Operating characteristics of mechanical seals for easy vaporization medium pumps-

Operating characteristics of mechanical seals for easily vaporized medium pumps

　2020-06-30　　

The operating characteristics of mechanical seals for easily vaporized medium pumps:

In the petrochemical industry, the safe operation of pumps for easily vaporized media (such as pumps for light hydrocarbon media) mainly depends on the reliability of the shaft seal. Because most light hydrocarbon media are flammable and explosive dangerous substances, such as methane, ethane, propane, butane, ethylene, propylene, propylene, etc. These media not only have the characteristics of low boiling point and easy vaporization, but also have the characteristics of high vapor pressure, low viscosity, low ignition point, and high pollution. For mechanical seals that seal such media, it is difficult to maintain a stable fluid film between the end faces due to the easy vaporization of the fluid, which is in a gas-liquid mixed phase unstable friction state, and phenomena such as 'steam spray' and 'screaming' occur, and even the seal end faces are completely opened However, severe leakage or severe dry friction will cause the seal to fail early. Therefore, the sealing problem of light hydrocarbon pumps has always been a practical problem that plagues long-term safe production.

At present, for pumps for conveying light hydrocarbons and other easily vaporized media, the domestic main adopts single-end, double-end, tandem and other contact mechanical seal structures.

For light hydrocarbon pumps with single-end face contact mechanical seals (as shown in Figure 9-9), there is not a complete liquid film between the sealing end faces. On the one hand, due to the large amount of frictional heat generated at the friction end surface, the temperature of the sealing surface increases, which promotes the end surface

Figure 9-9 Single end face seal structure

The inter-liquid film vaporizes, turning the liquid into vapor and undergoes a phase change; on the other hand, because the saturated vapor pressure of light hydrocarbons is higher than the ambient atmospheric pressure (Figure 9-10), light hydrocarbon media flows through the gap between the seal end The pressure drop inevitably vaporizes at a certain position, that is, a phase change occurs. Therefore, according to the phase state of the fluid film on the end surface, it can be divided into liquid phase seal, gas phase seal or gas-liquid mixed phase seal.

Figure 9~10 Variation curve of vapor pressure and temperature of petrochemical medium

①Liquid phase seal. As shown in Figure 9-11(a), a layer of liquid film is filled between the sealing end faces, and the leakage may appear in the form of liquid. This type of seal is a liquid phase seal. Its characteristic is that the film pressure coefficient is a certain value (Figure 9-12), the working state is stable, the friction coefficient is small, and the leakage is small.

Figure 9-11 End face fluid film model

② Gas-liquid mixed phase sealing. As shown in Figure 9-11(b), the sealing end faces are filled with a layer of mixed film formed by liquid and steam, and the leakage occurs in the form of gas phase. This type of seal is a gas-liquid mixed phase tight seal. Its characteristic is that the film pressure coefficient changes with temperature (Figure 9-12), indicating that the working state is unstable, prone to 'steam spray

③Gas-phase sealing. As shown in Figure 9-11(c), the seal end faces are filled with a complete vapor film formed by steam, and the leakage occurs in the form of gas phase. This type of seal is a gas phase seal. Its characteristic film pressure coefficient is a certain value (Figure 9-12), working state: stable state, but the friction coefficient is large, the amount of wear is large, and the service life is short.

Figure 9-12 Variation of pressure coefficient of end face film with temperature

In the petrochemical industry, most of the single-end contact mechanical seals used for vaporizable light hydrocarbon pumps do not fail due to normal wear, mainly because the friction pair is in a vapor-liquid mixed phase unstable friction state, which is prone to phase state. Early failure occurs due to instability. In engineering, the pressure of the medium in the sealed box or many cooling measures are often taken to keep the seal in a stable friction state as much as possible, but the effect is limited.

In addition, no matter what the friction state, there will be leakage of the sealed medium, and the light hydrocarbon medium basically has relatively high pollution and danger, and it is difficult to meet the modern environmental protection requirements.　　1136.html,

What are the common mechanical seals for separating temperature medium pumps?　　 Typical mechanical seal for easy gasification medium pump