Mechanical Seals for Pumps
Due to undesirable characteristics of a conventional stuffing box, especially when the leakage has to be an absolute minimum and under high pressures, an entirely different type of seal had been designed, with wearing surfaces other than the axial surfaces of the shaft and the packing. This form of seal is called a "mechanical seal", and has proved to be very suitable in almost all extreme conditions.
The design of mechanical seals may differ in various physical respects, but all are fundamentally the same in principle. The sealing surfaces are located in a plane perpendicular to the shaft and usually consists of two highly polished surfaces running adjacently, one surface being in contact with the shaft and the other to the stationary portion of the pump. The polished and lapped surfaces which are of dissimilar materials are held in continual contact by a spring, forming a fluid-tight seal between the rotating and stationary members with very small frictional losses.
Types of Mechanical Seals
There are two types of mechanical seals:
- The internal assembly, in which the rotating element is located inside the box and is in contact with the liquid being pumped out, and
- The external assembly, in which the rotating element is located outside the box.
The pressure of the liquid in the pump tends to force the rotating and stationary faces together in the inside assembly and to force them apart in the external assembly. Both types have three primary points at which sealing must be accomplished:
- Between the stationary element and the casing.
- Between the rotating element and the shaft (or shaft sleeve).
- Between the mating surfaces of the rotating and stationary seal elements.
To accomplish the first, conventional gaskets or some form of a synthetic O-ring are used. Leakage between the rotating element and the shaft is stopped by means of O-rings, bellows, or some form of flexible wedges. Leakage between the mating surfaces cannot be stopped completely but can be help to an insignificant amount by maintaining a very close contact between these faces.
It is preferable on vertical pumps to have shaft sealing at the pump upper end only. This allows for:
- Adjustment; if soft packed,
- Observance of conditions (i.e. leakage) if mechanically sealed,
- Containment of liquid in the pump, over long idle periods, for wetting of bearings, etc. on start.
- Seating ring
- Seating (Stationary)
- Seating (Rotating)
- Spring retainer
- Spring retainer
While the stationary seating can be of bronze or stainless steel, the rotating seating can be of carbon, bronze or stainless steel, possibly with a monel or stellite surface.
It is important that cooling / lubricating liquid is led to mechanical seals from the lowest point on the pressure side of the pump, to ensure that some liquid reaches them, even when priming. They must not run in an air-pocket and care must be taken to prevent ingress of foreign materials. Also, most mechanical seals incorporate a carbon face and there is a possibility of electrolytic action. For these reasons, soft packing has advantages in sea water pumps.