Shrink fit vs sold piece components 1

[YoYo2000]

All:

Which is better,having seal face inserts shrink fitted in seal face holders or having one solid piece of C/SiC? I have seen both from different seal vendors, just wanted to know pros and cons of each and which is better.

 

[JJPellin]

There is one main problem with an interference fit insert. For high temperature seals, it is necessary to try and maintain the interference even at operating temperatures. There are three ways to address this. Each of them has some drawback.

1. Construct the metal shell from an alloy that has about the same thermal growth properties as the insert. I don’t recall the most common alloy used for this. But, it has poor corrosion resistance. For sour services, the shell corrodes away and the interference fit is lost. Some seal manufacturers attempt to control this by chrome plating the shell.
2. Construct the shell from a corrosion resistant alloy and install the insert with an excessively high interference fit. This limits the options for insert material. Carbon cannot take this much interference. And, even with this design, it is very difficult to maintain face flatness from cold to hot. Some of these designs require that the face is lapped under one interference fit condition and then mounted into the shell. This results in a face that is not flat when cold and is supposed to rotate into flatness at operating temperature. It is difficult to predict and controlled this thermal rotation.
3. Construct the shell and face in a “canned” configuration. The insert is sealed to the shell with a high temperature gasket and then a retainer is welded to the shell to retain the insert in the “can.” This was common with one manufacturer many years ago. I don’t know if anyone still does this.

For low temperature applications, most of these problems don’t exist. The decision to make the entire face of one solid block versus the use of a small insert in a retainer is based on the design. I prefer a smaller insert for T-C or Si-C, since it is difficult to manufacturer a complex shape from these materials. For a carbon face, it is probably easiest just to make the face out of solid carbon rather than use an insert.

For bellows seals, an insert will obviously be needed in all designs.


Johnny Pellin

 

[YoYo2000]

Thanks for a detailed answer JJPellin. But from thermal and pressure distortion, which would be better?

 

[flexibox]

Johnny has raised some good point I will give you my point of view. It depends on what pressure you have. If its below 20bar then you can use inserted faces as there are designs out there that are suited above that you can experience pressure distortion. Using solid face will minimise face distortions. This depends on materials used. In high pressure designs convex lapping is common for example. When you consider temperature alone for example most high temperature bellows use shrink fitted inserts. These will be limited to pressure although there are high pressure designs. Combining high pressure and high temperature becomes an engineered seal.
Do you have some duty conditions to give you a specific answer to your application?

If this is a general question both designs work if applied correctly. Sometimes the insert is loose in the carrier and driven by pins or dimples to get away from shrink fits. The way the profile is designed to grip the insert is important to ensure minimal distortion and bending moments at the face.

So you see that to answer your question more detail is required because both designs work and are applied in the seal industry today.

Trust this helps a little

 

[YoYo2000]

Flexibox - Thanks for the response. It was only a generic question. Great answers folks! Thanks again!