For flanges, specifications and best practices historically have focused on ½-in. to 24-in. diameter ANSI pipe sizes. However, many process plants also have larger flange connections, typically associated with process equipment. Yet, flanges bigger than 24 inches in diameter remain largely untouched by standards — and thus over the decades generally have fallen outside the purview of specifications teams and Six-Sigma-like radar screens. Ironically, often these flanges are part of the most vitally important equipment in the entire processing operation, e.g., reactors and distillation columns.
As a result, it’s common to have specialized gasket constructions for larger flanges, with these often custom-fabricated offsite. To avoid the risk of lengthy lead-time delays, plants often invest in costly inventories of gaskets to accommodate each specific flange. In this size range, a single critical application gasket could cost hundreds if not thousands of dollars.
In addition, custom gasket construction raises the risk of variation in quality and performance. Large pre-cut gaskets also are difficult to ship and store. Moreover, these gaskets add complexity to the installation process, as they often require delicate handling to avoid functional damage. Frequently, cranes are needed to hoist them into place.
Large systems and equipment and their related flanges sometimes are hydro-tested by the original equipment manufacturers using a generic, elastomer gasket type. While this is fine for checking vessel integrity, it often leaves the user plant without clear direction when it comes to gasket selection in actual service.
For actual service, a user should choose gasket materials based on multiple criteria, including: the kind of media being processed, temperatures, flange loading, operation dynamics and installation methods.
Moreover, while elastomer-based materials provide an effective short-term seal for the hydro-test, they frequently can mask hidden imperfections in larger flanges. Once users move to more-traditional gasketing choices for real plant operating conditions, even the slightest flange imperfections quickly can come to light. That’s because metal, polytetrafluoroethylene (PTFE) and other commonly used gasket materials often sacrifice conformability (relative to elastomers) for higher chemical and temperature resistance, and durability.
Due to these layers of complexity, and with few specification guidelines or shared best practices to go on, it’s no wonder that plants often use many different custom-fabricated gaskets. This can result in some surprising situations. For example, one large vessel might have a specific gasket construction that works very well for it. Situated right next to that vessel may be a similar one that relies on a dramatically different gasket construction. In many cases, no one currently in the plant can explain the reason for the disparity, the rationale having been lost over time.
The Current Situation
While each processing facility has its own unique characteristics, we can generalize to a significant degree about common practices regarding gaskets for large equipment flanges.
For large gaskets that are custom-fabricated offsite, first, the critical dimensions are measured and mapped to enable a gasket fabricator to create the specialty size or shape required for the application. The more-sophisticated gasket fabricators archive these dimensions — and have transitioned from cardboard cutouts to digital design files.
Because these gaskets are specialty items, generally only when a plant places an order does the construction process begin. This is why a gasket often requires a lengthy lead time — sometimes as much as four to eight weeks before delivery.
To cope with this, many processing facilities have learned to order their supplies early. This can create significant challenges and stress on the plant’s procurement and turnaround planners. Typically, storerooms are encouraged to keep multiple extra gaskets for specialty units in inventory “just in case” of a supply pinch or to serve as backups should gaskets get damaged in transit, storage or installation — an all-too-common occurrence with large specialty gaskets.
Visit Your Storeroom
When was the last time you took a look at your plant’s gasket stores and checked out the large gasket situation?
If yours is like many processing facilities, you’ll find a multitude of material types — each selected to accommodate the various plant operations. Maybe some have been procured as the result of “fighting fires” over the years in desperation to seal a finicky unit.
PTFE is a favored gasketing material for chemical service; it comes in wide number of varieties, ranging from 100% pure PTFE (including expanded) to versions filled with glass, barium sulfate, or even ceramics. You’ll likely find a multitude of gasket constructions, too. Some large gaskets are welded together from smaller pieces, some are sandwich constructions with metal for added stability, and others may have a PTFE envelope surrounding components of other materials.