PTFE gaskets, due to their excellent chemical resistance, high-temperature resistance, and low coefficient of friction, are widely used in flange connections of pipelines, valves, pumps, and other equipment as static sealing elements. When connected to metal flanges, the following key points must be noted to ensure sealing performance and service life.
Preparation before installation is crucial. Carefully check the specifications of the PTFE gasket to ensure it matches design requirements and that dimensions (inner diameter, outer diameter, thickness) are accurate. Also, check that the gasket surface is smooth and flat, free from scratches, cracks, wrinkles, bubbles, or any form of damage. For modified or filled PTFE gaskets, observe whether the filler is evenly distributed. Furthermore, the sealing surfaces of the mating flanges must be thoroughly cleaned to ensure they are smooth, free from radial scratches, corrosion pits, old gasket residue, and any foreign matter. Ideally, the surface roughness of the sealing surface should be ≤Ra3.2μm; grinding may be necessary. Simultaneously, check the parallelism and alignment of the flanges to avoid uneven stress caused by misalignment or excessive opening.
Careful placement and alignment of the gasket are essential. Carefully place the intact PTFE gasket between the two flange sealing surfaces, ensuring it is centered and its inner diameter is aligned with the inner diameter of the pipe or equipment. Avoid allowing the gasket to extend into the flow path, creating obstruction or unnecessary turbulence. For flanges without locating steps, the operator must manually align the gasket, ensuring it is neither skewed nor extends into the valve cavity or rests on the shoulder. The gasket's inner diameter should be larger than the inner diameter of the sealing surface, and its outer diameter should be slightly smaller to ensure even pressure distribution.
Bolt tightening is a crucial technical aspect of the connection process. A calibrated torque wrench must be used to ensure even bolt load application that meets design specifications. The use of impact wrenches or tightening by feel is strictly prohibited. The tightening sequence should follow the "cross-symmetric" principle. First, tighten all nuts with a wrench until they contact the flange (hand-tight). Then, complete the tightening in at least 3-4 cycles. Gradual loading allows the PTFE material time to relax and flow, better adhering to the flange surface and preventing localized overload that could damage or crush the gasket. It is recommended to rotate the bolts by 90° each time, with an initial torque of approximately 50% of the standard value, gradually increasing to 80%-100%. Never tighten them completely in one go.
During bolt tightening, it is also crucial to control the tightening force. Overtightening will exceed the gasket's pressure capacity, causing excessive compression or even crushing of the pure PTFE gasket, or damaging the internal skeleton of reinforced gaskets, resulting in loss of resilience and permanent damage and leakage. Always strictly adhere to the manufacturer's recommended tightening stress or bolt torque values.
Post-installation inspection is just as important as subsequent management. For newly installed or repaired systems, when introducing media for the first time or conducting a pressure test, the pressure should be increased slowly as specified, and the connection points should be closely monitored. Due to the potential for stress relaxation in PTFE gaskets and the varying coefficients of thermal expansion among components after system heating, it is necessary to perform a hot inspection and re-tighten the bolts in the original sequence after the system reaches its initial operating temperature and has been running stably for a period (usually 24 hours). This is particularly important under high-temperature conditions.
Furthermore, the impact of environmental factors on the connection must be considered. At high temperatures, bolts undergo high-temperature creep, resulting in stress relaxation and increased deformation, potentially leading to leaks at the gaskets, requiring hot tightening. Conversely, at low temperatures, bolts shrink, requiring cold loosening. Additionally, avoid using unheated gaskets in low-temperature environments (below -185°C) to prevent them from becoming brittle and damaged.
In daily use, regular inspection and maintenance of the connections are essential. Check the tightening force quarterly and tighten as needed, especially in environments with large temperature fluctuations. Store gaskets away from direct sunlight to prevent aging; spare gaskets should be sealed in airtight packaging to maintain their performance stability.
