NO.1 The basic elements of sealing
Mechanical seals use the principle of friction between two planes to achieve the purpose of sealing. The rotating sealing surface is installed on the main shaft of the Rotary Lobe Pumps, emulsion pumps and other liquid pump, while the fixed sealing surface is installed inside the sealing gland. Due to the fact that one sealing surface is moving while the other sealing surface is stationary, this type of seal is called a dynamic seal.
The latter two types of leakage paths generally adopt static sealing, as there is no relative movement between the two parts. This part of the seal is usually referred to as a tertiary seal, and its sealing material is a gasket or an O-ring suitable for the process liquid.
Most mechanical seals are designed with softer materials to create a rotating surface that rotates and rubs against a harder stationary surface. For many years, the most common combination has been to use carbon materials as rotating surfaces to run on ceramic stationary surfaces. This type of material is still widely used, but the stationary surface is made of stainless steel or harder materials, such as tungsten carbide or silicon carbide.
Regardless of the material used, a liquid film must be maintained between the contact surfaces to provide lubrication. However, in the packing box, a combination of spring load and liquid pressure can provide a good sealing effect between the sealing surfaces. But if the sealing pressure is too high, it will affect the formation of a liquid film between the contact surfaces, leading to increased heat and premature wear. If the sealing pressure is too low, the gap between the contact surfaces will increase, which can easily cause liquid leakage.
Sealing manufacturers are constantly striving to improve the flatness of the contact surface, using special polishing plates for grinding. Then, a grating plate with a monochromatic light source is used to detect it. From this perspective, it is very important to handle these sealing contact surfaces carefully and strictly follow the installation instructions to ensure that the sealing surfaces are properly protected and correctly seated.
NO.2 Flexible options for sealing
The traditional sealing design used in the chemical industry, where the sealing pressure is applied to the rotating surface, is called a rotary seal because the spring or corrugated sealing device rotates with the spindle. A relatively novel design is with a spring or corrugated sealing device installed on a stationary surface. In current mechanical seals, both of the above sealing methods have common applications, which provide a certain degree of flexibility for installation.
For many highly corrosive applications, the most common design is to use metal bellows sealing devices. The corrugated pipe is welded from a series of metal discs to form a corrugated sealing device that prevents leakage.
By using this device, the sealing pressure between the sealing surfaces can be more uniform, and there is no need to add a secondary seal on the sealing surface, thus naturally avoiding any corrosion and wear phenomenon.
Generally speaking, although its main sealing pressure depends on the pressure of the packing box itself, the spring and bellows can compensate for the insufficient pressure caused by the spindle movement during the starting and stopping processes of the Rotary Lobe Pumps, emulsion pumps and other liquid pump, so that the sealing surface always maintains a certain sealing pressure.
NO.3 Corrosion and wear problems
Due to various reasons, such as bearing tolerances, shaft end clearance, vibration, and spindle deviation, the main shaft of the hydraulic pump can experience radial and axial motion. In addition, it is very difficult to maintain absolute parallelism between the contact surfaces, so it is normal for the internal movement of the mechanical seal itself to occur. This type of movement is often caused by equipment and installation tolerances, thermal expansion, pipeline stress, or improper spindle adjustment.
In order to maintain mutual coordination between the sealing surfaces at all times, the spring plays a constant adjustment role between the mechanical seal and the moving spindle. When an artificial rubber seal is used between the rotating surface and the spindle, the elastomer will move back and forth on the spindle. This repeated friction action will erode the anti-corrosion material on the spindle, lose the oxide film protection layer of the spindle, and ultimately form wear grooves on the friction surface of the spindle, causing liquid leakage from the grooves and increasing necessary maintenance workload or even replacing the spindle. To solve this problem, a replaceable shaft sleeve is usually installed inside the packing box.
However, the only permanent solution to the problem of corrosion and wear is to remove the dynamic seal inside. Nowadays, most major seal manufacturers produce non corrosive wear type seals to prevent corrosion and wear of liquid pump parts.
NO.4 Balanced and non-balanced seals
The balance of mechanical seals has a significant impact on the sealing pressure of the contact surface. This sealing pressure depends on the effective cross-section of the seal itself and the pressure inside the packing box.
The cross section on the opposite side of the rotating surface of the non balanced seal is completely exposed within the pressure range of the packing box, which can generate high sealing pressure between the sealing surfaces, thereby increasing the working temperature and accelerating the wear rate. Under high-temperature working conditions or when liquids have significant corrosion and friction, the service life of mechanical seals can be greatly reduced.
Balancing mechanical seals can reduce sealing pressure and extend the service life of the seals. Generally, the main shaft and shaft sleeve with steps are used to reduce the effective cross-section of the rotating surface to achieve the above objectives. However, do not adjust the net sealing pressure to a level close to zero, as doing so may result in unstable working conditions between the sealing surfaces and may cause the seal to blow open due to sudden changes.
The answer to these sealing problems may be to use unbalanced seals, and for some services, using unbalanced seals may achieve better results. For example, in certain application areas, safety issues caused by liquid leakage may be more emphasized than the service life of seals. In this case, the choice of sealing may also be understood as a preference for a higher sealing pressure. Similarly, when selecting seals with colder liquids, the increase in operating temperature may be negligible.
Regardless of the considerations, it is generally recommended to use a balanced seal when the pressure of the packing box exceeds 50psi.
NO.5 Inner seal and outer seal
The most common practice is to install the seal inside the stuffing box. This approach requires dismantling the wet end of the Rotary Lobe Pumps, emulsion pumps and other liquid pump during maintenance, and its main advantage is that the sealed environment is hygienic and easy to control.
The installation method of the outer seal is to reverse the direction of the static sealing surface, and the rotating unit on the spindle is located outside the packing box gland. The main benefits of external sealing are as follows:
1. Easy installation;
2. The cost is relatively low;
3. Can continuously monitor and clean;
4. Suitable for small packing boxes that cannot be sealed internally;
5. Due to its proximity to the bearing, there is less difficulty in affecting the spindle deviation.
Its main drawback is that centrifugal force will throw solid particles from below the seal towards the contact surface of the seal. Therefore, this type of seal is mainly suitable for clean and abrasive free liquids.
In recent years, separated seals have become another important additional feature in external seals. The separated seal is a complete assembly installed between the packing box and the bearing sleeve. This design eliminates the need to disassemble the Rotary Lobe Pumps, emulsion pumps and other liquid pump every time the seal needs to be replaced. This type of sealing is gradually being developed in conjunction with other design standards. Due to the ease of replacing seals in this design, it is important to resist the temptation to only replace seals without further investigation of the root cause of the malfunction.
As a non-Newtonian fluid transportation equipment research and manufacturing enterprise, Durrex Pumps has obtained 56 national patents, including 11 invention patents. The company has developed Rotor pumps, Homogeneous pumps, grinding pumps, rubber lobe pumps, magnetic pumps and other products, providing fluid transportation equipment and technical services for more than 10000 clients worldwide.
