What is 'It' of an impeller of centrifugal compressor 1

[mandygrewal]

I am an intern in a rotating equipment company.
Can i know what 'It' of an impeller is?

I know it is some kind of inertia. Ip is different and it is Polar moment of inertia. Ip in a solidmodelling software like UG is Iz.

Please advise on 'It'

 

[cbrn]

Hi,
already saw this as referred to "Inertia - T-orsional" instead of "Ip". But who knows, maybe it's the right explanation: are you dealing with foreign-country equipment, where for example the terms "It" and "Ib" (for Inertia - B-ending) could be used? Note that in some cases the "calculation men" use even more cryptical denominations depending on the conventions they use for shaftline analyses: for example, I1, I2 and I3 where generally "direction 1" is the shaftline axis (so I1 = Ip = Izz)... As a further example, for me-myself, direction-1 is the machine-axis regardless of if it's vertical or horizontal... I use Izz when I interface with the designers, and Ip when I "speak" with externals... I use Ib when speaking about bending inertia of axi-symmetric shapes (so that Ixx and Iyy are exactly the same), but I'm very far from knowing if these are standard terminologies or not!

Regards

 

[mandygrewal]

Hi cbrn

Actually we are buying a high speed motor from a supplier in UK. the acceptable specifications for the impeller for that motor are
Ip = .00123 kgm2
It = .000xyz kgm2
and CG lies t mm from back end of impeller

here we use Ip=Izz, but have no idea about "It"

so we have to design the impeller within these...
I can contact supplier to know what is It, but I am afraid to hear ..."you dont know this"..you know I am just an intern ....you can just tell me what you would assume if somebdy sends this to you...i will then have some idea before i talk to the supplier..

 

[cbrn]

Hi,

if I were you and if I hadn't anybody else to ask to, I wouldn't be too worried about asking the motor manufacturer: you can always "hide" behind the fact that the terminology you are used to is completely different (and nobody can argue, since it's a matter of fact that every company has its own terminology: try to compare Alstom, Siemens, GE, Andritz,...).

To return to your question: the "xyz" you have written after the ".000" in "It" is a bit misleading: in fact, if you considered the impeller as a thin disc, the bending inertia would be half the value of the polar inertia, so it would be interesting to see WHICH are the numbers written for this "It"; it could easily be "Inertia, T-ilting", no? - a specification like this would have to do with bending critical speeds, just like the polar one certainly has to do with torsional eigenfrequencies. Strange they don't specify a maximum residual unbalance (which would be in units of [force*distance])... If vibrations are a design point for you, then you will have to check this by yourself.

Another thing about terminology: what do you mean by the fact that "you're only an internal"? (Sorry, I'm not a native speaker...)

Regards

 

[mandygrewal]

Dear cbrn
Thanks a lot for guidance. I will get back to you after I talk to supplier.
---- by "intern" I mean, I am a Graduate Mechanical Engineering Intern at my present company. I am pursuing this internship and hope it turns into a full time job after I am done with my MS degree. hope so!!!!!!!

Again thanks a lot for your help.....