In practical applications of heat transfer oil valves, although bellows seals are used to some extent, packing gland structures remain widely adopted, among which graphite packing is the most common. However, numerous usage cases show that leakage often occurs around graphite packing, and most valve manufacturers fail to accurately identify the root cause. To address this issue, it is essential to analyze the properties of heat transfer oil, its application scenarios, and production processes in depth.
Due to its excellent thermal stability, high heat transfer efficiency, and low volatility, heat transfer oil is widely used in various industries as a "heat energy carrier," playing an irreplaceable role in many fields:
Chemical Industry: For heating and cooling reactors, and in processes such as distillation, evaporation, and drying. For example, in petrochemical applications, heat transfer oil provides stable thermal energy to distillation towers for separating crude oil into fractions at different temperatures.
Textile and Dyeing Industry: Used to heat dyeing vats, ovens, and other equipment to ensure uniform dyeing and fabric drying. In thermal transfer printing, heat transfer oil-heated ovens ensure the pattern is transferred clearly under proper temperature and pressure.
Food Industry: Employed in baking, frying, and steaming processes. For instance, in bread baking, ovens heated with heat transfer oil offer even and stable heat, giving the bread a golden color and crispy texture.
Paper Industry: Supplies thermal energy for drying and calendering, accelerating moisture evaporation and improving the drying efficiency and quality of paper.
Building Materials Industry: Used in the production of tiles, glass, insulation materials, etc. In tile manufacturing, heat transfer oil-heated kilns ensure even heating of tile bodies during firing to meet performance indicators such as color and strength.
Pharmaceutical Industry: Supports processes such as synthesis, drying, and concentration. For instance, in the concentration of traditional Chinese medicine extracts, heat transfer oil evaporators can evaporate solvents at low temperatures, avoiding damage to active ingredients from overheating.
These examples demonstrate the extensive industrial use of heat transfer oil. Therefore, to solve its sealing issues, one must first understand the material characteristics and production process of heat transfer oil.
The production temperature of heat transfer oil varies depending on the type, which indirectly affects the operating conditions of sealing materials.
Derived from petroleum distillates such as paraffinic or naphthenic base oils, the production process includes:
Base oil fractions are typically separated at 200–500°C (e.g., atmospheric and vacuum distillation), with heat transfer oil base cuts concentrated at 300–450°C for high-boiling, thermally stable fractions.
Refining processes (e.g., hydrogenation, solvent refining) occur at 150–300°C to remove impurities (sulfur, nitrogen, aromatics) and improve thermal stability.
Mixing with antioxidants, corrosion inhibitors, viscosity modifiers, etc., usually takes place at 60–150°C to avoid additive degradation or base oil oxidation.
Synthesized from chemical intermediates such as benzene, olefins, or siloxanes, synthetic oil production varies by type:
Alkylbenzene-based (e.g., dibenzyltoluene):
Alkylation reactions occur at 100–200°C with catalysts, followed by distillation and purification at 250–300°C.
Hydrogenated Terphenyl-based:
Synthesis and hydrogenation involve high temperatures (200–300°C) and high-pressure hydrogenation, with refining at 250–350°C.
Silicone-based (e.g., polydimethylsiloxane):
Ring-opening polymerization typically occurs at 150–250°C with catalysts, and purification through removal of low-boiling components may reach 200–300°C.
Overall, mineral-based oils range from 60–450°C, while synthetic oils generally operate between 100–350°C, with some high-temperature reactions reaching 300–400°C. These harsh thermal conditions impose strict demands on sealing materials.
From this perspective, the choice of high-temperature resistant graphite packing appears reasonable. However, in actual use, such packing often begins leaking after a period of operation. The core issue lies in the characteristic of flexible graphite being "hydrophobic but not oleophobic".
Flexible graphite sheets are made by compressing expanded graphite worms without any binder, preserving high heat resistance. However, this process creates numerous micron-sized pores. When in contact with oily media such as heat transfer oil, these pores absorb oil via capillary action like a sponge, with oil continuing to penetrate into the graphite structure.
During frequent valve stem movements, the absorbed oil causes repeated friction with the graphite layers—similar to ink-grinding—gradually wearing away graphite particles. With increasing valve operation cycles, the packing loosens due to wear, the gap enlarges, and ultimately leads to leakage and sealing failure.
This process is essentially the result of the interaction between the oleophilic porous structure of flexible graphite and dynamic frictional conditions, revealing an inherent limitation of traditional graphite packing in oil-based sealing applications.
In response to the particular conditions of oily media like heat transfer oil, a specially designed anti-permeation graphite packing has been developed. This packing overcomes oil infiltration by innovating in both structure and material composition, with the following breakthroughs:
Replacing the layered structure of traditional graphite sheets, the new design adopts a three-dimensional braided structure. Using high-purity graphite fibers, a tightly woven mesh skeleton is formed, eliminating interlayer gaps and blocking oil infiltration paths. This is especially effective in dynamic sealing scenarios such as valve stems and reciprocating shafts under fluctuating pressure.
After braiding, the packing is fully impregnated with special polymers (e.g., fluorocarbon resins, silicone polymers):
The polymer penetrates into graphite pores via capillary action, curing to form a nanoscale hydrophobic and oleophobic film on pore walls—preventing direct contact between graphite and oil. This reduces surface energy by more than 90%, effectively blocking oil absorption.
Controlled resin content ensures a balance of rigidity and flexibility—preventing brittle graphite from excessive wear during valve stem movement and improving resistance to extrusion. This allows the packing to maintain sealing stress under high temperatures (200–400°C) and high pressures.
Tests show that the modified packing gains less than 0.3% mass after 72 hours of immersion in heat transfer oil (compared to >15% for traditional graphite), breaking the cycle of internal softening and particle loss.
In simulated valve cycling (100,000 cycles at 300°C and 1.6 MPa), leakage is <5 drops/hour—while traditional packing exceeds 50 drops/hour. Service life is 3–5 times longer.
With excellent high-temperature stability (up to 450°C) and resistance to various oil-based media (lubricants, hydraulic oils, hydrocarbon solvents), the packing is ideal for demanding dynamic sealing environments in chemical, refining, and textile industries.
The anti-permeation graphite packing, through dual innovations in structural sealing and material modification, fundamentally solves the problem of oil infiltration and erosion in graphite seals. It offers a more reliable solution for long-term stable operation of industrial equipment using heat transfer oil.
