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.
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.
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.
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.
Washers prevent outer flange surfaces from scoring during tightening, as well as protect against embedment. When a bolt or stud is under tension, a stress concentration develops on the underside of the nut. Because nut and bolt materials are harder than the flange, the softer material can yield, allowing the nut to embed. This contributes to loss of bolt elongation and increased joint relaxation. A washer distributes the load over a larger area.
Replace soft wrought or non-through-hardened washers with through-hardened pieces. When soft-wrought flat washers are used, embedment and friction problems are a risk. These washers often compress and deform under significant load, contributing to additional compression loss. As indicated in Fastener Facts, "a bolt can lose as much as 30,000 psi clamping force for each 0.001 in. bolt relaxation per loaded inch." Use of through-hardened flat washers maintains joint service life by keeping bolts at right angles to the gasket and sealing surfaces, distributing load evenly and preventing surface damage.
Recently, a large chemical producer found that re-used nuts and soft washers can bring unexpected production problems. This facility operated a 1,000-gallon glass-lined reactor sealed with one-inch J-clamps torqued to 125 ft-lbs. After a routine gasket replacement, an expensive failure occurred. When the leak was first discovered, the maintenance crew increased the torque to the ceiling value of 250 ft-lbs. The leak continued, which led to a unit shutdown. Initially, the cause was diagnosed as a faulty gasket, not withstanding the fact that this gasket type had operated reliably for many years. Upon further investigation, the leak was traced to reused nuts and soft wrought washers. The investigation revealed evidence of abrasive wear, indicating severe friction during assembly. The washers had been coarsely gouged, dished and deformed. After cleaning the J-clamp threads properly, replacing the used nuts, adding through-hardened steel washers and following correct assembly procedures, the original specification of 125 ft-lb. was again effective.
Follow the recipe
Follow certain installation procedures to maximize gasket effectiveness. First, clean the joint and inspect the surfaces. Clean the outer flange surfaces where the nuts, washers or bolt head will rest. If necessary, file or grind them to ensure a flat bearing surface. Surface debris, such as product build-up, scale, corrosion or weld spatter, affects bolt perpendicularity and reduces the load transmitted to the gasket. Next, clean the inner sealing surface to ensure proper gasket seating. Remove surface debris, such as grinding dust, scale, dirt, rust, weld spatter, corrosion inhibitors, machine bluing, paints, previously used gasket material and residual process media. Surface contamination or obstructions inhibit proper gasket seating and make sealing difficult.
After the flange surfaces are clean, inspect sealing areas to determine if repairs are necessary. Then, select an appropriate gasket material of a thickness that will compensate for surface variations. All things being equal, a thinner gasket provides a smaller profile exposed to the hydrostatic end force, which acts to separate the flanges. It is easier to blow out a thicker gasket than a thinner one. Keep in mind that using a thicker gasket not only requires additional bolting force but is subject to more potential relaxation. Excessive gasket relaxation decreases bolt loads and jeopardizes joint integrity.
Next, check for flange misalignment, parallelism and flatness. Even the slightest angle on bolts or studs can reduce the assembly preload and contribute to joint relaxation. More importantly, equipment should be aligned so that bolts and studs pass through the boltholes using only finger pressure. Hammering bolts through damages threads, making it near impossible to achieve a safe preload. Everything must be aligned properly throughout the assembly. If not, many of the benefits of other good practices will be negated and installation labor time will be wasted.
Lubrication is another consideration. Use lubricants where they are applicable and permissible because they allow more torque to go into tensioning rather than combating friction. Moreover, a lubricant promotes accurate, consistent and uniform bolt loading. This, in turn, leads to a higher, more uniform gasket loading. Apply lubrication to the bolt or stud threads, nut thrust bearing surface and the washer face that contacts the nut's bearing surface. When it isn't possible to use washers, apply lubricant to the outer flange and nut's bearing surface.
On new equipment, it's acceptable to lubricate threads before inserting them through the boltholes. On older equipment, or those exposed to the elements, apply lubrication after insertion to ensure that dirt and debris does not contaminate the lubrication on threads as they pass through the holes.
Above all, remember that the objective is to attain a uniform load on the gasket. Avoid mixing old bolts and nuts with the new. This defeats uniform gasket loading from bolt to bolt.
Some nuts have raised identification markings. Install them so the markings face outward. Otherwise, the bolts or studs will not be perpendicular to the adjoining flanges and, when turned, the high spots will score and drag, thus increasing friction and damage. This will, in turn, consume much of the effort during the tightening process intended to place the bolts in tension. It also can cause a considerable amount of surface damage, especially if impact tools are used.
Proper gasket handling involves keeping gaskets free of surface nicks and irregularities that interfere with sealing performance. Ensure that gasket dimensions are accurate. Most importantly, perform a final pre-assembly inspection to prevent a dimensionally incorrect or inappropriately positioned gasket from causing problems at system startup.
The finishing touch
The torquing procedure for a small-diameter gasket differs from that used for large gaskets. For the smaller sizes, begin with a low torque or turning force and follow a staggered, cross-tightening pattern. Use multiple steps, with each pass yielding ever-higher torque values. A minimum of three passes is recommended, followed by a final reverse or chase pattern.
For larger-diameter flanges and critical-duty applications, where failure can be costly or catastrophic, use additional tightening passes. There is no predetermined number of passes, but in each case, follow a multiple cross-tightening, incremental procedure. Cross-tightening may be performed on individual bolts, multiple bolt groups or by using multiple tightening teams to expedite the assembly process. Experience advocates using more passes on larger vessels, reactors and similar equipment. Additional passes provide these benefits:
- It results in a more parallel alignment of the converging flange faces.
- There is less bolt load scatter or crosstalk (elastic interaction).
- It can reduce the amount of relaxation that occurs after bolt tightening.
Because bolt-tightening methods and speed influence joint relaxation, more incremental tightening passes translates into higher bolt load efficiency (less scatter) and more uniform gasket loading.
Hard vs. soft gaskets
Some suggest starting the joint assembly process with 50 percent of the final torque. However, this practice isn't appropriate for soft gasket materials (Shore Type A Durometer below 100), which require a minimum of three steps in 1/3 increments, with one or two circular passes for small diameters and more passes for larger diameters. Softer gaskets deflect more under a high initial torque--such as 50 percent--than harder materials. High initial torque pulls converging flanges out of parallel. Starting with lower torque levels and using multiple incremental steps achieves more uniformly loaded and parallel joints.
Re-torque or bolt re-tightening after initial assembly is advisable on equipment that experiences aggressive thermal or pressure gradients. One way around re-torquing may be to use conical spring washers. While this solution can be expensive, it may be the only option in areas where you can't re-torque. Re-torque either after the first 24 hours or one process cycle. That's when most relaxation has already occurred. Equipment should be at ambient temperature and pressure.
Following good bolting techniques, adhering to correct installation practices, and understanding sealing system components are the keys to successful joint sealing. Following the outlined techniques and procedures eliminates many of the problems associated with joint sealing.
Rich Rengers is a field application sealing specialist and is also involved with product R & D and testing at W. L. Gore and Assoc.--Sealant Technologies. He can be reached at 410-392-3200 or [email protected].