DISSIMILAR METAL CONNECTION AND METHOD FOR OPERATING SUCH A DISSIMILAR METAL CONNECTION

Abstract

A dissimilar metal connection arrangement comprising 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 behavior, and having a second end, wherein the first end of the first member is fixedly connected to the second end of the second member. An increased lifetime of the connection is achieved by arranging a heating means at said dissimilar metal connection for controlled heating of said dissimilar metal connection.

Claims

1. A dissimilar metal connection arrangement comprising: a first member made of a first metal; a second member made of a second metal different from the first metal at least with regard to its metallurgical behavior; a dissimilar metal connection between the first member and the second member; and a heat source arranged at the dissimilar metal connection and configured to control heating of the dissimilar metal connection.

2. A dissimilar connection arrangement according to claim 1, wherein the first member and second member are sections of a pipe, through which a hot fluid flows when in use.

3. A dissimilar metal connection arrangement according to claim 2, wherein the pipe is a steam outlet pipe of for an air cooler in for a combined cycle power plant.

4. A dissimilar metal connection arrangement according to claim 3, wherein one of the first members and the second member is made from austenitic stainless steel, while the other of the first members and the second member is made of ferritic steel.

5. A dissimilar metal connection arrangement according to claim 2, wherein the dissimilar metal connection comprises a welding joint.

6. A dissimilar metal connection arrangement according to claim 1, wherein the heat source surrounds said dissimilar metal connection.

7. A dissimilar metal connection arrangement according to claim 7, wherein the heat source is powered by an electric current.

8. A dissimilar metal connection arrangement according to claim 8, wherein the heat source comprises a heating blanket.

9. A dissimilar metal connection arrangement according to claim 1, wherein the dissimilar metal connection arrangement is located in a combined cycle power plant.

10. A method for operating a dissimilar metal connection comprising: providing a dissimilar metal connection between a first member and a second member, wherein the first member is made of a first metal and the second member is made of a second metal different from the first metal at least with regard to its metallurgical behavior; providing a heat source arranged at the dissimilar metal connection; heating the first member and the second member from a first low starting temperature to a second high operating temperature; and preheating the dissimilar metal connection using the heating source before and/or during a transition of the first member and the second member from the first low starting temperature to the second high operating temperature.

11. A method according to claim 10. wherein the preheating by the heat source is from said first low starting temperature to a temperature close to said second high operating temperature, and the preheating begins before the transition of said first and second members from said first low starting temperature to said second high operating temperature occurs.

12. A method according to claim 10, wherein the temperature of a first end of the first member and the temperature of a second end of the second member are monitored by a temperature sensor.

13. A method according to claim 12, further comprising sensing the temperature using thermocouples.

14. A method according to claim 10, wherein the dissimilar metal connection arrangement is part of a combined cycle power plant having a power plant control unit, and the heat source is connected to the power plant control unit.

15. A method according to claim 10, wherein the heating using the heat source is implemented into the operation logic of the combined cycle power plant, and the heat source is switched on automatically, and then switched off automatically, as soon as the required preheating temperature is reached.

16. A dissimilar metal connection arrangement according to claim 2, wherein the dissimilar metal connection comprises a flanged connection with bolts.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.

[0028] FIG. 1 shows a simplified scheme of a combined cycle power plant (CCPP), where a dissimilar metal connection is implemented;

[0029] FIG. 2 shows an embodiment of a dissimilar metal connection in form of a welded pipe joint surrounded by a heating blanket in a general view (a) and in a more detailed view (b);

[0030] 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

[0031] FIG. 4 shows a control scheme for a pipe joint according to FIG. 2 being used in a power plant according to FIG. 1.

DETAILED DESCRIPTION

[0032] Stresses in a dissimilar metal connection or joint resulting from of a high temperature gradient in a transient state can be easily reduced with application of a heating blanket or equivalent heating means.

[0033] 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).

[0034] 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.

[0035] The heating blanket 26 must be connected with a 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 can be switched on automatically without any operator engagement. The heating blanket 26 is 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, to control the temperature of the weld seam 25. As soon as the required temperature is reached, the heating blanket 26 can be switched off automatically.

[0036] Pre-warming or preheating of the dissimilar metal connection can also 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, one of the biggest problems is overloading the bolts 31 and flanges 30a,b during start-up of the plant caused by uneven heating of the flanges 30a,b and the bolting. Bolts 31 which are not subjected to the steam flow heat up much slower and suffer very high stresses by being stretched by the already hot flanges 30a,b. By using a heating blanket 26′, the bolts 31 are uniformly heated up together with the flanges 30a,b before the steam is admitted to the pipeline.