To the consumers of graphite flat gaskets. The main subject here is expanded graphite flat gaskets.
What will be discussed in detail here? The product of Frenzelit NOVAPHIT SSTC is being studied in detail. Every technical detail is explained and illustrated with interesting and important tests.
Since the prohibition of asbestos, NOVAPHIT SSTC material has been used more and more every year - in power plants, chemical plants, shipbuilding and other areas. A wide variety of different uses have been found for this material. Problems with other materials could be solved by applying NOVAPHIT SSTC. This material has been able to replace asbestos and non-asbestos fiber spacers, as well as other graphite and even metal spacers.
The consulting services provided by specialists in regards to Frenzelit application have shown that many issues need to be discussed over and over again. This work is intended to discuss all these issues and give consumers the relevant documentation.
It is obvious that not all issues are of equal interest to everyone. For the aspects that interest you the most, we advise you to consult the index.
These pages will not cover all the problems of gasket and sealing technology. The facts of principle do not require detailed explanation. In fact, this report focuses on issues arising from daily practice. For example, you can come across some comparisons with graphite gaskets on punched sheet metal, because they can be found quite often. We have brought together all technical descriptions.
Our NOVAPHIT SSTC material is one of the best materials available that can replace others without any problem. In general, this makes flange connections excellent. So NOVAPHIT SSTC material influences the reduction and standardization of different but required materials - that is, you only need one graphite flat gasket material to cover all your needs. This fact justifies the title of the booklet: "Highest quality at a decent price."
Why use graphite as a gasket material?
First of all, it is necessary to look closer at why graphite has become so important as a gasket material. In fact, graphite has achieved such a triumph only after the prohibition of asbestos.
Asbestos-free elastomer bonded fiber spacers have exhausted themselves relatively quickly due to their limiting factors in the general range of applications, especially with regard to heat resistance.
However, after the ban, no asbestos-free fiber substitute has been able to fully cover the entire spectrum of asbestos gaskets, as the fiber content in them was much lower. In thermally expanded graphite, individual layers of graphite crystals undergo changes, mainly under the influence of temperature. Such changes give the material qualities that make the material more and more important.
At first, as is always the case with new developments, the success was very moderate and certain difficulties arose. For example, the mechanical strength of the thermally expanded graphite foil was rather weak. This led to the development of grades that were reinforced with stainless steel, which remain in demand to this day. Among other things, the purity of the early graphite films was very low. Like asbestos, graphite is a natural raw material that requires intensive processing and cleaning in order to meet the requirements of the gasket material.
In other chapters, it will become clear that not every graphite foil is of the required quality. With good quality foil, thermally expanded graphite offers a number of qualities required in gasket technology.
The most important quality is its excellent thermal stability. Moreover, its good chemical resistance to a wide range of environments makes graphite versatile in its use. The following chapters present the test data for the individual properties of this material.
About NOVAPHIT SSTC
NOVAPHIT SSTC consists of the following components. The high quality thermally expanded graphite foil not only has a purity of at least 98% but also meets high requirements in other respects. Inside there is stainless steel (1.4404, ANSI 316L) which is a characteristic of the design and qualities of this product. A thin layer of glue on it provides excellent handling of the gasket.
Stretched perforated metal properties
Many of the properties, that make NOVAPHIT SSTC stand out in comparison with other products on perforated or smooth sheet metal, have been achieved thanks to the use of the special expanded material. The illustrations below show the structure and manufacture of stretched metal.
The base material is acid resistant stainless steel (1.4404). The production provides a 3D geometry that has no undercuts and projects a height of approximately 0.5mm. This material creates a kind of chambers in the core of the gasket.
So it is not a separate layer, but a structure that allows the top and bottom graphite foil to bond. The biggest advantage of this kind of core can be seen as soon as the user picks up the gasket: safe handling! These gaskets remain flexible and will not break or warp.
Be that as it may, when gaskets warp and crack, in general, the NOVAPHIT SSTC is very tolerant for rough handling. Higher graphite concentrations created by pressure on one side of the gasket can transfer graphite to the other side during installation because perforated stretched metal has open structure.
Thus, the gaskets are more tolerant to mishandling and installation. One advantageous aspect, not necessarily of a sealing function, is that the material has significantly reduced the risk of physical damage when handling edges inherent in materials with perforated and smooth sheet metal inside, the thickness of which is 0.1 and even 0.05 mm.
Many of the bends in the core are almost impossible to change because the steel is mechanically hardened. This means that the three-dimensional structure of the metal core is retained after installation. At high surface pressures, for example at tenon and groove flanges, the core provides the gasket with recoverability. This plays an active role in the sealing process.
Other advantages will be discussed in subsequent chapters.
Evaluation and permission tests
NOVAPHIT SSTC has the following approvals and ratings:
DVGW - German Water and Gas Association, approved for use in gas pipes
KTW - Artificial materials in potable water, approved for use with drinking water and food
BAM - Federal Office for Testing Materials, approved for use in oxygen up to +200°C and 130 bar, both in gaseous and liquid forms.
Fire Resistance Test - Lloyd's Report is registered in BS6755 Part 2 and API Standard 607
TRD401 - approved for use with oval covers in steam boilers with a maximum class d (250 ° C / 40bar)
TRD401 approval refers to the NOVAPHIT SSTC TRD 401 version, which has been tested for use with self-sealing oval covers by TUV Suddeutschland. In this segment, the NOVAPHIT SSTC TRD 401 has been recommended and has been used for assembly lines and secondary markets since the end of 2000. This will be discussed in more detail below.
Adaptability compared to asbestos gaskets
The deceiving quality of graphite gaskets is the overall density of the material. A normal fiber-based liner has a density of 1.4 to 1.8 g / cm3, while a graphite foil has a density of about 1.0 g/cm3.
Moreover, both types of liners have different structures. In asbestos fiber spacers, it is mainly the fibers, in general, dense filler and pre-vulcanized rubber that determine the properties of the spacer.
Thus, the gaskets give the impression of being "solid". In contrast to them, graphite gaskets are "soft" and are more delicate from the point of view of mechanical action. On the other hand, their adaptability to all kinds of surface imperfections is much higher.
The following Figure shows the behavior of a fiber liner in a cold installation (novapress universal) compared to NOVAPHIT SSTC. The following TEMP-test consists of two parts, of which the first one relates directly to this aspect.
The liner ring (size 55x75x2.0 mm) is compressed at room temperature up to 50 MPa. The decrease in thickness is measured simultaneously.
The curve data shows how strong the cold compression is on the tested pad. Novapress universal, fiber liner can be compressed to about 10% at room temperature under a load of 50 MPa. Under the same conditions, the graphite gasket has a compression ratio of 45-50%.
An important finding of this test concerns the determination of the required pad thickness. Graphite gaskets are much more suitable for use with rough flange surfaces than fiber gaskets. The same can be said for non-parallel flanges. This is a very important aspect for all old factories using gaskets. Even large sizes with a thickness of 1.5-2.0 mm are completely suitable.
Conduct under the influence of temperature
To explain the differences in fiber liners, we suggest referring to the second part of the TEMP-test.
After compressing the gasket with 50MPa, as described in the previous paragraph, the temperature begins to rise at a constant pressure. Simultaneously, the decrease in thickness due to heat exposure is measured.
The test is carried out in more extreme conditions than in real life. As you can see from the graph, fiber liners start to fail with increasing temperature. Depending on the tested material, the moment of failure of the gasket is in more or less in one critical temperature spectrum.
Graphite gaskets do not lose their working qualities, no matter what temperature is used. This means that when the temperature rises, the NOVAPHIT SSTC material practically does not crawl. This explains the excellent compatibility of graphite gaskets in high temperature applications.
The technical documentation for the standardization of graphite gaskets shows an upper temperature limit of 450 - 550°C. In an inert atmosphere, much higher temperatures (over 2000°C) can be achieved, since graphite will not react with oxygen.
Graphite tends to undergo oxidation and thus the heat resistance of the gaskets also depends on the quality of the foil. This phenomenon is discussed in detail in subsequent chapters.
In reality, NOVAPHIT SSTC can be used perfectly at temperatures up to 710 ° C (exhaust manifold of a high-speed diesel ship engine).
Oxidation and loss on calcination
In evaluation the quality of the graphite foil and its behavior at high temperatures, loss on calcination is the decisive factor.
It is not recommended to use graphite gaskets at high temperatures if their oxidation factor is too high.
In the test described below, the weight of liner rings (size 90x50x2.0 mm) with various grades of graphite foil was determined. Then they were placed in an oven and heated to 550°C. This procedure was repeated every day or two. The weight of the expanded metal core was not taken into account when measuring the loss on calcination.
It is useless to photograph foil 3 since the only thing left is the stainless core. For further information on the effect of temperature on material, the same test was carried out at +450°C.
The figure below shows the results with many different types of foil, among which there are also products that regularly appear on the market.
NOVAPHIT SSTC compared to other grades of graphite foil
The importance of this test should not be undermined in practical applications, even if only the edges of the gasket after installation would be exposed to oxygen. With prolonged exposure, low quality graphite foil burns out and the gasket can fail.
NOVAPHIT SSTC is manufactured using the best annealing material.
Leakage - laboratory and reality
One of the important aspects in assessing the quality of a gasket is its leakage potential. It should be as low as possible.
Each gasket material has limiting factors in terms of tightness classes. In order to guarantee comparability and reproducibility, the leakage is mainly tested according to specific test standards (DIN 28090).
Further consideration is needed with regard to whether the results can be applied in practice. When measuring the possibility of leakage when exposed to high temperature, for example, fiber gaskets with a binder rubber, it can be much lower than that of graphite ones.
However, in reality, the same fiber liner can fail in a relatively short time due to the constant exposure to high temperatures.
Similar observations are made with metal gaskets such as Kammprofile or spiral wound gaskets. Under ideal conditions, they have less chance of leakage in terms of surface pressure than graphite gaskets.
If some carelessness is committed during installation, such as the eight flange bolts not being tightened sufficiently, the increase in the chance of leakage for the metal gasket is much higher than for thermally expanded graphite.
The quality of flange surfaces is another aspect that cannot be completely ignored. The fact that conditions in real life are very different from laboratory ones is more than obvious. It should be emphasized that metal gaskets especially show the highest leakage potential when installed in such conditions.
The following chapters describe the collector test and the possible leak curves are shown. Test environments are nitrogen and helium. All tested gaskets were prepared according to prEN 13555 and DIN 28090.
It should be mentioned that the effect of surface leakage of the graphite gasket can be ignored in laboratory tests. All the difference in leakage is created by the so-called cross leakage.
Leakage - graphite influence
This test measures the effect of graphite foil on leakage. The following gasket pressure versus internal pressure diagram shows the constant leakage curve.
Comparison of the tightness classes L0.01 especially shows a significant difference in the performance of the material.
With an initial pressure of 40 bar, 40 instead of 83MPa, a surface pressure of 0.01mg / (s*m) is required. NOVAPHIT SSTC is designed with a denser graphite foil (foil 2).
This test clearly shows the significance of the effect of graphite foil on the possibility of leakage. The above graphs show that the piercing performance of the real NOVAPHIT SSTC foil is also excellent. To complete the picture, it should be mentioned that the mechanical properties of both types of foil are identical.
Leakage - influence of the punched-out stretched material inside
Of course, the selected core also affects the possibility of leakage. In order to prove this effect, several samples of gaskets with the same paper, produced by the same company from the same batch, were subjected to a separate test.
Perforated sheet metal gaskets were compared with smooth sheet metal gaskets, perforated sheet metal gaskets and coreless gaskets.
The photographs below show why the possibility of leakage in a material with expanded sheet metal is lower. On gaskets 2.0 mm with thickness with perforated stretched sheet metal and with perforated sheet metal, a surface pressure of 25 MPa was applied by means of a laboratory hydraulic apparatus. The same Fuji film of medium sensitivity (10 to 50 MPa) shows pressure propagation.
Perforated sheet metal, perforated stretched sheet metal
The photo on the left clearly shows the zones of increased pressure along the folds of the nubs. As a result, the pressure is lower in the areas near the bulges. The real disadvantage is that there are continuous areas of reduced pressure, which are obvious weak points in relation to the existing constant pressure.
The pressure distribution over the perforated stretched metal is also uneven, which is the result of a core thickness of 0.15 mm. In contrast to perforated material, in this situation there are closed high pressure lines. The gasket "optimizes" itself. This fact clearly reduces the leak factor.
Leakage – comparison of serial products
The diagram below shows a comparative analysis of serial gaskets as sold in the market.
NOVAPHIT SSTC was produced with two different types of foil and was compared with the standard as well as with the impregnated graphite perforated gasket.
It is interesting to note that NOVAPHIT SSTC produced with lower grade foil with surface pressure between 18 and 28 MPa had lower leakage than the two perforated versions.
The advantage of stretched metal was evident primarily in the area of reduced surface pressure.
Leakage - effect of liner’s width
For all comparisons, standard 90x50x2.0 mm liners were used. In practice, the difference between standard flanges and special designs with a gasket width of a few millimeters.
This critical application needs to be discussed in more detail. In the following diagram, the leak is measured with an internal pressure of 40 bar and a surface pressure of 20 MPa is applied depending on the width of the gasket with helium as the test environment.
The tested liners were of the following dimensions: 100x40, 90x50, 81x60 and 75x65 mm.
The curve shows that the gasket width has a decisive influence on the leak factor: "The wider the spacer, the better the sealing function" is the conclusion of this test provided there is sufficient pressure on the surface.
A general recommendation to maintain a minimum ratio of 1:10 thickness to width of the liner is confirmed by this test. Not only for mechanical reasons, but also for density imaging, this rule must be strictly followed in order to achieve optimal conditions.
In direct comparison with a perforated graphite gasket, an interesting aspect arises with regard to the width of the gasket.
The following diagram shows an increase in leakage with a decrease in surface pressure from 40 to 10 MPa. The test environment is helium again.
The response of an expanded graphite gasket to pressure reduction is much more severe than that of a punched gasket. In practice, this pressure reduction due to the work process must be calculated, especially for DIN flanges. Thus, this result is very important.
This test shows another advantage of expanded metal over punched metal, which is a reinforcement for graphite gaskets.
Gaskets with internal pores
There are various reasons for producing internal pore gaskets. In addition to mechanical aspects such as increased blowing resistance and helps to avoid environmental contamination, the effect of internal pores can also be checked at the leakage level.
Even routine tests show a mixed picture. With the use of internal pores, both the rise and fall in the leak rate can be monitored. The surface structure of the cushioning material plays an important role.
The relationship between these factors was tested at Frenzelit. In addition to high-precision estimates on a helium spectrometer, new facts were found. However, it is difficult to deny the fact that internal pores affect the installation process. The initial deformation of pores with internal pores requires increased preloading.
Внутренние поры могут оказывать негативное влияние особенно на фланцы низкого уровня давления. Здесь можно наблюдать частичное повышение в уровне течи. В данном контексте существует разница между просечной графитовой прокладкой NOVAPHIT SSTC. Пластина из расширенного металла согласуется с внутренними порами гораздо лучше, чем вставка из просечного металла. Причиной такого явления является разная проектируемая высота вставок. Расширенный металл никогда не бывает тоньше, чем 0,50 мм. Высота в просечном металле прим. 1,75 мм, на два 0,15 мм, то есть толщину внутренней поры должна быть добавлена. Пора также как и графитовая фольга должна быть деформирована против давления которые должны быть согнуты. Следующие фотографии показывают микроснимок сжатой внутренней поры увеличенной в 30 раз. Фотография микроснимка сжатой поры у просечного металла ясно показывает деформацию поры.
Expanded metal Perforated metal
Oversized gaskets made segmentally
NOVAPHIT SSTC can be supplied both in standard sheets of 1000x1000 mm and in XL sizes of 1500x1500 mm. In equipment for the chemical and general industry, flanges with an outer diameter of more than 1500 mm can be easily found
In such cases, fiber-based gaskets are supplied with beveled gaps. In the process, the corners fit into segments. They are glued together.
However, one must be careful when using this method with graphite gaskets - in practice this is hardly possible. In practice, the segments are often simply stacked next to each other. This aspect is, of course, a weak point in flange connections.
In general, the biggest problem is the overlap of metal inserts. Such overlaps on one side and gaps on the other side will definitely lead to unsuccessful use of the gasket.
NOVAPHIT SSTC production segmentally shows another advantage due to the expanded metal insert. The open mesh structure allows graphite to close gaps. The situation can be remedied by inserting a small, thin graphite plate (0.35 mm) into one side of connection.
The following figures show the different behavior of expanded metal compared to grooved or smooth in the gap.
Perforated and smooth metal are two different layers. However, it is not possible to fill the gap around the insert sufficiently.
It is entirely possible to optimize the connection by using a button-like connection in liners with a minimum width of 24 mm. The following figures show the current version of this optimized technology, which has been developed from advanced developments.
It is all up to the buyer: whether the gaskets will be supplied in segments or in the specified dimensions.
In the production of ready-to-fit gaskets, an additional doubling of the gasket has been proven.
When this method is selected, segments of 1.0 and 1.5 mm thickness are produced. These segments are connected and glued between layers.
We recommend supplying large spacers in segments and assembling them on site because shipping in segments is much easier. With segments, it is not necessary to double spacers.
The following graph clearly shows that NOVAPHIT SSTC is suitable for creating liners in segments. In practice, this solves several problems.
Impregnation - pros and cons
In graphite gaskets, the graphite foil is glued to the metal insert and actually performs the function of a seal. The importance of high quality foil has been explained in the previous paragraphs.
There are only a few companies offering an impregnated version besides pure graphite foil. It is possible to soften the graphite by applying temperature-sensitive additives - just like the impregnation - and change the properties of the gasket.
Does impregnation reduce leakage? Yes, it does.
In standard laboratory test times, this effect can be understood depending on the type of impregnation used. After a longer installation at a high temperature - this corresponds to the purpose of using graphite gaskets - the effect is canceled or the opposite effect is obtained. Organic impregnation reacts under temperature, thus changing the structure of the installed strip.
Does impregnation increase the stability or resistance to damage to the surface of the gasket? Yes, it does.
Impregnation of the gasket surface has the effect of a protective film on the graphite. This is not necessary for graphite gaskets. Scratches or even small dents on the surface, which are by no means decorating, are destroyed even with low pressure on the surface.
The following photos show a tested pad with various defects and the same pad after being under 20 MPa pressure:
However, the leakage was measured on the previous tested ring in nitrogen at 40 bar. The result does not show any deviation compared to the gaskets without defects. Up to a depth of 350, the defects were reasonably balanced due to the good flow properties of the graphite.
Can impregnation prevent moisture absorption? No, it cannot.
This effect, which is often associated with impregnation, does not actually exist.
For testing purposes, the impregnated punched graphite and NOVAPHIT SSTC gaskets were dried at 100°C and then stored at 25°C and 90% humidity for 24 hours.
Moisture absorption has been measured.
The above table shows that none of the pads absorbed a lot of moisture.
Testing the gaskets for their maximum surface pressure did not reveal much of a difference compared to dry gaskets.
In conclusion, we can say that there are no advantages in impregnating graphite gaskets.
In addition, impregnated gaskets are more susceptible to inaccurate handling or bending because impregnation reduces the micro-flow properties of graphite.
Cutting options with a plotter
In addition to the most important technical properties of gaskets, it is also important to pay attention to the material behavior during cutting.
The main purpose is to cut gaskets with special geometries, such as heat exchangers, than larger quantities of standard gaskets.
NOVAPHIT SSTC stands out in this respect also when compared to other graphite materials.
In addition to cutting the liners by hand using knives, special scissors, it is also possible - and much more convenient - to use a plotter.
Regular tests have shown that the plotters made by Aristo (Hamburg, Germany) are especially suitable for cutting our material.
NOVAPHIT SSTC. Even extremely small spacer widths and narrow radii can be punched out without any problem from a 2D drawing.
Application - SIHI thermal oils for pumps
Environment: synthetic oil heating tank
Installation: pump gasket made of GGG30, theoretical surface pressure 50 MPa.
Temperature: from room temperature to 360°C with varying loads.
Pressure: 4 to 9 bars
Dimensions: (e.g.) 216x229x1 mm
Until now: fiberglass spacers up to 290°C
Problem: Above 290 ° C, all (graphite) gaskets with perforated and smooth metal inserts failed during test runs and simulation of varying loads.
Solution: NOVAPHIT SSTC passed the entire test program in 1997
Application - steam crushers in chemical plants
Since the revision in March 2001, NOVAPHIT SSTC gaskets have been used in over 300 nozzles in steam crushers up to PN63.
Once the NOVAPHIT SSTC was installed, the frequently encountered gasket problems were forgotten. Until then, many different materials were used from graphite groove gaskets in Kammprofile gaskets.
Application - steam
Environment: steam / superheated steam
Installation: Sealing the hose connections on the steam turbine
Temperature: from room temperature to 425°C with varying loads
Pressure: up to 23 bar
Problem: Due to a leak in one of the multilayer, laminated gaskets installed on one of the four turbines, it had to be replaced every 6 to 8 weeks.
Solution: NOVAPHIT SSTC was installed on the first turbine in the summer of 1997 and is still running without any problems. (Prevention: 5 years).
Reason: multilayer coating of smooth metal structures is only possible at a limited level in order to even out damage on the gasket surface due to the comparatively thin outer graphite layer.
NOVAPHIT SSTC, however, is much more durable as the insert does not serve as a separating layer.
Some types of gaskets have a relatively thin layer of material that acts as a seal. In sealing surfaces without any defects, the leakage is very low.
In practice, the sealing surfaces look different.
They are strangers in the system of sealing materials.
However, gasket materials used to be tolerant such factors.
The following graph shows a ''perfectly'' compressed 2.0 mm graphite gasket (compressed at about 50%) and symbolizes the effect of damaged flanges.
Multilayer smooth metal delaminates the expanded metal.
In multilayer smooth metal, only the outer graphite layers are laminated to serve the purpose of leveling uneven surfaces. Where the flanges are damaged, the compression of the original 0.5mm thick graphite layer is significantly lower.
Due to the open and unseparated structure of the expanded metal - the core. NOVAPHIT SSTC is fully compressed liner, thickness can be adjusted to compensate for unevenness. The compressibility in the damaged areas is only slightly lower than in other areas.
The following pictures show the condition of the sealing surfaces at a glance, as they appear in practice.
The pressure during installation, the increase in the output of the installation tasks and the increase in costs will apply to the flanged surfaces and other components, the difference in appearance was minimal than shown above, regardless of the standards and regulations used. This is another argument for NOVAPHIT SSTC.
When choosing a graphite material, it is necessary to take into account not only laboratory conditions, but also the “harsh” reality. All of the above explains all the benefits of NOVAPHIT SSTC - how to get the highest quality at a decent price. Any graphite gasket can be replaced with NOVAPHIT SSTC.
An added benefit is the exceptional ease of use during installation.
Any graphite gasket can be replaced with NOVAPHIT SSTC. Nevertheless, standardization is a decisive factor in reducing costs and increasing the level of safety in the flange connection.