Cracks in impeller of three stage integrally geared centrifugal Air compressor

Hi friends ,

Back ground of the machine :

Integrally geared centrifugal air compressor of 03 stage with two inter coolers of capacity 12000 NM3/Hr at 10.6 Kg/cm2G pressure.

Commissioned on : 2012,   Since commissioning it was running all of the sudden on Nov 2016 it was got tripped on high 3rd stage vibration and It was opened for impeller inspection purpose. It was found that 096 no's vanes have been broken.

Refer the attached images  of the impeller. Can you give any inputs about Root cause?3rd st damage vanesIMG_20161124_1039272nd st DP test [6)2nd st DP test [4)

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Original Post

Looks like FOD, for sure something comming from downstream  hitten the blades, pull out the intercooler bundle to inspect.

Sometimes the nuts and rods that are used in the bundle assembly came loosen or broken and hit the blades.

also,

the inlet pipe need be inspected, mostly if turning vanes were considered during the compressor manufacturing. corrosion atack damage the vanes and flow vibration broke up it

 

Best Regards

 

 

Thanks Mr.David

As no foreign material hitted, we have been inspected up stream intercooler, there was nothing , it was very neat and clean ..But as we observed the crack profile of the vane ...there is the half thickness of the vane is corroded since long back and half looks like fresh crack...and I observed that where is the fouling is high since long 4 years once we clean the fouling and deposition and go for DP test ..there are the small pitting marks found ...same are found in other similar machines....

interesting is that this impeller material is 17-4PHSS CAST/MACHINED and we had one last failure of same type another make compressor of KOBELCO STEEL CO in that case compressor has breakdown due to no lube oil supply in the case of total power failure..in which all 4 impellers got rubbed with casing for 2 minutes at RPM 30000 BUT STILL NO CRACKS (MOC 17-4PH) happened only plastic deformation has been observed ..refer the attached images of another failure for comparison....I feel that it is due to low thickness of 1.2 mm approx. and casted method of construction with metallurgical impurities. IMG_20170606_115108 

  

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Dear Mr. Kanzariya,

I have seen a similar type of failure in our 4 stage integrally geared compressor but the vane damage was near the impeller inlet (at its minimum diameter). This looks like FOD. In our case our inlet air filter element (aluminum material) was damaged and it got into the compressor. It is not necessary that you will find the foreign object in the after cooler. It may have long passed it and somewhere in the downstream. 

I don't think corrosion did this. Because in the first pic you attached one of the vane is not broken but its bent. Pitting or any other phenomenon doesn't cause a bend like this. So something got there in my opinion. Since its 3rd stage you better look at the intercooler for any damage there which might have found its way into the 3rd stage.

Dear Yasaswy

Thanks for sharing information.

Bent...I think the bent one vane due to previous broken piece got chocked between diffuser and impeller that could damage the consecutive vanes....If it could be FOD that there might be any hitting marks in impeller eye but there is nothing in eye...Another way my point to share is that  when there is long time deposits and fouling on impeller vanes that creats pitting on impellers ...same has been experienced in same make other compressor of same size and we found that every a year impeller cleaning should be done ...that improves the vibrations values and also increases the Radial bearing life....as these are the very high speed machines...What do you say ??  We have total 10 air compressor (Integarally geared type ) Atlas copco, FS Elliott ,Kobelco makes....with capcity 12,000/10000/3000 NM3/HR ....

what else problems are you facing in integrally geared compressors? 

Regards  

 

Dear Mr. Kanzariya,

We too clean the impellers on a regular basis because we observe coke dust deposition on the impellers and it causes some unbalance in the rotors and cleaning does improve the vibration. We observed some erosion (not corrosion) on the impeller vanes. This could be due to moisture or coke dust on these high speed machines. And in one case pitting at the impeller backside where it sits on the tie rod.

Don't just eliminate FOD. I have seen many cases where the foreign object will do the damage and wont leave a trail. You just need a small metal piece to do the damage on these high speed machines. The pitting has to be so severe for this type of damage to occur as seen in your case. But in my opinion its less probable.

Don't eliminate anything until you find strong evidence. Explore all possibilities.

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