Page 22 - Valve Selection Handbook

Fundamentals 9

where
r = radius of capillary
AP = capillary pressure
T = surface tension
6 = contact angle between the solid and liquid

Thus, if the contact angle formed between the solid and liquid is
greater than 90°, surface tension can prevent leakage flow. Conversely, if
the contact angle is less than 90°, the liquid will draw into the capillaries
and leakage flow will start at low pressures.
The tendency of metal surfaces to form a contact angle with the liquid
of greater than 90° depends on the presence of a layer of oily, greasy, or
waxy substances that normally cover metal surfaces. When this layer is
removed by a solvent, the surface properties alter, and a liquid that previ-
ously was repelled may now wet the surface. For example, kerosene dis-
solves a greasy surface film, and a valve that originally was fluid-tight
against water may leak badly after the seatings have been washed with
kerosene. Wiping the seating surfaces with an ordinary cloth may be suf-
ficient to restore the greasy film and, thus, the original seat tightness of
the valve against water.
Once the leakage capillaries are flooded, the capillary pressure
becomes zero, unless gas bubbles carried by the fluid break the liquid
column. If the diameter of the leakage capillary is large, and the
Reynolds number of the leakage flow is higher than critical, the leakage
flow is turbulent. As the diameter of the capillary decreases and the
Reynolds number decreases below its critical value, the leakage flow
becomes laminar. This leakage flow will, from Poisuille's equation, vary
inversely with the viscosity of the liquid and the length of the capillary
and proportionally to the driving force and the diameter of the capillary.
Thus, for conditions of high viscosity and small capillary size, the leak-
age flow can become so small that it reaches undetectable amounts.

Sealability Against Gases

The sealability against gases is determined by the viscosity of the gas
and the size of the gas molecules. If the leakage capillary is large, the
leakage flow will be turbulent. As the diameter of the capillary decreases