Selection and maintenance points of mechanical seals and packing seals for screw

As a typical representative of volumetric fluid conveying equipment, the sealing performance of screw pumps directly affects the stability of equipment operation and the risk of medium leakage. Mechanical seal and packing seal are two commonly used sealing forms for screw pumps. The selection should be based on the characteristics of the working conditions, the properties of the medium, and the maintenance cost. Maintenance should develop targeted strategies based on the characteristics of the sealing structure. The following analysis will be conducted from two aspects: selection principles and maintenance points.

1、 Sealing form selection: working condition adaptation and performance balance

1. Key points for selecting mechanical seals

Mechanical seals achieve sealing by fitting the end faces of the moving and stationary rings, and have the advantages of low leakage, good service life, and long maintenance cycle. They are suitable for high temperature, high pressure, and corrosive media conditions.

Media compatibility: The sealing material should be selected based on the characteristics of the media. For example, when transporting highly corrosive media, the moving and stationary rings need to be made of non corrosive materials such as silicon carbide, ceramics, or hard alloys; When transporting media containing particles, it is necessary to apply a coating on the sealing surface or use a double end sealing structure to prevent particle embedding and seal failure.

Temperature and pressure adaptability: High temperature working conditions require the use of high-temperature resistant elastic components (such as fluororubber or perfluoroether rubber) to avoid aging and deformation of the sealing ring; Under high-pressure conditions, it is necessary to increase the spring specific pressure or adopt a balanced sealing structure to reduce the sealing surface pressure and extend the service life.

Speed matching: Screw pumps need to use low friction coefficient materials (such as a combination of carbon graphite dynamic rings and hard alloy static rings) to reduce heat generation; At the same time, it is necessary to optimize the flushing plan by using external flushing or self circulating flushing to remove frictional heat and prevent the sealing end face from overheating and failure.

2. Key points for selecting packing seals

Packing seal is achieved through radial compression between packing and shaft sleeve, which has the characteristics of simple structure, reasonable cost, and convenient replacement. It is suitable for low-pressure, low-temperature, and non corrosive medium working conditions.

Material selection for fillers: When conveying clean water or low viscosity media, cotton yarn fillers or flexible graphite fillers can be used; When transporting media containing particles, strong carbon fiber fillers or PTFE fillers should be selected; When transporting high-temperature media, aramid fiber fillers or ceramic fiber fillers should be used, and the fillers should not carbonize at high temperatures.

Packing box structure: The packing box needs to reserve sufficient compression capacity, usually with a packing height of two-thirds to three-quarters of the packing box, to avoid leakage caused by loose packing or wear of the shaft sleeve caused by tight packing. For high-pressure conditions, multiple packing combinations or the addition of retaining rings can be used to improve sealing performance.

Adaptability assessment of working conditions: Packing seal is suitable for screw pumps with low speeds. If the speed is too high, the friction between the packing and the shaft sleeve will generate heat, which will accelerate the wear of the packing. It is necessary to reduce the speed or use mechanical seals to solve this problem.

2、 Sealing maintenance strategy: prevention oriented and condition based maintenance

1. Key points for maintaining mechanical seals

Regular inspection and replacement: After each operating cycle, it is necessary to check the wear of the sealing end face. If scratches, cracks, or uneven wear occur, the dynamic and static rings should be replaced immediately; At the same time, check whether the elastic components (such as springs or bellows) have fatigue failure. If the compression of the spring decreases or the bellows ruptures, they need to be replaced synchronously.

Maintenance of flushing system: For external flushing mechanical seals, it is necessary to regularly clean the flushing pipeline to prevent impurities from blocking; For self circulating flushing seals, it is necessary to check the flow rate and pressure of the flushing fluid to ensure that the flushing fluid can timely remove heat and particles from the sealing end face. If the flushing solution is contaminated, it is necessary to replace the flushing solution and clean the system.

Installation and alignment calibration: When installing mechanical seals, it is necessary to keep them clean to avoid impurities entering the sealing surface; After installation, it is necessary to check the fit between the moving ring and the stationary ring, and contact traces can be detected by the lead pressure method or red lead powder; At the same time, it is necessary to calibrate the coaxiality of the pump shaft and the sealing shaft, as excessive deviation can cause seal wear.

2. Key points for maintenance of packing seal

Adjustment of packing compression amount: During the initial operation, it is necessary to regularly check the leakage amount of the packing. If the leakage amount increases, the packing gland bolts should be gradually tightened. The tightening amount each time should be such that the packing leakage amount is significantly reduced and the temperature of the shaft sleeve is not significantly increased; If the packing has hardened or worn, the gland should be loosened and replaced with new packing.

Lubrication and cooling management: Friction between the packing and the shaft sleeve generates heat, which needs to be lubricated to reduce wear. Lubricating grease or oil can be injected into the packing box to form an oil film and reduce friction; For high-temperature conditions, a cooling water jacket should be installed outside the packing box to remove heat through circulating cooling water.

Monitoring of shaft sleeve wear: After long-term operation, grooves may appear on the surface of the shaft sleeve due to friction with the packing. It is necessary to regularly check the diameter changes of the shaft sleeve. If the wear exceeds the allowable value, the shaft sleeve needs to be replaced and the packing compression amount needs to be readjusted to prevent seal failure caused by shaft sleeve wear.

3、 Collaborative optimization of selection and maintenance

The selection of screw pump seals needs to form a closed loop with maintenance strategies: mechanical seals should be selected for high temperature and high pressure conditions, but a matching flushing and monitoring system is required; Packing seals can be used for low-pressure and intermittent operating conditions, but maintenance cycles need to be shortened and lubricated for cooling. Regardless of the sealing form used, a sealing status file must be established to record operating parameters, maintenance records, and fault phenomena, providing a basis for subsequent selection and optimization. By adapting to working conditions, upgrading materials, and maintaining them in detail, the sealing performance of screw pumps can be significantly improved, reducing the risk of unplanned shutdowns.