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

Abstract

A method to limit or exclude operational creep of the bolts and gaskets of Bolted Flanged Connections (BFCs) is disclosed. The method consists in the use of bolts and gaskets manufactured from the same Shape Memory Alloys (SMAs) having temperature intervals of reverse martensitic phase transformation close to operational temperatures of the BFCs. These bolts and gaskets of the BFCs are shape-memorized to the compression (bolts) and “swelling” (gaskets) during (1) the formation of stress-induced martensite under temperature of martensite state of the SMAs while bolt preload application, or (2) during the formation of stress-induced martensite while loading-unloading previously stretched bolts and previously compressed gaskets under temperature of martensite state resulting in residual bolt elongations and residual gasket contraction followed by bolt preload application. Constrained shape recovery of initial length of the bolts and initial thickness of the gaskets under operational temperatures of the BFCs is accompanying by appearance of reactive shape-recovering stresses having direction inverse to the direction of operational creep of the bolts and gaskets that limits or excludes their operational creep.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

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.

FEDERALLY SPONSORED RESEARCH

None

SEQUENCE LISTING OR PROGRAM

None

BACKGROUND OF THE INVENTION

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.

JP 1255782 relates to a gasket of SMA that is extended or contracted by heating or cooling under preset temperatures. The gasket body is heated by a heater, and it is expanded to form a fully tight structure. The removal of the gasket is performed by cooling that contracts it. This invention relates to the well known assemblage procedure of the BFC described by some mentioned above patent publications.

JP 4073469 describes a gasket of SMA inserted between the sides of the flanges where the knife edges are formed. The gasket is heated with heaters inserted into the bodies of the flanges and restores the original shape ensuring higher tightness. When the gasket is released from connection and heated with another heating device, a recess due to the knife edge can be eliminated and the original shape of the gasket can be restored. Consequently, the gasket can be again used.

JP 6101762 proposes a spiral-shaped gasket of SMA that is set between the flanges, and sealing effect is performed even if the tightening force of the flanges and bolts lowers because of thermal expansion. This invention is very close to the one described above by JP 58180892.

JP 2005249123 describes the bolts from CuAlNi and FeNiCoTi SMAs. The expanded shape of the bolts is stored at normal temperature, and contracted shape is stored at high temperature. The fastening and unfastening work can be easily performed under normal temperature, and strong fastening force is exercised under high temperature. This invention relates to well known assemblage procedure of the BFC.

GB 2352768 proposes the bolts of NiTiCuFe SMA that connect the members of cryogenic satellite tank when the bolts are in their austenite state, but connection between the members is loosened when the bolts expand in their martensite state following cooling in space. This invention relates to assemblage-disassemblage procedure of the BFC.

U.S. Pat. No. 5,226,683 discloses a method to use a gasket of NiTi SMA under martensite state in order to fill the microspaces between the hard flange faces having microscopic surface irregularities that can prevent the fluid leakage between the faces and will allow further to reuse the gasket. The NiTi SMA of which the gasket is made “remembers” its original shape when it was formed in its austenite state. When this gasket is deformed under temperature of martensite state it fills the irregularities of flange faces under pressure exerted by hard clamping flanges. The shape memory effect is used when gasket resumes its original shape being released and heated to austenite state during the restoration step before reuse.

U.S. Pat. No. 6,435,519 presents a next attempt to use the NiTi SMA as a gasketing material in order to provide a seal between component parts of an imaginary generalized assembly. Unfortunately, this invention claims a well-known procedure to clamp the gasket between adjacent flange faces. As for application of the SMA, this invention claims a spring forces generated by bending of the gasket when it is in super-elastic state. It is easily to observe that SMA in its super-elastic state displays all mechanical properties of typical elastic material including the property of creep-relaxation while subjecting to elevated temperature and internal pressure.

Clearly, none of above-mentioned prior patent documents touch the problem of leakage elimination by means of creep limitation of the bolts and gaskets of the BFCs. Accordingly, it is a principal object of the present invention to form a method of creep limitation of the bolts and gaskets that provides an “active” behavior of the bolt-gasket system under critical operational conditions including internal pressures and a variety of operational temperatures. This invention is the first one introducing a new sealing philosophy based on active intervention of the bolts and gaskets into sealing process under critical operational conditions. This active intervention is called “negative creep” effect to satisfy the conditions as employed in the present invention.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to introduce a new sealing technology based on active intervention of bolts and gaskets of the BFCs into technological processes of engineering structures that operate under critical conditions due to variety of operational temperatures and internal pressures.

It is next object of the present invention to provide active intervention of bolts and gaskets of the BFCs into critical technological processes by means of significant limitation or complete exclusion of their operational creep.

It is another object of the present invention to provide a method of creep limitation of the bolts and gaskets by means of specific types of bolts and gaskets that are manufactured from SMAs and display “negative creep” effect under critical operational conditions. “Negative creep” effect of the bolts and gaskets results from reactive shape-recovering stresses that appear during constrained shape recovery of shape-memorized deformations of the bolts and gaskets under operational temperatures. Reactive shape-recovering stresses have direction inverse to the direction of operational creep of the bolts and gaskets that limits or excludes the bolt elongations and gasket contraction due to creep.

Method of creep limitation consists in application of bolts and gaskets manufactured from the same SMAs having temperature intervals of reverse martensitic phase transformation close to operational temperatures of the BFCs. The bolts and gaskets of SMAs are shape-memorized to the compression (bolts) and “swelling” (gaskets) by different ways including bolt preload force that produces elongation of the bolts and contraction of the gasket. These deformations are shape-memorized ones because they change the initial length of the bolts and initial thickness of the gasket. The bolts and gasket that are shape-memorized by bolt preload force will try to recover their initial length and initial thickness under operational temperature of the BFC, but this shape recovery will be blocked by rigid flanges with appearance of reactive shape-recovering stresses having direction inverse to the direction of operational creep of the bolts and gasket. Reactive shape-recovering stresses will block the bolt elongations and gasket contraction due to creep that corresponds to “negative creep”

The shape-memorized deformations of the bolts and gasket due to bolt preload force can be combined with their residual shape-memorized deformations obtained in advance during the formation of stress-induced martensite. Stress-induced martensite is formed while loading-unloading a work piece of SMA under temperature below than temperature of martensite state of the SMA, and this procedure results in appearance of residual shape-memorized deformation of the work piece that can be the bolt or gasket.

The recovery of initial length of the bolts and initial thickness of the gasket occurs after standard tightening of the bolts and clamping of the gasket followed by application of internal pressures and a variety of operational temperatures that are close to the temperatures of reverse martensitic phase transformation of the SMAs from which the bolts and gaskets are manufactured.

The SMAs on a basis of Cu, Fe, Al, Ma, Co, Ga, In, Ni, Ti, Zr, Pd, Pt, Hf, and other elements have large temperature intervals of reverse martensitic phase transformations that correspond to the variety of temperatures which provide the recovery of shape-memorized deformations. Hence, the operational temperature of the BFC must be close to temperature interval of reverse martensitic phase transformation of the SMA from which the bolts and gasket are manufactured. For example, the operational temperatures of technological equipment such as rectors, steam generators, heat exchangers, piping systems, and others used in Fossil Fuel and Nuclear Power Plants vary from dozens to hundreds degrees, and usually they remain stable for given type of equipment, so that temperature of reverse martensitic phase transformation of suitable SMA must be in the interval of operational temperatures of the assembly. This condition ensures the recovery of initial length of the bolts and thickness of the gasket, but this shape recovery will be blocked by rigid flanges of the BFC with appearance of reactive shape-recovering stresses having direction inverse to the direction of operational creep of the bolts and gasket that corresponds to “negative creep” effect.

Reactive shape-recovering stresses may be considerable depending on quantity of shape-memorized deformations and rigidity of opposed flanges of the BFC that provides significant limitation or inhibition of operational creep of the bolts and gasket. For example, reactive shape-recovering stresses of the SMAs on a basis of Ni—Ti compositions may be close to 800 MPa, and for some compositions of Ni—Ti—Hf they can reach 1300 MPa.

A principal advantage of the present invention is a new approach to the sealing technology based on “active” intervention of bolts and gaskets of the BFCs into technological processes by means of bolts and gaskets manufactured from SMAs having temperature intervals of reverse martensitic phase transformation close to operational temperatures of the BFCs.

Another advantage of the present invention consists in the use of reactive shape-recovering stresses generated by constrained recovery of shape-memorized deformations of the bolts and gaskets manufactured from SMAs and shape-memorized to the compression (bolts) and “swelling” (gaskets). The reactive shape-recovering stresses appear under operational temperatures of the BFCs providing the “negative creep” effect that limits or excludes operational creep of the bolts and gaskets.

It is a next advantage of the present invention to provide a continuous automatic tight contact between flange and gasket surfaces under critical operational conditions including high internal pressures and a variety of operational temperatures. This contact is provided by constrained shape recovery of the gasket thickness of SMA that is shape-memorized to the “swelling”. Reactive shape-recovering stresses due to constrained recovery of initial thickness of the gasket ensure this contact that significantly increases a leak-tightness of the BFC and maximizes efficiency of critical technological equipment.

Further brief description of applied drawings followed by detailed description of the invention is intended to provide a necessary basis for understanding the nature and character of the present invention, and introduce the method to limit a creep of bolts and gaskets of the BFCs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a part of the BFC with the bolts and gasket of the same SMA having temperature interval of reverse martensitic phase transformation from A_s (start temperature) to A_f (final temperature).

FIG. 2 represents schematically constrained shape recovery of the bolt of SMA that is shape-memorized to the compression by bolt preload force providing contact shape-memorized deformation ε_c under temperature below than temperature M_f of martensite state of the SMA. Constrained shape recovery of the bolt is accompanying by appearance of reactive shape-recovering stress σ_sr at temperature interval A_s≦T≦A_f of reverse martensitic phase transformation of the SMA.

FIG. 3 represents schematically constrained shape recovery of the bolt of SMA that is shape-memorized in advance to the compression during the formation of stress-induced martensite while loading-unloading the bolt under temperature below M_f followed by application of bolt preload force that provides contact shape-memorized deformation ε_c due to stress σ_c. The constrained shape recovery is accompanying by appearance of reactive shape-recovering stress σ_sr at temperature interval A_s≦T≦A_f of reverse martensitic phase transformation of the SMA.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross-sectional view of a part of BFC having the flanges 1 and 2 of conventional structural steel, and bolts 3 and gasket 4 manufactured from the same SMA having temperature interval A_s≦T≦A_f of reverse martensitic phase transformation that is close to operational temperatures of the BFC. Bolts and gasket are shape-memorized to the compression (bolts) and “swelling” (gasket). The constrained shape recovery of stretched bolts and compressed gasket at temperature interval A_s≦T≦A_f produces reactive shape-recovering stresses σ_sr having direction inverse to the direction of operational creep of the bolts and gasket. Hence, FIG. 1 illustrates a main principle of “negative creep” effect that limits or excludes elongation of the bolts and contraction of the gasket due to their operational creep under temperature interval A_s≦T≦A_f of the BFC.

FIG. 2 is a schematic representation of constrained shape recovery of the bolt of SMA that is stretched by bolt preload force from point O to point K under temperature below than temperature M_f of direct martensitic phase transformation of the SMA producing contact shape-memorized deformation ε_c due to stress σ_c. This procedure is shown at the rear graph of “stress σ—strain ε—temperature T” diagram. The recovery of initial length of the bolt constrained by rigid flanges occurs under temperature interval A_s≦T≦A_f that produces the reactive shape-recovering stress σ_sr while heating the BFC from point K to point H shown at the front graph in FIG. 2. Similar procedure may be applied to the gasket compressed by bolt preload force under the same temperature conditions. In this case it is sufficient to change directions of the axes σ and ε. Reactive shape-recovering stress σ_sr will block the bolt elongations and gasket contraction due to their operational creep that corresponds to “negative creep” effect.

FIG. 3 is a schematic representation of constrained shape recovery of the bolt of SMA that is shape-memorized in advance to the compression during formation of stress-induced martensite while loading from point O to point K followed by unloading from point K to point L under temperature below M_f. After this procedure bolt receives some quantity of residual elongation defined by location of the point L. Application of bolt preload force leads to additional bolt elongation from point L to point G producing final contact shape-memorized deformation ε_c due to stress σ_c. This procedure is shown at the rear graph in FIG. 3. The recovery of initial length of the bolt constrained by rigid flanges occurs under temperature interval A_s≦T≦A_f that produces the reactive shape-recovering stress σ_sr while heating the BFC from point G to point H shown on the front graph in FIG. 3. Similar procedure may be applied to compressed gasket that receives final contact shape-memorized deformation ε_c.

The shape-memorized deformation of the bolts and gaskets obtained with described procedures are a basis to produce “negative creep” effect under operational temperatures of the BFCs resulting in appearance of reactive shape-recovering stresses having direction inverse to the direction of operational creep of the bolts and gaskets that limits or excludes their operational creep. This forms a basic idea of presented method.

CONCLUSION

Bolts and gaskets, manufactured from SMAs having temperature intervals of reverse martensitic phase transformation close to the operational temperatures of the BFCs and shape-memorized to the compression (bolts) and “swelling” (gaskets), display “negative creep” effect resulting in appearance of reactive shape-recovering stresses due to constrained recovery of initial length of the bolt and initial thickness of the gaskets under operational temperatures of the BFCs. The reactive shape-recovering stresses having direction inverse to the direction of operational creep of the bolts and gaskets limit significantly or exclude completely bolt elongations and gasket contraction due to their operational creep.

Described procedures of “negative creep” formation are the basis of new sealing technology and method to limit operational creep of bolts and gaskets of the BFCs that is a principal object of the present invention. This method provides an active behavior of the bolts and gaskets under critical conditions of internal pressures and a variety of operational temperatures. This active behavior may be considered as a new sealing philosophy based on active intervention of the bolts and gaskets into critical processes providing significant increase of the leakage tightness of the BFCs and maximizing the efficiency of pressure vessels and piping systems used in critical engineering structures.

Changes may be made by those skilled in the art in matters of shape, size, and arrangement of the bolts, gaskets and flanges of the BFCs without exceeding the scope of the invention described by appended claims.

Claims

1. A new sealing technology based on active intervention of bolts and gaskets of the Bolted Flanged Connections (BFCs) into technological processes of critical engineering structures by means of creep limitation or complete creep exclusion of bolts and gaskets.

2. The new sealing technology according to claim 1 wherein said creep limitation or complete creep exclusion of said bolts and gaskets results from bolt contraction and gasket “swelling” at operational temperature of said BFCs.

3. A method to limit or exclude a creep of bolts and gaskets of the BFCs wherein bolts and gaskets are manufactured from the same SMAs having temperature intervals of reverse martensitic phase transformation close to operational temperatures of the BFCs.

4. The method according to claim 3 wherein said bolts and gaskets of said same SMAs are shape-memorized to the compression (bolts) and “swelling” (gaskets) during the bolt preload under temperature below than temperature of martensite state of the SMAs.

5. The method according to claim 3 wherein said bolts and gaskets of said same SMAs are shape-memorized in advance to said compression (bolts) and “swelling” (gaskets) when the bolts and gaskets, being previously stretched (bolts) and compressed (gaskets) under temperature below than temperature of martensite state of the SMAs, obtain some quantity of residual elongations and contractions after unloading.

6. The new sealing technology and method according to claims 1 and 3 wherein said bolts and gaskets manufactured from the same said SMAs and shape-memorized to said compression (bolts) and “swelling” (gaskets) display “negative creep” effect resulting from constrained recovery of initial length of said bolts and initial thickness of said gaskets followed by appearance of reactive shape-recovering stresses having direction inverse to the direction of operational creep of bolts and gaskets under said operational temperatures of the BFCs that are close to temperature intervals of reverse martensitic phase transformation of said SMAs from which said bolts and gaskets are manufactured.