DISSIMILAR PIPING JOINT AT HIGH TEMPERATURE, HIGH PRESSURE TRANSIENT AND UNDER CYCLIC LOADING

Abstract

Dissimilar piping joint arrangement, including a first pipe section and a second pipe section and a dissimilar piping joint between the pipe sections, the first and second consecutive pipe sections being made of first and second metallic materials respectively with different material behavior and properties. An improved lifetime and extended applicability is achieved in that said dissimilar piping joint is a coupling joint, said first pipe section made of said first metallic material is provided at one end with a first coupling made of said first metallic material, said second pipe section made of said second metallic material is provided at one end with a second coupling made of said second metallic material, and the first coupling and the second coupling are bolted together, whereby a first seal is established by direct metallic contact between the front faces of the first coupling and the second coupling.

Claims

1. A dissimilar piping joint arrangement comprising a first pipe section comprising a first metallic material; a second pipe section comprising a second metallic material; a dissimilar piping joint defining a coupling joint, wherein said first pipe section is provided at one end with a first coupling made of said first metallic material, said second pipe section is provided at one end with a second coupling made of said second metallic material, and the first coupling and the second coupling are bolted together; and a first seal that is established by direct metallic contact between the front faces of the first coupling and the second coupling.

2. A dissimilar piping joint arrangement according to claim 1, wherein said first and second couplings are welded to their respective pipe sections.

3. A dissimilar piping joint arrangement according to claim 1, wherein said front faces of said couplings are slightly conical.

4. A dissimilar piping joint arrangement according to claim 1, wherein said front faces of said couplings are conical with an angle of aperture in a range between 178° and 179.9°.

5. A dissimilar piping joint arrangement according to claim 1, wherein said first material is a Ni-based alloy and said second material is a ferritic/martensitic alloy.

6. A dissimilar piping joint arrangement according to claim 5, wherein said first material is an Alloy 625 Grade 2 and said second material is an alloy.

7. A dissimilar piping joint arrangement according to claim 1, wherein said first pipe section is welded at the other end from the first coupling to a third pipe section made of a third metallic material different from said first and second metallic materials.

8. A dissimilar piping joint arrangement according to claim 7, wherein said third metallic material is steel.

9. A dissimilar piping arrangement according to claim 3, wherein said first and second couplings each have a central bore, said first seal is established adjacent to said central bore of said couplings, and a second seal is provided, which surrounds said first seal and keeps said dissimilar piping joint tight in case of a failure of said first seal.

10. A dissimilar piping joint arrangement according to claim 9, wherein said second seal comprises a metallic seal ring placed in an annular space being made up by lining grooves in said front faces of said couplings.

11. A dissimilar piping joint arrangement as claimed in claim 1, wherein said first and second couplings have outer dimensions substantially smaller than those of standard ASME B16.5 couplings.

12. A combined cycle power plant comprising: a dissimilar piping joint arrangement comprising a first pipe section comprising a first metallic material; a second pipe section comprising a second metallic material; a dissimilar piping joint defining a coupling joint, wherein said first pipe section is provided at one end with a first coupling made of said first metallic material, said second pipe section is provided at one end with a second coupling made of said second metallic material, and the first coupling and the second coupling are bolted together; and a first seal that is established by direct metallic contact between the front faces of the first coupling and the second coupling.

13. A combined cycle power plant according to claim 12, wherein said first and second pipe sections connect an air cooler of a gas turbine of said combined cycle power plant and a heat recovery steam generator of said combined cycle power plant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Embodiments of the present invention are now to be explained more closely by means of different embodiments and with reference to the attached drawings.

[0030] FIG. 1 shows a simplified scheme of a combined cycle power plant CCPP;

[0031] FIG. 2 shows an exemplary prior art pipe connection between air cooler and heat recovery steam generator HRSG comprising a critical mixed weld seam;

[0032] FIG. 3 shows an embodiment of a coupling Dissimilar Piping Joint according to the embodiments of the invention;

[0033] FIG. 4 compares the size of a coupling according to an embodiment of the present invention with the size of a coupling with identical inner diameter according to ASME standard;

[0034] FIG. 5 shows details of a coupling with slightly conical front face according to an embodiment of the invention; and

[0035] FIG. 6 shows a longitudinal section of a coupling joint with plural seals according to an embodiment of the invention.

DETAILED DESCRIPTION

[0036] The problems of a connection of stainless steel pipe sections and ferritic steel pipe sections in a piping used in a combined cycle power plant are: An optimization of CTE (coefficient of thermal expansion) mismatch; Fulfillment of operational pressure and temperature ratings; Steep transient conditions during plant start up; Consideration of creep, fatigue and their interaction; Lifetime and allowable number of cycles; Maintenance free till end of lifetime—no interference of operation regime; No mixed weld seams on site; External forces and moments; Easy site replacement of existing arrangement; ASME and PED certification;

[0037] According to embodiments of the invention a dissimilar metal coupling joint itself considering different mechanical properties of material involved, is used to provide the critical material transition between the coupling faces that have different material behavior and properties, without the need of fusion of material (mixed weld seam), which can achieve the required lifetime even considering the combination of boundary conditions of high pressure, high temperature, high cycling, high creep and external forces and moments.

[0038] FIG. 3 shows an embodiment of a coupling Dissimilar Piping Joint 28 according to embodiments of the invention. A first pipe section 29 made of material (metal) M4 is connected to a second pipe section 30 made of material (metal) M5 by means of a less-critical (material 30 is the integral part of the coupling) weld seam 32. Second pipe section 30 and a third pipe section 31 made of a third material (metal) M6 are connected by means of a coupling joint comprising couplings F1 and F2. Coupling F1 is made of the same material as second pipe section 30, i.e. material M5. Coupling F2 is made of the same material as third pipe section 31, i.e. material M6. Couplings F1 and F2 are connected by means of suitable bolts 33 and nuts 34.

[0039] The coupling joint F1, F2 represents a direct material transition from material M5 to material M6.

[0040] Due to this arrangement of material M5 and M6 in Dissimilar Piping Joint 28 only homogenous welding work on site is necessary (no mixed weld seam on site). The optimization of different thermal expansion rate is done by the right choice of material M5 and material M6.

[0041] The external dimensions of couplings F1 and F2 can substantially deviate from standard coupling dimensions according to ASME B16.5 standard. FIG. 4 shows a comparison of the outer dimensions of a coupling 35 with welding neck according to ASME B16.5 standard and a (compact) coupling 36 with same inner diameter according to an embodiment of the invention. As can be seen from FIG. 4, the overall height h2 of coupling 36 is less than half of the overall height of ASME standard coupling 35. The coupling height h1 of coupling 36 is about half the coupling height of ASME standard coupling 35. The outer diameter d of coupling 36 is approximately ⅔ of the outer diameter of ASME standard coupling 35.

[0042] Thus, compact coupling 36 has only about 60% material volume compared to a traditional (standard) welding neck coupling 35.

[0043] This reduction in material volume of the couplings F1 and F2 offers various advantages some of which are: Improved thermal stress behavior during transients; Lower weight; Negligible impact on piping support system 4 no modification on support concept necessary.

[0044] Another advantage is related to the specific design of the couplings F1 and F2 with regard to their front faces. According to FIG. 5 the couplings F1 and F2 have slightly conical front faces 42 the conicity or tapering of which is defined by two different angles α and β. Angle β defines the conicity of the main part of front face 42 (inside a circular groove 40) while angle α is related to the conicity of a rim part outside the connecting bores 37 and circular groove 40. Angles α and β are related to the angle of aperture θ of the conical front face 42 by formula

θ=180°−2α or θ=180°—2β, respectively.

[0045] With angle α ranging between 0.05° to 0.75° and β ranging between 0.08° to 1.00° the angle of aperture θ can be said to range between 178° and 179.9°. Furthermore, the back side of couplings F1, F2 also has a conicity with an angle γ ranging between 0.04° and 0.8° (angle of aperture between 179.92° and 178.4°).

[0046] The two-stage two-angle design with angles α and β leads to an optimized lifetime of the joint. The conicity with angle β of the main front face defines a contact pressure at inner bore (heel) 38 of coupling F1, F2 (see seal S1 in FIG. 6)

[0047] Elastic deformation of the coupling faces due to optimized bolt pretension force close faces with their conicity angle α at outside diameter (see seal S3 in FIG. 6).

[0048] In any case, shear forces due to different expansion of ferritic (material M6) and Ni-base coupling material M5 have to be considered (CTE mismatch optimization).

[0049] A coupling Dissimilar Piping Joint according to an embodiment of the invention is shown in the connected state in a longitudinal section in FIG. 6. Couplings F1 and F2 are connected by bolts 33 extending through connecting bores 37 (FIG. 4) and nuts 34.

[0050] Compact couplings F1, F2 have no compressed soft gasket, which is directly influencing bolt pretension. Due to metal-to-metal contact at the front faces 42 of couplings F1 and F2 a defined surface pressure is established. Thus, a bolt pretension loss can only be driven by metal behavior and not by any gasket compression loss.

[0051] As shown in FIG. 6 the compact coupling design comprises two main seal areas with a first metallic face-to-face seal S1 adjacent to central bore 38 (heel) of couplings F1, F2. A second seal S2 surround the first seal S1. Second seal S2 comprises a hollow annular space 41a made up by opposing lining grooves 39 in the front face of each coupling F1, F2. A metal seal ring 41 is inserted into said annular space 41a and compressed in radial direction when the couplings F1, F2 are connected.

[0052] The outer second seal S2 is only in operation when inner first seal S1 (heel area) has opened resulting in a double sealing instead of one main seal only. Metal seal ring of second seal S2 is self-energized. The gasket is compressed by bolt force only. Third seal S3 acts as an environmental seal.

[0053] The characteristic features of the pipe transition according to embodiments of the invention and its various embodiments can be summarized as follows: A dissimilar metal coupling joint (F1, F2) itself considering different mechanical properties of material involved, is used to provide the critical material transition between the coupling faces 41 that have different material behavior and properties, without the need of fusion of material (mixed weld seam), which can achieve the required lifetime even considering the combination of boundary conditions of high pressure, high temperature, high cycling, high creep and external forces and moments. CTE (Coefficient of thermal expansion) mismatch of the involved materials is optimized via material selection which covers the given boundary conditions on one side and offers the smallest possible difference in CTE. One embodiment involves the design of coupling face angles α and β and pre-stressing, which are optimized to control creep and fatigue behavior of the whole coupling system to achieve the target values of lifetime and load cycles. Number and diameter of bolts 33 are optimized considering creep, fatigue and stresses due to external forces and moments. The bolts loading has very high pretension (ranging up to 120-160 kN) to keep coupling joint 28 together considering high loss of pretension expected during service. The bolt loading is applied with special hydraulic tools to achieve tension only and no additional stress due to torsion. Double sealing with primary and secondary seals S1 and S2 is used as a typical feature of the coupling design: For the primary seal S1, the mechanical integrity calculations are used to control local stress, creep and fatigue behavior via proper pre-stressing the whole system in order to achieve given load cycles and lifetime. In such a way, the primary seal S1 is still in sufficient contact after intended lifetime. The function of the secondary seal S2 is not required even at the end of intended lifetime. Secondary seal S2 will hardly see any contact with fluid and pressure, but is considered as an additional safety measure against leakage. Even in case primary seal S1 would lose sufficient contact pressure, the secondary seal S2 could overtake the full tightness function. This results in leak free design and is 100% EHS compliant.

[0054] The benefits of the solution according to embodiments of the invention are: Prevention of mixed weld seams to be performed on site. Achievement of extended lifetime considering creep, fatigue and their interaction. The components involved are designed for increased operational intervals (as high as up to 50,000 EOH). They do not require maintenance in between, which would disturb operation. Therefore, maintenance activities are lower. Double sealing against leakage makes sure that even in case of primary seal losing sufficient contact pressure; secondary seal could overtake the full tightness function. No major modification expected to piping design and support concept which is beneficial for existing service fleet. The new coupling concept has low weight impact which means the coupling installation does not require major modifications of the present piping as a working system, exposed to dead weight, external loading as forces and moments, thermal restricted expansion, wind and earthquake loads.