Comprehensive solution for flanged joints

If the leak recurs after replacing the gasket with an identical type, the cause almost certainly lies elsewhere. Most often, it is due to an unsuitable gasket material for the given operating temperature, incorrect or missing torque calculation, improper assembly procedure, or damaged sealing surfaces of the flanges.

Essential. Different lubricants have different coefficients of friction, which also vary depending on the measurement methodology used. If the torque specified in the drawing documentation does not specify the lubricant for which it was calculated, and you use a different one, the resulting clamping force can be up to three times higher or lower than required. This directly affects the specific pressure on the seal and the reliability of the joint.

For flanges in lower pressure classes, specifically up to PN 40, long-term sealing is more complicated than it seems. It is not possible to derive sufficient specific pressure for the sealing corresponding to the dimensional standard EN 1514-1 for these flanges. The solution is to optimize the dimensions of the sealing based on calculations according to EN 1591-1, rather than simply substituting the product.

The tightness certificate is the result of a calculation according to EN 1591-1, which verifies whether the designed gasket and bolts will ensure sufficient specific pressure in all operational states of the joint - during assembly, operational pressure, and temperature. It is essential for critical joints, when changing operational parameters, when switching to a different type of gasket, and everywhere where joint failure poses a safety or operational risk.

The temperature resistance of fiber-reinforced rubber gaskets is limited by the elastomeric binders, not by the graphite or aramid fibers. The upper limit is approximately 150 °C. At higher temperatures, the binders harden, leading to loss of volume and elasticity, a decrease in bolt preload, and subsequent leakage. The argument that these materials can be used short-term at 350 °C cannot be reliably substantiated, as the term "short-term" is not defined in this context.

Teflon seals have a reported temperature resistance of up to around 260 °C, but from the perspective of ensuring sealing, their practical limit is approximately 180 °C. As temperatures increase, the undesirable flow of the sealing material, the release of bolt preload, and the gradual leakage of the joint also increase.

The key parameter is the degree of graphite oxidation, which depends on the purity of the deposit from which the graphite was extracted and also on the subsequent manufacturing process. The higher the content of chlorides, fluorides, or sulfur in the graphite, the greater the oxidation. At high temperatures in the presence of oxygen, there is a loss of sealing material, a decrease in specific pressure, and leakage. The quality of graphite can be objectively verified by TGA analysis, which measures the weight loss of the sample at high temperature over a unit of time.

The values indicated in the pT diagrams are approximate and based on ideal conditions. They do not take into account the specific dimensions of the flange joint, the condition of the sealing surfaces, the fasteners used, or the assembly procedure. A reliable selection of sealing material must be based on measured sealing curves and calculations with proof of tightness for the specific application.

Controlled tightening is a process in which bolts are tightened to a precisely specified torque or axial force, in a cross pattern over multiple steps, using calibrated tools. Unlike tightening by estimation, it ensures an even distribution of specific pressure on the gasket and a repeatable result. For critical joints, hydraulic wrenches are used, allowing simultaneous tightening of two or four bolts at the same time.

The tightening protocol documents the achieved values of torque or axial force on each bolt, the tools used, lubricants, and the assembly procedure. It serves as evidence of properly executed assembly, a basis for potential analysis in case of future leaks, and in some industries, it is also a requirement for operational or safety audits.

Up to 50% of flange joint leaks are caused by incorrect assembly. In most cases, installers do not have a specified tightening torque and tighten by estimation. Training according to the EN 1591-4 standard provides both theoretical and practical knowledge about assembly procedures, seal selection, lubrication, and the influence of tightening torque on sealing. Certified installers are able to check their work after completion.