HYDROSTATIC SEAL H033
There are presently two types of non-contacting seals available for fugitive emission and gas sealing:
- Hydrodynamic or lift off seals that float on a cushion of gas.
- Hydrostatic seals, where the faces are held at a predetermined small separation by controlling the opening and closing forces acting on the faces.
Non-contacting seals have a couple of advantages over conventional face seals:
- The product you are trying to seal does not have to be a lubricant. Gases or hot water are examples of typical non-lubricating fluids. A non-lubricant is defined as a fluid that will not maintain a film thickness of one micron (0.000039″) or more at its operating temperature and load.
- There is little to no heat being generated at the seal faces. Heat causes all sorts of expansion and other difficulties. The non-contacting seal eliminates many of these problems.
- Except for some possible erosion you should not experience any face wear.
- Dual versions of these seals can use an inert gas as a barrier fluid and eliminate the possibility of any fugitive emissions escaping to the atmosphere.
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 a previous part of this alphabetical section we discussed hydrodynamic sealing, but this section is all about hydrostatic sealing and you will find that the principle behind this type of seal is not too difficult to understand:
We will maintain a very small, constant separation between the seal faces regardless of any 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. Therefore force is also a rectangle.
Look at the following drawing. You are looking at a typical hydrostatic seal:
You are looking at a stationary version of this type of seal. Let’s check out at the individual parts:
- S = Spring loaded stationary seal face.
- R = Rotating face. It is held to the shaft shoulder by a clamping sleeve. A gasket would be located on either side of the rotating face to prevent leakage along the rotating shaft.
- G = Gland for the spring loaded stationary seal face.
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 the 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. We use the same technique that is used to lap seal faces flat within three helium light bands (one micron)
We are going to use this small recess 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:
- The force on the back of the stationary face (S) is represented by the rectangle formed when the pressure was multiplied by the area ( Closing force = P x A)
- This closing force is in addition to the spring load and is not affected by the axial position of the stationary face. The area remains a constant. The closing force changes only with the system pressure.
Now we will look at the force generated between the faces:
- The stationary face (S) has a larger area (A)
- The pressure between the seals (P) starts out the same as on the back of the stationary face (S) but:
- If the rotating face should try to come into contact with the stationary face the pressure would be felt to point (b) and then we would experience a pressure drop across the extended nose on stationary face (S). This would cause a larger force between the faces, causing the stationary face to move away from the rotating face.
- If the rotating face should move away from the stationary face too far a distance, the gap would widen and we would take a single pressure drop from point (a) to point (c). This would cause a reduction of the force between the faces causing the stationary face (S) to move towards the rotating face (R)
- Somewhere between these two extremes is where the opening and closing forces equalize. It is shown by the dotted line (a-d-c). In this position we take a slight pressure drop from (a-d) and another pressure drop from (d-c). It is at this point that the opening and closing forces are in equilibrium.
If the shaft moves axially and the seal 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 balance point.
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 should be very low and 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 compressor applications in the chemical process industry.
Although these were successful systems why have we not see more of these applications in recent years?
- The sealing of gas is the largest market for this application and until the chemical industry requirement for fugitive emission sealing came into popularity the application was limited to the smaller compressor market
- In past years we did not have the stable materials that were needed for the seal faces. Fluctuating temperature and pressure variations would cause the loss of the critical lapped dimension into the stationary face. Silicon carbide has changed all of that.
- Hydrodynamic sealing is the present fad. The hydrostatic concept was developed mainly in the aircraft industry with limited commercial application. Most of the major commercial seal companies either do not know about the concept, or have elected to ignore it.
Hydrostatic seals offer some real advantages over their “hydrodynamic cousins”:
- An important feature of this face geometry is that it is independent of shaft rotation. Most of the hydrodynamic, or lifting designs have to be engineered for clockwise or counter-clockwise rotation, and experience all kinds of “mix-up” problems on double ended pumps.
- Unlike hydrodynamic seals, these hydrostatic type do not require that the shaft be turning to function. They work just as well on stationary or slow turning shafts. Hydrodynamic seal designs require that the shaft has to be tuning at a reasonable rpm to provide the proper dynamic lifting forces. Many turbine driven pumps are rolled or rotated at a slow speed to keep the turbine and piping warm. This can cause destructive wear to the seal face geometry.