HU207386B - Method for producing sealing - Google Patents

Abstract

For producing calibrated, reliably functioning packings, which are universally usable in the entire valve-fitting-piston-pump area, of various use sizes, in particular for compressed gaseous or liquid oxygen with reduced expenditure, a dry PTFE-graphite yarn having a tensile strength of 0.9 to 1.0 g/den, to achieve a packing bulk density of about 1.1 to 1.2 g/cm<3>, is braided with a stress slightly beneath its tensile strength. The yarn is preferably composed of a double-ended PTFE-graphite yarn, which is preferably braided at an angle of about 55 to 60 DEG .

Description

BACKGROUND OF THE INVENTION The present invention relates to a process for making a seal, which is used mainly in the interior of valves, fittings, rotors and piston pumps, particularly in the case of compressed gas or liquid oxygen, by weaving dry the yarns through a sealing device.

As is known, some of the seals are so-called. serves as a resting seal and the rest as a moving seal. In particular, these latter seals are more demanding, either along rotary shafts or pins, for example in rotary fittings, or in the outlets of linear piston rods and valve pistons.

The stress on the seal is increased in the case of a pressurized interior. This increases the risk of leakage in compressed air or other gases as well as in high-pressure fluids. For high-speed rotary shafts and high-stroke piston rods, the frictional warming of the seals should also be considered. With increasing heat load, some sealants may experience heat shrinkage, resulting in leakage losses. A further requirement for the seals is that the seal should be precisely aligned with the shaft or rod to be sealed at the point of installation, and that there should be no deformation between the inner surface of the seal and the shaft or rod. In other words, there should be no protrusions or bends in the inner diameter of the seal.

In valves used in high-pressure valves, particular attention must be paid to the risk of cold flow and, consequently, of gaps. The sleeve space is often deep, in which several sealing rings made of relatively hard sealing material must be placed so that the surface pressure is also transmitted to the lower sealing rings towards the high pressure medium.

In such applications, the seal should be of optimum density and fine surface. In the case of compressed gas or liquid oxygen, gaskets in fittings and other equipment have been found to have a density of approximately 1.5 g / cm ³ .

Due to the development of sealing technology, gaskets of a wide variety of materials and processes are known, starting from simple gaskets made of textile fiber material. Natural fibers of plant and animal origin can only be used as compression seals at temperatures of up to 80 ... 90 ° C, depending on the bedding material (rubber or plastic), so their use today is extremely limited.

Synthetic fibers (polyamide, polytetrafluoroethylene) are preferably used in the manufacture of gland seals due to their special chemical, corrosion-resistant or self-lubricating properties.

Polytetrafluoroethylene, also known as PTFE or commonly known as Teflon, has recently become a widely popular sealant due to its excellent chemical and heat resistance. Graphite is incorporated into the finely digested tefion fibers to improve their abrasion resistance and lubrication properties. Its friction properties are much better (0.028 ... 0.1 friction coefficient than conventional raw materials).

Teflon fiber can be made by a braiding process to form a continuous sealing cord, with or without gut fibers, and one or more multiple layers. It is easy to see that the more layers of braid, the finer, smoother the surface. The surface of the braided sealing cord, and thus its sealing property, also depends on the number of sections of the braid. The more yarns we make, the closer the outer surface of the yarn to the surface bounded by the flat plates, the closer it is to the square or rectangular cross-section required for the sealing rings.

Solutions for replacing one or more yarns with metal fiber, graphite or PFTE fiber are used to reinforce the braid or improve frictional conditions.

Another group of sealing rings are gland seals made of fabric strips embedded in impregnating or vulcanizing material.

Where asbestos graphite, pure graphite sealing rings, were previously used in areas exposed to higher heat loads. Healthy asbestos has been replaced by P1FE or PiFE-graphite seals because of the increased heat generation at high-speed shafts or high-speed piston rods.

At higher temperatures, sealing rings with a very good sealing effect are required due to the high pressure and high pressure. In addition to the low friction coefficient, these sealing rings require a long service life. Experience has shown that PTFE and graphite best meet these requirements.

Recently, PTFE and graphite seals have been featured in the catalogs of well-known seal manufacturers.

Various patent documents also disclose efforts to improve both the composition of the sealant and the impregnating agent used. A preferred sealing braid for use in the manufacture of a gland for rotating and sliding shafts is disclosed in GB 1288878. which comprises PF FF and graphite Alamment, and also includes filaments.

Two other patents also deal with braiding compositions, vulcanizing and / or impregnating materials, which are intended to improve the efficiency and service life of the seal. In one of these, U.S. Pat. U.S. Patent No. 4,123,125 relates to the arrangement of strands of TFE (tetrafluoroethylene) and high temperature nylon, low twist braid. The other, US 3,668,646. document describes a sealing material made of fibrous fiber and graphite casing braids and various impregnating pastes, fillers and solid lubricants (graphite),

None of the above descriptions refers to the process characteristics of the braiding and to the technical measures which would include instructions for producing the desired quality braiding.

HU 207 386 Β

Sealing rings for sealing rotating or sliding shafts in high-pressure space are produced by the known process in the following manner. In the first process step, a PTFE braid is first made at a medium density while the cavities are filled as a suspension. Then, a piece of braid (sealing cord) made in the running meter is cut to a length corresponding to the application length and pressed by a high-pressure press to the desired application size, approximately to the intended final density. In the third step, each pre-pressed seal is finally calibrated to a final, exact size.

The disadvantage of this process of three successive operations is that it is time-consuming and labor-intensive, and that each of the sealing rings is individually sized for its size. Storing these extrusion tools further increases costs.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an economical method for producing high pressure, gas tight, leak-tight seals which have a long service life at the above-mentioned installation sites and are free of the above disadvantages.

SUMMARY OF THE INVENTION It is an object of the present invention to properly select a substrate having favorable properties and to provide an optimization process, having regard to the characteristics of the substrate and the spinning process, to achieve the desired sealing density in a single operation rather than the multi-step compression-compaction process.

The inventive idea is based on the discovery in PTFE of the stretching effect of stretching, which is accompanied by the achievement of a more compact, shape-proof seal. With our series of experiments, we have found that, contrary to the previous professional position - that the yarn to be processed, the thread, the thread, etc. only tensile strengths of 30-50% should be stretched - in our case, the increased tensioning of the yarn during processing is particularly advantageous when braiding PTFE.

Typically, in a textile processing process, the yarn, fiber, and filament have a tensile strength limit of about 1 to about 10 percent. 1/3 of the tension. Measurements and tests are also included in János Gyimesi's "Physical Examination of Textile Materials" (Technical Publisher, Budapest, 1968). Figure 438 of the book shows tensile strength diagrams of various natural and artificial fibers.

It can be generalized in the case of known fibrous materials that the optimum stretching condition is a prerequisite for favorable processing, that is, if the fibrous material is subjected to a tensile stress of We stretch it to 50%. At this tension, the range of residual elongation is still relatively low, which was a fundamental aspect of the known processing methods.

The process of the present invention is based on the discovery of the PTFE tensile elongation diagram, by examining the relationship between tensile strength and residual strain, to obtain the densest seal of the desired density if at least 85% of the tensile strength value.

We recognize that the braid of sufficient density can be produced in a single operation without the need for subsequent compression and / or calibration of the finished braid in a separate apparatus.

According to an object of the invention, the process for producing a seal according to the invention is used mainly in the interior of valves, fittings, rotors and piston pumps, especially in the case of compressed gas or liquid oxygen, wherein the sealing device is woven dry from yarn . In the process of the present invention, the braids are braided from a dry PTFE-graphite yarn having a tensile strength of 8.1 cN / tex to 9 cN / tex at an angle of 45 ° to 60 ° with respect to the direction of the finished braid, while the tensioning elements to at least 85%, preferably up to a 90% stretching of density between 1.1 g / cm ³ and l, 2g / cm ³ is achieved, and then passed through the lumen of the braid calibration density value at least ^three compacted to 1.5 g / cm.

A further advantageous feature of the process of the present invention is that the gasket is braided from stretched film of the same proportion of PTFE and graphite.

According to another preferred process feature, the gasket is braided from double-stranded PTFE-graphite thread.

It is also advantageous to incorporate PTFE gut fibers into the braid of PTFE-graphite yarns.

The sealing process of the present invention already produces a sealing density near the final density during the spinning process. Thus, in the subsequent calibration process after braiding, only a small amount of compaction is required to achieve the desired sealing density of 1.5 g / cm ³ . This can be done simply by passing the braid out of the braiding machine continuously through a calibration cavity where the cavity has the same size as the final gasket size. At the same time, during this calibration process, the sealing braid is compacted to the desired final density. Thus, the usual high-pressure extrusion with special tools is missing. It is advantageous to use dry PTFE graphite yarn because the graphite directly and evenly embedded in the PTFE yarn structure stabilizes the thermal expansion of the PTFE yarn. The gaskets at the installation sites remain sealed, even when the valves are loaded and the rotors are started, despite their cold condition. Advantageously, a dry seal is formed which is completely self-lubricating and in which no "leakage" of auxiliary lubricants occurs. As a result, at the same time, the seal has a very good volumetric resistance, practically no readjustment is required. In addition, there is no dirt and a significant increase in lifetime of about 6 to 10 times compared to conventional seals.

The seal produced by the process of the present invention is generally applicable, has a longer service life, has little or no readjustment.

EN 207 386 Β required. Gasket replacement is less frequently required, resulting in reduced downtime.

Preferably, the PTFE-graphite yarns on the spinning machine are pre-stretched onto the dolls. It takes 8-24 yarns, or as many babies, to produce a sufficiently tight braid. By increasing the number of yarn branches, a finer, more evenly distributed surface is obtained and the desired square or rectangular cross-section can be approximated. It can be rarer to make braids with 32 babies, but here you need extra-fine, high-density Teflon yarn.

A more firm, "harder" seal is obtained by using so-called diagonal braiding instead of weft weaving. Moving the dolls can create 2, 3, or 4 times the diagonal braid. This determines the pattern on the surface, the higher this number, the stiffer the braid.

In the process of the present invention, if the yarns constituting the braid are stretched to at least 85% of their tensile strength during the braiding, then at a raw material density so far available of approx. 8 to 15% higher density is obtained, and the desired density of 1.1 g / cm ³ to 1.2 g / cm ^{3 is} achieved already during spinning.

Further advantages and features of the invention will be described in the following section, in which the properties of the seal according to the invention, the material testing experiments and the results obtained are described in more detail.

To determine the ignition temperature of the seal of the present invention in compressed oxygen, about 0.2-0.5 g of liquid test material applied to the shredded solid or ceramic fiber was placed in a chromium-nickel-lined alutoclave having an autoclave volume of about 34 cm ³ . After the autoclave was sealed, it was filled with oxygen _{to a} pressure of p and heated inductively in a low-frequency heater while the temperature rose almost linearly at about 120 ° C / min. The temperature change at the sample site was recorded with the aid of a thermocouple and a smoothing recorder. The pressure change was measured and recorded simultaneously with the pressure gauge. As the temperature in the autoclave increased, the oxygen pressure continued to increase. The inflammation of the sample became apparent from the sudden, steep rise in temperature and pressure. The p _e oxygen pressure at the ignition temperature was calculated. Specifying _the oxygen pressure p _e is important because there are substances whose ignition temperature decreases with increasing oxygen pressure.

In five experiments, p _a = 5 sec _at oxygen pressure resulted in an ignition temperature of 446 ° C ± 3 ° C. The oxygen pressure p _{e was} about 12.5 MPa.

In the following experiment, the seal produced according to the invention was tested for resistance to aging from compressed oxygen as follows. In a gas-tight container made of chromium-nickel steel, a weighed sample of the test substance was exposed to compressed oxygen at a higher temperature for 100 hours. The filling pressure of oxygen at 20 ° C was calculated to be 10 MPa at the test temperature. With this artificial aging, we examined whether the sample was reacting to oxygen or suffering any detectable lesion. The criterion for resistance to oxygen in the present experimental conditions was the degree of retention of the appearance and properties of the sample, the weight of the sample, and the ignition temperature, within certain tolerances - after aging.

After aging at about 200 ° C and 10 MPa oxygen pressure, the sample remained externally unchanged. The weight of the sample decreased by about 1.05%. The ignition temperature of the aged sample was 444 ° C ± 4 ° C, thus remaining essentially the same as that of the non-aged sample.

In a further experiment, the seal produced according to the invention was tested for its reactivity under oxygen pressure shocks. To perform the assay, about 0.2-0.5 g of liquid test material, shredded on solid or ceramic fiber, was placed in a heated steel sleeve having an internal volume of 15 cm ³ . The steel sleeve was connected to an oxygen pressure vessel through a 750 mm long and 14 mm inside diameter pipe and a pneumatic actuated quick-opening valve. After fölmelegítése to acélhüvelynek t "test temperature and filling _{of the} pressure of oxygen in the tube and the acélhüvelynek p fast-opening valve is opened, so that a temperature of 60 ° C preheated oxygen p is _the pressure, shock-like flowed into the tube and the acélhüvelybe. In this way, the oxygen in the tube and the steel sleeve practically adiabatically changes p _from pressure to p _e and heats up. If, therefore, a recognizable reaction of the test substance with oxygen occurs at a rapid rise in temperature in the steel sleeve, then the experiment is continued or continued at reduced p _e / p at _the pressure ratio. However, if after a waiting time of 30 seconds the reaction of the test substance with oxygen was not recognizable, the steel sleeve was depressurized again and the experiment continued (for example four times) until the reaction occurred.

Each experiment consisted of a maximum of five experiments. In each experiment, the same substance was treated under the same conditions. If you came up reaction to the current series of experiments fifth attempt than not, the experiments were always conducted by _the pressures of nine samples greater pg / min, until finally gotten the pressure relationship, which is not yet up reactions in five individual experiments involving a test series. If the repetition of the nine-sample series of experiments yielded the same result, the study could be terminated or another study at _the experimental temperature could be continued.

The results of the tests are shown in the following table.

HU 207 386 Β

Temperature of the samples oxygen Pressure Behavior under pressure Pa P _e 60 ° C 0.1 MPa 6 MPa Inflammation at pressure 2 60 ° C 0.1 MPa 5 MPa Inflammation at pressure 3 60 ° C 0.1 MPa 4 MPa No reaction at pressure 5 60 ° C 0.1 MPa 4 MPa No reaction at pressure 5 100 ° C 0.1 MPa 4 MPa No reaction at pressure 5 100 ° C 0.1 MPa 4 MPa No reaction at pressure 5 150 ° C 0.1 MPa 4 MPa No reaction at pressure 5 150 ° C 0.1 MPa 4 MPa No reaction at pressure 5 200 ° C 0.1 MPa 4 MPa No reaction at pressure 5 200 ° C 0.1 MPa 4 MPa No reaction at pressure 5

Finally, the reactivity of the seal produced according to the invention with liquid oxygen in the event of impact is also examined. To perform this test, approximately 0.5 g of liquid or shredded solid test material was always placed in a cup-shaped sample container of 0.01 mm thick copper plate (size h-10 mm; 0-30 mm), and the test substance was poured into it. Exposed to a 5 kg hammer impact. The drop height of the drop hammer was variable. The sample container was provided with a steel anvil with a support plate made of chromium-nickel steel. The anvil, which weighs approximately eight times the weight of the drop hammer, was supported by four shock absorbers mounted on the steel frame of the experimental device. The experimental device itself was on a concrete foundation.

The reaction of the sample to be examined with liquid oxygen is usually recognized by the formation of flames and, more generally, by a loud explosion. By varying the drop height of the drop hammer, we determined the impact energy at which no reaction has yet occurred. This result had to be confirmed by performing the experiment ten times under the same conditions. The experiments were interrupted when impact energy of 125 Nm or less (corresponding to a drop height of 0.17 m or less) was observed. In this case, the material in question is unsuitable for equipment containing liquid oxygen for safety reasons.

As a result of the experiments, it was found that up to a drop height of the drop hammer of 0, 83 m (impact energy of 625 Nm) in 10 individual experiments, neither explosion nor other reaction of the substance with liquid oxygen could be detected. First, at a drop height of 1.0 m (impact energy-750 Nm), two out of seven experiments produced a flame reaction.

Summarizing the results of the experiments, it is apparent that the dry seal produced by the process of the invention can be used reliably and safely in equipment or parts of equipment containing liquid oxygen for use at oxygen operating temperatures up to 200 ° C and up to 4 MPa oxygen pressure. It can be stated that the seal produced by the process according to the invention has additional effects compared to the prior art seals.

The application ranges from chemical, pharmaceutical and petroleum industries to power plants, power supply equipment and the food industry, and is also suitable for use in the nuclear field due to its low liquid content. Special applications include horizontal mixer for granules and powders, application to materials to be cured and bonded, use in flue gas valves in power supply equipment, sealing of gas phase of mixture and autoclaves, etc.

Claims (4)

A method of making a seal, which is used mainly in the interior of valves, fittings, rotors and piston pumps, particularly in the case of compressed gas or liquid oxygen, wherein the seal is braided dry from yarns and then passed through a calibration ring, the braids are made of dry PTFE-graphite yarn having a tensile strength of 8.1 cN / tex to 9 cN / tex, the yarns being woven in an angle of 45 ° to 60 ° to the finished direction, while the yarns are at least 85% of their tensile strength preferably stretched to 90% to a density of 1.1 g / cm ³ to 1.2 g / cm ³ and then, through the calibration cavity, compacted to a braid density of at least 1.5 g / cm ³ . Method according to claim 1, characterized in that the seal is braided from double-stranded PTFE-graphite thread. The method of claim 1, wherein the seal is braided from an equal amount of stretch film of PITE and graphite. 4. The method of claim 1, wherein the PTFE-graphite yarn is braided with PTFE enteric fiber.