BASIC KNOWLEDGE OF PUMP SEALS (2)

There are two main types of sealing devices for pumps: one is static seals and the other is dynamic seals. Static seals usually include gasket seals, O-ring seals, thread seals and other types. Dynamic seals mainly include soft gland packings, oil seals, labyrinth seals, spiral seals, power seals and mechanical seals. This article mainly introduces O-ring seals and gasket seals.

1. O-ring seals

Rubber O-rings are commonly used in pumps. Because of its simple shape, it is easy to manufacture and is low in cost. No matter how large the overall size of the O-ring is, its cross-sectional size is very small (only a few millimeters). Therefore, it is light in weight, consumes little material, and is easy to use. The more outstanding advantage is that the O-ring has good sealing ability and a wide range of use.

The working pressure of the static sealing can reach more than 100MPa, and the working pressure of the dynamic sealing can also reach 30Mpa. The applicable temperature is -6- 200℃, which can meet the requirements of various media. Therefore, it is more and more widely used in the design of pump seals.

The O ring seal is installed between the groove and the sealed surface, with a certain amount of compression. The resulting rebound force gives the sealed smooth surface and the bottom surface of the groove an initial compression stress, playing a sealing role.

When the pressure of the sealed liquid increases, the deformation of the O-ring also increases, so that the pressure transmitted to the sealing surface also increases, and the role of the seal also increases. This is why the O-ring seal has good sealing ability.

2. Gasket Seals

Sealing methods commonly used in chemical pumps include gasket seals. Gaskets are the basic components of static seals of centrifugal pumps, and are widely used. The selection of gaskets is mainly determined by factors such as delivery medium, temperature, pressure and corrosion.

1) The Sealing Mechanism of Gaskets

Leakage is the undesirable phenomenon that the medium flows from the inside of the limited space to the outside, or enters the inside of the limited space from the outside. The medium flows through the interface between the inside and outside spaces, and the leakage occurs. The root cause of the leakage is due to the gap in the contact surface, and the pressure difference and concentration difference on both sides of the contact surface are the driving force for the leakage.

Due to the influence of factors such as the form of the sealing surface and the processing accuracy, it is inevitable that there is a gap in the sealing surface, which will cause the sealing surfaces to not completely be matched, thus causing leakage. To reduce leakage, it is necessary to fit the contact surfaces to the maximum extent, that is, to reduce the cross-sectional area of the leakage channel, increase the leakage resistance, and make it larger than the leakage driving force.

Applying a compression load to the sealing surface to produce a compression stress can improve the contact degree of the sealing surface. When the stress is increased enough to cause significant plastic deformation on the surface, it can fill the gap of the sealing surface and block the leakage channel.

2) Selection of Gaskets

The gasket selection of chemical pump seals. The medium delivered by the chemical pump has the dangers of corrosion, volatilization, explosion, etc. The requirements for the sealing performance of the pump body are much higher than other types of pumps.

When the chemical pump conveys the medium and the temperature and pressure are not high, non-metallic gaskets are generally used. When the medium conveyed by the chemical pump is medium-pressure and high-temperature, non-metallic and metal combination gaskets are used. Non-metallic gaskets are most commonly used on pumps, and their materials are generally paper, rubber, and Teflon. When the temperature does not exceed 120℃ and the pressure is below 1.0Mpa, green shell paper or molded paper gaskets are generally used.

If the medium is oil and the temperature is at -30-110℃, nitrile rubber with better aging resistance is generally selected. When the temperature is at -50-200℃, the choice of fluororubber is more appropriate, because in addition to its resistance to oil and heat, its mechanical strength is also large. If the conveyed medium of chemical pumps is corrosive, PTFE is generally used as the gasket material.

As chemical pumps are more and more widely used, and the types of media being conveyed are more and more various, when selecting gasket materials, you should search relevant information or carry out experiments, and then make the right choice.

Certainly, in addition to the above two types of seals, you can also use spring seals, elastomer bellow seals, etc.