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Endless flexibility

Long-life cover seal
Endless flexibility

Self-sealing cover seals used in high-pressure valves operating at high temperatures have proven their effectiveness in applications such as in the power station sector. Seals with a rectangular cross section are designed for general use and can be purchased by the meter. The new packing design was developed as a variation of the braided Brettschneider seal. It is very service friendly and is ideal for use in maintenance and repair work.

Hubert Müller, Dipl.-Ing. Beatrix Isermann

Braided seals are a familiar solution on self-sealing high-pressure valve covers. These seals are also known as Brettschneider seals (which are named after the inventor). This type of packing, which was used primarily in power stations, was initially made of braided asbestos fiber. After asbestos was banned in 1992, there was no equivalent alternative, and this led to the development of pressed, pure graphite cover seals. These rings, which are manufactured to fit the exact dimensions of the valve, are now available in many special versions. They often have metal reinforcement or stainless steel caps for use in special applications. Graphite rings proved to be a good engineering solution and quickly gained wide market acceptance. However compared to braided packing, this alternative also has disadvantages.
When maintenance has to be performed on high-pressure valves, replacement of graphite rings causes extended down time, and this leads to serious problems. When maintenance work is neces-sary, the dimensions of the seal have to be ascertained before a spare can be procured. Especially on older valves, it is not safe to assume that the dimensions have remained unchanged, since the valve may have been refurbished in the meantime.
This makes it necessary to open the valve to measure the dimensions. Rework is neces-sary if ovality or damage is detected in the area around the cover. On graphite rings with stainless steel caps, the caps can become wedged into the seal and thereby cause damage. The seal geometry changes after rework and a cap seal cannot compensate for the difference.
The new seal cannot be obtained until the status has been ascertained and any neces-sary rework on the valve has been carried out. Seals often cannot simply be ordered from stock if rework has been carried out. The seal has to be specially made, and new tools are usually needed to fit the dimensions of reworked valves. All of this means that the plant operator has to cope with significant downtime.
Back to an old reliable solution
Ongoing improvements in packing materials at Burgmann have enabled the company to develop a new version of the braided Brettschneider seal. The new packing design offers a significant reduction in downtime compared to pure graphite seals, and it also offers greater flexibility as well as easier handling.
Statotherm-V-Flex is a braided packing made of permanently elastic, expanded pure graphite foil (>96 %) which contains no binding agents. An inner Inconel wire braid increases the seal’s resistance to pressure. The packing is very adaptable and does not harden. Compared to other solutions such as metal seals, the new seal can be installed and removed without damaging the seal seat, and it is suitable for both new valves and retrofit jobs. The application of operating pressure activates the deformation and sealing action. Because the material is flexible, it can compensate for minor damage, scaling and ovality. It provides a good fit on any geometry, eliminating the need for expensive rework. The seals are easy to install properly, and all of these advantages add up to a significant savings potential.
The proven service-friendly solution
Statotherm-V-Flex is available in all standard rectangular profiles. The material is pre-calibrated and pre-compressed. Rectangular profiles can also be used on covers with internal or external bevels, because the expanded pure graphite adapts very well to the bevel. Material deformation at the beveled surface leads to better sealing by the graphite in the sealing zone, and this results in improved sealing action and enhanced safety. As is the case with all other types of packing, Statotherm-V-Flex is not dependent on the diameter and is supplied by the meter. The length needed for the diameter is calculated and cut from the reel. A 30o to 45 o slanted cut is made at the ends. Several rings can be installed depending on the height. The ends must be offset during installation. Three or four sizes will normally suffice for all the various dimensions that are found at a customer site. Statotherm-V-Flex has proven to be particularly effective in self-sealing covers used on high-pressure valves in power stations, refineries and petrochemical plants. During a general refurbishment project at Dow Chemicals in Böhlen, for example, these self-sealing covers were installed on the cracker. This solution enabled the operator to reduce downtime, and this led to significant cost savings. The system is still running satisfactorily.
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Reducing downtime during maintenance work
Statotherm-V-Flex enables operators and service firms to significantly reduce downtime during maintenance work, because there are no long lead times since material can be procured in advance. This alternative sealing technology also offers other cost benefits:
  • There is no need to hold a large stock of finished, pre-formed graphite rings for the most widely used diameter sizes. A stock of three to four sizes is normally enough to cover all of the requirements.
  • Because the material is very adaptable, operators can avoid costly, time-consuming rework.
  • Special tools are needed to produce pre-formed graphite rings. These costs are eliminated with Statotherm-V-Flex cover seals, which are formed in the sealing space.
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