Technical Information Data

Maximizing Service Life

The Topog-E Gasket is designed to offer specific and aggressive resistance to the hostile atmosphere of live steam and to offer a minimum of deflection under load. We recommend a limit of 180 psi steam (12 bar) and 380*F. (193*C.) in steam boiler applications. This is not stated as a maximum upper limit, but since we have no control over the mounting of the gaskets in the field, it is a statement of reasonable expectation of service life. The following precautions, if observed, can help maximize the service life of Topog-E Gaskets.

1. The curvature of the cover plate should match the curvature of the boiler shell.

2. Observe good housekeeping; let clean metal surfaces bear on the gasket if at all possible. Be sure that no encrusted matter is on the boiler shell.

3. Machine the crabs to a true 90 degrees from the vertical axis so that an absolutely
balanced stress acts on the gasket.

4. Press the gasket into firm contact with the cover plate, and the bolt gasket, if used, into firm contact with the bolt head.

5. Center the gasket cover plate exactly on the opening, then apply the crab. Tighten only enough to hold the assembly in place.

6. Start making steam. As the gasket heats, stress relaxation occurs and leakage commences. Tighten only enough to stop serious leakage and ignore hissing. Follow this procedure in gentle stages until the internal pressure itself effects the seal.

7. Keep the crab snug. In the event of shutdown, inverse pressure will displace a loose cover plate, violating the seal. Once an escape path has been established for the steam, it is difficult to reestablish a seal having complete integrity.

8. Minimize as much as possible the amount of outside air that reaches the annulus of rubber that is exposed and subject to oxidation. Properly centering the cover plate will help significantly in protecting this vulnerable area of the gasket.

9. Never overtighten!! A properly mounted gasket is held in place by internal pressure, and is secured from shifting by the crab. In this manner, the rubber never witnesses greater pressure than steam pressure, and thus its elastic memory will not be exceeded.

10. Always follow the advice of a qualified water treatment specialist. The proper use of boiler water and steam line treatment formulations is essential for efficient boiler operation. Over the years, we have analyzed the chemicals that are used on a regular basis in treatment formulations. Data compiled from solubility parameters, chemical resistance data, suppliers' guidelines, customer feedback, and input from treatment specialists, suggest that there should not be a noticeable effect on the service life of a properly installed Topog-E Gasket from the normal use of these water and steam line treatments when used in accordance with treatment specialist's guidelines. Boiler operators should always seek out and follow religiously the wise counsel of a reputable water treatment specialist, as doing so increases system efficiency, prolongs the life of expensive equipment, and minimizes the potential for chemical compatibility problems with various system components (e.g. gaskets).

Stress Relaxation

Stress relaxation is a visco-elastic response of an elastomer when impressed into service. This relaxation is promoted by such stresses as heat, compression, elongation, and shear. If this relaxation proceeds at a measured pace (see steps 5 & 6 above), very little impairment is caused to the physical properties profile of the gasket. When a combination of stresses, one of which is heat, is visited on rubber, relaxation is specific on this heat. The compressive stress must be coordinated with heat relaxation, but if applied at a rapid rate and maintained (i.e. by prematurely overtightening), irreversible damage is caused to the gasket, and a shortened service life can be expected. Remember, overtightening a properly aligned gasket in the cold condition can seriously impair service life, but overtightening a misaligned or ill-fitting gasket in the cold condition can often generate enough shearing force to cut a rubber gasket in two even before a boiler comes up to its operating pressure.

The Environment of Steam Pressure Vessels

Even under ideal conditions, Topog-E Gaskets will deteriorate over time. The three principal factors that cause this natural degradation in organic elastomeric material, oxygen, stress, and heat, are interdependent and synergistic (i.e. working in concert these factors create a more hostile environment than they do when acting independently of each other).

1. Atmospheric oxygen is probably the cardinal agent of harm to the rubber gasket in service, and its effect increases significantly in the presence of the other two elements.

2. Stress is the compressive effect - the reduction in thickness brought about by the steam pressure of the boiler.

3. Heat and steam pressure are gradients of each other in an exponential relationship. As a rough generalization, it may be stated that the speed of a reaction doubles for each 18*F. (10*C.) rise in temperature. Refer to the Topog-E Steam Temperature Calculator to calculate specific temperatures and pressures (we will be happy to send you one upon request).

Topog-E Gasket material has specific and aggressive resistance to the hostile environment of live steam. If very low oxygen content is present in the steam, trouble rarely arises inside the boiler with a properly mounted Topog-E Gasket. The real test is outside the boiler where a thin periphery of the gasket is exposed to the atmosphere that contains 21% oxygen. This component is highly corrosive to rubber, an effect which increases with heat, pressure, and also with the amount of gasket surface area directly exposed to the outside atmosphere.

The situation is further complicated by the fact that rubber under stress degrades more rapidly than rubber that is not under stress. Unperturbed, survival of a rubber component is measured in years, while under stress, it is most often measured in terms of months. This is why it is important to follow the installation instructions and the precautions listed above in order to insure that the optimal service life of Topog-E Gaskets is obtained.

An organic system like the one used in Topog-E Gaskets can seal effectively for periods averaging approximately one year in a steam pressure vessel, even though the outer surface of the gasket will gradually crack and degrade under the unremitting and combined attack of oxygen, stress, and heat. The 180 psi (12 bar) level does not represent an absolute upper service limit for Topog-E Gaskets. Rather it represents the approximate highest and continuous pressure level at which one can expect to obtain an average service life of twelve months from properly installed Topog-E Gaskets. Customers have reported using Topog-E Gaskets successfully in steam pressure vessels operating at pressures moderately higher than 180 psi (e.g. 200-250 psi), and it is important to note that doing so is not imprudent or inherently unsafe, especially if the Topog-E Gaskets are carefully installed. However, it must be remembered that the higher the steam pressure and corresponding temperature, the shorter the expected service life of the gaskets becomes.

Topog-E Gasket Company invests heavily in research and development efforts to insure that Topog-E Gaskets live up to their worldwide reputation for quality, durability, and ease of use. Topog-E Gaskets are tested in-house at steam pressures and temperatures far exceeding the 180 psi and 380*F. levels below which most Topog-E Gaskets are used in steam applications. Carefully installed and monitored Topog-E Gaskets are routinely subjected for extended periods of time to steam pressures and temperatures of up to 360 psi and 438*F. However, we do not actively recommend service in boilers at these elevated testing levels because we have no control over how our gaskets are installed and because there are always unique application specific factors that can affect both the installation and service life obtained with Topog-E Gaskets.

Other Applications

Topog-E Gaskets have been used successfully around the world for over twenty-five years. In addition to using them in steam pressure vessels, customers also use Topog-E Molded Gaskets and Sheets with great success in many other applications, including: water softeners, hot water heaters, steam humidifiers and cookers, water purifiers and demineralizers, refrigeration units, liquid treatment vessels, carbon absorption and filtering vessels, dryer cans in paper mills, water hydrants, various types of mixing tanks, compressed air tanks, various types of dryers, air starters and receivers, and hatch covers on railroad tank cars and river barges. In general, any type of industrial pressure vessel or tank that has inspection openings is a potential application where Topog-E Gaskets can be used as a cost effective sealing device.

Because there are many application specific factors that can affect service life, it is always advisable to first test Topog-E Gaskets in a particular application to determine their ultimate suitability. Please contact your distributor or Topog-E Gasket Company if you have questions regarding a specific application.

Boilers make steam by boiling water. That's about the most oversimplified statement by boiling water . That's about the most oversimplifed statement we could make. Some very interesting things happen when water is boiled. At atmospheric pressure water expands 1,600 times its original volume when its turns to steam. Which explains why a tea kettle may put off a plume of vapor half the morning before running dry.

Water at atmospheric pressure boils at 212*F. The boiling water in an open vessel measures 212*F, the steam coming off the surface of the water also measures 212*F. All the while, heat is being applied to the vessel but nothing is increasing in temperature. The applied energy is going into changing the water into steam. The steam will give up this energy before returning to liquid. This energy is called Latent Heat and for most purposes is considered the useable energy of steam.

Now, below the boiling point, the heat applied to the vessel goes into the water and raises its temperature. We are able to sense this temperature change with a thermometer and we call this heat Sensible Heat.

Steam is so familiar to all of us that we easily forget what a marvelous thing it is. Steam will carry twenty times the BTU's per pound that Freon 12 will, nearly fifteen times the BTU's per pound of F22, and over twice that ammonia. Even when we use nuclear energy for power, it is only to heat water to make steam. For all of this, we tend to take steam for granted without much thought to how it really is produced or how it works best for us.

For most purposes for which steam is used, it is the Latent Heat of the steam that we utilize, for after the Latent Heat is given up, the steam recondenses into a liquid. Wherever possible, we return this condensed steam, or condensate, back to the boiler for reuse. This conserves a large part of the Sensible Heat we had to apply to raise the temperature of the water up to 212*F. Even if the condensate has come into contact with the contaminating materials that make it unsuitable for return to the boiler, it can often still be used to help heat new incoming water through a heat exchanger and thus reduce fuel requirements.

So far we have only talked about an open vessel for boiling. Close off the vessel to the atmosphere and, as we continue to add heat above the boiling point, the expanding steam causes the pressure to rise. This increase in pressure, however, causes the temperature at which water boils to rise. This calls for a further supply of Sensible Heat to get the water to its new, higher, boiling point. At the same time though, the Latent Heat required to convert the higher temperature water into steam is reduced. The net result is only a slight increase in the Total Heat required for each pound of steam. But since that same pound (by weight) of steam will occupy 26.8 cubic feet of space at atmospheric pressure and only 2.14 cubic feet at 200 lbs. per sq. inch pressure, it is more handily packaged at higher pressure. While occupying only 1/8 the volume, 200 P.S.I. steam has only required 4 1/2% more heat per pound. Smaller piping will serve the same system and will reduce piping heat loss as well.

For steam engines or turbines, it is often part of the power unit's design to use superheated steam. This is a specialized application of steam utilizing the entropy of steam (which needs to be treated separately.) Suffice it to say that after expansion through the engine, this steam has nearly all of the heat units that were put in it, and can often be put to additional use in process applications.

Steam can be in one of three forms, wet, dry, or superheated. Wet steam contains small water droplets entrained with it. These droplets contain no Latent Heat and therefore have before becoming returned condensate. Just 6% of water particles at 200 P.S.I.G. reduces the Total Heat in a pound of dry steam at atmospheric pressure.

Dry Steam requires the elimination of water particles from the steam line. This can be done mechanically with baffles in the steam flow designed so that water particles are deposited and left behind, or by removing the wet steam from the water surface and applying additional heat to drive all the water particles into steam. Flue gas waste heat is increasingly used for this additional heat.

Superheated steam occurs when enough additional heat is applied to raise the steam temperature to any level above that of saturated dry steam. Some superheat is sometimes required to make sure that dry steam is available at the end of a long steam line. It would seem that superheat would be the course to take in all cases as a means to transfer more heat. In reality, the amount of additional heat transferred is not great. One pound of steam at 200 P.S.I. superheated 100*F. more has gone from 1200 B.T.U.'s Total Heat to 1260 B.T.U.'s a gain of only 4.8%. While that small change was taking place, the saturated steam started acting like a perfect gas and expanded in volume from our original 2.14 cubic feet to 2.45 cubic feet, a volume change of 14.5%. The heat content per cubic foot of steam has actually decreased by superheating.

Reflecting then, we got eight times the volume of steam in the same space by raising the pressure from atmospheric to 200 P.S.I.G. but we lost 14.5% volume efficiency by superheating another 100*F. at the same 200 P.S.I.G. Superheat is therefore and advantage only when the heat requirement can be met by the small amount of energy in the superheat and where the higher temperature gradient from the hotter steam is required by the process job.

We have not attempted to discuss some certain areas of steam use. For example, we have said nothing about steam trapping, a very necessary consideration in most steam systems. But we have zeroed in on steam's basic simplicity, its great heat transfer capabilities and the versatility of its application. When these are taken together, it is obvious that steam will always continue to be a very important tool to industry. It will not become obsolete for there is nothing else that even comes close to having these properties.