The controlled gap seal is a seal frequently used on centrifugal pumps between rotating and stationary pump components. The gap's width (clearance gap width) and shape are designed to minimise the mass flow which can pass through it (see Clearance gap loss)
Examples are the sealing of the individual stages of multistage pumps against one another or in balancing devices and the sealing of the impeller's suction side against the discharge side. (see Clearance gap pressure).
The width of the controlled gap seal affects both the economic efficiency and the operating reliability of a centrifugal pump and depends on the following factors: shaft deflections, vibrations (including self-induced vibrations), nature of fluid handled, degree of contamination, grain size of the dirt particles (see abrasion), temperatur. If the thermal expansion of the rotating components differs from that of the stationary components, the clearance gap width may change and under certain conditions the casing may even be distorted.
The main types of controlled gap seals used in centrifugal pump engineering are smooth controlled gap seals, stepped controlled gap seals and labyrinth seals. Convex (curved towards the outside) and conical (in the shape of a cone) gaps have proved their worth in applications such as bottle cleaning systems where the removal of labels presents a risk of clogging.