'No leak' gasket cuts fugitive emissions and saves money

Paul Boughton

Andreas Schmiedel looks at how plant operators and suppliers have to rely on their own assessment of gasket materials when it comes to fugitive emission regulations.

The EU IPPC Directive, the US Clean Air Act or the Kyoto Protocol are just some of the regulations companies have to deal with. It is not a question whether or not, it is a question how to comply in the day-to-day business with these stringent regulations and the demand to apply BAT (Best Available Techniques) sealing solutions for a sustainable development in the industry.
In fact the situation in Germany is already crystal clear: since October 2002 plant operators have had to observe the drastically tightened threshold values on diffuse emissions -- that is what the revised German Fugitive Emission Regulation TALuft requires which have thus been adjusted to the new European regulation (Council Directive96/61/EC) as well as to new environmental and technical standards.
This German regulation is leading the way for the other European member states and will most probably be adapted and transformed into national laws in other European member states, latest until 2007.
So far, so good. Practice, however, shows: It is not sufficient to blindly rely on certificates presented. It is true, those confirm that this important criterion on leak rates is kept.
A general statement on temperature and chemical resistance or on tolerance to improper installation of the gasket materials, however, completely lacks in such certificates.
The most important references of the German fugitive emission regulation are VDI [Association of German Engineers] 2440 (Emission Reduction in Oil Refineries) as well as the VDI2200 (working title: Tight Flange Connections) the preliminary version of which is to be published within short.
On the one hand they stipulate very low leak rates of 10-4mbar*l/(s*m) in components tested with a differential pressure of 1bar helium after heat treatment. On the other hand blow-out resistance is a must.
Experts, however, emphasise that these criteria are in no way sufficient especially in the highly sensitive areas of the chemical and petrochemical industries and in plant construction to guarantee maximum safety and availability.
Inadequate installation of the gaskets or lacking maintenance of the necessary tooling, for example, as well as insufficiently trained personnel or too low tolerance to installation errors of the gasket materials used can deteriorate the functionality of state-of-the-art gasketting solutions to the same extent as can insufficiently cleaned or disproportionately damaged flange surfaces.
The increasing share of external assembly work caused by the outsourcing of labour-cost intensive maintenance work makes the above aspects ever more important. A fool-proof one-fits-all solution is desperately needed to meet the new standards.
In view of these facts one may wonder: What is the actual behaviour of newly developed gasket materials in practice?

Putting them to the acid test

The following materials dare the comparative test: novaphit SSTCTA-L and a second, commercially available material. Both materials are certified to the German fugitive emission regulation TA Luft and consist of expanded graphite with metal insert. What we would like to know: How high is the adaptability of both aspirants under realistic test conditions?
In order to examine this criteria radial grooves were applied onto the flanges, which had an original roughness of <5µm. Once they were millcut (exact setting of depth and width), and once they were carved -- as this frequently happens in practice.
Result: The millcut version results in even depressions all over the flange, while there is an additional flash to the left and to the right of the depression when carved (Fig.1) -- a real challenge to any gasket material.
The test conditions: Four flanges with millcut radial grooves of different widths and depths (eg Fig.2). Assembly of the gaskets with a surface pressure of 30MPa. Removal of the gaskets after five minutes. Measurement of the permanent deformation of the gasket material above the grooves via 2D measuring device.
The result: novaphit SSTCTA-L filled the gaps in a much better way than did the second material (Fig.3).
One question remains: What is the effect of the different degrees of adaptability on the leak rates in a real module?
The aspirants are: novaphit SSTCTA-L without inner eyelet; novaphit SSTCTA-L with inner eyelet made of AISI316 (1.4571); a second graphite material also tested to TALuft with a modified inner eyelet.

Test conditions

The test conditions during the leakage test: test method similar to German standard (DIN28090-2): Gasket assembly with a surface pressure of 30MPa via calibrated micrometer screws; exposition of the module to 300°C for 24 hours; determination of the leak rates after cooling with an internal pressure of 40bar nitrogen via the pressure drop method.
Smooth flanges show leak rates below the limit. Grooves of 60µm depth and 115µm width do not constitute any problem either.
In the third test (depth 95µm, width 150µm), however, the versions equipped with inner eyelets show clear weaknesses.
While there is only a very low increase in the leak rate of the novaphit SSTCTA-L the increase in ovaphit SSTCTA-L with inner eyelet is already significant.
Looking at the competitor's material the results are really staggering: The leak rates are so high that they can hardly be measured. On top of it more than half of the tests of this type blew out (Fig.4).
There are good reasons for the superior victory of novaphit SSTCTA-L:
Firstly, the expanded metal insert consists of cid-proof chrome-nickel steel -- the basis for low leak rates. This is millionfold tried and tested in its asister' called novaphit SSTC. Secondly, its gradual density.
The higher densified core considerably increases the material's cross-sectional tightness .The density of the outer layers remains on a typical value for graphite -- for a maximum degree of adaptability.
Thirdly, its internal impregnation which is capsuled in the remaining pore volume of the graphite and thus further increases the material's cross-sectional tightness. In the competitor's material a modified inner eyelet is to increase cross-sectional tightness.
As Fig.5 clearly shows that the surface of the inner eyelets mainly consists of this graphite foils put on smooth metal inserts. The adaptability of such a material is clearly limited in practice.

Conclusion

Experience shows that the cases as constructed above quite frequently occur in practice. Despite an explicit forbiddance sharp objects are used for cleaning the flange surfaces. It is definitely not possible to check all flanges beforehand.
This result is also valid independent of flange irregularities as well as in view of conditions as can be found in practice of one of the flange roughness Rz of up to 160µm (form B and C) as laid down in standards, eg in DIN2526 and spiral grooves. It becomes evident that the competitor's material tested does not fill these grooves.
The result will be excessive leakage. Such a gasket material can definitely not bring any advantages in practice despite being accredited to fugitive emission regulations. Under real conditions an unfavourable surface adaptability equals an increase in diffuse emissions in the plants.
Despite a certificate the competitor's material shows inadequately high leak rates -- and cannot bring any advantages in practice.

Economic advantages

If the user decides in favour of novaphit SSTCTA-L he will have further, decisive advantages: Standard as well as non-standard gaskets can be cut out of the sheet material.
Another asset: The cutting of novaphit SSTCTA-L with plotters, water-jet cutters, electronic scissors or even manual devices does not hinder further steps such as equipping the gaskets with eyelets. But this is not compulsory. This product offers a maximum degree of flexibility. ovaphit SSTCTA-L is a real all-round graphite gasket material -- and is able to replace all classic flat gaskets as used by the customers up to now, dependent, of course, on the area of application.


Andreas Schmiedel is with Frenzelit-Werke GmbH & Co KG, Bad Berneck, Germany. " target="_blank">www.frenzelit.com

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