12/17/2011
Following simple procedures makes all the difference in successful sealing. Today is not a time to be wasteful. Ensuring pipes do not leak is one of the easiest ways to cut costs and improve productivity in the current economic environment. According to the PVRC Study-Post Mortem Observations and Findings, improper gasket installation and loose bolts account for 41 percent of failed seals. Damaged and misaligned flanges account for another 37 percent of failures. In other words, approximately 3/4ths of all failed seals-and the waste, downtime and expense associated with these failures-can be avoided by understanding the ways sealing system components influence the integrity of a connection. Further productivity improvements can result from optimizing gasket performance. Thankfully, this does not have to be overly complicated; it is a matter of choosing the right material and ensuring that installers are properly trained. By understanding sealing system components and their functions, correct gasket installation and proper joint assembly, many potential problems can be avoided. 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
Flange Surface & Gasket Selection
The flange's surface condition is the first consideration in achieving a reliable seal. Well-machined surface finishes secure a seal and provide resistance to gasket extrusion. Since many companies today are delaying equipment replacement, flange surfaces are commonly radially scored, scratched, pitted, corroded or contaminated. These irregular surfaces can prevent proper gasket seating. (Seating is the compressive stress required to deform the gasket sufficiently to prevent leakage.) The chosen gasket material must be pliable enough to conform to these uneven flange surfaces.The Gasket
Select the proper gasket material carefully. Its thickness is as important as the more obvious factors of temperatures, pressures and media. Do not use "more gasket than you need." In fact, 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 increases this 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 while maintaining dimensional integrity. Standardizing on a single gasket material for all operations can go far in improving productivity, first by reducing the level and cost of inventory and by minimizing the risk of selecting the wrong gasket. This cuts maintenance time while improving reliability and safety.Bolts or Studs
All pieces of tightening hardware-including nuts, washers and bolts or studs-perform a critical role. They not only apply compressive load, but also maintain load while resisting mechanical or thermally-induced changes that result from process cycling, shock, vibration, prying and shear. Reusing hardware can be risky because it is often hard to see whether bolts or studs have been strained beyond their yield point. However, if hardware is reused, ensure that undamaged threads are wire-wheeled or brushed. Diligently remove scale, debris and process fluid or media, including old, contaminated or weathered lubricant. If a nut does not 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 nut is the most critical and dangerous component to reuse. Studies have repeatedly shown that a reused fastener is not strong enough to develop sufficient force.
Replacing nuts is far less costly than dealing with a leak.Washers
Another important yet overlooked component is the washer. It acts as a bearing surface to reduce friction and distribute the load evenly. Through hardened steel washers are recommended, but not often used. 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, protect against embedment and distribute the load over a larger area. 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. Use of through hardened flat washers maintains joint service life by keeping bolts at right angles to the gasket and sealing surfaces, thereby distributing the load evenly and preventing surface damage.Follow the Recipe
Follow 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 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 that can 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. Check for flange misalignment, parallelism and flatness next. 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 bolt holes using only finger pressure.Lubrication
Lubrication is another important 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 is not possible to use washers, apply lubricant to the outer flange and the nut's bearing surface. On new equipment, it is acceptable to lubricate threads before inserting them through the bolt holes. On older machinery or equipment exposed to the elements, apply lubrication after insertion to ensure that dirt and debris do not contaminate the lubrication.Other Factors
Remember that the overall objective is to attain a uniform load on the gasket. Avoid mixing old bolts and nuts with new ones. Failing to do 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. 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 start-up.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. Additional passes provide these benefits:- More parallel alignment of the converging flange faces
- Less bolt load scatter or crosstalk (elastic interaction)
- Reduced relaxation after bolt tightening
Cross-bolt tightening pattern for smaller flanges.
Cross-bolt tightening pattern for larger flanges.
