Sealing Factors

Sealing Factors

March 28, 2022


Seals can be formed by many different means. Two contacting metal surfaces can hold incredible pressure if machined, finished, and fastened properly. Since this is very difficult to achieve, one of the most common sealing methods is to place a gasket made from soft material, such as rubber or cork, between two mating surfaces. When compressed, the gasket fills microscopic imperfections in each of the sealing surfaces to block any potential leak paths. While this seems like a simple solution to prevent leakage, researchers have found that there are over 100 variables present in a joint that can affect sealing performance. When designing sealed joints, performing routine maintenance, or experiencing leaks, the following should be considered:

Fasteners

Sealing joints are often fastened using nuts with either studs or bolts. These fasteners can be manufactured to one of many standards that dictate the factors below and affect their performance.

  • Tightness: When properly tightened, the nuts force the studs to stretch slightly and act as springs, pulling the flanges together and compressing the gasket. Predictably, if the fasteners are not tight enough, the gasket will be prone to leaking. On the other hand, if the fasteners are too tight the studs can deform over time and relieve some of the gasket compression or, in extreme cases, break entirely. Depending on the gasket material, too tight fasteners can also crush the gasket and cause it to extrude out of the joint or into the internal flow. In all cases, it is recommended to use a calibrated torque wrench when installing a gasket.
  • Size: Intuitively, the diameter of a stud directly relates to its strength: larger means stronger. Increased diameter means an increased material area over which a load can be distributed. Fastener stress and strength are measured in pounds per square inch (psi), so if a uniform force is applied to two studs of different diameters, the larger stud will distribute the load more and therefore experience less stress.
  • Material: While industrial fasteners are generally made from steel, not all steel is created equal. Depending on manufacturing processes and material additives, the yield strength (how much the stud can stretch before deforming) can vary from 30,000 to over 100,000 psi.
  • Condition: When fasteners are tightened around two flanges, the total tightening load is unevenly distributed across the fastener threads so that the first two engaged threads hold greater than 50% of the load. Depending on the tightness, size, and material, these threads can deform and weaken, and the effect worsens with each reuse. If not properly stored and maintained, the fasteners can also rust, reducing the strength of both the threads and shank. For these reasons, new fasteners (both nuts and studs/bolts) should be used each time a sealing joint is reassembled.

Flanges

Industry standard flanges are rated for the amount of internal pressure and fastener load that they can withstand, and can be sorted into two classes: flat face (having a uniform surface across the entire flange area) and raised face (having a protruding ring around the center hole to compress the gasket).

  • Size: Standard flanges are rated for performance based on their inside diameter and thickness. If the internal pressure is greater than the equipment rating, it is possible for the flanges to separate or for the attached pipes to burst. From the outside, fasteners that are too tight can deform the flanges and cause them to lose strength. This is particularly important when using raised face flanges, as over-tightening can warp the sealing faces to reduce the total sealing area, and put bending stress on both the flanges and the fasteners. Using larger flanges can help alleviate these problems.
  • Material: Just like fasteners, flanges can be made from many different materials. Each material will react differently to internal pressure, fastener compression, bending stress, and temperature. Particular care must be taken when using non-metallic flanges as these materials will deform and crack easier than metals. Across the board, it is important to use washers with all fasteners to distribute the fastener load more evenly around the bolt holes.
  • Condition: While gaskets are meant to fill imperfections in the flanges, there are limits to their ability. Particularly large pits, gouges, and radial scratches in the flange faces can present easy leak paths, so flanges must be inspected before gasket installation and resurfaced if necessary.

Lubricants/additives

  • It is always recommended when installing a gasket to use lubricant on the fasteners, whether pre-coated or applied manually. When fasteners are lubricated, the torque required to reach a specified bolt load is reduced by 50% or more. This means less work is required to form a reliable seal.
  • On the other hand, lubricant should never be manually applied to a gasket. An off-the-shelf lubricant leads the gasket to slide along the flange surfaces to the point that the inner and outer gasket surfaces may extrude out of the sealing area; this action is called creep, and can cause the gasket to tear. If not chosen carefully, liquid lubricants can also chemically degrade the gasket material and quickly lead to leakage. If lower gasket surface friction is required for installation, it is possible for some raw materials to be impregnated with lubricating compounds which are formulated to avoid the dangers of manually applied lubricants.
  • Other products such as electrical insulation kits and toxic/corrosive/high pressure leak shields are also available. These work in conjunction with the gasket to improve the lifespan and safety of industrial piping systems.

Physical space/stack-up

  • For the best performance, gaskets should be loaded uniformly across the sealing faces. This means that, in addition to making sure the fasteners are tightened evenly, the flange faces should also be parallel to each other. Pipes that are misaligned by more than a few degrees cause uneven stress on the gasket, flanges, and fasteners, and failure of any of those components can lead to leakage.
  • Gaskets must be compressed by a specific amount in order to work properly. For this reason, the compressed gasket thickness, rather than the original thickness, must be considered when designing piping systems. A gap due to compression that is not accounted for will grow with every installed gasket, so that an initial gap of a fraction of an inch can grow to multiple inches in long piping systems.

It is easy to see that while a gasket is integral to forming a leak-tight seal, it comprises only a fraction of the total equation. Throughout the last 60 years, Sealing Devices has cultivated strong relationships with leading gasket material manufacturers, like Klinger Thermoseal, that offer a wealth of knowledge including what is presented here. To maintain status as not just a supplier, but an invaluable resource to our customers, Sealing Devices regularly leverages these resources to develop the experience base of all employees; our sales and engineering teams are poised not just to provide recommendations on the gasket, but also to help solve the total sealing equation. Click here to contact us.