The parts of the seal inside the pump are subjected to an axial force from the pressure in the pumped liquid.
What is 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 forces
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.