Page 21 - Valve Selection Handbook

8 Valve Selection Handbook

SEALING MECHANISM

Sealability Against Liquids

The scalability against liquids is determined by the surface tension and
the viscosity of the liquid.
When the leakage capillary is filled with gas, surface tension can
either draw the liquid into the capillary or repel the liquid, depending on
the angle of contact formed by the liquid with the capillary wall. The
value of the contact angle is a measure of the degree of wetting of the
solid by the liquid and is indicated by the relative strength of the attrac-
tive forces exerted by the capillary wall on the liquid molecules, com-
pared with the attractive forces between the liquid molecules themselves.
Figure 2-2 illustrates the forces acting on the liquid in the capillary.
The opposing forces are in equilibrium if

Figure 2-2. Effect of Surface Tension on Leakage Flow through Capillary.