Mechanical seals are widely used in rotating equipment. The sealing effect of mechanical seals directly affects the operation of the entire machine, and in severe cases, it can lead to major safety accidents.
This paper analyzes several factors affecting the sealing effect of mechanical seals from the perspective of internal and external conditions, and proposes reasonable measures to be taken.
1. Principles and requirements of mechanical seals
Mechanical seals, also known as face seals, are shaft sealing devices for rotating machinery. They consist of at least one pair of end faces perpendicular to the axis of rotation, which, under the influence of fluid pressure, the elastic force (or magnetic force) of a compensating mechanism, and the cooperation of auxiliary seals, remain in contact and slide relative to each other to prevent fluid leakage. Their main function is to transform easily leaking axial seals into more leak-resistant face seals. They are widely used for sealing the rotating shafts of pumps, reactors, compressors, and other similar equipment.
Mechanical seals typically consist of a rotating ring, a stationary ring, a clamping element, and a sealing element. The rotating ring rotates with the pump shaft, and the rotating and stationary rings fit tightly together to form a sealing surface, preventing media leakage. The rotating ring's end face is pressed against the stationary ring's end face by the pressure of the liquid in the sealing chamber, generating an appropriate specific pressure and maintaining a very thin liquid film on both ring end faces to achieve a seal. The clamping element generates pressure, ensuring that the end faces remain in contact even when the pump is not running, guaranteeing no leakage of the sealed medium and preventing impurities from entering the sealing end face. The sealing element seals the gap between the rotating ring and the shaft, and the gap between the stationary ring and the gland, while the elastic element buffers pump vibrations and impacts. In actual operation, the mechanical seal operates in conjunction with other pump components, and its normal operation is greatly influenced by its own performance and external conditions. However, we must first ensure the performance of its own components, the technical requirements of the auxiliary sealing devices, and the installation to ensure the mechanical seal performs its intended function.
2. Failure symptoms and causes of mechanical seals
2.1 Failures of Mechanical Seal Components During operation, rotating equipment often experiences wear, thermal cracking, deformation, and damage to the sealing end face. Springs, over time, can also loosen, break, and corrode. Auxiliary sealing rings may also develop cracks, twisting, deformation, or breakage.
2.2 Causes of mechanical seal vibration and overheating failures
During equipment rotation, the contact surfaces of the moving and stationary rings become rough, and the gap between the moving and stationary rings and the sealing cavity becomes too small. This causes collisions due to oscillation, resulting in vibration. Sometimes, poor corrosion resistance and temperature resistance of the sealing surface, insufficient cooling, or particulate impurities trapped on the surface during installation can also cause vibration and heat generation in the mechanical seal.
2.3 Causes of mechanical seal media leakage
(1) Leakage during static pressure test. Due to carelessness during installation, mechanical seals often suffer damage, deformation, or failure to clean properly, resulting in particulate impurities. Loose positioning screws, insufficient gland tightening, or inadequate machine or equipment precision can also cause leakage, leading to incomplete sealing. If the shaft sleeve leaks, it's because the shaft sleeve seal ring wasn't properly tightened, insufficiently compressed, or damaged during assembly. (2) Periodic or intermittent leakage. Periodic vibration or excessive axial movement of the mechanical seal's rotor assembly can cause leakage. A mechanical seal requires a certain specific pressure to function effectively. This necessitates a certain compression of the spring to provide a thrust to the sealing surface, generating the required specific pressure during rotation. To ensure this specific pressure, the pump shaft must not have excessive axial movement, generally within 0.25mm. However, in actual design, due to improper design, the pump shaft often exhibits significant axial movement, which is detrimental to the mechanical seal's performance. (3) Frequent leakage of mechanical seals. There are many reasons for frequent leakage of mechanical seals. First, frequent leakage is caused by defects in the sealing end face. Second, frequent leakage is caused by auxiliary sealing rings. Third, leakage is caused by defects in the spring. Other reasons include leakage caused by rotor vibration, leakage caused by poor quality or looseness of transmission, locking and thrust parts, leakage caused by mechanical seal auxiliary mechanisms, and frequent leakage caused by medium problems. (4) Excessive vibration of mechanical seals. Excessive vibration of mechanical seals will eventually lead to loss of sealing effect. However, the reasons for excessive vibration of mechanical seals are often not only due to the mechanical seal itself, but also due to other parts of the pump, such as unreasonable pump shaft design, processing reasons, insufficient bearing precision, poor parallelism of coupling, large radial force, etc.
3. Measures taken to handle the fault
If a component of the mechanical seal malfunctions, it needs to be replaced or its machining precision improved. Improving the machining precision of the mechanical seal itself and other pump components is highly beneficial to its performance. To enhance the sealing effect, high smoothness and flatness are required on the friction surfaces of the rotating and stationary rings. The width of the friction surfaces of the rotating and stationary rings is relatively small, generally between 2 and 7 millimeters.
3.1 Handling of vibration and heat generation in mechanical seals
If the gap between the dynamic and static rings and the sealing cavity is too small, the inner diameter of the sealing cavity should be increased or the outer diameter of the rotating part should be decreased to ensure a gap of at least 0.75mm. If the friction pair is improperly matched, the materials of the dynamic and static rings should be changed to be more temperature-resistant and corrosion-resistant. This will reduce the vibration and heat generation of the mechanical seal.
3.2 Handling of Mechanical Seal Leakage
Mechanical seal leakage can be caused by a variety of reasons, and each case must be addressed individually. To minimize leakage, mechanical seals must be installed strictly according to technical requirements, and the following points should also be noted.
(1) Assembly must be clean and smooth. The mechanical seal components, tools, lubricating oil, and wiping materials must be very clean. The sealing end faces of the dynamic and static rings must be wiped with soft gauze. (2) Trim and round the chamfers. The chamfers of the shaft, sealing end cover, etc., must be trimmed smoothly, and the relevant rounded corners of the shaft and end cover must be sanded and polished. (3) When assembling the auxiliary sealing ring, the rubber auxiliary sealing ring must not be soaked or washed with gasoline or kerosene to avoid swelling, deformation, and premature aging. After the dynamic and static rings are assembled, press the compensation ring by hand to check whether it is in place and flexible; check whether the elastic open ring is reliably positioned. After the dynamic ring is installed, it must be ensured that it moves axially on the shaft flexibly.
3.3 Handling large pump shaft misalignment
A reasonable axial force balancing device should be designed to eliminate axial displacement. To meet this requirement, for multi-stage centrifugal pumps, the design scheme is: a balance disc plus an axial thrust bearing. The balance disc balances the axial force, and the axial thrust bearing provides axial restraint for the pump shaft.
3.4 Add an auxiliary flushing system
If the sealing medium in the sealing cavity contains particles and impurities, it must be flushed. Otherwise, the spring of the mechanical seal will malfunction due to crystal precipitation and the deposition of particles and impurities. If particles enter the friction pair, it will lead to the rapid destruction of the mechanical seal. Therefore, the auxiliary flushing system of the mechanical seal is very important. It can effectively protect the sealing surface and play a role in cooling, lubrication, and flushing away debris.
3.5 Measures to address pump vibration
During the manufacturing and assembly of pump products, standards and operating procedures must be strictly followed to eliminate vibration sources. During on-site installation of auxiliary equipment such as pumps, motors, bases, and field pipelines, strict quality control must be implemented to eliminate vibration sources.
Having briefly examined the application and failures of mechanical seals in rotating equipment, when encountering mechanical seal malfunctions in the future, we must first consider the influencing factors inherent to the mechanical seal itself, and then also consider external influencing factors. For example, when analyzing the causes of mechanical seal quality accidents, we must consider the impact of other pump components on the mechanical seal's operation and take measures to continuously improve the mechanical seal's effectiveness.
