Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation

 

 

 

Machine Design - Case No. 96: Achille's Heal of a Vanstone Metal Bellows with Different Materails of Construction

 

Vanstone Metal Bellows

Vanstone metal bellows are versatile. The bellows wraps around the flanges, and in so doing, eliminates two design weaknesses: 1) the flange TIG weld, and 2) exposure of the flange to the process. There is an "Achilles heal" though, so beware.

An expansion joint that is constructed with a Vanstone metal bellows is shown in the picture to the left. The last convolution gives the appearance of being oversized. This is where the flange is located, behind the bellows. The flange is not welded to the bellows. The lip of the bellows forms a gasket seating surface, the outer diameter of which is located along the red dots in the picture. The flange is allowed to rotate freely with respect to the bellows. This allows easy bolt-hole alignment with existing flanges. Another advantage is that in corrosive service, a flange of a lower alloy material can be used when trying to reduce costs.

So then, what is the weak point, or Achilles heal, of this seemingly versatile expansion joint? To the untrained eye, the expansion joint looks just like any other expansion joint, and that's the problem.

Straight Stainless Steel Metal Bellows With Carbon Steel FlangesIn the case of the failed expansion joint shown to the left, can you tell whether it is a Vanstone bellows or a bellows welded to two flanges? To the trained eye, the first remark is that one cannot see the flange faces in this picture. Well in real life, out in the plant, you can't see the flange faces. All you can see is the gasket crushed between what appears to be a raised face flange. There is no way to tell that the raised surface is part of a Vanstone bellows.

The scenario probably went something like this. The expansion joint with a Vanstone bellows, having been in service for years, probably started leaking and the foreman was in a hurry to replace it. Being quite aware that almost everything was made out of stainless steel in the plant, the conscientious foreman takes a magnet and verifies the materials of construction. The bellows is non-magnetic, so it must be stainless steel. The flanges are magnetic, so they have to be carbon steel. That information alone is good enough for the foreman, and the stainless steel bellows is ordered and expedited with carbon steel flanges.

The new expansion joint is installed. Seven days later, not seven years, but seven days later it starts leaking. The first words out of people's mouths is that the supplier did a lousy job. "Hang the supplier!" Well maybe they didn't say exactly that, but you know darn well they were thinking it. Inspection of the failed joint revealed that the process had corroded the wetted part of the carbon steel flanges until there was no more flange. The pictures to the lower left and right show the stainless steel TIG weld suspended in mid air. The weld is attached to the stainless steel bellows on one side but is not attached to anything on the other side. The area that is corroded on the carbon steel flange is where the gasket ended, allowing the process to come into contact with the steel flange. The red corner lines in the picture to the lower left depict how much of the carbon steel flange was missing.

Corrosive Attack of Carbon Steel Flange Corrosive Attack of Carbon Steel Flange Leaving Stainless Steel TIG Weld
How this mongrel got into the system is somewhat of a mystery. Why the mismatch in materials did not send up warning signals to the salesperson on the other end of the phone, and why that salesperson did not ask whether a Vanstone bellows was desired, is also a mystery. However, a thought came to mind, the salesperson probably has a sign hung up on the wall that says "The customer is always right." So no questions were asked, "Just give the customer what they want."

 

 

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