Guide To Gasket Replacement (March 26, 2010)
Proper gasket removal and replacement can reduce maintenance and increase uptime.
Proper gasket removal and replacement can reduce maintenance and increase uptime.
The gaskets that keep flanged joints in piping systems from leaking are relatively low-cost but high-consequence components. They can spell the difference between efficient, profitable operations, and unscheduled outages, lost production and penalties for non-compliance with environmental regulations. Even so, they often do not receive the attention they warrant from plant operations and maintenance personnel.
Gaskets perform two basic functions — creation of an initial seal and maintenance of that seal over an extended period of time. To perform these functions effectively, they must retain their integrity during handling and installation, and be sufficiently deformable to flow into imperfections in flanges. And they have to be strong enough to resist crushing under applied loads and blowouts under system pressures.
Moreover, gaskets in flanged joints have to be chemically compatible with system fluids, withstand extreme temperatures, and be resilient and creep-resistant to maintain adequate load. Finally they must not contaminate the system or promote corrosion of the seating surfaces, and must be able to be easily and cleanly removed at the time of replacement.
However, the removal of spent gaskets can be a tedious, laborious and sometimes dangerous task that can damage equipment and injure workers. Flanged joints are often in difficult-to-access areas that require working in awkward positions in confined spaces near other equipment that can result in burns and other injuries.
In addition, flanged joints can be hard to separate, making it difficult to see where the gasket is located, and they may have to be removed when there are flange gaps of only 0.5 in. or less.
Under such conditions, new gaskets are sometimes installed directly over old ones — with predictably poor results. Even a small fragment of gasket left on a flange can make it impossible for the new gasket to conform properly, resulting in an immediate leak. Or it might produce a high stress point, around which sufficient load cannot be developed to maintain an effective seal for the useful life of the gasket.
Besides causing premature gasket failure, such fragments can break loose from the flange and contaminate the fluid in the system or impair the performance of downstream equipment such as pumps and valves.
Avoid certain lubricants
In some cases, installers apply bolt-thread lubricants to gaskets to facilitate removal. If the gaskets contain non-oil-resistant binders, such lubricants can chemically attack them, softening the binders and reducing their crush strength.
These lubricants also reduce the retentive friction of gaskets, causing them to extrude and eventually blow out. Metal in the lubricants can bond to flanges and fill in surface serrations that bite into the gaskets and hold them in place. In addition, the lubricants can enter the process stream and contaminate the system fluid, and they can bake off at elevated temperatures, leaving a problematic void between the gasket and flange.
Some installers use caulk to affix gaskets to flanges or to compensate for damaged or irregular flange surfaces. However, some caulks contain acetic-acid-based cure systems that can attack elastomeric gaskets and gaskets containing rubber binders. Because of their lubricity, caulks also can cause gaskets to shift within the flange assembly and, as with the lubricants discussed previously, can lead to the same loss of friction, crush strength and blowout resistance.
Gaskets should be installed using only products specifically designed to compensate for flange inconsistencies and designed to hold them in place.
Many gaskets, except those made of PTFE, which do not require it, are coated with anti-stick agents. If it is necessary to apply more of these compounds to a gasket before installation, it is always advisable to use dry materials, such as talc, graphite or mica. Metallic-based agents should never be used because, as with certain lubricants, metal particles may accumulate in surface serrations, making the flange surface too smooth to be effective.
A proprietary, anti-stick agent is now available to speed removal of gaskets from flanged joints.
The new material is a high-temperature, inorganic coating that dramatically reduces the time and effort required to remove gaskets after extended service — in most cases seconds versus hours and with just a fraction of the force required to remove untreated gaskets. Unlike most anti-stick agents, the material is fused to the surface of the gaskets and does not contain chemicals that can cause them to crack or otherwise degrade.
In the process of developing the material, it was learned that the binders in compressed sheet gaskets act as viscoelastic materials that tend to flow at elevated temperatures and pressures. As the binders ‘wet out’ and make intimate contact with the metallic face of a flange, chemical adhesion, mechanical interlocking and other modes of adhesion come into play. The new anti-stick compound acts as a barrier that prevents the binders from ‘wetting out’, and because they can be removed intact, gaskets treated with it are easier to dispose of properly.
Proper gasket removal
Replacement of used gaskets begins with the removal of all flange fasteners, including bolts, studs, nuts and washers, and replacement of any that are worn, corroded or otherwise damaged. The flanged joint should be carefully opened using a special flange spreading tool or soft wedges so as not to damage the flange seating surfaces. The old gasket can then be removed.
This is best accomplished by using an aerosol gasket remover or, if necessary, a brass scraper that will not nick, scratch or gouge the flanges, the surface of which is critical to achieving the necessary friction for an effective seal.
After removal of the old gasket, the flange facings should be inspected for imperfections that can adversely affect the new gasket’s ability to seal. If surface damage exists, it may be necessary to re-machine or replace the flange. Acceptable surface finishes should range between 125 and 250 micro-inches.
Flanges should be free of foreign material. Residual debris can be removed from the serrations by scouring the surface of the flange with a brass wire brush in a rotary, not linear motion. After the old gasket has been removed and the flange faces cleaned and conditioned, the replacement gasket can be installed.
Selecting a new gasket
Selecting the new gasket depends upon a number of variables. Whenever possible, thinner gaskets should be used unless the flanges are warped, bowed or severely pitted, in which case a thicker gasket will be needed to compensate for these irregularities. It should be noted, however, that thicker gaskets require higher compressive comable pressive loads, which may not be obtainable obtain- in the application.
The use of ring gaskets is preferable to full-face gaskets, which typically cover twice the area. This extra material also requires greater torque to compress. Where one or both flange faces are flat and made of soft or brittle material, a full-face gasket might be needed to prevent flange rotation.
Once the replacement gasket has been selected, it should be inspected for correct I.D., O.D. and appropriate thickness. If it has cracks, gouges, folds or other surface damage, it should not be used.
For ease of installation, all fasteners should be lubricated with an oil-and-graphite mixture or other suitable thread lubricant, being careful not to allow it to come into contact with the gasket. Flanges with vertical seating surfaces should have at least two fasteners inserted into the bottom holes to support the gasket.
Proper gasket replacement
The gasket can now be inserted between the flange seating surfaces. To avoid damaging the gasket, do not use an instrument to push it into place. The flange spreader can then be carefully removed, allowing the flanges to come together, and the remaining fasteners inserted and tightened or ‘snugged’.
The pattern in which the bolts are tightened is extremely important. If done improperly, the flange can move out of parallel. Refer to the gasket manufacturer’s literature to determine the appropriate bolting pattern for the application. Using calibrated torque and box-end type wrenches, tighten each fastener to no more than one-third of the desired torque value to uniformly compress the gasket. Repeating the same pattern, increase the torque wrench setting to two-thirds of the desired value.
To achieve the final torque value, repeat the pattern again at the target torque value, and finish with a circular ‘check pass’, moving from one fastener to the next in a counter-clockwise sequence to assure each fastener is applying the same load. The gasket should now be properly installed and capable of operating at a high level of performance.
The process of removing and replacing used gaskets is every bit as critical as initial gasket selection and installation. Anti-stick agents, aerosol gasket removers and specialized tools all can facilitate gasket removal without damaging flanges or injuring workers.
Provided flanges have been cleaned, repaired or replaced as needed and the replacement gasket has been installed properly, the flanged joint should provide trouble-free performance until the next changeout is scheduled.
Tim Hurley is a senior product manager and Dave Burgess is senior applications engineer for Garlock Sealing Technnolgies. They may be contacted at 1-800448-6688; e-mail tim. firstname.lastname@example.org email@example.com.