SEALING HIGH SPEED APPLICATIONS SS001
High speed is defined as a surface speed in excess of 5000 fpm. (25 m/sec.) as measured at the middle diameter of the rotating seal face.
There are several problems involved in sealing at these speeds:
- It is easy to generate too much heat at the seal faces.
- Vibration is a common problem. The equipment sometimes passes through a critical speed on its way to the operating speed.
- Misalignment and dynamic balance problems are magnified.
- Lubrication between the lapped faces is critical because of potential slip stick vibration problems.
- You might exceed the tensile strength of the rotating face, and cause it to go out of flat.
- Centrifugal force will cause the rotating face of a rotating seal to become square to the shaft and pull away from the stationary face.
- Centrifugal force can also act on the single spring in some rotating seal designs causing the seal to loose its face loading and letting the lapped faces separate.
High-speed sealing requires:
- A lower spring load at the seal faces. The 10-30 psi is lowered to 8-15 psi. (0,7-2 bar is lowered to 0,5 – 1,0 bar)
- The hydraulic balance ratio is reduced to 60/40 from the conventional 70/30 because of potential heat problems at these higher speeds.
- Select low friction face combinations. Carbon-graphite vs. silicon carbide is a good one.
- Try to use faces with good heat conducting ability. Graphite impregnated silicon carbide vs. silicon carbide is very good.
- Stationary seal designs are necessary but great care must be taken with the cartridge version of these stationary seals. See stationary cartridge seals in the alphabetical section for a detailed description of the problems.
To calculate the surface speed at the mean or middle diameter of the rotating face:
In USCS units, feet per minute = 0.261 x face mean diameter x shaft rpm
In SI units, meters per second = 0.188 x face mean diameter x shaft rpm