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Method to limit a creep of bolts and gaskets of bolted flanged connections

Method to limit a creep of bolts and gaskets of bolted flanged connections description/claims

This Patent Application is continuation of earlier provisional patent application Ser. No. 60/925,840 filed Apr. 24, 2007, and patent application Ser. No. 10/834,955 filed Apr. 30, 2004 now abandoned.

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1. Field of the Invention

This invention relates to Bolted Flanged Connections (BFCs) with gaskets as sealing elements that must provide a leak-tight and durable joint between component parts of the pressure vessels, piping systems, and other engineering structures that operate under conditions of internal pressures and a variety of operational temperatures.

2. Background of the Invention

One of the most typical means to obtain a leak-tight and durable joint between component parts of boilers, reactors, steam generators, piping systems, and other engineering structures is an assemblage of their pieces with the BFCs. These types of connections have a wide applicability in petrochemicals, petroleum refining, aerospace, submarine shipbuilding, fossil fuel and nuclear power generation, and other critical industries.

There are millions of bolts and gaskets used in critical engineering structures, and the problem of the BFCs' structural integrity and plant/piping operational leakage reduction is very complex and involves many areas of applied mechanics and technological findings. Safe design of the BFCs from structural integrity point of view has been satisfactorily solved and standardized, but the leakage events remain an unresolved problem and a principal cause of bolt and/or gasket damages and failure that are attributed to the high level of corrosion combined with high level of stresses and strains due to cyclic internal pressures, external loadings, elevated operational temperatures, flow-induced vibrations, integral flow of neutrons, and other critical factors.

Statistic data show that, for example, piping system leakages alone conservatively cost each process industry hundreds millions of dollars annually in lost profits as a result of plant shutdowns, production penalties, maintenance rework activities, equipment repair or replacement, and so on. It is necessary to add the liquidation of consequences of possible fires, explosions, environmental pollution, and other disasters. Hence, the leak tightness of the BFCs used in critical plant/piping systems has a greater influence on their safe and extended service life, and highest priority in plant/piping reliability programs is to limit or exclude the operational leakages, thus protecting critical engineering structures from untimely degradation and failure.

One of the main causes of plant/piping leakages is a “passive” behavior of bolts and gaskets under critical operational conditions when nonlinear gasket response to the loading-unloading processes combined with creep of bolt and gasket materials lead to clamping force decrease and unavoidable joint opening followed by leakages. Generally, creep is accompanying by stress relaxation, and elongation of the bolts along with contraction of the gasket due to creep-relaxation is a very serious problem because it lead to bolt load and gasket stress losses that, in turn, increase the leakage rate.

It is known that creep-relaxation of the bolts and gaskets increases with elevated temperatures and load-induced stresses although room temperature creep-relaxation can be also significant even at relatively light loads. A plant maintenance practice includes periodical retightening or replacement of the bolts and gaskets subjected to creep-relaxation to prevent leakages, and, having in mind a great quantity of bolts and gaskets used in critical industries, the procedure involves an expensive time-consuming process, but it provides only temporary effect because the creep-relaxation increases rapidly after each additional retightening and the risk of leakage event relatively increases. Nevertheless, during the development of most design procedures, a little consideration has been given to operational creep-relaxation of the bolts and gaskets, and similar situation is observed with patent documents.

U.S. Pat. No. 6,199,453 by Steinbock entitled “High temperature bolting system” offers a sophisticated apparatus for maintaining a clamping bolt force between component parts of a steam turbine while operating at temperatures from 800° F. to 1200° F. However, the disclosed elongated stepped fastener shank manufactured from superalloy Inconel 718 having a thermal expansion coefficient similar to flange material cannot stop a creep-relaxation process and protect proposed bolting system from creep-relaxation that is an increase of bolt elongations and decrease of bolt stresses with time. Moreover, the combination of high level of stresses and temperatures will induce the high level of creep-relaxation of the bolts that defines their routine “passive” behavior under critical operational conditions.

EP Pat. No. 352608 discloses a method of fabrication of reinforced polytetrafluoro-ethylene (PTFE) gasketing materials “characterized by high strength, excellent recovery and superior creep-relaxation resistance”. However, these super characteristics did not exclude the creep-relaxation from 20% to 30% in dependence on thickness of proposed tested materials. Moreover, the tests were carried out under standard procedure during only 22 hours at only 212° F. Thus, the proposed gasketing materials demonstrate “superior” physical and functional properties when compared with previous PTFE gasketing materials described by cited prior art. The most important failing however is the fact that proposed gasketing materials copy a typically used approach to the fabrication of any known sealing materials based on their traditional “passive” behavior under critical operational conditions.

The attempts to use the bolts and gaskets manufactured from advanced Shape Memory Alloys (SMAs) were made in Japanese patent publications Nos. 59026668, 62188764, 63172064, 1255782, 4073469, 6101762, 2005249123 as well as in GB2352768, and U.S. Pat. Nos. 5,226,683 and 6,435,519.

JP 59026668 describes a gasket obtained by bending a plate material of SMA in an annular body having U-shaped cross sectional form. The gasket is so set that its width in the axial direction becomes larger than the depth of a fitting groove in a temperature range higher than the shape memory temperature, and becomes smaller than the depth of the groove in a temperature range lower than the shape memory temperature providing in the first case a sealing action between the members of the assembly. This invention relates to the use of two-way shape memory effect that may be completed under condition of preliminary continuous training of the gasket at two temperature ranges.

JP 62188764 discloses a method to manufacture a bolt of NiTi SMA that may be easily fastened and detached. This bolt is previously subjected to axial compression and aging treatment under specific high temperature while holding it under compressive strain. Thus-obtained bolt repeats reversibly the elongation in a length direction at a temperature of martensite transformation and the contraction at a temperature of inverse transformation. Owing to these characteristics, the length of the bolt is arbitrary changed, so that bolt may be firmly fastened or easily detached. Obviously, this invention relates to the assemblage procedure of the BFC based on two-way shape memory effect under temperatures corresponding to martensitic phase transformation of NiTi SMA. Additionally, the bolt fabrication procedure is based on compression of the bolt under high temperature of austenite state that increases a risk of the bolt's buckling.

JP 63172064 describes a gasket of SMA that remains in nearly flat shape at low temperature and forms a bead on the peripheral edge part of opening to be sealed when its temperature becomes above a defined value. This invention is close to the present invention, but the problem of creep-relaxation of the gasket is completely failed.

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