Variable end seal for CV lines
The production of medium and high voltage cores (MV and HV cores) is effected in continuous vulcanization lines (CV lines). In the process polyethylene is applied to the copper and aluminium core in the cross head. In order to meet the mechanical and electrical properties of the insulation polyethylene is cross-linked in the pressurised tube under inert gas. This cross-linking is made at a pressure of approx. 10bar and temperatures of approx. 300°C. After cross-linking the core is cooled down by water or nitrogen in the same pressurised tube.
The pressurised tube divided in heating and cooling section is sealed against the cross head by a telescopic tube. After the cooling sealing is effected by an end seal.
In order to start the production process, i.e. to seal the pressurised tube against the core, the telescopic tube has to be closed and the end seal has to pressed to the core that the core can pass the end seal from the pressure room without great leakages. "Impacts", which lead to measure deviations, have to be avoided.
For closing the end seal it is partly common still in these days to run the core down to the end seal with nominal value cross section, then to lay the seal and to start the production process (cross-linking process) (admission of CV tube with pressure and temperature).
This procedure results from the problem that at conventional end seal systems the diameter area does not allow any seal against deviating diameters respectively only in a very restricted dimension.
Resulting from this limitation the total pipe distance as starting length has to be rejected at each product change. Additionally the length to achieve stable process parameters has to be rejected as well.
To avoid scrap during the start of production there are different procedures and end seal systems leading to an economic production.
In the following the end seal systems are explained in detail.
Single end seal
The application of a single end seal permits to seal the cross-linking tube being under pressure against the core in a small diameter range. In doing so, usually the diameter is slightly varied through a hydraulically acting seal disc.
At product change outside the small diameter range the line has to be stopped and the end seal and the lead wire seal have to be adapted to the core diameter to be newly produced.
If one does not apply a lead wire adapted to the diameter, the complete CV line length has to be rejected as additional starting length.
Single end seal with following lead wire seal
Using this combination enables the sealing of CV tube with a (smaller) lead wire deviating from the diameter of the core.
The effective saving compared to 1 is working with a smaller number of lead wire (weight proportion for sag control). A correspondence product sequence allows the usage of the preproduction as lead wire (direct series of production lengths) corresponding to savings of time and setting-up.
The lead wire seal is not completely leakproof against water pressure or gas pressure but the control of water level respectively pressure control are not disturbed by leakages!
The application of this combination is recommended for catenary lines, particularly for numerous changes at the production of medium voltage cores since a change of the main seal (see 3. double end seal) is not a possibility at production speeds of more than 2m/min.
Double end seal
The double end seal are 2 main seals positioned in series. Originally this technology was used to counteract against wear at gas cooling in high voltage lines.
The 2nd seal is applied during production, the 1st seal is “open”. If the 2nd seal is worn, the 1st seal is applied during the time required for changing the membrane in the 2nd seal. Since with this method there is almost no wear of the 1st seal the production length is not determined / restricted by seal wear.
Installing the 1st and 2nd seal in a certain distance there are further possibilities realized by Troester already in the early Nineties.
Since the production speed during HV core production is usually below 2m/min the distance of both main seals allows to apply them with seal sets of different diameters. The application is as follows:
The 2nd seal is equipped with a smaller diameter adapted to a lead wire seal. The 1st seal is prepared for the diameter of the core to be produced. When starting the line the tube is charged with pressure and temperature from the start. If the core reaches the 1st seal this is applied, the 2nd seal is opened immediately and the seal set is removed so that the core can pass the 2nd seal.
Now the production can be continued in this configuration or the 2nd seal will be changed to the diameter of the core and applied. Here the advantage is that the total cooling length is at disposal again.
The handling corresponds approximately to the combination end seal and lead wire seal, however, it is advantageous during HV production that no excessive losses of gas occur. The line also runs quietly in sag and in the pressure control affecting he quality (diameter stability). This solution should be preferred for high voltage production.
Change from smaller to bigger diameters can also be carried out!
Double end seal with following lead wire seal
This solution is provided for combined MV/HV CV lines with a speed spectrum not permitting to change in any cases the seal seat of the 2nd main seal always within the time when the core diameter reaches the 2nd seal.
The advantages and disadvantages are described under 2. and 3. depending on the application.
End seal in combination with variable (lead wire seal) end seal
It has to be recorded that all systems described under 2 – 4 only allow to vary from small to bigger diameters. They are even limited to the diameter range of the used seal set. Seal 1 big seal set, seal 2 small seal set.
A production sequence both from a small diameter to a bigger respectively from a big diameter to a small one is not allowed by any of the mentioned end seal systems available on today's market!
By solving the problem to vary the end seal diameter without changing installed components (seals, supporting discs etc.) and consequently without production stop new possibilities occur.
If one applies such variable end seal as 2nd seal like the lead wire seal used up to now, it meets only the function of the lead wire seal, but with the advantage that no seal discs have to be mechanically applied for new other production diameters and the opening does not mean the abrupt pressure loss in the CV tube, as far as end seal 1 is not optimally installed, i.e. it provides a higher reliability of production process!
On the other hand, if one applies it as a 1st seal all possibilities for the production spectrum arise. The line can be converted from a bigger core diameter to be produced to a smaller and vice versa.
In this case the variable end seal is positioned in an adequate distance from the 2nd seal.
During seal function of the variable end seal the 2nd seal could be set up on he diameter of the core.
Outlook
If one achieves a tightness with the variable end seal that approximately corresponds to the traditional end seal, it would be theoretically imaginable to work only with one system (a variable end seal) and to reduce complex handling, number of applied devices and as a result costs. The actual evaluation of technical possibilities and previous tests, however, question the realisation. The application of the variable end seal in the described way, see also Scheme 5 b, however, shows further essential advantages compared to present systems in regard to savings of starting lengths.
At present, the system is in industrial testing and will be available for marketing presumably in 4 months.
Am Brabrinke 1-4, 30519 Hannover, Germany
Tel.: +49 511 8704-0
Fax: +49 511 864028
e-mail: bernd.pielsticker@troester.de