Study on the Influence of External Conditions on Pump Mechanical Seals
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Abstract of the paper: Through the practical application and theoretical analysis of the mechanical seal for pumps, it is proposed that the actual sealing effect of the mechanical seal is not only related to the performance of the mechanical seal itself, but also important to the conditions provided by other parts and the sealing auxiliary system. Therefore, when designing pump unit products, it is necessary to provide a good external condition for the use of mechanical seals.
At present, mechanical seals are widely used in pump products. With the improvement of product technology and the requirement of energy saving, the application prospect of mechanical seals will be more extensive. The sealing effect of the mechanical seal will directly affect the operation of the whole machine, especially in the petrochemical industry. Due to the presence of flammable, explosive, volatile, and highly toxic media, the leakage of the mechanical seal will seriously affect the normal production, and serious safety accidents will occur. When people analyze the causes of quality failures, they are often used to the mechanical seal itself.
1 The principle and requirements of mechanical seal
The mechanical seal relies on a pair of relatively moving ring end faces A (one is fixed, the other rotates with the shaft, see Figure 1). The small axial gap formed by the mutual fit serves as a seal. This device is called a mechanical seal.
The mechanical seal is usually composed of a moving ring, a static ring, a compression element and a sealing element. The end faces of the moving ring and the static ring form a pair of friction pairs. The end face of the moving ring are pressed against the end face of the static ring by the pressure of the liquid in the sealed chamber, and an appropriate specific pressure is generated on the end faces of the two rings and a layer of thin liquid film is maintained, and
the purpose of sealing is achieved. The compression element generates pressure, which can keep the end faces fit in a non-operating state of the pump to ensure that the sealing medium does not leak out and prevent impurities from entering the sealing end surface. The sealing element plays the role of sealing the gap B between the moving ring and the shaft, and the gap C between the static ring and the gland, and at the same time cushions the vibration and impact of the pump. The mechanical seal is not an isolated part in actual operation. It is combined with other parts of the pump to operate. At the same time, it can be seen from its basic principles that the normal operation of the mechanical seal is conditional, such as: pump shaft The amount of channeling cannot be too large, otherwise the end surface of the friction pair cannot form the normal required specific pressure; the pump shaft at the mechanical seal cannot have too much deflection, otherwise the end surface specific pressure will be uneven and so on. Only by satisfying the external conditions like this, coupled with the good performance of the mechanical seal itself, can the ideal sealing effect be achieved.
2 Cause analysis of the influence of external conditions
2.1 Excessive axial displacement of the pump shaft
The sealing surface of the mechanical seal must have a certain specific pressure in order to achieve a sealing effect. This requires the spring of the mechanical seal to have a certain amount of compression to give the sealing end surface a thrust and rotate to make the sealing surface produce the required specific pressure of the seal. In order to ensure this specific pressure, the mechanical seal requires that the pump shaft can not have too much displacement, generally within 0.5 mm. However, in the actual design, due to the unreasonable design, the pump shaft often produces large displacement, which is very unfavorable to the use of the mechanical seal. This phenomenon often occurs in multi-stage centrifugal pumps, especially when the pump is started, the displacement is relatively large.
It is the working principle of the balance disc method to balance the axial force. When the balance disk is working, the axial gap b between the balance disk and the balance ring is automatically changed, thereby changing the pressure difference between the front and rear sides of the balance disk, and generating a force opposite to the axial force to balance the axial force. Due to the inertial effect of rotor movement and the fluctuation of transient pump operating conditions, the running rotor will not stand still at a certain axial equilibrium position. The balance plate is always in a state of oscillating left and right. The axial displacement of the balance disc in normal operation is only 0105 ~ 011 mm, which meets the requirement of the allowable axial displacement of the mechanical seal of 015 mm, but the axial displacement of the balance disc may be possible when the pump is started, stopped, or when the working conditions are drastically changed. It greatly exceeds the axial displacement allowed by the mechanical seal.
After the pump has been running for a long time, the balance disc and the balance ring are frictional and abraded. As the gap b increases, the axial displacement of the mechanical seal continues to increase. Due to the axial force, the pressing force of the sealing surface on the suction side increases, and the wear of the sealing surface increases, until the sealing surface is damaged and the sealing effect is lost. For the mechanical seal on the discharge side, with the wear of the balance disc, the axial displacement of the rotor components is greater than the axial displacement required by the seal, and the pressing force of the sealing surface is reduced, which fails to meet the sealing requirements, and finally all the mechanical seals on both sides of the pump lose the sealing function.
2.2 Excessive axial force
The mechanical seal cannot bear 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 balance mechanism and the reasons of manufacturing, installation and use. The mechanical seal bears an axial force. During operation, the temperature of the sealing gland will be high. For polypropylene media, it will be melted at high temperature. Therefore, the sealing effect will be lost soon after the pump is started. When the pump is stationary, there will be intermittent leakage at the sealing end face.
2.3 Excessive deflection of pump shaft
Mechanical seal, also known as end face seal, is a contact dynamic seal with rotating axial direction. Under the action of fluid medium and elastic elements, the two sealing end faces perpendicular to the axial line closely fit and rotate relatively, so as to achieve the sealing effect. Therefore, it is required that the force between the two seals should be uniform. However, due to the unreasonable design of the pump product, when the pump shaft is running, the deflection at the installation of the mechanical seal is large, resulting in uneven stress between the sealing surfaces, resulting in poor sealing effect.
2.4 It is unreasonable there is no auxiliary flushing system or the auxiliary flushing system setting
The auxiliary flushing system of the mechanical seal is very important. It can effectively protect the sealing surface and play a role in cooling, lubricating, and flushing away debris. Sometimes the designer does not properly configure the auxiliary flushing system, and the sealing effect cannot be achieved; sometimes although the designer designs the auxiliary system, but due to impurities in the flushing fluid, the flow and pressure of the flushing fluid are not enough, and the flushing port position is unreasonable. , The sealing effect is also not achieved.
2.5 Excessive vibration
The vibration of the mechanical seal is too large, which will eventually lead to the loss of the 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 vibration, such as unreasonable pump shaft design, processing reasons, insufficient bearing accuracy, poor parallelism of the coupling, and large radial force.
2.6 Causes of pump cavitation
Due to unreasonable operation of the device system, poor cavitation performance at the pump inlet, and high speed of the pump, local cavitation occurs at the pump inlet. After cavitation occurs, there will be bubbles in the water, which will impact the mechanical sealing surface on the one hand, damage the outer surface; on the other hand, the flow film on the anastomotic surface of the dynamic and static rings will also contain air bubbles, which can not form a stable flow film, resulting in dry friction of the anastomotic surface of the dynamic and static rings, and damage the mechanical seal device.
2.7 Insufficient machining accuracy
There are many reasons for the insufficient machining accuracy. Some of the reasons are the insufficient machining accuracy of the mechanical seal itself. This reason is easy to attract people's attention and easy to find. But sometimes the processing precision of other parts of the pump is not enough. This is not easy to attract people's attention. For example: the increase precision of the pump shaft, shaft sleeve, pump body, and sealed cavity is not enough. The existence of these reasons is very detrimental to the sealing effect of the mechanical seal.
3 Measures to be taken
3.1 Measures to eliminate large displacement of pump shaft
Reasonably design the axial force balance device to eliminate the axial displacement. In order to meet this requirement, for multi-stage centrifugal pumps, there are two ideal design schemes: one is the balance plate plus the axial thrust bearing, the balance plate balances the axial force, and the axial thrust bearing performs the pump shaft axial limit; the other is the balance drum plus axial thrust bearing, the balance drum balances most of the axial force, the remaining axial force is borne by the thrust bearing, and the axial thrust bearing shafts the pump shaft to the limit. The key to the second scheme is to design the balance drum reasonably so that it can truly balance out most of the axial force. For other single-stage pumps, split pumps and other products, some measures are taken in the design to ensure that the displacement of the pump shaft is within the range required by the mechanical seal.
3.2 Measures to eliminate excessive axial force
Reasonably design the axial force balance mechanism, so that it can truly and fully balance the axial force, and create a good condition for the mechanical seal. For some important products used in the fields of power plants, petroleum, chemical industry, etc., before the products leave the factory, bench tests must be performed to detect and find and solve problems. For some important pumps, an axial force measuring ring can be designed on the rotor to monitor the axial force at any time, and solve the problem in time.
3.3 Measures to eliminate too large deflection of the pump shaft
This phenomenon mostly exists in horizontal multi-stage centrifugal pumps. The following measures are taken in the design:
(1) Reduce the distance between the bearings at both ends. The number of stages of the pump impeller should not be too many. In the case of high total pump head requirements, try to increase the head of each stage of the impeller and reduce the number of stages.
(2) Increase the diameter of the pump shaft. When designing the diameter of the pump shaft, do not simply consider only the transmitted power, but also consider factors such as mechanical seal, shaft deflection, starting method, inertial load, radial force and other factors. Many designers do not fully realize this.
(3) Improve the grade of pump shaft material.
(4) After the design of the pump shaft is completed, the deflection of the pump shaft shall be checked and calculated.
3.4 Add auxiliary flushing system
When conditions permit, try to design an auxiliary flushing system. The flushing pressure is generally required to be higher than the pressure of the sealed chamber 0107 ~ 011 MPa, if the conveying medium is easy to vaporize, it should be higher than the vaporization pressure of 01175 ~ 012 MPa. The pressure in the sealed chamber should be calculated according to the structure of each pump, system pressure and other factors. When the pressure in the shaft seal cavity is very high or the pressure is almost close to the maximum limit of the seal, the liquid can also be drawn from the seal cavity to the low pressure area, so that the shaft seal liquid can flow to take away the frictional heat. The recommended flushing volume is shown in Table 1.
According to the operating conditions of each pump, reasonably configure the pipelines and accessories. Such as coolers, orifices, filters, valves, flow indicators, pressure gauges, temperature, etc. In fact, the reliability and life of the seal depend to a large extent on the configuration of the seal auxiliary system.
3.5 Measures to eliminate cavitation at the inlet of the pump
(1) Improve the cavitation performance level of the pump to meet the requirements of the cavitation performance of the field device.
(2) The requirements of the field test device should match the pump cavitation performance level.
(3) Site installation and working condition adjustment should create favorable conditions for the pump.
3.6 Measures to eliminate pump vibration
(1) In the design process of pump products, the source of vibration must be fully analyzed to eliminate the source of vibration.
(2) During the manufacturing and assembly process of pump products, strictly follow the standards and operating procedures to eliminate vibration sources.
(3) When auxiliary equipment such as pumps, motors, bases, and on-site pipelines are installed on-site, strict checks must be made to eliminate vibration sources.
(4) During on-site production, operation, maintenance, and adjustment, strictly check and eliminate vibration sources.
3.7 Strictly implement design standards
The design of pump and mechanical seal must implement relevant domestic and foreign standards. In the process of product design, designers should earnestly implement the standards, deeply understand the specific meaning of each content of the standard, and implement the requirements of the standard content to the product. During the design process. So far, many designers have not understood the actual meaning of the standard, and have not strictly implemented the new standard, but blindly copied the old drawings and the experience of the old designers. This approach is very detrimental to improving the technological level of our products and entering the international market. Improving the awareness of standardization is an urgent problem for designers in the machinery industry.
4 Conclusion
When designing the mechanical seal for pump, we should consider not only the influencing factors of the mechanical seal itself, but also various external influencing factors of the mechanical seal. In practical work, we should pay attention to the following problems:
(1) In the design process of the pump product, the influence of other parts of the pump and other equipment on the use effect of the mechanical seal should be fully considered to create a good external condition for the mechanical seal.
(2) Increase the understanding of the important role of the mechanical seal auxiliary system, and be equipped with a complete mechanical seal auxiliary system as much as possible to improve the sealing effect.
(3) For the mechanical seals of important pump products, protective measures should be taken to improve the seal quality and reduce seal quality accidents.
(4) When analyzing the causes of mechanical seal quality accidents, it is necessary to fully consider the impact of other parts of the pump on the operation of the mechanical seal, and take measures to continuously improve the effect of the mechanical seal.
References
[1] Mou Jiegang 1 Design and research of propane pump 1 Water Pump Technology, 1999 (1): 9～131
[2] Shenyang Water Pump Research Institute 1 Vane Pump Design Manual 1 Beijing: China Machine Press, 19731
[3] How to improve the performance and life of mechanical seals for petrochemical pumps 1 Petrochemical Equipment Technology, 1994 (6): 23～261