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."



All Pictures and Text Copyright © 2001 - 2016 Contact Mr. Adler Adler Engineering LLC of Wyoming USA

Great care has been taken in the compilation of this article. However, no warranty, expressed or implied, including without limitation, warranties of merchantability or fitness for a particular purpose, are given in connection with this article or any article archived on this website. Although this information is believed to be accurate by the author, the author cannot guarantee favorable results will be obtained from the use of this article alone. This article is intended for use by persons at their sole discretion and risk. Since the conditions of product or material use are outside of the author's control, the author assumes no liability or obligation in connection with any use of this information. The author is not liable for special, indirect or consequential damages resulting from the use of this material.

No part of this article or any article archived in this website, or any part thereof, may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, without the prior written permission of the copyright holder R. H. Adler. Nothing contained in this article or any article archived in this website shall be construed as a grant of any right of manufacture, sale, use, or reproduction, in connection with any method, process, apparatus, product, composition, or system, whether or not covered by letters of patent, copyright, or trademark, and nothing contained in this article or any article archived in this website, shall be construed as a defense against any alleged infringement of letters of patent, copyright, or trademark, or as a defense against any liability for such infringement.