Why fretting corrosion happens in particular seals only? 3

[reallyme00]

Dear All,

Why fretting happens in particular seals only? For example, In a process industry, if we have a hundred seal almost 80% of the seal's secondary seals contacts the shaft/sleeve. But only in particular seals failure happens due to fretting at the shaft/sleeve.

One more intresting thing is, If it is a double seal, I/B seal fails due to fretting but the O/B does not?

How to avoid fretting? Is fretting associated with the pumping fluid? Is the fretting vulnerable in high pressure application?

Please drop your valuable answers.

Thanks to all.

 

[JJPellin]

The type of fretting damage that you describe most likely comes because of movement of the dynamic elastomers. The most likely cause is run-out on the seal chamber face. If the shaft is not passing squarely down the center of the seal chamber, the dynamic elastomer has to move with each rotation of the shaft. The quickly results in fretting damage to the sleeve. For a double seal with fretting only on the inner seal, there could be another cause. If the product is "sticky", there could be a slip/stick phenomenon resulting in relative motion. If you are describing a pressurized double seal (API plan 53) then this would be much less likely since both seals would be running on the same fluid. But if it is a tandem seal (API plan 52), then the pumped product could result in slip/stick but not the barrier fluid.

 

[bingopin]

While not a fix, you can get better results with a hard coating on the shaft/sleeve in the area where the dynamic seal runs. If properly applied, chrome oxide worked pretty good for me. Even it would not hold up to dynamic TFE vee rings.

How to avoid? Consider seals that do not use dynamic elastomers such as a bellows type seal or maybe a seal with a spring mounted stationary face.

 

[reallyme00]

Dear Sirs,

Thanks for your Post. As Mr.JJ mentioned, it was happened on the ApI 53 plan seal. The Pump has two Seals. On Drive end we find heavy fretting at I/B seal (diametrically,deep groove for 60% and relatively lesser at another 40%)and no fretting at O/B.

On NDE O/B seal got heavy fretting compared to I/B seal.

On all of these, DE's I/B seal got deepest and uneven grooving.

Really,we hadn't check the Stuffing box run out / face out due to urgency of the pump. Next time, sure I will check it.

As Mr.Bin said, I will discuss with my superior about the ceramic coating.

I wish you all have a Merry Christmas and Happy New Year.

 

[lgoochy]

YOu mention drive end and non drive end, I am therefore assuming you are looking at a split case type pump.
I also also seen this happen on seals where the shaft is whirling or bent due to the hydrodynamic loads inflicted on the pump.
More movement on the inboard seals than the outboard seals and this is what you are witnessing on the fretting front.

It is also advisable to go for stationary seals and have the O rings on top of the faces rather than underneath them.

I have a drawing of a typical seal you could use if you want me to PM it.

 

[reallyme00]

Dear Igoochy,

Yes, it is vertical split liquid ring vacuum pump.
It will be definitely helpful to me if you could send me that drawing. How could I contact you? please give me the details. Sincere thanks to you.