speed range

Rohit02 ·
Sorry for lack of data.. but this balancing activity was carried out at vendor works and no plots were available. If I manage to get any plots in next such opportunity i will definitely share them. Thanks to all for inputs. Some questions were asked in the thread: Rated RPM-5200 Phase change 100º during critical Max amp during critical-29 microns Before critical amp-12 microns At rated speed amp-10 microns
Reply

Re: Critical speed range

William_C._Foiles ·
If the balance was carried out in a high speed balance setup, then I would not expect the criticals to be the same insitu.
Reply

Re: Failure Mode Effect Analysis (FMEA)

Shurafa ·
Do recommend a specific product/reference that I can buy /refer to with libraries of FMEA for rotating equipment?
Reply

Re: Failure Mode Effect Analysis (FMEA)

Rav0501_EAM ·
If you can specify your requirement, it will be easy to suggest. Rotating equipment category applies to a range of equipment majorly pumps and fans. Failure modes are different for each type. Also circumstances in which equipment is working also to be considered. Please specify whats your requirement.
Reply

Re: Failure Mode Effect Analysis (FMEA)

Shurafa ·
Good to know there many options and I need to be more specific to pick up the best for my case. Let's focus on centrifugal pumps and compressors that are typical designs for industrial applications like power, chemicals, O&G.
Reply

Re: Failure Mode Effect Analysis (FMEA)

Erik Hupje ·
Good summary, you can find a more detailed outline on FMEA in this article .
Reply

Re: Raw Mill Bag House Fan Vibration Critical Issue

CM Inspector ·
Dear Rizwan , This is common in cement industries,Particularly in Preheater Fans due to high sulphur content and various other Process parameters. But in case of Raw mill Bag house it is the question of effectiveness of the fabric filters, first course of action would be through checking of the filter bags.second would be asking your process people to measure the dust loading and flow measurements to know whether any change happened. if both are ruled out , then if coating is the real issue...
Topic

high absolute vibration in critical speed in a gas turbine

m-afshar ·
Hello all dears, A gas turbine having few problems has been shut down for about 4 years in an industrial plant. Now, after maintenance of the gas turbine and replacing of the turbine shaft  and running it, the absolute vibration of journal...
Topic

High speed balance question

Hesam267 ·
Hello everybody 1. when do we need high speed balance exactly in industry? 2. is it possible a balanced rotor in 40000 rpm (above second critical speed), show unbalance in 3000 rpm balance machine?
Reply

Re: Why do we need criticality analysis as part of a reliability improvement effort?

candy ·
To focus different strategies based on criticality. To identify the safety-critical elements. To manage risk = consequence x probability x detectability
Reply

Re: Why do we need criticality analysis as part of a reliability improvement effort?

Reliability ·
Thanks Candy. Can we use criticality to determine condition monitoring coverage? Do you have any examples?
Reply

Re: Why do we need criticality analysis as part of a reliability improvement effort?

USMC 08 ·
We are running into this with a client right now. Huge gaps and holes in their asset criticality documentation. Makes it hard to make a recommendation on any type of maintenance action. Should we let it RTF or should we install an online monitoring system? Now an informal assessment based on tribal knowledge is the only basis we have for making a decision. Because...well...a big drawn out study is going to cost a lot of money. Just ask Joe or Bill...he has been here for 39 years...he knows...
Reply

Re: high absolute vibration in critical speed in a gas turbine

Ngo Dinh ·
It is likely the rubbing when high vibration happen which show that it is dominant in 0.5X, the 2x in normal situation is due to misalignment but it is also by some other effects. how about the rotor? it is from OEM? how about a residual unbalance? The bent shaft after long time remaining in horizontal storage is able to be a case in this situation. But where the transducer probe located? how about the diagram of machine arrangement? type of bearings? how about clearance? type of...
Reply

Re: high absolute vibration in critical speed in a gas turbine

John from PA ·
Originally Posted by m-afshar: 1-What kind of problem may the machine have according to the FFT graphs? 2-Is it possible the bend-shaft has occurred? 3-Is there more suitable condition monitoring system to measure relative vibration of the shaft? Thanks M. farhadi Afshar 1. We would need the "FFT Graphs" to better answer this question. 2. Yes, most likely the rotor was stored horizontally, taking on a bow. Did you have the rotor rebalanced prior to installation, or did you take it out of...
Topic

ESP Fan Critical Issue

Garry Kristen ·
An ESP Fan of a Cement Plant is badly worn, there found a swear wear and tear on its all 12 vanes same like as you can see in this image. Need your suggestions/opinions to resolve the issue??? Impeller specification are 7684 kg weight , double plane centrifugal fan RPM 745, Diameter 2600 mm. I am gonna decide to rebuild the vanes by Hardox plates having each plate 2 to 3kg, But I am little confused Could it be possible to Balance the Fan after recovering it by too much mass ??
 
×
×
×
×