Standardized sealing for large gaskets tames chemical plant complexity

Dec. 1, 2015

Still, gasket selection poses a huge challenge for chemical manufacturers and other processing facilities – mostly due to the significant complexity that follows from the wide range of operations, chemicals, vessels and flange types used.

When flanges will not seal, most of the time the blame goes to the gaskets. But how often is it a fair accusation?

A post mortem observations and findings research study by the Pressure Vessel Research Council found the most common sealing-failure causes are improper installation (26 percent), flange damage (25 percent), defective gaskets (22 percent), loose bolts (15 percent) and flange misalignment (12 percent).

Gaskets made of form-in-place ePTFE can be customized onsite and enable a quick and easy installation, even for special configurations.

However, this capacious complexity can be brought to order using a sealing material that covers all applications and requirements.

Specifying by service

Day-to-day and by means of strategic initiatives, executives and managers unlock hidden resources, improve operations and mitigate risk. Success follows from focus and finding practical solutions to complex challenges.

However, fluid sealing is more complex than plant operations managers realize. Even selecting gaskets for common or routine applications remains highly dependent on process conditions. What’s needed to address those conditions is often anything but routine.

For flanges, the relevant specifications and best practices focus on standard piping from 1/2-inch to 24-inch diameter ANSI pipe sizes. Most process plant flanges fall within this range of standard dimensions, which provides a solid foundation for further specification by service.

However, dimensions outside that range remain an area where this type standardization is unrealized.

Large stands out

In fact, flanges larger than 24 inches in diameter fall outside the purview of specifications teams and Six Sigma-like radar screens. Ironically, these are the very flanges often found on a process operation’s most important equipment. As a result, specialized gasket constructions are common for larger flanges (see custom-fabricated flanges in Figure 1).

To avoid lead-time delays, plants stock special flanges. A single critical-application gasket can cost thousands of dollars. Further, gaskets often require delicate handling while needing to be moved by crane.

Large flanges are commonly associated with specialty operations such as reactors, distillation towers, mixers and tanks. The flange size in these situations is usually dictated less by a specified convention than the chemical engineering involved.

Large system and equipment sealing and related flanges are sometimes hydro-tested by original-equipment manufacturers using generic, often low-grade gaskets. While these measures are fine for checking vessel integrity, they often leave users without clear direction regarding gasket selection. For actual service, they must select gasket materials based on criteria that include the kind of media processed, temperatures, flange loading, operation dynamics and installation methods.

While rubber-based materials are an effective short-term seal for the hydro-test, they can mask hidden imperfections in larger flanges. Metal, polytetrafluoroethylene (PTFE) and other common gasket materials often sacrifice conformability relative to elastomers – even as they deliver the higher chemical resistance, durability and temperature resistance users seek.

Window of opportunity

Gaskets made of form-in-place ePTFE can be customized onsite and enable a quick and easy installation, even for special configurations. Courtesy W.L. Gore & Associates Inc.

Rigorous criteria for large gasket selection can mean lead times of up to four to eight weeks once an order is placed. That’s why plants stock gaskets. Storerooms keep extra gaskets for specialty units and to have a window of opportunity in the event of a supply pinch or a damaged replacement.

Many companies store a multitude of gasket constructions. Some large gaskets are welded together from smaller pieces, some are sandwich constructions with metal for added stability, and others have a PTFE envelope surrounding other materials.

PTFE, a popular gasketing material for chemical service, is available in varieties from 100 percent pure PTFE (including expanded) to gaskets filled with glass, barium sulfate or ceramics.

Finally, non-standard applications, such as heat exchangers, might require special gasket materials and constructions. Plants stock large gaskets for heat-exchanger configurations that vary by vessel size and by crossbar placement or number. The fate of each gasket is tied to a specific flange. Once it is purchased and sits on a shelf, it represents dormant value.

Images of inefficiency

Taken all together, these practices are highly inefficient. A better way for large-flange gaskets starts with expanded PTFE (ePTFE) technology. The ePTFE varieties eliminate questions of whether gaskets will withstand the media to be sealed.

The sole areas of chemical protection weakness for ePTFE involve molten alkali metals or significant amounts of elemental fluorine. The material also has enhanced strength and creep (cold flow) resistance under high loads and temperatures, making it an ideal material for industrial flanges, and it is significantly more conformable than other PTFE-based gasket materials.

Benefits of ePTFE gaskets include:

  • They can be customized into any shape while being installed on the flange, regardless of size or configuration.
  • Metal is not needed for gasket rigidity because it adheres to the flange surface without further intervention.
  • Materials subject to chemical compatibility are reduced from many to one.
  • A single gasket can seal imperfect or damaged flange surfaces, reduce emissions and improve plant reliability.
  • Inventory investment is reduced.
  • Chemical plant operations can be simplified.

What about gasketing tape?

The latest form-in-place gasket tapes enable performance and simplicity. Gasketing tape, as opposed to a pre-cut gasket, is more flexible and easy to install for large gaskets. The gasket-tape microstructure delivers greater reliability and enhanced operational tightness – even under extreme operating conditions.

In addition, ePTFE gaskets come as form-in-place to eliminate lead time, can be customized onsite, and enable a quick and easy installation even for special configurations, shafts and bundles and vertical flanges (see Figures 2 and 3). These sealing solutions are easier to ship and store and allow forf reduced inventory.

A misperception about form-in-place ePTFE gasket tapes arises because the material is not installed as a continuous one-piece unit. Accordingly, some fear it might leak at the overlapped section. Third-party testing shows the overlapping material does not cause leaking. In fact, ePTFE gasket tapes can deliver more tightness than other sealing solutions. 

5 steps to more gasket standardization

How do you know if your plant is ready for gasket standardization?

  • Check out the plant’s gasket storeroom to see what sort of complexity can be found in the large gasket selection.
  • Find out how much money is hanging on the walls in inventory.
  • Ask the maintenance teams to tell their large flange war stories. Chances are, they will have at least a few of them to share.
  • Find out whether cranes are ever needed to hoist large gaskets high into the air.
  • Ask how much time and trouble it is to remove and then reinsert heat exchanger tube bundles – simply to fit a one-piece gasket behind them.

These steps will tell a lot about whether consolidating on a single, more reliable solution for large gaskets has practical potential in a plant.

Rob Haywood is a product specialist at W. L. Gore & Associates Inc.

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