WO2004085981A1

WO2004085981A1 - Thermowell and a method of measuring the temperature of a pipe

Info

Publication number: WO2004085981A1
Application number: PCT/EP2003/003268
Authority: WO
Inventors: Emil Edwin; Tore Arnesen; Kim Reidar HØVIK
Original assignee: Borealis Technology Oy
Priority date: 2003-03-28
Filing date: 2003-03-28
Publication date: 2004-10-07
Also published as: AU2003212383A1

Abstract

A thermowell for use with thermocouples and a method of measuring the temperature of a pipe using a thermowell. The thermowell comprising a solid cylinder with an axially extending cavity for receiving a thermocouple. The cavity being of the cylinder towards the surface of the thermowell which is to be attached to the pipe.

Description

THERMOWELL AND A METHOD OP MEASURING THE TEMPERATURE OF A PIPE

The present invention relates to thermowells for use with thermocouples, in particular for taking gas temperature measurements in relation to petroleum crackers .

In a petroleum cracker, large hydrocarbon molecules such as ethane and propane from natural gas, or heavier liquids such as naphtha and gas oil from petroleum are split into smaller molecules. This is often done to provide olefins such as ethylene that are useful in themselves, or may be used in polymerisation processes.

In the case of ethane and propane, the gas is heated to above about 1500°P at which point bonds within the molecule break, producing a range of smaller molecules. The desired products are then separated out. The same principle applies when cracking heavier substances, but since the molecules are much larger, a far greater range of smaller molecules is provided.

In a typical ethane cracker plant, the cracking takes place in a pyrolysis section. Here, ethane is pumped through a maze of 4-6 inch diameter metallic tubes located within a furnace where it is heated up to about 1500°F and cracks.

Ethane is pumped through the pyrolysis section at a very high rate. The residence time of any individual molecule is a few seconds or less in older plants and less than a one tenth of a second in more modern plants. It is important that the flow rate is kept this high in order to prevent the gas from overheating, which would lead to the cracking process running away. If it were to do so, the ethane would crack not into the desired products, but into methane or even carbon (coke) and hydrogen. A further measure that is taken to control the possibility of runaway is the mixing of steam with the ethane before it is fed to the furnaces . This lowers the temperature necessary for the cracking to take place.

It is important to monitor and control the temperature of the gas during the cracking process in order to ensure that optimum results are obtained. For reasons of practicality, the temperature may be read from pipes leading from the furnace, rather than from those actually inside it.

Thermocouples are commonly used for measuring at high temperatures . Thermocouples work by exploiting the Seebeck effect,. When two different metals are placed in contact with each other the voltage generated is dependent upon the temperature at the junction. By placing one junction near the area to be monitored (the ^' hot junction) and creating another reference junction (the cold junction) , which is kept at a constant temperature, the potential difference between the two junctions can be obtained and from this the temperature difference is deduced. In order to measure the temperature of liquids and . gases flowing through pipes, both internal and external thermocouple sensors are known.

Internal thermocouple assemblies penetrate the pipe to bring the junction close to the substance to be measured. However, the need to provide an opening in the pipe is undesirable as it weakens the pipe and can lead to corrosion and damage. Also, the thermocouple itself corrodes more quickly and therefore requires frequent replacement. Alternatively, external thermocouples can be used. One such thermocouple can be seen in US Patent No. 4,338,479. Two wires of dissimilar metal extend axially through the centre of a metal sheath. The wires are electrically insulated from each other and the sheath by a suitable insulation material. At the sensing end, the insulation is stripped away and the wires are bent and welded to the inside wall of the sheath to create the hot junction. Once the junction has been formed, it is surrounded by insulation material and sealed with a preformed segment of solid metal. This preformed segment is then welded directly onto the pipe which contains the substance of interest. In this way, a short direct heat path is created between the thermocouple junction and the pipe surface.

There are disadvantages with this type of thermocouple however. As the thermocouple is directly welded to the pipe, it is not easy to connect and replace. Also the thermocouple is exposed to the external environment which may lead to corrosion.

To overcome these problems, the thermocouple may be located in a sheath-shield pipe or thermowell. This is a steel member such as a cylinder that is welded to the outside of the pipe. A central bore is provided within the thermowell into which the thermocouple may be inserted. Heat is conducted to the thermocouple via the pipe wall and the body of the thermowell . A disadvantage of this arrangement is that due to the extra distance between the pipe and the thermocouple the measurements are not as accurate. Also, dirt such as oxidised metal tends to gather in the tube, which further distorts the readings by reducing the heat conduction to the thermowell . These problems can lead to an error margin of as much as 100°C in some instances.

According to one aspect of the present invention there is provided a thermowell for attachment to a pipe comprising a solid cylinder with an axially extending cavity for receiving a thermocouple, the cavity being offset from the axis of the cylinder towards the surface of the thermowell which is to be attached to the pipe.

The cylinder of the thermowell can have a base of any shape, e.g. hexagonal, triangular etc, but preferably it is circular.

The asymmetrical positioning of the cavity allows the thermocouple to be placed nearer to the surface to be measured and thus creates a shorter heat path between the two. Consequently, the present invention provides. a thermowell which allows a greater accuracy of temperature measurement. Preferably, the cavity is positioned proximate to the exterior surface of the cylinder in order to give a short heat path between the thermocouple and the measured surface. To create a sufficiently short heat path, the cavity is preferably positioned 5mm or less, for example less than 3mm, from the exterior of the cylinder.

However, in order to ensure that the thermowell wall is strong enough to prevent breaching of the cavity during attachment and use of the thermowell it is preferable that the cavity is positioned 0.5mm or more from the exterior of the cylinder.

Therefore, in order to accommodate the need for a short heat path and also to provide a thermowell with sufficient strength, it is preferable that the cavity is positioned between 0.5 and 5mm and most preferably 1mm from the exterior.

It is preferable that the diameter of the cavity is just slightly larger than that of the thermocouple so that the thermocouple can be inserted but such that there are no air gaps to impede heat conduction to the thermocouple.

The cavity and thermocouple can be arranged to provide an interference fit for the thermocouple, in which case no further attachment may be required. However, this may be difficult to achieve and so preferably the thermocouple is securely welded into position.

In either case, there is no need for any further fixing means. However, preferably the closed end of the cavity creates a stop for the thermocouple head to abut against. This ensures the correct positioning of the thermocouple within the thermowell. A biasing force can be applied to push the thermocouple against the stop and thus aid its retention within the thermowell . In this instance only a small amount of weld (if any), e.g. a spot weld, needs to be applied to the thermocouple to secure it and also to prevent dirt entering the cavity. Alternatively, the weld itself can be used as the sole means of fixing the thermowell in place in which case a larger weld may be necessary. Preferably the thermocouple sheath is welded to the cylinder end by the cavity entrance.

When the above-described preferred forms of the invention are provided an error due to heat transmission loss of less than 5% can be achieved.

In order to give adequate protection to the thermocouple head, and to keep it securely retained in the cavity, it is preferable that the cavity extends along between 60 and 90% of the length of the cylinder. Most preferably it extends along approximately 80% of the length of the cylinder. Thus, in a typical embodiment of the invention, the cavity length is preferably between 30mm and 55mm in length.

Although it may be clamped or otherwise held in place, the cylinder of the thermowell is preferably welded onto the pipe surface at the desired measuring point. This would normally be done prior to the insertion of the thermocouple . Preferably the welding is thick and continuous along the length of the cylinder. Multiple thin beads of weld are applied to build up a thick weld. A thick weld is desirable as it increases the thermal conduction between the pipe and the thermowell. Typically the completed weld extends up and around the thermowell to between one third and two thirds of the thermowell ' s height .

Preferably the thermocouple sheath extends along and in contact with the surface to be measured prior to entering the cavity. This ensures that the thermocouple sheath is kept at the same temperature as the thermocouple sensor in order to prevent heat conduction along the wires away from the junction, which would lower the thermocouple reading. Preferably the sheath is in contact with the measuring surface for between 5 and 300mm, most preferably around 150mm.

Where the thermowell is applied to a pipe that is not subjected to external heating, such as the outlet pipe from a cracker furnace, preferably the measuring surface is insulated and this insulation also covers the cylinder. This reduces the effect of the external temperature on, the thermocouple .

As mentioned above, it has been recognised that a further problem with thermowells is that dirt often collects within them. This can result in poorer heat conduction and therefore distorts the temperature readings .

Preferably therefore, the thermowell also comprises a blow hole which connects the base of the cavity to the exterior of the cylinder. This creates a thermowell that can be easily cleaned to remove dirt.

By blowing air into the blow hole dirt which has ^•gathered in the cavity is dislodged, thus preventing the thermocouple's readings from being distorted. While the blow hole can have any orientation to the cavity it is preferable that it extends axially from the cavity base and is preferably coaxial with the cavity. This simplifies its manufacture and gives the best angle for dislodging dirt. Preferably the blow hole's diameter is less than that of the thermocouple in order to prevent the thermocouple from slipping into it. Also, a blow hole of this diameter would expose the thermocouple end unnecessarily to the external environment. Preferably the blow hole is between 0.5 and 4 mm in diameter and has a length of between 2 and 15cm. Viewed from another aspect, the invention provides a thermowell comprising a solid cylinder with an axially extending cavity for receiving a thermocouple and a blow hole which connects the base of the cavity to the exterior of said cylinder.

While it is preferable to provide a cylindrical thermowell, it is possible for other shapes of thermowell to be used, such as asymmetrical shapes.

Therefore, viewed from another aspect the invention provides a thermowell for attachment to a pipe comprising a solid member with an axially extending cavity for receiving a thermocouple, wherein the cavity is offset from the centre of the solid member towards the surface of the thermowell which is to be attached to the pipe. Preferably the cavity is positioned between 0.5 and 5mm from the surface of the thermowell which is to be attached to the pipe .

Preferably the surface of the solid member for attachment to the pipe is undercut to allow space for weld to be inserted to create a strong, thermally conductive bond between the thermocouple and the pipe.

The invention also extends to a method of measuring the temperature of a pipe using a thermowell according to the aspects of the invention hereinbefore described, wherein the method comprises the steps of fixing the thermowell to the exterior of the pipe and inserting the thermocouple into the cavity.

A preferred embodiment of the invention is now described, by way of example only, with reference to the accompanying drawings, in which:

FIG 1 shows a thermowell according to an embodiment of the present invention in use against a cracker pipe,- FIG 2 shows a cross-section along the line A-A of FIG 1; and FIG 3 shows a cross-section along the line Al-Al of FIG 1. FIG 1 shows an outlet pipe 1 such as would be used in a cracker furnace. The outlet pipe carries the cracked gases to a fractionation column to allow the different gases to be separated. Prior to this step the heated gas is often passed through a heat exchanger in order to cool the gas .

As gas is pumped through the pipe 1 prior to reaching the heat- exchanger, the pipe's temperature is measured by a thermocouple 2 to give an indication of the temperature of the gas flowing thought it . The temperature is, monitored in order to control the cracking taking place in the furnace .

The thermocouple 2 consists of two wires (not shown) encased in a metallic sheath 2c. The wires are connected together to form a junction at the sensing end 2a of the thermocouple and also at the reference end 2b. The reference end 2b is kept at a constant temperature . The thermocouple sensing end 2a is inserted into a thermowell 3 which is welded onto pipe 1. The pipe 1 and the thermowell 3 are both enclosed in suitable insulation material 10.

The thermowell can be seen in more detail in FIG 2. The thermowell comprises a solid metal cylinder with a cavity 4 which extends axially through the cylinder. As can be seen in FIG 3 this cavity 4 is offset from the centre of the cylinder so that it lies close to the cylinder's exterior surface. The thermocouple 2 is inserted into this cavity until the sensing end 2a abuts the base of the cavity 4a. When the thermowell 3 is welded onto the outside of pipe 1, it is positioned so that the cavity 4 lies flush against the pipe 1. In this way, a short heat path is provided between the thermocouple sensing end 2a and the pipe 1. This increases the accuracy of the measurements over those of standard thermowells.

The weld 9 provided extends along the length of the thermowell 3 and fills the air gap between the pipe 1 and the thermowell 3. The welding forms a wedge on either side of the thermowell 3 which reaches half way up the thermowell ' s exterior. Therefore, the half of the thermowell 3 containing the cavity 4 is completely encased in weld 9.

The weld 9 creates a good heat path between the thermowell 3 and the pipe 1, providing good thermal conduction to the thermocouple sensing end 2a.

In order to prevent thermal conduction from the thermocouple sensing end 2a along the wires due to a temperature gradient, the sheath 2c extends along the pipe 1 for approximately 150mm prior to entering the thermowell 3. After this distance the sheath 2c bends through the insulation 10 and is connected to a terminal head 12 where the cold junction 2b is formed.

Once the thermocouple 2 has been inserted into the thermowell 3 it is welded into position. Weld 19 is applied to the sheath 2c at the point where it enters the thermowell 3. Only a small amount of weld is needed as the snug fit between the cavity 4 and the thermocouple 2 reduces the likelihood of the thermocouple 2 working lose. Also, a small force can be applied to the thermocouple to bias the sensing end 2a towards the cavity base 3a. Only having a small fixing weld 19 between the thermocouple 2 and the thermowell 3 is advantageous as, when necessary it is easier to remove the thermocouple 2. This allows the thermocouple 2 to be replaced quickly and simply as compared to the situation when the thermocouple is welded directly to the pipe 1.

During the operational life of the thermowell 3, dirt such as oxidised metal gathers at the cavity base 4a and can distort the thermocouple readings .

In order to prevent this a blow hole 5 is created which connects the cavity base 4a with the exterior of the thermowell 3. This provides a passage through which air can be blown to dislodge any dirt which has collected at the cavity base 4a.

This procedure can be done each time the thermocouple 2 is replaced, i.e. after the old thermocouple has been removed and prior to the insertion of the new one .

The blow hole 5 has a diameter of approximately half that of the cavity 4 and is coaxial to the cavity 4. This allows a sufficient volume of air through the blow hole while also keeping a stop upon which the thermocouple 2, can rest.

Thus it can be seen that the new thermocouple protection tube of the present invention at least in its preferred forms provides a more accurate means of measuring the temperature of a surface; allows thermocouples to be removed and replaced quickly and simply without the need for extensive welding; and provides the means to keep the thermocouple reading from being distorted due to dust build up.

Claims

1. A thermowell (3) for attachment to a pipe (1) comprising a solid cylinder with an axially extending cavity (4) for receiving a thermocouple (2), the cavity being offset from the axis of the cylinder towards the surface of the thermowell which is to be attached to the pipe .

2. A thermowell as claimed in claim 1, wherein the cylinder base is circular.

3. A thermowell as claimed in claim 1 or 2 , wherein the cavity (4) is positioned proximate to the exterior surface of the cylinder.

4. A thermowell as claimed in claim 3 , wherein the cavity (4) is positioned between 0.5 and 5mm from the exterior of the cylinder.

5. A thermowell as claimed in claim 4, wherein the cavity (4) is positioned 1mm from the exterior of the cylinder.

6. A thermowell as claimed in any preceding claim, wherein the diameter of the cavity (4) is just slightly larger than that of the thermocouple (2) .

7. A thermowell as claimed in claim 6 wherein the cavity (4) and thermocouple (2) are arranged to provide an interference fit for the thermocouple.

8. A thermowell as claimed in any preceding claim, wherein the closed end of the cavity (4) creates a stop (3a) for the thermocouple head to abut against.

9. A thermowell as claimed in any preceding claim, wherein the cavity (4) extends along between 60 and 90% of the length of the cylinder.

10. A thermowell as claimed in claim 9, wherein the cavity (4) extends along approximately 80% of the length of the cylinder.

11. A thermowell as claimed in any preceding claim, wherein the cavity length is between 30mm and

55mm.

12. A thermowell (3) as claimed in any preceding claim, further comprising a blow hole (5) , which connects the base of the cavity (4) to the exterior of said cylinder.

13. A thermowell as claimed in claim 12, wherein the blow hole (5) is coaxial with the cavity (4) .

14. A thermowell as claimed in claim 12 or 13, wherein the blow hole's diameter is less than that of the thermocouple (2) .

15. A thermowell as claimed in claim 14 wherein the blow hole (5) is between 0.5 and 4 mm in diameter.

16. A thermowell as claimed in any of claims 12 to 15 wherein the blow hole (5) has a length of between 2 and 15cm.

17. A thermowell comprising a solid cylinder with an axially extending cavity for receiving a thermocouple (2) and a blow hole (5) which connects the base of the cavity to the exterior of said cylinder.

18. A thermowell as claimed in claim 17, wherein the cavity (4) is offset from the axis of the cylinder.

19. A thermowell for attachment to a pipe (1) comprising a solid member with an axially extending cavity (4) for receiving a thermocouple wherein the cavity is positioned between 0.5 and 5mm from the exterior surface of the thermowell which is to be attached to the pipe.

20. A thermowell as claimed in claim 19 wherein the cavity is positioned 1mm from the exterior surface of the thermowell .

21. A thermowell as claimed in claim 19 or 20 wherein the surface of the thermowell for attachment to the pipe is undercut .

22. A method of measuring the temperature of a pipe (1) using a thermowell (3) as claimed in any preceding claim comprising the steps of; fixing said thermowell to the exterior of said pipe (1) ; and inserting a thermocouple into the cavity (4) .

23. A method as claimed in claim 22, further comprising the step of extending the thermocouple along and in contact with the pipe prior to inserting it into the cavity.

24. A method as claimed in claim 23, wherein the thermocouple (2) is in contact with the pipe for between 5 and 300mm.

25. The method as claimed in claims 21, 22 and 23 wherein the thermowell (3) is welded onto said pipe exterior.

26. The method as claimed in 25 wherein the welding is thick and continuous along the length of the cylinder.

27. The method as claimed in any of claims 22 to

26, further comprising the step of securely welding said thermocouple into position.

28. The method as claimed in any of claims 22 to

27, further comprising the step of insulating the pipe and thermowell .

29. The method as claimed in any of claims 22 to 28, wherein the measured temperature is used to control an olefin production process.

30. A thermowell as claimed in any of claims 1 to 21 for use in an olefin production plant.