Section 5 — Piping

IPE-EP-5-11-3

IPE Engineering Practice IPE-EP-5-11-3

Document number: IPE-EP-5-11-3 · Section: 5 — Piping

CoInafsletcatilo nA Prouinbta E nEgningeeirninege Prriancgtic Pesractices On-Stream Leak Repairs of Piping Components

CoInafsletcatilo nA Prouinbta E nEgningeeirninege Prriancgtic Pesractices On-Stream Leak Repairs of Piping Components 5.1.2 Flange clamps are split rings designed to encircle the flange, see Figure 1. The clamp is installed by bolting the two halves of the clamp together. The clamp then forms a sealant cavity between the flange faces and provides the means of injection of sealant through injection ports in the clamp ring. 5.1.3 The design of the clamp includes a seal on the inside of the ring. The seal is designed to make contact with the flange when the clamp halves are bolted together. The purpose of the seal is to contain the sealant and prevent extrusion during injection. The sealant can then fill the sealant cavity and seal off the leak completely. 5.1.4 There are two distinctly different designs of flange clamps: the insert clamp and the peripheral (outside diameter fitting) clamp. The size of the gap between the two flanges is the primary determining factor in selecting the type of clamp. Peripheral clamps are used where the flange gap is insufficient to accommodate an insert clamp blade or tongue. 5.1.5 The insert clamp is manufactured to fit precisely in the gap between the flanges. Some key advantages to this type of clamp are: • Little or no mechanical work is required on the flange. Flange studs change–out is not typically required, injection holes need not be drilled into the flange, and little or no peening is required. • Being manufactured to fit, the clamp can be tolerant of flange misalignment, out–of– roundness, or differing diameters. • Many applications permit the use of standard clamp rings held in readiness in a finished or semi–finished state, requiring that only the insert be machined to the measured gap size. • Clamps are self–relieving and will vent should flange separation take place as a result of bolt relaxation. As the relaxation occurs, the leak will probably restart and serve as a warning that additional actions are necessary. • The quantity of sealant is reduced. • Pressurized area between flanges is reduced. • A high degree of success has been experienced. 5.1.6 Peripheral clamps can be used as a simple ring to enclose the outside of the flanges with injection ports through the ring. The outside of the flanges can also be enclosed with packing that is compressed onto the flange periphery when the bolts on the clamp split line are tensioned. Additionally, these clamps are frequently used in conjunction with a low–viscosity compound to establish a homogeneous molding within the narrow flange gap. The major advantages of the peripheral clamp are: • The clamp can be used for narrow gap flanges where an insert clamp is impractical. • The technique requires fewer injection points when used with low viscosity compound. • Leak sealant work on site is reduced. • No drilling or peening of flange surfaces is required. 5.1.7 Peripheral clamps require careful and accurate flange measures, and subsequent marking to correct flange misalignment. Also, because there is not direct means of pressure release, it is advisable to check the ability of the joint fasteners to withstand full–line pressure over the total flange area. The maximum injection pressure should be analyzed by the LRC in relation to bolt stress to ensure that overloading does not occur.

CoInafsletcatilo nA Prouinbta E nEgningeeirninege Prriancgtic Pesractices On-Stream Leak Repairs of Piping Components 5.11.3 High Stud Tension - Excessive stud tension can cause flange leakage by either damaging the gasket or the flange itself. An initial bolt tension equivalent to a stress of 40,000 to 50,000 psi is normally required to prevent flange leakage. Higher bolt tension can lead to excessive loading of certain types of gaskets to a point where they no longer have sufficient elasticity to prevent leakage even under minor changes of temperature or pressure. In addition, high bolt tension can also overload the flange itself and cause excessive rotation. Large flange rotations can redistribute the flange loading across the gasket width and reduce the seating area, giving a shorter leakage path and possibly crushing the outer edge of the gasket. Hot bolting tightening of already over–tensioned bolts can increase leakage by further damaging the gasket and flanges and consequently is not recommended. 5.11.4 Damage to Flange Faces and Gaskets - Defects to flange faces and gaskets such as scratches and gouges can create leakage paths and allow fluid to escape. In some cases, erosion or corrosion of the gasket and/or the flange face can occur which will eventually lead to the formation of a leakage path. For example, external steam rings have caused erosion/corrosion of solid soft iron gaskets that subsequently led to major leakage. Many steam leaks are initiated by low bolt tension, and as soon as a leakage path is formed, erosion of the flange face and gasket will occur and result in greater leakage. In the event of damage to flange faces and gaskets, hot bolt tightening is unlikely to prevent leakage and another technique should be employed. However, a careful evaluation of the possible consequences of not fully repairing a leak where erosion/corrosion could occur is required. 5.11.5 Varying Operating Conditions - Alterations in operating temperature and pressure cause changes to the gasket seating load and bolt tension. These changes can sometimes lead to flange leakage. Rapid temperature changes commonly produce flange leakage as they can significantly alter the bolt load and gasket seating load. However, very often the leakage stops when the flange and bolts reach steady state conditions. In some units operating procedures have been altered to reduce the rate of temperature changes to flanged joints and thereby solve leakage problems. Reducing temperature changes to a rate of less than 100°F/hr. should prevent most cases of leakage due to temperature transients. However, in the long term, severe cyclic temperatures will lead to leakage since the joints may be over–stressed and a loss of elasticity eventually leads to leakage. Tightening of the bolting on–stream is frequently successful in preventing flange leakage due to long term temperature cycling. Care should be taken when using other techniques to prevent leakage to ensure that any leakage that occurs as a result of an operating condition change does not go undetected. 5.11.6 Improper Flange Alignment - Improper alignment, especially lack of flange face parallelism, can cause uneven bolt loading and gasket compression, local crushing and subsequent leakage. Proper centerline alignment of flanges is also important to assure even compression of the gasket. Flange alignment criteria are cited in EP 5–5–3. 5.11.7 Improper Gasket Centering - A gasket that is installed so that its centerline does not coincide with the flange centerline will be unevenly compressed with the possibility of subsequent leakage. Spiral wound and double jacketed asbestos gaskets are provided with a centering ring or gasket extension to the inside diameter of the bolt circle to facilitate gasket centering. Even so, the gasket should be centered with respect to the bolt circle. Sheet gaskets should be cut so that the outside diameter extends to the inside diameter of the bolts for centering considerations.

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