Sealing elements, or seals, are components or constructions which help to limit or prevent the unintentional transition of substances from one space to another.
Sealing elements in centrifugal pumps are designed to ensure the separation of internal and external spaces within and around the pump in a leaktight manner, or with minimal leakage. Sealing elements are still referred to as sealing elements even if absolute leak tightness is not achieved (e. g. in the case of throttling). Degrees of sealing range from absolutely leak-tight sealing elements (welding together of sealing faces), to non-welded sealing elements (e. g. gaskets, ground sealing elements, O-rings) and clearance gap constructions with a given leakage rate.
A large number of different sealing methods and elements (from elements sealing off the pump connection to the piping to diverse internal pump seals) are available to provide solutions which meet the numerous requirements of pumping applications.
Sealing element types
- Stationary mating sealing faces
- Sealing faces which slide against one another
- Sealing faces which adjust themselves in relation to one another within given limits
- Sealing faces which rotate against one another
Centrifugal pumps are mostly connected to the piping by means of sealing elements with stationary mating sealing faces, although other seal types are also used (sealing elements whose faces adjust themselves in relation to one another within given limits and whose sealing faces rotate against one another).
Stationary mating sealing faces
Sealing element designs must be selected according to the functions stationary mating sealing faces are required to perform (i.e. "sealing", "centring" or "aligning"). Considerable forces are often necessary to perform the "sealing" function. The axial contact pressure required by axially sealing O-rings is relatively low; axially sealing gaskets necessitate more, and the highest contact pressure is required by sealing elements with ground metal sealing faces. Radially sealing O-rings, by contrast, require only a low radial preloading force for the sealing function.
Sealing elements at locations such as drain plugs function only to seal, and can therefore be very flexible because they perform no alignment function. Sealing elements which are not exposed to a significant differential pressure, e. g. at bearing covers of bearing brackets, can employ a simple design.
Rotating seals fitted between stationary sealing faces (e. g. shaft protecting sleeve against shaft) are subjected to both centrifugal and differential pressure forces. A suitable design (e. g. confined gaskets) ensures that these can be absorbed.
Differentiation between sealing elements with stationary mating sealing faces:
- Non-separable (welded flanges and sealing faces)
- Separable (gaskets, flexible materials)
- Confined (O-rings etc.)
- Lens gasket (metal sealing elements)
- Sealing elements fed with barrier fluid (for toxic or explosive fluids, or if there is a risk of air penetrating at pressures below atmosphere)
Sealing faces which slide against one another
Sealing faces which slide against one another are used to compensate for thermal expansion during installation or operation due to temperature fluctuations, an example being two concentric pipes sealed against one another by a gland packing. In the case of mechanical seals, compensation for the seals' axial movement is ensured by means of O-rings or rubber bellows.
Sealing faces which adjust themselves in relation to one another within given limits
Sealing faces which adjust themselves in relation to one another within given limits are sealed by means of diaphragms, bellows and expansion joints (to limit the forces acting on the nozzles or to provide noise insulation between the pump casing and the piping). This type of sealing element also includes the connection and sealing by hoses.
Sealing faces which rotate against one another
In the case of sealing faces which rotate against one another, the task is to seal rotating machine components against non-rotating parts in such a way that leakage or the penetration of substances from the outside are reduced to a given extent and possible wear of the sealing faces is minimised (See also Shaft seal).