Sleeve compensator Material classification

　　The basic requirements for sealing materials used in sleeve compensators are: sufficient elasticity and plasticity, the ability to convert axial pressure into significant radial working pressure; good chemical structural stability; low permeability; resistance to abrasion; temperature resistance; ease of replacement; and low cost. Sealing materials can be categorized into molded packing and non-molded packing. The materials currently in use and their performance characteristics are as follows:

　　1) Preparation of filler products, including oil-immersed asbestos packing, flexible graphite packing, and composite graphite molded seals, as well as their modified materials. The term “modified” refers to the incorporation of other relevant materials from around the world into these products to enhance their performance—for example, asbestos fillers combined with molybdenum disulfide and PTFE emulsion, resulting in a friction coefficient as low as 0.02–0.02. To achieve optimal sealing performance, it is essential to understand the properties of these sealing materials and use them appropriately. The density of expanded graphite significantly affects its performance: when the density increases from 0.7 g/cm³ to 1.2 g/cm³, the tensile strength rises from 2.2 MPa to 6.2 MPa, and the rebound rate also improves. Since graphite in China has a markedly higher electrode potential than other metals, it is prone to chemical corrosion (even stainless steel can be corroded). Therefore, expanded graphite with corrosion-inhibiting properties should be selected.

　　2) The long-term service temperature range for polytetrafluoroethylene (F-4) plastic products made from F-4 material is -195 to 250 degrees Celsius. After adding fillers such as MoS₂, SiO₂, bronze powder, and graphite to F-4, in addition to maintaining the original performance characteristics of the material, its dimensional stability under load can be improved by a factor of 10, and its wear resistance can be enhanced by a factor of 400 to 800 times. The static and dynamic coefficients of friction between steel and this material are both 0.04. This material has been successfully used in sleeve compensators for 7,000 cycles without any external leakage. It can thus be referred to as a leak-free sleeve compensator.

　　3) The rubber products are made of silicone rubber and fluororubber, offering excellent water resistance, steam resistance, aging resistance, and air tightness. They can withstand temperatures up to 200°C under normal conditions and up to 350°C under high-temperature conditions. In the past, these materials were used for sealing piston rods in steam locomotive cylinders, but they were costly.

　　4) Sealing oil

　　1. Use a high-pressure oil gun to inject sealing oil into the stuffing box of the sleeve compensator. This method of leak prevention is inspired by conventional atmospheric-pressure leak-stopping techniques. However, the sealing oil is not a leak-sealing adhesive—though leak-sealing adhesives, also known as sealants, are sometimes referred to as such. Sealants are used to inject a viscous material into leaking areas of pipeline equipment during operation (under pressure), achieving static joint sealing through their movement. After the sealing oil (which may also be in the form of an adhesive) is injected into the sleeve compensator, it seals the system by increasing the radial pressure exerted on the core pipe by the sealing material; yet the core pipe remains capable of axial expansion and contraction. The formulation of this sealing oil varies from one manufacturer to another. Once the oil seal is applied, the pipeline can continue operating with minimal leakage, and the sleeve compensator can achieve a new level of sealing performance.

　　2. The sealing system of sleeve compensators typically employs either packing seals or mechanical seals. A packing seal is a contact-type seal in which an elastic sealing material is packed between the outer casing and the inner tube. After being compressed by an axial force, the packing material closely adheres to the surface of the inner tube, leveraging its elastic deformation to compensate for any imperfections on the sealing surfaces, thereby reducing wear and preventing medium leakage. Due to the differing elasticity and plasticity of various packing materials, the radial force (Py) that the axial force (Px) can generate within the packing along its depth direction is not uniform; rather, Py = KPx, where K is the pressure coefficient. Because of this characteristic, modern packing designs have evolved from using single packing materials to employing combined packing structures, including “oil”-sealed packing configurations.