DISSIMILAR METAL CONNECTION AND METHOD FOR OPERATING SUCH A

DISSIMILAR METAL CONNECTION

BACKGROUND OF THE INVENTION

The present invention relates to the technology of metal joints between members of different metallurgical behaviour, which are exposed to transient thermal operating conditions, especially in power plants of the CCPP type. It refers to a dissimilar metal connection according to the preamble of claim 1 .

It further refers to a method for operating such a dissimilar metal connection.

PRIOR ART

The use of different metals/alloys, which differ in their metallurgical behaviour and which are called in this context "dissimilar metals" in a joint being exposed to high temperature, high pressure loading, high cyclic, and high transients with external forces and moments, can pose problems of stress and reduced lifetime of this joint.

Fig. 1 shows the basic scheme of a combined cycle power plant (CCPP) 10. The combined cycle power plant 10 of Fig. 1 comprises a gas turbine (GT) 1 1 connected to a water/steam cycle 12via a heat recovery steam generator (HRSG) Gas turbine 1 1 comprises a compressor 14, which aspirates air through an air inlet 13 and delivers the compressed air to a combustor 15, where it is used to generate hot gas by burning fuel 16. The hot gas drives a turbine 17, and the exhaust gas 18 of the turbine 17 passes through the heat recovery steam generator 19 and finally exits as flue gas 20.

Heat recovery steam generator 19 generates steam for a steam turbine 21 . In addition, water from the heat recovery steam generator 19 is fed to an air cooler 22 and used to cool down compressed air from the compressor 14, which is fed to the turbine for cooling purposes. While the water is supplied through a water inlet pipe 24, the generated steam flows back to the heat recovery steam generator 19 via steam outlet pipe 23. A more detailed plan of a combined cycle power plant is shown in document DE 196 45 322, for example.

The high pressure air cooler 22 of GT 1 1 usually needs to be made of austenitic stainless steel to avoid high temperature corrosion product entering the hot gas path parts of the turbine 17. At the same time, the remaining water/steam side of the plant, which the cooler is connected to, is made of ferritic steel. The weld connection at the steam outlet pipe 23 of the cooler 22 is a dissimilar metal joint or weld of the kind explained above, and thus experiences reduced lifetime issues. One of the main factors leading to premature failure of such a dissimilar metal connection is the very high temperature gradient in the wall of the pipe during start-up of the plant. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a dissimilar metal connection for the use in an environment, especially in a combined cycle power plant, where it may be exposed to high temperature, high pressure loading, high cyclic (e.g. repeated use cycles that change load or temperature), and/or high transients (e.g. fast changes of temperature) with external forces and moments, which dissimilar metal connection avoids the disadvantages of the prior art connections, is easy to install, and has an increased lifetime.

It is another object of the present invention to disclose a method for operating such a dissimilar metal connection in order to reap all the advantages of the dissimilar metal connection according to the invention. These and other objects are obtained by a dissimilar metal connection according to Claim 1 and an operating method according to Claim 10.

The invention is directed to a dissimilar metal connection arrangement. In some embodiments, the dissimilar metal connection arrangement can be used in an environment, such as a combined cycle power plant, where it may be exposed to high temperature, high pressure loading, high cyclic, and/or high transients with external forces and moments. The dissimilar metal connection arrangement comprises a dissimilar metal connection between a first member made of a first metal and having a first end, and a second member made of a second metal different from said first metal with regard to its metallurgical behaviour, and having a second end, whereby the first end of the first member is fixedly connected to the second end of the second member .

It is characterized in that the dissimilar metal connection arrangement comprises a heating means arranged at said dissimilar metal connection for controlled heating of said dissimilar metal connection. An embodiment of the dissimilar metal connection arrangement according to the invention is characterized in that said first and second members are pipe sections of a pipe, through which a hot fluid flows when in use. Specifically, said pipe is a steam outlet pipe for an air cooler in a combined cycle power plant.

More specifically, one of said members is made from austenitic stainless steel, while the other of said members is made of ferritic steel.

Another embodiment of the dissimilar metal connection arrangement according to the invention is characterized in that said dissimilar metal connection comprises a welding joint. A further embodiment of the dissimilar metal connection arrangement according to the invention is characterized in that said dissimilar metal connection comprises a flanged connection with bolts.

Just another embodiment of the dissimilar metal connection arrangement according to the invention is characterized in that said heating means surrounds said dissimilar metal connection.

Specifically, said heating means is powered by an electric current. More specifically, said heating means comprises a heating blanket.

In an inventive method for operating a dissimilar metal connection arrangement according to the invention, said first and second members are heated from a first low starting temperature to a second high operating temperature, whereby said dissimilar metal connection is preheated by said heating means before and/or during the transition of said first and second members from said first to said second temperature. Even just preheating during the heating period can help avoid the dissimilar metal connection being subjected to short periods of rapid heating if periods of rapid heating occur during the heating of the first and second members.

An embodiment of the method according to the invention is characterized in that said dissimilar metal connection is preheated (preferably slowly) by said heating means from said first low starting temperature to a temperature close to said second high operating temperature, and that preferably said preheating begins before said transition of said first and second members from said first low starting temperature to said second high operating temperature starts.

Another embodiment of the method according to the invention is characterized in that the temperature of the first end of the first member and the temperature of the second end of the second member are each monitored by temperature sensing means.

Specifically, thermocouples are used as the temperature sensing means.

A further embodiment of the method according to the invention is characterized that said dissimilar metal connection arrangement is part of a combined cycle power plant having a power plant control unit and that said heating means is connected to said power plant control unit.

Specifically, the operation of said heating means is implemented into the operation logic of the combined cycle power plant, and that said heating means is switched on automatically, and then switched off automatically, as soon as the required preheating temperature is reached. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.

Fig. 1 shows a simplified scheme of a combined cycle power plant

(CCPP), where a dissimilar metal connection according to the invention may be advantageously used; Fig. 2 shows an embodiment of a dissimilar metal connection according to the invention in form of a welded pipe joint surrounded by a heating blanket in a general view (a) and in a more detailed view (b); Fig. 3 shows another embodiment of a dissimilar metal connection

according to the invention in form of a flanged pipe joint with bolts surrounded by a heating blanket; and

Fig. 4 shows control scheme for a pipe joint according to Fig. 2 being used in a power plant according to Fig. 1 .

DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS OF THE INVENTION

The general idea of the present invention is that stresses in a dissimilar metal connection or joint being a result of a high temperature gradient in a transient state can be easily reduced with application of a heating blanket or equivalent heating means.

When the joint is a welded pipe joint between an air cooler (22 in Fig. 1 ) and a water/steam cycle (12 in Fig. 1 ) in a combined cycle power plant CCPP (10 in Fig. 1 ), a heating blanket placed around said pipe joint would slowly heat up a weld seam in said joint to a temperature close to the operating temperature, before the steam generated in the cooler starts to flow inside of the pipe (steam outlet pipe 23 in Fig. 1 ).

A respective pipe joint configuration is shown in Fig. 2: The steam outlet pipe 23 comprises two pipe sections 23a and 23b made of a dissimilar metal and being joined at their ends by a welding joint 25 (welded joint or weld). The welding joint 25 and the adjacent end sections of the two pipe sections 23a and 23b are surrounded by an electrically powered heating blanket 26, which is in thermal contact with the pipe ends and the welding joint 25.

The heating blanket 26 must be connected with the control room of the power plant 10 and its operation implemented into the operation logic of the plant, as shown in Fig. 4, where the heating blanket 26 is connected to a blanket power supply 33, which is controlled by a temperature control circuit 34 being in connection with a power plant control unit 35. This way the heating blanket 26 would be switched on automatically, without any operator engagement. The heating blanket 26 would be fitted with thermocouples 27, 28 on both pipe ends (see Fig. 2(b) and Fig. 4), which are connected to the temperature control unit 34 via a temperature measuring circuit 32, for control of the temperature of the weld seam 25. As soon as the required temperature is reached, the heating blanket 26 would be switched off automatically. The concept of pre-warming or preheating of the dissimilar metal connection can be also used for a flanged joint or flanged connection 30 between pipe section 29a and 29b of a dual material pipe 29, as shown in Fig. 3, which may be used as an alternative to a weld seam. In a flanged joint 30 the biggest problem is possible overloading of the bolts 31 and flanges 30a, b during start-up of the plant as a result of uneven heating of the flanges 30a, b and the bolting. Bolts 31 being not subjected to the steam flow heat up much slower and suffer very high stresses being stretched by the already hot flanges 30a, b. By using of a heating blanket 26', the bolts 31 would be uniformly heated up together with the flanges 30a, b before the steam is admitted to the pipeline.

LIST OF REFERENCE NUMERALS

10 combined cycle power plant (CCPP)

1 1 gas turbine (GT)

12 water/steam cycle

13 air inlet

14 compressor

15 combustor

16 fuel

17 turbine

18 exhaust gas

19 heat recovery steam generator (HRSG)

20 flue gas

21 steam turbine

22 air cooler (e.g. OTC)

23 steam outlet pipe

23a,b pipe section

24 water inlet pipe

25 welding joint

26,26' heating blanket

27,28 thermocouple

29 dual material pipe

29a,b pipe section

30 flanged connection

30a,b flange

31 bolt

32 temperature measuring circuit

33 blanket power supply

34 temperature control circuit

35 power plant control unit