Successful joint sealing

It's easy to point a finger when a joint won't seal or when a sealed joint leaks. Usually, that finger gets pointed at the gasket. But, while the gasket plays an important part in the seal, other factors may have caused the failure. The PVRC Study--Post Mortem Observations and Findings documents why seals fail:

• Flange misalignment (12 percent).
• Loose bolts (15 percent).
• Gasket defective (22 percent).
• Flange damaged (25 percent).
• Improper installation (26 percent).

As it indicates, damaged components, misaligned equipment and improper joint assembly, as a whole, caused more seal failures than did gaskets. That's why plant professionals should familiarize themselves with the ways sealing system components influence the integrity of a connection, and acquire a set of practices for optimizing gasket performance, solving problems and sealing joints successfully.

A wealth of published technical information and resources is dedicated to proper manual closure on bolted connections. However, Providence Bolting Technologies captures the essence of successful joint sealing in a technical assembly training manual, Bolting to Reduce Fugitive Emissions. Its author states, "It has been my experience that most problems can be solved by adhering to good basic practices. Most often I find that controlling the friction, applied torque, cross-torque and relaxation prevents the leak. And all of these are well within the boundaries of the craftsman's abilities."

So, the issue is not overly complicated. Rather, it's installer training that can make the difference. By understanding sealing system components and their functions, correct gasket installation and proper joint assembly, one can eliminate many potential problems.

Know your ingredients
Three main objectives in the static sealing of piping and equipment connections are:

• Convert the bolt tightening force into gasket load.
• Distribute the developed load evenly.
• Ensure sufficient load to counter relaxation and external forces which may act upon the joint.

The three elements of the connection are the flange surfaces, the gasket and the tightening hardware. Each plays a part in how well a joint seals. Only by understanding each can one achieve consistent, successful joint sealing.

Flange surface and gasket selection
The flange's surface condition plays its part first. Well-machined surface finishes secure a seal, as well as provide resistance to gasket extrusion. Conversely, radially scored, scratched, pitted, corroded or contaminated surfaces prevent proper gasket seating. "Seating" is the compressive stress required to deform the gasket sufficiently to prevent leakage. Whatever gasket material is selected, it must be able to adapt. Neglecting the surface finish or condition can be costly.

The gasket
Select the proper gasket material carefully. Its thickness is as important as choosing on the basis of temperatures, pressures and media. Don't use more gasket than you need. A thinner gasket is more desirable than a thicker one--provided it can adapt to surface irregularities. Under a compressive load, gaskets flow plastically and change dimensions. Temperature simply increases the effect. Therefore, under load, the gasket should deform sufficiently to fill surface irregularities, exhibit low or tolerable extrusion or creep, and be able to withstand crushing and maintain dimensional integrity.

Bolts or studs
Tightening hardware includes the nuts, washers and bolts or studs. Each performs a critical role. They not only apply compressive load, but maintain load while resisting mechanical or thermally-induced changes that result from process cycling, shock, vibration, prying and shear.

Reusing hardware is risky. It's a gamble because one can't tell visually whether bolts or studs have been strained beyond their yield point. However, if they are reused, at least ensure that undamaged threads within the useable length are wire-wheeled or brushed. Be diligent in removing scale, debris and process fluid or media, including old, contaminated or weathered lubricant. Don't contaminate new lubricant by applying it over the old. If the nut doesn't spin freely by hand, restore the threads with a die or re-threading tool. Bolts or studs should be the same diameter, type and grade.

Nuts
The most critical and dangerous component of a system to reuse is the nut. Many published articles, as well as The International Fastener Institute, support this view. Extensive studies have shown that a reused fastener is not strong enough to develop sufficient force.

A recent study performed at our labs on SAE J429, 3/4-10 Grade 5 bolts found that, after six reuses, the torque needed to achieve the 18,440-lb. target load increased from 230 to 334 ft-lb. This was almost 1.5 times the initial torque. However, when the worn nut was replaced, the original 230 ft-lb. was again sufficient. The point is that nuts are important but inexpensive component to replace. Too often, reused or worn nuts are not recognized as a critical risk factor. Certainly, exchanging these components costs much less than dealing with a leakage and an unexpected production interruption. So, replace nuts or other components if they appear worn or damaged, or have been used multiple times. Of course, make sure that they are compatible and the same size, type and grade.

Washers
Another important yet overlooked component is the washer. It acts as a bearing surface to reduce friction and distribute the load evenly. While using through-hardened steel washers is highly recommended, it is typically disregarded. Hardened washers provide a flat thrust-bearing surface and promote bolt perpendicularity. They also can promote greater load uniformity from bolt to bolt, which means more uniform gasket loading.