RU2097646C1 - Method of prevention of development of flaws in pipe line walls - Google Patents

Abstract

FIELD: mechanical. SUBSTANCE: detachable coupling consisting of two halves is mounted on pipe line at preliminary filling of flaw with hardening noncorrodible polymer material, after which coupling halves are welded together. Invention gives mathematical dependence of pressure drop versus geometry of pipe and stresses in it. EFFECT: enhanced efficiency. 2 cl, 3 dwg

Description

 The invention relates to a method for local reduction of ring stresses from internal pressure in the places of pipeline defects (corrosion, cracks, dents, etc.).

 The reduction of ring stresses is carried out by mounting and welding on the pipeline a steel full-sleeve coupling with a given wall thickness for a given pressure reduction in the pipeline or under certain compression conditions of the coupling on the pipeline by external hydraulic chain clamps.

 Couplings that unload the supporting pipe can also be made in the form of multilayer thin-sheet structures wound around the pipe with a decrease in pressure in the pipeline.

 The tension in the coupling, compressing the carrier tube, is created when the pressure in the pipeline increases to the working pressure or when the coupling halves are compressed before welding with clamps.

 Known methods for coupling pipes.

 Closest to the invention is the welding on the pipeline covering couplings (Pipeline Maintenance Welding Practices. API Recommended practice 1107, 1991.) of various patches that do not create a dosed tensile state of compression in the wall of the pipelines (Typical design for the safe conduct of hot work at gas facilities of Mingazprom. VNIIGAZ 1988).

 The objective of the invention is the local reduction of annular tensile stresses in the wall of the pipeline at its working pressure to prevent the growth of detected defects, especially crack-like, and subsequent destruction of the pipe.

 The essence of the invention lies in the fact that to eliminate damage to the wall of the pipeline, it can be completely blocked, and then freed from the product, cut the damaged area and weld the coil.

 However, blocking the pipeline can cause significant losses due to interruptions in the supply of consumers, and sometimes it is impossible.

 In world practice, there are solutions to this problem in the form of imposing various emergency clamps, patches.

 Circular couplings superimposed on the defect site in the form of a welded ring of sheet metal have also found application. Such rings, usually of two halves, increase the thickness of the pipe at the place of the defect, as well as prevent the development of deformation before failure, which delays the destruction itself.

 However, the simple application of the coupling to the pipeline operating under working pressure does not yet give enough effect, since this does not reduce the stress state of the pipe wall in the place of the defect and, therefore, does not inhibit its development, especially for crack-like defects.

 The coupling can give a more complete effect if it takes on part of the ring stresses acting in the wall of the pipe with a defect.

 This is possible if, before applying the coupling to the defective area in it, reduce the pressure by a certain amount.

From simple mathematical calculations, it follows that the reduction of annular stresses in the carrier pipe will depend on the relative decrease in pressure at the time of installation of the coupling and the relative wall thickness of the coupling as follows:
σ / σ _o = P _y / P _o + (1-P _y / P _o ) :( 1 + δ _m / δ) (1)
Where
σ ring tensile stresses in the pipe wall under the installed sleeve at a working pressure P _about ;
s _o ring tensile stresses in the wall of the pipe without coupling at operating pressure P _o ;
P _{y the} pressure in the pipeline when installing the coupling;
P working pressure in the pipeline;
δ _m coupling wall thickness;
δ _m pipe wall thickness.

In FIG. 2 graphically shows the dependence (1) regarding the reduction of the stress state of the pipe wall σ / σ _o under the coupling on the degree of pressure reduction during installation of the coupling (P _у / P _о ) and its relative thickness (σ _m / σ)
As can be seen from the graph, applying a sleeve to a pipeline without pressure can relieve the wall with a thick sleeve (δ _m / δ = 4) by almost 80% when the pressure increases to P _о
With an optimum pressure drop in the pipe wall P _у up to 50 65% of P _о, ring stresses that are sufficient from the safety point of view can be reduced by 20–40% depending on the thickness of the coupling wall
To transfer force from pipe to sleeve between the sleeve and the pipe in places of loose contact, a quick-hardening substance such as epoxy must be applied to the surface or poured through the holes made in the sleeve.

 Such crimp couplings can prevent the development of defects only in the direction of the generatrix of the pipeline.

 If there is damage located along the ring (for example, a defect in an annular weld), the crimp sleeve will prevent the defect from moving in the direction along the pipeline, preventing avalanche damage. To prevent the discovery of a defect in the direction of the axis of the pipeline, the coupling should be welded to the pipeline at the ends with two flank seams (Fig. 2).

 Since the strength coefficient of flank joints is usually ≈ 0.7 of the strength of the butt joint, the leg of the flank joint should be at least 1.4 δ, which will ensure equal strength of the coupling with an annular weld in the direction of the axis of the pipeline.

 The creation of compressive stresses in the carrier pipe can be achieved not only by lowering the pressure in the pipeline, but also by compressing the coupling halves in the pipeline using hydraulic chain clamps (Fig. 3), creating significant compressive forces and tightly pressing the coupling halves to be welded with horizontal seams to the defective section of the pipeline .

 Couplings can be made of multilayer wound fiberglass or thin steel sheet. Sheets of multilayer couplings must be glued together with polymer materials. Such couplings can prevent the development of defects only along the axis of the pipeline and do not absorb axial loads.

 The use of couplings locally compressing ring tensile stresses only in places of defects in the wall of pipelines will in many cases allow continuing trouble-free operation of pipelines without cutting and replacing the defective section.

Claims (2)

1. A method of preventing the development of crack-like and corrosion defects of the walls of the pipelines along the pipe generatrix, which consists in determining the location and nature of the defect, opening the pipe, removing the insulation coating and stripping the defect, installing a detachable sleeve with pre-filling the defect with a hardening non-corrosive polymer material flush with the pipe contour or injection of this material into the space between the coupling and the pipe, welding with horizontal longitudinal seams of the coupling halves, distinguishing Take into account that the coupling is installed and welded on the pipeline at a pressure drop specified below, lower than the working pressure, while the value of the reduced pressure in the pipeline is established from the relation

where σ are the annular stresses in the pipe under the installed sleeve at an operating pressure of P ₀ ;
σ _o - ring stresses in a pipe without a coupling at a working pressure P ₀ ;
P _{y the} pressure in the pipeline when installing the coupling;
P ₀ working pressure in the pipeline;
δ _m - wall thickness of the coupling;
δ is the wall thickness of the pipe.  2. The method according to claim 1, characterized in that the sleeve at both ends is welded to the pipeline with flank seams with a leg of at least 1.4 pipe wall thickness.

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