SUBJECT: Hydrostatic sealing 12-2
There are presently two types of non-contacting seals available for fugitive emission and gas sealing:
Non-contacting seals have a couple of advantages over conventional face seals:
Be careful about selecting the rotating "back to back" dual seal design as shown on the left.
Centrifugal force will throw solids under the inner seal faces restricting their movement, and in many instances damage the faces.
Of course there is a down side to non-contacting seals. You are going to experience some leakage either into the atmosphere, or your product. The trick is to keep the leakage within acceptable limits. Most of the time we are talking about leakage in the order of a portion of a standard cubic foot per hour (not per minute).
In another paper we will discuss hydrodynamic sealing. This paper is all about hydrostatic sealing and the principle behind this type of seal is not too difficult to understand:
Hydrostatic sealing means that you will maintain a very small, constant separation between the seal faces, regardless of any axial shaft movement, thermal expansion, or face distortion caused by pressures that might be present. We will accomplish this by controlling the opening and closing forces between the seal faces to maintain the desired separation.
To understand hydrostatic forces you must first understand that any time you multiply two numbers together you are describing a rectangle. Look at the following line drawing. Here we are demonstrating that if you multiply two things by four things you get eight things, and as you can see it is a rectangle.
Force is pressure times area. Force is a rectangle.
Look at the following drawing. You are looking at a typical hydrostatic seal:
This is a stationary version of a hydrostatic seal seal. Let's check out at the individual parts:
Although this drawing looks like a conventional mechanical face seal, we will learn that the seal faces never do come into contact. In the next sketch we will look at a detail of the stationary face.
The thing to notice in this sketch is the width of he channel leading to the stationary nose piece. As you can see we are talking about a distance that is not visible to the human eye.
The smallest object that can be seen with the human eye is forty (40) microns and we are talking about a distance of one micron. This dimension is lapped, not machined into the stationary face in the same way we lap conventional seal faces.
We are going to use this small width to develop a two stage pressure drop across the seal face. This is different than a conventional mechanical seal where we experience one pressure drop from the outside to the inside of the extended nose.
In the next drawing we will look at the forces acting on the stationary face and learn how we are able to obtain the desired face separation by experiencing two pressure drops.
Let's look at the force generated on the back of the stationary face:
Now we'll look at the force generated between the faces:
If the shaft moves axially, and the hydrostatic faces try to come together the opening force builds up and separates the faces, but as they begin to separate we lose the two pressure drop concept and take a linear pressure drop between the faces, causing them to close again. In practice the faces do not move once they have found the correct separation.
The result of all of this is a very stiff and stable system. If the fluid you are sealing is an inert gas the leak rate will be very low, in the order of a portion of a standard cubic foot per hour (not minute). This is more than acceptable in most applications.
I saw this system first used in early 1960 for the sealing of compressor air in an aircraft application. Compressor air is very expensive and worth conserving. The concept was later used in commercial compressor applications in the chemical process industry.
Although these were successful systems, why do we not see more of these applications in recent years?
Hydrostatic seals offer some real advantages over their hydrodynamic cousins:
For information about my CD with over 600 Seal & Pump Subjects explained, click here
Link to Mc Nally home page