influence Mechanical seal for kettle What are the external conditions of?

Mechanical seal usually consists of moving ring, stationary ring, pressing element and sealing element. The end faces of the moving ring and the fixed ring form a pair of friction pairs. The moving ring presses its end face on the end face of the fixed ring through the pressure of the liquid in the sealing chamber, and generates appropriate specific pressure on the end faces of the two rings to maintain a very thin liquid film to achieve the purpose of sealing. The pressure generated by the pressurizing element can keep the end face fitted when the pump is not running, thus ensuring that the sealing medium does not leak and preventing impurities from entering the sealing end face. The seal seals the clearance B between the moving ring and the shaft, and the clearance C between the stationary ring and the gland, while buffering the vibration and shock of the pump.

The mechanical seal is not an isolated part in actual operation, it operates together with other parts of the pump. At the same time, it can be seen from its basic principle that the normal operation of the mechanical seal is conditional, for example, the leakage of the pump shaft cannot be too large, otherwise the friction pair end face cannot form the normal required specific pressure; The pump shaft should not have too much deflection at the mechanical seal, otherwise the end face specific pressure will be uneven, etc. Only by meeting such external conditions and good mechanical sealing performance, can the ideal sealing effect be achieved.

2、 Cause analysis of external conditions

1. Large axial displacement of pump shaft

The sealing surface of the mechanical seal must have a certain specific pressure so that it can play a sealing role. This requires that the spring of the mechanical seal must have a certain amount of compression, give a thrust to the sealing face, and rotate to make the sealing surface produce the specific pressure required by the seal. In order to ensure this specific pressure, the mechanical seal requires that the pump shaft should not have too much displacement, which should generally be within 0.5mm. But in the actual design, due to the unreasonable design, the pump shaft often produces a large displacement, which is very unfavorable to the use of mechanical seals. This phenomenon often occurs in multistage centrifugal pumps, especially in the process of starting the pump, with a large amount of channeling.

When the balance disc works, it automatically changes the axial clearance b between the balance disc and the balance ring, thus changing the pressure difference between the front and rear sides of the balance disc, and generating a force opposite to the direction of the axial force to balance the axial force. Due to the inertia effect of rotor shifting and the fluctuation of transient pump working condition, the running rotor will not rest at a certain axial balance position. The balance disc is always in the state of shifting from left to right. The axial displacement of the balance disc in normal operation is only 0105~011 mm, which meets the requirement of 015mm of the allowable axial displacement of the mechanical seal. However, the axial displacement of the balance disc may greatly exceed the allowable axial displacement of the mechanical seal when the pump starts, stops, and the working condition changes.

After the pump has been running for a long time, the balance disc and the balance ring are worn. With the increase of clearance b, the axial displacement of the mechanical seal increases continuously. Due to the axial force, the compression force of the sealing surface on the suction side increases, and the wear of the sealing surface intensifies until the sealing surface is damaged and loses its sealing effect. For the mechanical seal at the discharge side, with the wear of the balance plate, the axial displacement of the rotor parts is greater than the axial displacement required by the seal, and the compression force of the sealing surface is reduced, failing to meet the sealing requirements, so that the mechanical seals at both sides of the pump lose their sealing effect.

2. The axial force is too large

The mechanical seal cannot bear the axial force during use. If there is axial force, the impact on the mechanical seal is serious.

Sometimes, the axial force of the pump is not balanced due to the unreasonable design of the axial force balancing mechanism, manufacturing, installation, use and other reasons. The mechanical seal bears an axial force, and the temperature of the seal gland will be higher during operation. Polypropylene media will be melted at high temperature, so the sealing effect will be lost soon after the pump is started. When the pump is stationary, the seal face will have intermittent leakage.

3. The deflection of pump shaft is too large

Mechanical seal, also known as end face seal, is a kind of rotary axial contact dynamic seal. Under the action of fluid medium and elastic elements, two seal faces perpendicular to the axis line closely fit and rotate relatively to achieve the sealing effect. Therefore, the force between two seals is required to be uniform. However, due to the unreasonable design of pump products, when the pump shaft is running, the deflection at the mechanical seal installation is large, which makes the force between the sealing faces uneven, leading to poor sealing effect.

4. There is no auxiliary flushing system or the auxiliary flushing system is set unreasonably

The auxiliary flushing system of mechanical seal is very important. It can effectively protect the sealing surface and play the role of cooling, lubrication and flushing away debris. Sometimes the designer does not reasonably configure the auxiliary flushing system, which can not achieve the sealing effect; Sometimes, although the designer has designed the auxiliary system, due to impurities in the flushing fluid, insufficient flow and pressure of the flushing fluid, unreasonable design of the flushing port position and other reasons, the sealing effect is also not achieved.

5. Excessive vibration

The mechanical seal vibrates too much, resulting in the loss of sealing effect. However, the reason for the excessive vibration of the mechanical seal is often not the reason of the mechanical seal itself. Other parts of the pump are the source of the vibration, such as the unreasonable design of the pump shaft, the reason for machining, the insufficient bearing precision, the poor parallelism of the coupling, and the large radial force.

6. Insufficient machining accuracy

There are many reasons for the insufficient machining accuracy, some of which are the insufficient machining accuracy of the mechanical seal itself, which is easy to attract people's attention and find.

But sometimes the machining accuracy of other parts of the pump is not enough, which is not easy to attract people's attention. For example, the enlargement accuracy of pump shaft, shaft sleeve, pump body and sealing cavity is not enough. The existence of these reasons is very unfavorable to the sealing effect of mechanical seals.

7. Causes of pump cavitation

Due to improper operation of the device system, poor cavitation performance at the pump inlet, and high pump speed, local cavitation occurs at the pump inlet. After cavitation occurs, bubbles will appear in the water, which will impact the external surface of the mechanical seal surface on the one hand, causing damage to its surface; On the other hand, the flow film on the mating surface of the moving and stationary rings will also contain bubbles, which will not form a stable flow film, causing dry friction on the mating surface of the moving and stationary rings, and damaging the mechanical seal device.