Fault Cause and Preventive Measures of Valve (Two)

The third fault cause and preventive measures  

Another common failure of the valve is leakage of the sealing surface, leakage of the packing and the valve body bonnet joint. The cause of this fault must first be addressed from the sealing mechanism. 

(1) pressure of test gas

The difference between gas and liquid lies in the distance between molecules and the force between molecules. Gas pressure is produced by the collision of molecules on four walls. The pressure depends on the speed of molecular motion and the number of molecules per unit volume. The higher the speed of molecular movement, the greater the pressure occurs. The more molecules in unit volume, the greater the pressure occurs. When the gas density is small, the volume of the molecule itself and the attraction between the molecules can be ignored, and it is considered as an ideal gas. Nitrogen is the ideal gas in normal state. 

During the test, the temperature of the test gas is the same as the ambient temperature, and it follows the Boule-Marie law of ideal gas. The product of volume and pressure is a constant quantity. Namely,

P · V = C, where V is the volume of gas at a certain temperature. 

Low pressure gas seal test can replace high pressure water seal test. Because the molecular density of liquid is much higher than that of gas, and the molecule is in equilibrium. When it is compressed, the molecular spacing decreases and the repulsion force increases. In a single region, the number of liquid molecules is large and the number of gas molecules is small. According to previous analysis, the impact force of gas molecules is greater than that of liquid molecules. Therefore, the low pressure gas seal test can replace the high pressure water seal test. 

For high and low pressure gas, if the above conditions are met, the above conclusions will also be drawn.

In addition, the positive and negative charge centers of water molecules do not coincide. Because opposites attract each other, the same electrician repels each other, and the direction of two electric dipoles tends to be the same. The attraction of opposite charges will exceed the repulsion of the same charges, so the net attraction of two molecules is not easy to escape from the edge. It also helps seal.

(2) Characteristics of Fluid Pressure Generation

1. Characteristics of Gas Pressure

According to the theory of molecular motion, the magnitude of gas pressure is related to two conditions.

(1) the higher the molecular impact velocity, the higher the pressure is.

(2) the more molecules hit the wall surface per unit time, the greater the pressure is.

According to the law of Boule-Marie, PV = C. For example, at 0 ℃, the molecular effect of nitrogen at 1 atmospheric pressure is basically the same at 100 atmospheric pressure. Regardless of the gas type, the molecular effect at a given temperature is the same. Therefore, the gas pressure at a certain temperature is only related to the molecular number N per unit volume.

2. Characteristics of Liquid Pressure

According to the theory of molecular motion, it is known that there is an interaction between molecules. When the liquid is pressed, the distance between molecules decreases, and the repulsion force between molecules is expressed as pressure. That is, the pressure is expressed in the form of energy required for expansion. The pressure of the liquid will also produce the position energy generated by the height of the liquid column. This position energy will always exist at the height of the liquid column and cannot be eliminated.

Causes of Fluid Leakage

1. Causes of Gas Leakage

When the nitrogen pressure in the vessel is 100kgf / cm2 (9.8MPa) and the outside of the vessel is at atmospheric pressure of 0.098mpa, the gas molecules in the vessel will escape to the gas at the low pressure side, which will involve the leakage gap per unit length of the gas vessel escaping at the same time. Molecules, it is assumed that molecules with closely aligned linear formation escape at the same time in unit length, but molecules outside each unit length collide with the leakage gap molecules smaller than those escaping from the container, and conflict with the outside. In the gap between the molecules leaking from the container, the molecules in the container still leak to the outside of the gap.

2. Causes of Liquid Leakage

The leakage of liquid is caused by intermolecular repulsion, which causes the molecules at the inner edge of the sealing surface to wedge in and pass through the sealing surface. Or the diameter between the sealing surfaces is larger than the gap between the molecular diameter and the outer edge, which causes the molecules to extrude the sealing surface under the action of repulsion force.