The Equilibrium Ratio Control of the Mechanical Seal
The nominal contact area between the effective area and the sealing surface under mechanical seal fluid pressure is called mechanical seal area ratio and was known as equilibrium ratio on abroad. Equilibrium ratio has an important influence on the performance and life span of mechanical seal. Deviation of equilibrium ratio from normal operational allowables caused by improper equilibrium ratio selection or change of operation conditions (such as sealing medium pressure changes) will ultimately lead to seal failure. The most common failure modes associated with equilibrium ratio are the opening failure of sealing face, early wear, dry running, hot crack, blister, phase instability, oblique instability, fluid film instability, liquid film rupture, vaporization and sealing loss. The following are several reliable and effective mechanical sealing technologies that ensure sealing effects by controlling equilibrium ratio, i.e. constant equilibrium ratio seal, variable equilibrium ratio seal and zero equilibrium ratio seal.
Constant equilibrium ratio seal includes two situations, equilibrium ratio is constant and equilibrium ratio is zero. This type of seal can adapt to the variable pressure condition and ensure that the equilibrium ratio remains unchanged.
In order to meet the requirement of the job, changing hydraulic pressure on the static ring back, variable equilibrium ratio seal to change the sealing surface load and make the frictional states of sealing face transform from contact to contactless or vise versa.
Zero equilibrium ratio seal is sealed with a narrow edge of the sealing surface, its internal and external diameter is similar and equal to the effective diameter of the bellows, therefore, the equilibrium ratio is close to zero. The advantage of this seal is that because the sealing surface is very narrow, it can limit the formation of solid debris, and the material or fiber formed can be cut off by the sharp edge. The sealing surface load will not change with the pressure of the sealing medium. This kind of seal has been used in some rubber factories in China.