What could be the reason for high axial displacement on a centrifugal blower? 3

[Mechanical guy]

Hi all ,
i have a problem in one of our Plant centrifugal blowers , when it was stopped it gave a HH alarm for axial displacement.

can anyone help me to know what could be the root cause for this problem ??

The blower was offline for around 1.5 months for some low lube oil pressure then oil issues solved by operation team but when they start the blower the HH alarm for axial displacement came up.

the blower was stopped and we rotated the blower by hand and the alarm was reduced from high high to high only. ( from > + 0,55 mm reduced to > + 0,5 mm )but that's it , we need to reduce it more but we don't know what can we do further ,

if you can give me some hints of what can i check or do to solve this i would really appreciate it ,,,,,,,

many thanks in advance ,,,

 

[Tmoose]

No details provided about the fan bearing construction , so .............

I'd be concerned the undefined lube issues caused the thrust bearing to wipe/wear.

Regardless an inspection of the bearing and displacement probes is in order, by some folks well versed in the fan bearing construction, etc.

 

[JJPellin]

What type of thrust bearing does it use? What system is used to read and alarm on axial displacement? I will assume the answers to my questions.

If it uses a circulating lube oil system with a hydrodynamic, thrust pad bearing and axial displacement is measured with non-contacting proximity probes, then I would tend to agree with Tmoose. You probably wiped the thrust bearing on shutdown. Other possibilities are that someone moved the thrust probes while it was down and did not regap them correctly. In either case, I would measure the total axial float and compare that to original specifications. Depending on the results of that test, I would pull the thrust bearings for inspection as already suggested by Tmoose, above.

Johnny Pellin

 

[byrdj]

I would suspect coupling.
Either alignment or the "means" for offset is compromised.
The spool looks like it could be a "gear tooth" design with grease pack. (maybe the grease has dried) The ones I worked like that have been replaced with the dry flex shim design

 

[Mechanical guy]

Thank you so much for your support guys ! tomorrow we are going to drop the coupling & measure the axial float . I will tell you about the findings ,,,

 

[Artisi]

Existing on going problem, or something new?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[byrdj]

does the fan and motor have their own thrust? checking the rotors float to verify thrust bearing condition

do you know the thermal offsets for coupling alignment, drive to driven?
If you can check it when just coming out of service, then no need to set offset

while "laser align" systems say you don't need to seperate couple. I have had serious error, but it was due to a problem with the coupling (grease had gone solid)

If both drive and driven have thrust, then coupling hubs should have "float" to allow for axial thermal
If only one has thrust, then It might be advantage to limit the float in the coupling hubs. (some coupling sells a bumper kit)

 

[Mechanical guy]

quick update :

axial float found 1.6mm

old info : bearing temp went to 119 deg. C during the trip .

also old info : lube oil pump pressure dropped to low level before the trip.

so we are assuming thrust bearing failure.( maybe the journal as well )

We are planning to open the bearings and see.( maybe we will start next Sunday )

i will tell you what we find once we open the bearings ! this is little pet a big job for us as we don't have much technicians. our company just keeps reducing the man power

anyhow ..
you guys are amazing ! & i am so happy to talk to you ,,,,,,

 

[Tmoose]

I would not undertake the bearing inspection without detailed maintenance information, which should be included in the O&M manual.
This is true even for senior technicians who do not have training on this machine.

Sometimes there are "tricks" that will blindside even folks broadly experienced with turning wrenches and "good with their hands."

 

[Mechanical guy]

UPDATE:

we found the thrust bearing pads white metal melted away.

we are suspecting our main oil pump is having internal damage , looking to the oil pressure trends , the oil pressure dropped before the trip and the bearing temp , went to 110 degree C.

now we are stuck up after replacing the bearings because we don't have the alignment specs and we don't have any clearances specs or even the range for the axial float.

we replaced the bearing pads with new and we measured the axial float and find it 0.24 mm total float.

our reliability engineer said it cannot be correct but he also cannot support his statement with OEM reference. we are only referring to the usual axial float in any other rotating machine which is usually 0.5 mm , so 0.24 is kind of too small ..

any how we are communicating with the OEM and i will let you know further updates.

it is so rigid here is work so i am happy to talk to you guys.

 

[JJPellin]

I am currently in the process of overhauling 6 large centrifugal compressors during a partial plant shutdown. All of them use tilt pad thrust bearings. The design clearance of our bearings is generally in the range of 0.25 to 0.35 mm

Johnny Pellin

 

[Mechanical guy]

is it MHI ones ?

 

[Mechanical guy]

update :

the shaft is too heavy to push !!

we are dying to reach 0.45mm ( required axial float .

the location is too small you cannot fit your body and push.

i got 2 guys , one big strong Indian guy and another small Filipino guy ( who is also very good rotating machines guy ), but i need minimum 2 strong men cus the Indian is too tired now.

i am thinking of fabricating some special tool to be able to push the hub with decent force .


the good thing is we already got 0.4 mm axial float instead of 0.24 earlier , & now i know whats the issue ( no enough applied force )


,,,,,,,,,,,,,,,,,,,,,,,,,,,

 

[JJPellin]

No. Our compressors are Roots/Dresser and Elliott.

Johnny Pellin

 

[Strong]

"the shaft is too heavy to push !!"
For journal bearings, it can be much easier to rotate shaft to get oil film established; followed by moving shaft axially in both directions to get the axial float.

I did not see mention that you conducted a vibration analysis.

Blower operation well below BEP or a clogged inlet filter can cause pulsation that overloads thrust bearing. I expect that root cause of thrust bearing failure is beyond the axial float issue!

Walt

 

[Mechanical guy]

Maybe I did not mentioned the story behind all of this which I'm still digging it because i just joined this plant 1 month ago and I'm struggling to get history about this issue.

I found an investigation report said this :
on July 2018 ,The blower main oil pump had a major failure.
it was reported to have a sudden major oil leak.
internal parts found damaged and all pump feet shims found scattered everywhere.

I'm going to check the trend before and after that case.

maybe that was the initial issue which led to reduce the bearing life significantly .


now we are waiting to get the alignment specification from the vendor to finish the blower side and troupe shoot all the lube oil unit .

Note : i checked the MOP at site , it was not fixed properly , mounting bolts in bad shape and not the standard ones , i think because the old ones got broken and they just put anything there !

 

[Mechanical guy]

Last update :

hi there , I'm happy to tell you that I finally managed to finish this minor overhauling job. I did the following:

1- replaced the Journal bearing ( DE ) .
2- replaced the combined ( journal/thrust bearing) (NDE).
3- replaced the main oil pump ( MOP) which i discovered that it was the same pump that earlier had a major failure and was not replaced with new ( was only temporary repaired ) , so i replaced it with new.
this pump by the way was delivering only 2.5 bar oil pressure , while it should deliver more than 5 bar.
4- i replaced the MOP feet bolts ( i found some none standard bolts installed so i replaced all with new).

I contacted with the vendor and he gave me the required specs ( axial float range , radial clearances for the bearings , alignment specs ... etc. ) .

after start up, everything went smooth ,, now it is running since 3 days and all good.

I conclude the bearing failure was due to the MOP lack of pressure , ( old one only delivered 2.5 bar while the new one delivered 6 bar ) .


thanks guys for your support , I am happy to be part of this community

 

[Tmoose]

There is no low oil pressure alert/alarm/ shutdown ?

 

[Mechanical guy]

Hi Tmoose , sorry for late response ,, actually there was .

 

[Tmoose]

In OP Mechanical Guy asked " can anyone help me to know what could be the root cause for this problem ?? '

From the latest (15 Jan, 2020) post it sounds like the "root cause" was folks over-riding or ignoring the various low oil pressure protections.