EP3598845A1

EP3598845A1 - Trace heating system for a fluid flow machine

Info

Publication number: EP3598845A1
Application number: EP18186484.4A
Authority: EP
Inventors: Alexander GOSTOMELSKY; Yevgen Kostenko; Karl Funken
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2018-07-19
Filing date: 2018-07-31
Publication date: 2020-01-22

Abstract

The invention relates to a trace heating system (10) for a fluid flow machine (2), the fluid flow machine (2) having an outer casing (4) and an inner casing (6), comprising: at least one mineral insulated cable (12, 14) extending inside the outer casing (4), at least cable without mineral insulation (16), the cable extending at least partially outside the outer casing (4) and a joint (20) between the mineral insulated cable (12, 14) and the cable without mineral insulation (16). The joint (20) is designed as high temperature resistant electrical joint (20) by sealing a welding joint (22) using insulating ceramic material. The invention enables the use of a more flexible cable without mineral insulation (16), which is led out of the outer casing (4).

Description

The invention relates to a trace heating system for a fluid flow machine. The invention further relates to a fluid flow machine with a trace heating system as well as to a method for connecting cables inside an outer casing of a fluid flow machine.
One of the methods to improve efficiency of gas or steam power plants is to achieve better thermal match between the rotating and the stationary components using a trace heating system. When such electrical trace heating system is installed e.g. on the compressor vane carrier (CVC) to actively control the temperature of the compressor vane carrier, several technical challenges must be overcome. Some of these difficulties are related to realizing of the electrical wiring, in particular the transition between mineral insulated (MI) cables and cables without such insulation.
As part of an electrical trace heating system, a first mineral insulated cable is mounted at the inner casing and heats it up. The temperature increase is based on the physical principal of electrical current through electrical resistance, i.e. the mineral insulated cable. Mineral insulation coating is used in the first place to insulate the electrical conductor (wire) and besides to transfer the heat energy coming from the electrical conductor to the inner casing to increase its temperature. Usually, the electrical trace heating system also comprises a second mineral insulated cable with an identical structure to the first mineral insulated cable, but with a larger core diameter, and consequently of less electrical resistance. After the second mineral insulated cable is led out from outer casing, there is a transition to conventional electrical wires without mineral insulation which do not require operating under high temperatures as it is the case inside a compressor, a gas turbine or a steam turbine. This transition between the second mineral insulated cable and the conventional cable is realized by a hermetic sealing which insulates the mineral insulated cable core from outside. Since the first mineral insulated cable is very good attached to inner casing, the transition between the inner and the outer casing is the weakest link and is more likely to be damaged, e.g. due to relative motion between the casings, vibrations, etc.
The disadvantage of this design is that the conventional potted seals cannot operate under high temperatures and can only be used outside the casings. This leads to the necessity to use the second mineral insulated cable with a larger diameter for transition from the inner casing to outside the outer casing. This cable is relatively thick and inflexible, so that there is a high risk of damage.
The potential damage of the mineral insulated cable at the transition area between the inner casing (i.e. the compressor vane carrier) and the outer casing of a gas turbine or a steam turbine was identified as one of the main concerns when using electrical heating systems.
The object of the present invention is, therefore, to enable the use of a more flexible cable without mineral insulation, which is connected to a mineral insulated cable inside a fluid flow machine and is then led out of the outer casing.
The object of the invention is achieved by the independent claims 1 and 9. The dependent claims describe advantageous developments and modifications of the invention.
In accordance with the invention there is provided a trace heating system for a fluid flow machine, the fluid flow machine having an outer casing and an inner casing, comprising: at least one mineral insulated cable extending inside the outer casing, at least one cable without mineral insulation, the cable extending at least partially outside the outer casing, and a joint between the mineral insulated cable and the cable without mineral insulation, wherein the second joint is designed as high temperature resistant electrical joint by sealing a welding joint using insulating ceramic material.
In accordance with the invention there is also provided a fluid flow machine comprising an outer casing and an inner casing, further comprising such trace heating system.
And last but not least, in accordance with the invention there is also provided a method for connecting cables inside an outer casing of a fluid flow machine using insulating ceramic material to produce a high temperature resistant electrical joint between a mineral insulated cable extending inside the outer casing and a cable without mineral insulation extending at least party outside the outer casing.
The advantageous features of the trace heating system, the fluid flow machine and the method for connecting cables can generally be combined.
A fluid flow machine within the meaning of the present invention has an outer casing, an inner casing (when the fluid flow machine is a compressor, the inner casing is the compressor vane carrier) and a gap between the outer casing and the inner casing. The function of the trace heating system is to actively control the temperature of the inner casing and of the rotor. For this purpose, at least one mineral insulated cable is mounted on the inner casing. The mineral insulated cable extends only inside the outer casing. This mineral insulated cable is connected to a cable without mineral insulation, wherein the cable without mineral insulation extends partly inside the outer casing and partly outside the outer casing.
The essential idea of the present invention is that it is only possible to use a cable without mineral insulation inside the outer casing of a fluid flow machine and to lead it out of the outer casing, when a high temperature resistant electrical joint is provided that is eligible for connecting the cable without mineral coating with the mineral insulated cable. The high temperature resistant electrical joint allows a sealed connection between the mineral insulated cable and the cable without mineral insulation, so that the cable without mineral insulation is protected both against high temperatures and moisture. The high temperature resistant electrical joint composition is based on a ceramic formula and is widely used for other applications at ambient temperature environment. The ceramic material is preferably applied in powder form and is then melted or pressed together. The material properties of the composition for the high temperature resistant electrical joint allow withstanding harsh environments inside a compressor, a gas turbine, a steam turbine or another kind of fluid flow machine while fulfilling the necessary function, i.e. electrical sealing of the inner core of mineral insulated under high temperature loading and harsh environment. Especially, in comparison to conventional connections based e.g. on epoxy resin, the high temperature resistant electrical joint connection based on ceramic formula can be used at higher temperatures and at the same time it provides better protection against moisture penetration.
In a preferred embodiment, the trace heating system comprises: a first mineral insulated cable extending inside the outer casing, the first mineral insulated cable having a first diameter, a second mineral insulated cable extending inside the outer casing, the second mineral insulated cable having a second diameter which is larger than the first diameter of the first mineral insulated cable, wherein the high temperature resistant electrical joint connects the second mineral insulated cable and the cable without mineral insulation. Both the first and the second cable extend just inside the outer casing, only the cable without mineral insulation extends partly inside and partly outside the outer casing. The technical realization of the high temperature resistant electrical connection between the cable without mineral insulation and the mineral insulated cable with the larger diameter proved to be easier in practice.
In another preferred embodiment, the high temperature resistant electrical joint is located inside the outer casing. The electrical joint can be located either on the surface of the inner casing or in the gap between the outer and the inner casing.
In yet another preferred embodiment, the trace heating system comprises an electrical connector inside the outer casing connecting one cable without mineral insulation to another cable without mineral insulation. In this case either there are two cables without mineral coating connected in row or these are two parts of the same cable without mineral insulation. In both cases one of the cables or cable parts is located only inside the outer casing the other one extends to the outside. The electrical connector can be any kind of an electro-mechanical device used to join the cables and create an electrical circuit. The usage of an electrical connector that is directly attached to the inner casing, to connect electrical wires coming from outside of fluid flow machine has a few advantages. In the first place it makes assembly and disassembly procedures of the inner casing with the electrical trace heating system easier. During the assembly process, the inner casing can be assembled as usual (no electrical cables are "hanging" around). After the assembly is completed and the outer casing is closed, the outside cables can be attached to the inner casing cables by using the electrical connector. During the disassembly process the outside cables can be detached and removed first, after which the usual disassembly procedure can take place. In particular, when the fluid flow machine is turbine with horizontal joint frame e.g. compressor, the usage of the intermediate electrical connector allows easier roll-out of inner casing e.g. compressor vane carriers without rotor lift.
The usage of an electrical connector also simplifies repair and replacement procedures in service cases. Normally, the likelihood of damage, e.g. cable break, caused by vibration and/or relative movement between the inner and the outer casings is significantly higher at the transition area between the inner casing and the outer casing. The cables including the connector parts that are directly installed at the inner casing can be attached and fixed in a way that would almost eliminate relative movements and with it the main source for potential damage. The cables that are connected from outside to the ones at the inner casing can be relatively easy disconnected and replaced or repaired if necessary.
In order to facilitate the connection of the cable without mineral insulation, preferably, the high temperature resistant electrical joint and the electrical connector build together an electrical connection unit, in particular in form of a plug-in/plug-out connector.
Preferably, the mineral insulation contains magnesia. In particular, the mineral insulation of the at least one cable is composed of magnesia. Magnesium oxide or magnesia belongs to the group of high-temperature ceramic materials. It has good electrical insulation properties at high temperatures and very good thermal conductivity.
In a preferred embodiment, the first and the second mineral insulated cables are connected to each other using a welding joint. Both cables are made as inseparable parts due to special laser welded joint, which makes assembly and disassembly difficult.
Preferably, a service opening is provided on the outer casing. The service opening is used by an operator or an operating device to connect the cable without mineral insulation coming from outside the outer casing to the cables inside the outer casing. The service opening can be used during installation as well as during service and maintenance.
Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings, of which:

FIG 1: shows a fluid flow machine with a first embodiment of a trace heating system; and
FIG 2: shows a fluid flow machine with a second embodiment of a trace heating system.

The illustration in the drawings is in schematic form. It is noted that in different figures, similar or identical elements may be provided with the same reference signs.
FIG 1 shows a fluid flow machine 2, e.g. a compressor or a turbine of a gas or steam turbine plant which is not shown in the drawing. The fluid flow machine 2 comprises an outer casing 4 and an inner casing 6. Between the outer casing 4 and the inner casing 6 there is a gap 8. The fluid flow machine 2 comprises also a trace heating system 10 mounted on the surface of the inner casing 6 inside the outer casing 4. The trace heating system 10 comprises a first mineral insulated cable 12, two second mineral insulated cables 14 connected to the ends of first mineral insulated cable 12 and two cables without mineral insulation 16 which are connected each to one of second mineral insulated cables 14. The mineral insulation of the first and the second mineral insulated cables 14, 16 consists of magnesia (MgO). The first and second mineral insulated cables 14, 16 extend only inside the outer casing 4. The first mineral insulated cable 12 has a first diameter and the second mineral insulated cables 14 have a second diameter which is larger than the first diameter.
Both cables without mineral insulation 16 have a part 16a extending inside the outer casing 4 and another part 16b extending outside the outer casing 4. The external ends of the cables without mineral insulation 16 are connected to a termination box 18. Inside the outer casing 4 each of the cables without mineral insulation 16 is connected to one of the second mineral insulated cables 14 by means of a high temperature resistant electrical joint 20. The high temperature resistant electrical joint 20 is a welding joint sealed by silicon-based insulating ceramic material. Thus, the high temperature resistant electrical joint 20 protects the connection between the cables without mineral insulation 16 and the second mineral insulated cables 14 from both heat and humidity. The first and the second mineral insulated cables 12, 14 are connected to each other by means of another welding joint 22, which is not necessarily a high temperature resistant connection.
The embodiment of a trace heating system 10 according to FIG 2 differs from the embodiment according to FIG 1 by implementing an electrical connector 24 located inside the outer casing 4. The electrical connector 24 connects one part of the cable without mineral insulation 16a to another part of the same cable 16b. The high temperature resistant electrical joint 20 and the connector 24 build together an electrical connection unit 26.
A service opening 28 is provided on the outer casing 4. The service opening 28 is used for access inside the outer casing 4 to connect the outer part 16b of the cable without mineral insulation 16 to the inner part 16a.
In general, the transition between the inner casing 6 and the outer casing 4 is potentially subject to vibration fatigue due to harsh environment (air or steam high velocity flow). It is, therefore, the most susceptible to operation flaws. According to the state of the art, in case that repair would be required, the entire electrical cable circuit of the trace heating system 10, potentially including the inner casing 6, will need to be replaced. All wires or cables are made inseparable, so that the repair of only one segment of cables, e.g. only the second mineral insulated cables 14, is at present impossible. This is why both the assembly and the disassembly procedures create significant risk of damaging of system components such as wires, laser welded joints etc.
In contrast, the electrical connector 24 is used in order to provide a more convenient way to lead the cables without mineral insulation 16 out of the outer casing 4 during installation or during maintenance. The assembly sequence can be as follows:

Step 1: the first mineral insulated cable 12 is installed on the inner casing 2;
Step 2: while assembling of the inner casing 6, the second mineral insulated cables 14 as well as the conventional without mineral insulation 16 are rolled to the side;
Step 3: the second mineral insulated cables 14 is led out though one or more openings in the outer casing; and Step 4: the outer casing 4 is closed.

Claims

Trace heating system (10) for a fluid flow machine (2), the fluid flow machine (2) having an outer casing (4) and an inner casing (6), comprising:
- at least one mineral insulated cable (12, 14) extending inside the outer casing (4),

- at least one cable without mineral insulation (16), the cable extending at least partially outside the outer casing (4), and

- a joint (20) between the mineral insulated cable (12, 14) and the cable without mineral insulation (16), wherein the joint (20) is designed as high temperature resistant electrical joint (20) by sealing a welding joint using insulating ceramic material.
Trace heating system (10) according to claim 1, comprising:
- a first mineral insulated cable (12) extending inside the outer casing (4), the first mineral insulated cable (12) having a first diameter,

- a second mineral insulated cable (14) extending inside the outer casing (4), the second mineral insulated cable (12) having a second diameter which is larger than the first diameter of the first mineral insulated cable (12),

- wherein the high temperature resistant electrical joint (20) connects the second mineral insulated cable (14) and the cable without mineral insulation (16).
Trace heating system (10) according to any of the preceding claims,
wherein the high temperature resistant electrical joint (20) is located inside the outer casing (4).
Trace heating system (10) according to any of the preceding claims,
comprising an electrical connector (24) inside the outer casing (4) connecting one cable without mineral insulation (16a) to another cable without mineral insulation (16b) .
Trace heating system (10) according to claim 4,
wherein the high temperature resistant electrical joint (20) and the electrical connector (24) build together an electrical connection unit (26), in particular in form of a plug-in/plug-out connector.
Trace heating system (10) according to any of the preceding claims,
wherein the mineral insulation contains magnesia.
Trace heating system (10) according to any of the claims 2 to 5,
wherein the first and the second mineral insulated cables (12, 14) are connected to each other using a welding joint (22).
A fluid flow machine (2) comprising an outer casing (4) and an inner casing (6), further comprising a trace heating system (10) according to any of the preceding claims.
A fluid flow machine (2) according to claim 8,
wherein a service opening (28) is provided on the outer casing (4).
Method for connecting cables (14, 16) inside an outer casing (4) of a fluid flow machine (2) using insulating ceramic material to produce a high temperature resistant electrical joint (20) between a mineral insulated cable (14) extending inside the outer casing (4) and a cable without mineral insulation (16) extending at least party outside the outer casing (4).
Method according to claim 10,
wherein inside the outer casing (4) an electrical connector (24) is used to connect one cable without mineral insulation (16a) to another cable without mineral insulation (16b).