Subject: An overview of metal bellows sealing.12-10

Metal bellows seals perform many functions. Unfortunately all the functions are not available in the same seal.

The individual bellows convolutions can be formed in different ways:

In the following drawing we will learn the names of the individual parts of a typical "nested convolution" bellows seal

Please take a look at the following diagram for some more bellows terminology;

The driving end of the bellows seal can be attached to the shaft in several ways:

  • It can be welded to a sleeve, and the sleeve clamped and held to the shaft by the impeller. A stainless steel gasket can prevent leakage between the sleeve and the shaft. This method is also used to attach the bellows to a stationary gland.
  • Soft Aluminum gaskets have been tried in this location, but they never worked out very well

 

  • The end fitting can be sealed to the shaft with a combination of set screws and a graphite wedge or V rings.
  • This is a popular attachment method in the chemical industy. Sometimes a Teflon® wedge is substituted for the graphite wedge

 

  • The seal can be held and sealed to the shaft with hydraulic force. As shown in the sketch, when you tighten the cap screw the expanding fluid exerts a holding and sealing force on the thin metal section touching the shaft.
  • This method shows a lot of promise for elevated temperature applications, but should not be used in cryogenic applications.

 

You have a choice of different metals for the bellows plates:

There are several ways to retain the seal face in the bellows end fitting ;

 

  • Shrink fitting the carbon in a metal holder is not usually a good idea. Both the holder and the face are out of round to some degree. When the holder is expanded and allowed to shrink around the seal face it will put uneven stresses on the face outside diameter causing it to go out of flat.
  • If you install the carbon face this way you will have to stress relieve the assembly to keep the seal face flat. This can be done by taking the assembly through a series of temperature transients or leaving the assembly on the shelf for several months to relax and then relap the seal face.

 

  • A press fit makes sense with carbon because the carbon will shear, to conform to the "out of roundness" of the harder metal holder.

 

Metal bellows seals have been used successfully since the late 1950s, but they are not trouble free. If they were, we would use them all the time. Here are some of their limitations:

® Dupont Dow elastomer

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