Gasket selection after asbestos
Not too many years ago, selecting the right gasket was easy. It was asbestos. But once asbestos was identified as a health hazard, the gasketing field started to change radically and the gasket selection process hasn’t been the same since.
At one time, most piping joints were sealed with asbestos, available primarily in sheets or cut gaskets. Asbestos was a wonderful gasketing material — it was inexpensive and it worked well. At least we thought it worked well. Environmental and safety regulations were few and nonspecific, and fugitive emissions were a relatively obscure concept. Gasket performance was seldom an issue.
In the 1980s, asbestos became identified as a health hazard and environmental concerns drew attention to flange connections as a source of dangerous fugitive emissions. The search was on for other materials that would perform as well as asbestos, yet would be safe.
Since all plants operate under budget constraints, a gasket can now be selected for a particular application based on matching up requirements while balancing costs.
What tests tell
Today, there are a multitude of gasket performance tests that are performance driven — and a vast improvement over old testing methods. These tests can determine the viability of any gasket material for a particular application.
Current test results should help engineers make informed decisions about the best possible gaskets for their applications.
It’s one thing to possess all this important test data. It’s quite another to put it to use to determine the best gasket material for an application. Also, it doesn’t help that many gasket suppliers are misusing the data, showing comparisons between gasket materials within very narrow contexts where their gasket material might appear to provide superior results.
Basically, the criteria for proper gasket selection boils down to what can be referred to as TAMP: Temperature, Application, Media, and Pressure. How well does a particular gasket material stand up to the temperatures to which it will be exposed, the application (this relates primarily to flange material and type, conditions, joint/bolting design, assembly practices, etc.), the media it will be required to resist, and the pressure to which it will be subjected?
However, there may be additional criteria related to certain gasket applications that should also be considered, such as fire or explosion hazards. Engineers should also take into account all of the application-specific criteria that may be present in the plant.
In a refinery, for example, resistance to fire should be factored in, so as not to exacerbate a problem by using a gasket that might melt or break down, thereby creating an open connection and spewing flammable liquid (Fig. 1).
Gasket materials maze
Asbestos is remembered as being cheap, mechanically strong, and effective in almost all applications. And, in its heyday, there weren’t very many other gasket materials on the market.
Today, the situation is different. A profusion of gasket materials provides a wealth of choices, but this also complicates the selection process.
Basically, gasket choices can be grouped into five classifications:
Compressed synthetic fiber (CSF)
Within each classification there are additional variations. Some gaskets are a combination of materials (e.g., envelope gaskets, with a PTFE envelope surrounding CSF or other compressive material and a metal core). To make the best selection for each specific application, the characteristics and performance properties of each gasket option must be analyzed carefully.
As newer gasketing materials were developed during the 1980s, engineers began combining PTFE with other materials to achieve better creep resistance. But it wasn’t until expanded PTFE sheets were introduced in the early 1990s that creep relaxation was significantly reduced.
Today, numerous expanded PTFE gaskets are available. Just as it is important to consider the performance comparisons among the different gasket materials, it’s also critical to look at the performance testing comparisons of the various expanded PTFE brands as well.
As if the gasket selection process weren’t complicated enough, there is yet another element to consider, gasket type. In this matter, decisions are based on flange diameter and material, access to the flange surface, gasket inventory, and cost — with cost playing a major role.
The decision facing the maintenance engineer is often whether to use a cut gasket, gasketing tape, a spiral-wound gasket, or an envelope gasket.
On a large-diameter flange, gasketing tape is often easier to install and less costly than a cut gasket, since you’re not paying for the scrapped material often lost with a large pre-cut gasket.
If sealing equipment needs to be completely disassembled in order to install a cut gasket, installation costs can be reduced by using a tape product (Fig. 2).
Process temperature may dictate using a spiral-wound metal gasket material. Or, if the flange is fragile, either an envelope gasket or gasketing tape might be used.
Since budgets inevitably play a key role in the gasket selection, do not just consider the price of the gasket alone. Instead, look at total installed cost.
A good illustration of this point is a large reactor with a mixing shaft. An envelope or cut gasket may demand that the reactor cover be completely removed and the mixing shaft disassembled from the motor. A crane might even be required. Installation could take days.
Using a gasketing tape for this application would eliminate the crane and specialized labor, while reducing installation to a matter of hours. Thus, the total installed costs would be dramatically lower.
With a constant eye towards reducing costs in industry today — and with the proliferation of available gasketing materials — it may make sense to try to standardize and select one primary gasket material. A quick look at inventory will likely reveal gaskets of all types and materials (Fig. 3).
If one material could be standardized, not only could money be saved, the possibility of selecting the wrong gasket for an application and causing a serious problem would be avoided, something that could cause an unexpected shutdown, or worse.
Examine all gasketing applications. See if there isn’t one material that would work for the majority of them. Even if the price of that one material might be higher than what is currently being used in some less critical applications, standardization would lower overall costs by reducing inventory, freeing up storage space, simplifying the purchasing decision and administrative processing.
Based on experience, it is believed that expanded PTFE, which can be used in every sealing application in the plant under 600 F, could be a candidate for a primary, standard gasket material.
While selecting the right gasket is critical to a successful seal, it’s not the only factor in sealing a pipe or equipment joint successfully. A recent study showed that most seal failures were the result of worn or damaged flange and tightening hardware or improper assembly techniques.
Once the correct gasket material is chosen, three objectives must be met if the joint is to be sealed successfully:
Transfer the load developed during bolt tightening to the gasket
Make sure that the bolts are perpendicular to the gasket and the load is uniform
Ensure that there is sufficient pre-load to counter any relaxation and/or external forces which may act upon the joint.
These factors are all within the purview of the maintenance mechanics working on the equipment.
Actual gasket selection should be left to a specifications expert, who must keep abreast of the latest materials and their performance, and match the right gasket to each application.
The selection process has become more of a science now than ever before. It behooves manufacturers and suppliers to work with plant engineers to evaluate materials and present accurate data and then work with maintenance personnel to ensure that the gaskets are installed properly.
More Info : If you have any questions about gasketing contact Rob Haywood at 410-506-7563 or firstname.lastname@example.org . Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, email@example.com