Packing Seal: Principles, Problems and Optimization

In industrial production, sealing technology is a key link in ensuring the normal operation of equipment and preventing leakage. As a widely used contact sealing method, packing seal plays an important role in many fields due to its simple structure and low cost. However, this sealing method also faces many challenges such as friction and wear. This article will explore in depth the working principle of packing seal, its existing problems, and optimization strategies to help readers better understand and apply this technology.

Packing seal is a device that achieves sealing through the close contact between packing and the shaft (or rod). It usually consists of packing, stuffing box, and gland. The packing is loaded into the stuffing box, and pressure is applied by the gland, causing the packing to closely adhere to the shaft, thereby preventing fluid leakage. Compared with other sealing methods, packing seal has a relatively large contact area and requires a certain amount of compression force. This makes friction and wear problems more prominent during operation.

In practical applications, friction and wear are key issues affecting the sealing performance and service life of packing seals. Friction not only increases energy consumption but also causes wear of both packing and shaft, thereby affecting sealing effectiveness. Wear will further exacerbate leakage and may even lead to equipment failure.

Gland pressure is one of the important factors affecting friction and wear of packing seals. Proper pressure ensures sufficient contact between packing and shaft, thus achieving good sealing. However, excessive pressure causes over-compression of packing, increases friction force, and accelerates wear of both packing and shaft. Conversely, insufficient pressure cannot guarantee sealing, and leakage is likely to occur.

The longer the operating time of a packing seal, the more severe the wear usually is. Prolonged operation gradually causes the packing to lose elasticity and the surface to become rough, thereby increasing friction with the shaft. In addition, prolonged friction leads to the loss of lubricants inside the packing, further aggravating wear.

The number of packing rings also affects friction and wear. From a friction perspective, fewer rings are better because more rings mean larger contact area and greater friction force. However, from the standpoint of sealing performance, a sufficient number of rings is necessary to ensure proper sealing. Therefore, in actual applications, the number of rings should be reasonably selected based on specific working conditions.

The roughness of the shaft surface has a significant impact on friction and wear of packing seals. A rough shaft surface increases friction between packing and shaft, intensifying wear. At the same time, a rough surface may lead to poor sealing and leakage. Therefore, during design and installation of packing seals, efforts should be made to ensure smoothness of the shaft surface.

The type of packing material also has a significant impact on friction and wear. Different packing materials have different coefficients of friction. For example, the coefficient of friction between PTFE packing and steel is only 0.04, while that of cotton packing against steel is 0.6–0.7, nearly 20 times higher. Therefore, packing material should be selected based on specific working conditions and medium characteristics to reduce friction coefficient and minimize wear.

Wear is one of the most prominent problems in packing seals. It not only reduces sealing performance but may also trigger equipment failure. Therefore, understanding the manifestations of wear and adopting effective solutions are crucial to extending the service life of packing seals.

Wear is a prominent issue in packing seals. Almost every machine using packing seals is equipped with spare parts for this reason. Normally installed packing tends to wear evenly, with greater wear at the gland and gradually decreasing inward. However, poorly installed packing experiences rapid wear near the gland while the inner packing remains unworn. In addition, wear is closely related to whether corrosion occurs on the shaft surface. When stainless steel shafts use graphite-lubricated packing, severe corrosion occurs. This is because, in a conductive medium, graphite becomes the cathode and the stainless steel shaft becomes the anode, causing electrochemical corrosion. The metal on the shaft surface dissolves, creating a very rough surface and accelerating wear between shaft and packing.

Optimize packing installation: Correct installation is critical in reducing packing wear. During installation, packing should be evenly distributed to avoid high-stress zones. A step-by-step installation method can be used, compressing slightly after each ring is placed to ensure close contact with the shaft. In addition, spacer rings may be installed in the middle of the packing, serving both as lubricant injection points and leakage monitoring channels. When leakage exceeds limits, sealant can also be injected through them.

Select appropriate packing materials: Different packing materials exhibit different wear resistance. Carbon fiber packing offers the best wear resistance, while asbestos packing causes the most shaft wear, nearly 50 times more. However, asbestos packing impregnated with PTFE is very close in performance to carbon fiber packing. Thus, impregnants greatly influence wear resistance. Packing material and impregnant should be selected according to specific working conditions and medium properties to enhance wear resistance.

Adopt lubrication and cooling measures: Lubricants play a vital role in packing seals. They reduce friction between packing and shaft, decrease wear, and dissipate friction heat, thus reducing thermal wear of packing. Therefore, in conditions of high temperature, high pressure, or high speed, forced lubrication and cooling measures should be adopted. For example, lubricants can be supplied externally, or the leaking fluid can be used directly as a lubricant. In addition, a washer can be added below the packing with a spring underneath. When packing wears, the spring pushes it tight again, maintaining the seal. This method effectively prolongs packing life and reduces maintenance costs.

In packing seal applications, lubrication and cooling are key factors in ensuring efficient operation and extending service life. Proper lubrication significantly reduces friction between packing and shaft, lowering wear, while effective cooling removes heat generated by friction, preventing accelerated aging or deformation of packing caused by high temperature.

Lubrication has a major impact on wear and directly affects the service life and sealing ability of packing. Most braided packings are impregnated with various lubricating and heat-resistant materials, and sometimes lubrication is supplied externally or directly through the leaking fluid. In addition to lubrication, lubricants dissipate friction heat and reduce thermal wear of packing. Thus, forced lubrication and cooling measures play an extremely important role under conditions of high temperature, high pressure, and high speed.

Lubricant selection should be based on specific working conditions and medium characteristics. Generally, lubricants should meet the following requirements:

Good chemical stability: lubricants should not react with the medium to form precipitates or solid particles that may affect sealing. Good impregnating performance and lasting retention: lubricants should easily penetrate small gaps in packing fibers and slowly seep out under pressure to maintain lubrication. No acceleration of electrochemical corrosion: lubricants should not act as electrolytes; insulation properties are preferable to prevent electrochemical corrosion at the sealing surface. Good self-lubricating and temperature resistance: commonly used lubricants include graphite, molybdenum disulfide, mica, and PTFE, all of which have good self-lubricating and temperature-resistant properties suitable for various working conditions.

Animal fat: suitable for cold water, especially fiber packing, but released fatty acids corrode shafts.

Castor oil: suitable for water and acid-salt media, but soluble in petroleum-based mineral oils.

Glycerin: insoluble in petroleum-based mineral oils, particularly suitable for petroleum products, especially gasoline packing. Also applicable to rubber packing for steam.

Graphite: chemically stable, excellent lubricant, and a commonly used solid lubricant. However, special attention must be paid to electrochemical corrosion of sealing surfaces since it is a good conductor.

PTFE: both a filler and a lubricant. It performs well at low temperatures, usable in the range of -200 to 250°C.

It has good lubricating properties though not as good as graphite.

Being an insulator, it prevents electrochemical corrosion and resists various chemical media.

As a widely used contact sealing method, packing seal plays an important role in industrial production. However, due to its prominent issues of friction and wear, a series of optimization strategies must be adopted to improve its performance and service life. By reasonably selecting packing materials and structure, employing automatic compensation devices, strengthening lubrication and cooling measures, and optimizing installation and maintenance, packing wear can be effectively reduced, sealing performance improved, and maintenance costs lowered. It is hoped that this article will help readers better understand and apply packing seal technology, providing reliable sealing assurance for industrial production.