SUBJECT : The sealing of liquid slurries 3-4

A slurry is defined as: Solids suspended in liquid that cannot be dissolved by controlling the temperature and/ or pressure. The solids may or may not be abrasive. It does no good to try to identify the number of solids or their size because no one knows how these numbers relate to slurry related seal problems. Whenever you deal with slurries there are several problems you must consider:

It's generally believed that the main problem with slurries is that they penetrate between the lapped seal faces and cause damage. Although this is true, it's also true that they cannot penetrate until the seal faces open.

Seal faces should be lapped flat to within three helium light bands. That is a distance just a little bit shy of one micron. Compare this to the fact that the smallest object that can be seen with the human eye is forty microns in size, and you'll appreciate the technology used in the manufacture of mechanical seals. As a matter of comparison, look at a common coffee filter. It filters out particles larger than ten to fifteen microns.

All of this means that the seal is in fact a superior filter and as long as you can keep the two lapped faces in contact, there's little chance for solids to penetrate the faces and do any type of damage.

There are three approaches to the sealing of solids :

Let's look at each of the approaches, and in the process learn a sensible method of sealing a slurry.

Build a seal with non clogging features.

Create a clean sealing environment.

Give the seal as much radial room as possible. You can either bore out the packing chamber or install a large bore sealing chamber. Try to give yourself at least 1 inch (25 mm. ) radial space if possible. The more room you can provide for the seal the better off you're going to be.

Try to remove the solids from the sealing area. There are a number of techniques for doing this. Some work and some do not. Let's look at each of them. First we will look at the solutions that do not work very well and comment on their problems :

Now we will look at some methods that do work:

Good Solution #1.

Good Solution #2.

Good Solution #3.

Good Solution #4.

Compensate for the fact that the rotating unit will go out of balance.

The pump will lose its efficiency and experience more shaft movement as close tolerances wear.

A few more thoughts about the sealing of slurries :

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