GB2106321A

GB2106321A - Mounting thermocouple junction

Info

Publication number: GB2106321A
Application number: GB08227370A
Authority: GB
Inventors: Peter Martin Godfrey
Original assignee: Babcock Power Ltd
Current assignee: Doosan Babcock Ltd
Priority date: 1981-09-25
Filing date: 1982-09-24
Publication date: 1983-04-07

Abstract

To connect a thermocouple junction to a metal member, especially a metal tube that is to be used in rigorous conditions, the junction is embodied in a pad (2) that is connected to the tube (1) by a D.C. welding process, in particular by capacitor discharge welding. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the connection of a thermocouple junction to a metal member This invention is concerned with the connection of the hot junction of a thermocouple to a metal member of which the temperature is to be measured and arose from a particular consideration of the problems met in seeking to connect a thermocouple to a tube when the tube is part of a helical heat exchanger used in a nuclear boiler plant.

The geometry of such heat exchanges can make it difficult to form an adequate attachment of a thermocouple to a tube of the heat exchanger in the first place and that, and the conditions of use, may make it impossible to replace the thermocouple during the working life of the heat exchanger. It may, nevertheless, be desirable to be able to monitor the performance of the heat exchanger throughout its working life and this emphasises the value of being able to effect a reliable attachment in the first place. Further complications can be presented by the facts that the conditions existing during the connection of the thermocouple junction to the tube should not damage the junction and that the tube to which the thermocouple junction is to be connected may be thin-walled.When the tube is thin-walled then, generally, the dimensions of the thermocouple should be small with the result that they may be less able to withstand onerous conditions whilst the connections is being effected.

According to the present invention, in a method of connecting a thermocouple junction to a metal member, the thermocouple junction is embodied in a metal pad and the metal pad is connected to the metal member by a D.C. welding process.

The invention also includes a metal tube having a thermocouple junction connected to the metal tube, the junction being embodied in a metal pad and the pad having been welded to the tube by a D.C. welding process.

By way of example, the connection of a thermocouple to a tube embodying the invention will now be described with reference to the accompanying diagram.

In the diagram, there is indicated at 1 the cross-section of part of a helical 9%Cr~1 %Mb tube to be included in the heat exchanger of a nuclear reactor plant. The tube is intended to lie within an outer tube. At 2 there is indicated a pad of Inconel steel in which a Nickel Chromium v Nickel Aluminium thermocouple has been embodied by the use of a plasma flame. The pad may typically be Smmx3.Smmx 1 2mm. The use of an electron beam method is to be considered as an alternative. The pad 2 is held with a light pressure against the tube 1 by the upper electrode 3 and the lower electrode 4. The operative surface, at least, of the electrodes are of cupro-nickel to prevent their adhesion during the D.C. resistance welding process that will now be described.

The electrodes 3 and 4 are connected in parallel with a capacitor bank 5 across a supply 6.

A control system 8 is included to determine the level, in the order of 2000V, to which the capacitor bank 5 is charged and a pulse control system 9 is effective to permit the discharge of the capacitor bank 5 only when the bank is appropriately charged and the physical pressure existing between the electrodes 3 and 4 is appropriate. The system 9 ensures that the discharge (which may be between 1000 and 5000 joules) is effected at a low voltage but with a high current.

When the desired conditions have been achieved, the system 9 is operative to effect discharge of the capacitor bank 5 across the pad 2 and tube 1 to weld the two together. As a result of the use of a high current for a short period, the pad 2 will be bonded to the tube 1 with little risk of either becoming overheated. Since the period of discharge is comparatively short and the energy control reliable, there is small risk of overheating.

In so far as the metal of either component becomes molten, the capacitor discharge is set to bring the surfaces to a temperature just sufficient to ensure adequate fusion of the surfaces. By the avoidance of excessive melting, grain growth in either the tube 1 or pad 2, which (by causing porosity and so allowing the ingress of moisture) is a cause of trouble with other welding processes that have been proposed, can be avoided. At the same time, the pad is not subjected to temperature conditions that would damage the thermocouple junction.

The surface of the pad 2 that is welded to the tube 1 may initially be flat or curved better to conform with the tube 1. whilst the thermocouple may be embodied in the pad 2 before the D.C.

welding process is carried out, it may initially simply fit within the pad, and the D.C. welding process may be relied upon to effect the embodiment of the thermocouple within the pad.

Claims

1. In a method of connecting a thermocouple junction to a metal member, the thermocouple junction is embodied in a metal pad and the metal pad is connected to the metal member by a D.C.

welding process.

2. A method as claimed in claim 1 in which the metal member is a metal tube.

3. A method as claimed in either of claims 1 and 2 in which the thermocouple junction initially lies within a metal pad and the embodiment of the thermocouple junction in the metal pad is effected by the D.C. welding process.

4. A method as claimed in any of claims 1 to 3 in which the welding process includes holding the pad to the tube under pressure.

5. A method as claimed in any of claims 1 to 4 in which the circuit for the welding process is provided by the discharge of a capacitor through 2000V.

6. A metal tube having a thermocouple

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in or relating to the connection of a thermocouple junction to a metal member This invention is concerned with the connection of the hot junction of a thermocouple to a metal member of which the temperature is to be measured and arose from a particular consideration of the problems met in seeking to connect a thermocouple to a tube when the tube is part of a helical heat exchanger used in a nuclear boiler plant. The geometry of such heat exchanges can make it difficult to form an adequate attachment of a thermocouple to a tube of the heat exchanger in the first place and that, and the conditions of use, may make it impossible to replace the thermocouple during the working life of the heat exchanger. It may, nevertheless, be desirable to be able to monitor the performance of the heat exchanger throughout its working life and this emphasises the value of being able to effect a reliable attachment in the first place. Further complications can be presented by the facts that the conditions existing during the connection of the thermocouple junction to the tube should not damage the junction and that the tube to which the thermocouple junction is to be connected may be thin-walled.When the tube is thin-walled then, generally, the dimensions of the thermocouple should be small with the result that they may be less able to withstand onerous conditions whilst the connections is being effected. According to the present invention, in a method of connecting a thermocouple junction to a metal member, the thermocouple junction is embodied in a metal pad and the metal pad is connected to the metal member by a D.C. welding process. The invention also includes a metal tube having a thermocouple junction connected to the metal tube, the junction being embodied in a metal pad and the pad having been welded to the tube by a D.C. welding process. By way of example, the connection of a thermocouple to a tube embodying the invention will now be described with reference to the accompanying diagram. In the diagram, there is indicated at 1 the cross-section of part of a helical 9%Cr~1 %Mb tube to be included in the heat exchanger of a nuclear reactor plant. The tube is intended to lie within an outer tube. At 2 there is indicated a pad of Inconel steel in which a Nickel Chromium v Nickel Aluminium thermocouple has been embodied by the use of a plasma flame. The pad may typically be Smmx3.Smmx 1 2mm. The use of an electron beam method is to be considered as an alternative. The pad 2 is held with a light pressure against the tube 1 by the upper electrode 3 and the lower electrode 4. The operative surface, at least, of the electrodes are of cupro-nickel to prevent their adhesion during the D.C. resistance welding process that will now be described. The electrodes 3 and 4 are connected in parallel with a capacitor bank 5 across a supply 6. A control system 8 is included to determine the level, in the order of 2000V, to which the capacitor bank 5 is charged and a pulse control system 9 is effective to permit the discharge of the capacitor bank 5 only when the bank is appropriately charged and the physical pressure existing between the electrodes 3 and 4 is appropriate. The system 9 ensures that the discharge (which may be between 1000 and 5000 joules) is effected at a low voltage but with a high current. When the desired conditions have been achieved, the system 9 is operative to effect discharge of the capacitor bank 5 across the pad 2 and tube 1 to weld the two together. As a result of the use of a high current for a short period, the pad 2 will be bonded to the tube 1 with little risk of either becoming overheated. Since the period of discharge is comparatively short and the energy control reliable, there is small risk of overheating. In so far as the metal of either component becomes molten, the capacitor discharge is set to bring the surfaces to a temperature just sufficient to ensure adequate fusion of the surfaces. By the avoidance of excessive melting, grain growth in either the tube 1 or pad 2, which (by causing porosity and so allowing the ingress of moisture) is a cause of trouble with other welding processes that have been proposed, can be avoided. At the same time, the pad is not subjected to temperature conditions that would damage the thermocouple junction. The surface of the pad 2 that is welded to the tube 1 may initially be flat or curved better to conform with the tube 1. whilst the thermocouple may be embodied in the pad 2 before the D.C. welding process is carried out, it may initially simply fit within the pad, and the D.C. welding process may be relied upon to effect the embodiment of the thermocouple within the pad. Claims

welding process.

2. A method as claimed in claim 1 in which the metal member is a metal tube.

6. A metal tube having a thermocouple junction connected to the metal tube, the junction being embodied in a metal pad and the pad having been welded to the tube by a D.C. welding process.

7. What is claimed in claim 2, any of claims 3 to 5 when appendent to claim 2, or claim 6 in which the tube is curved.

8. What is claimed in claim 2, any of claims 3 to 5 when appendent to claim 2, or claim 6 or 7 in which the tube is a helical heat exchanger tube.

9. What is claimed in claim 8 in which the tube lies within another helical heat exchanger tube.

10. A method of connecting a thermocouple junction to a tube substantially as described with reference to the accompanying drawing.