In its most basic form, a shaft seal combines a rotating part with a stationary part. When properly designed and installed, the rotating part rides on a lubricating film, only 0.00025 mm in thickness.
Should the film become too thick, the pumped medium will leak. If the film becomes too thin, the friction loss increases and the contact surfaces overheat, triggering seal failure.
The stationary part of the seal is fixed in the pump housing. It consists of a stationary seat and a stationary secondary seal. The secondary seal prevents leakage between the stationary seat and pump housing. It prevents the seat from rotating in the pump housing as well.
The pumped liquid (A) is generally in contact with the outer edge of the rotating seal ring (B). When the shaft starts to rotate, the pressure difference between the liquid (A) in the pump housing and the atmosphere (D) forces the liquid to enter the sealing gap (from B to C) between the two flat rotating surfaces. A lubricating film is generated. The pressure in the sealing gap is reduced from B to C, reaching the pressure at D. Leakage from the seal will appear at out in the air.
The parts of the seal inside the pump are subjected to a force emanating from the pressure within the pump. The axial component of this force, together with the spring force creates the closing force of the seal.
Shaft seal forces
The parts of the seal inside the pump are subjected to an axial force from the pressure in the pumped liquid. If the differential pressure between the pumped medium and the atmosphere is above 20 bars, the closing force becomes so strong that it prevents the formation of an adequate hydrodynamic lubricating film. The result is that the seal faces begin to wear.
Reducing the area of the hydraulic pressure will automatically affect the axial force on the shaft seal. As a result, the hydraulic force of the primary seal faces as well as the closing force of the seal is reduced.
The pump pressure acting on the area Ah causes a closing force to be exerted on the seal. The area Ah of the unbalanced mechanical shaft seal is larger than the area As, and the balancing ratio k is larger than 1.
The contact pressure in the sliding face area exceeds the pumped liquid pressure. In the low-pressure range of the pumped liquid, the unbalanced mechanical shaft seals are sufficient. The area Ah of the balanced mechanical shaft seal is smaller than the area As, and the balancing ratio k is smaller than 1. The area Ah can be decreased by a diameter reduction of the shaft.
In the high-pressure range of the pumped medium or at high speed, the balanced mechanical shaft seal is used. The contact pressure in the sliding face area can be smaller than the pumped medium pressure. The balancing ratio is often chosen to be around 0.8.
Grundfos offers a wide range of special-purpose shaft seals and shaft seal arrangements for varying liquid and operating requirements.