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HOW TO GET A GASKET WITH A LONG SERVICE LIFE
For flanged joints, gaskets are used to ensure optimum sealing. If you want to avoid leakage and an unnecessarily short service life, there are several factors that you need to focus on.
MOST COMMON ERRORS
Several errors should be avoided when sealing flange joints with gaskets. The most common defects are incorrect choice of gasket material, incorrect roughness of the flange surface, unclean or greasy flange surfaces, lack of lubrication of bolts, nuts, and washers, and lack of use of washers.
We regularly experience misunderstandings when it comes to the use of lubricants in connection with fitting gaskets at flange joints. In the worst-case scenario, incorrect use of lubricants can create the exact opposite effect of the desired effect, resulting in a leaking flange joint and, on top of some gaskets with a very short service life.
CORRECT AND INCORRECT USE OF LUBRICANT
The four columns show different applications of lubricant and nut, bolt, and washer, respectively, when mounting gaskets on flange joints. In all four trials, bolts have been tightened to 100 newton metres.
The loss of force in the bolt is linear with the loss of flat pressure on the gasket.
BOLTS AND NUTS
Thoroughly clean bolts and nuts and lubricate with Molykote 1000 paste. Also remember to use greased washers. It is extremely important to be careful about this. Figure 1 shows how much surface pressure is lost by not lubricating bolts or not using washers.
Bolts, nuts, and washers must be used, all lubricated with Molykote 1000 paste.
AVOID COPPER GREASE
Do not apply copper grease to the gasket, as it will be easier to replace. Grease creates a “slide” for the gasket, which is destroyed by no friction. As a result, the unit pressure on the seal drops by up to 70%. Be aware, however, that fiber gasket sheets have been treated with an anti-stick surface, so this type of sheet requires a minimum of cleaning when replacing.
LACK OF DEGREASING
Another problem is the lack of degreasing and cleaning flange surfaces before mounting gaskets. Particularly with oil pipes, problems occur regularly when flanges are not cleaned before gaskets are fitted. The flanges must be cleaned, otherwise, the gasket may need to be replaced again.
ROUGHNESS ON THE FLANGE SURFACE
The gasket should be mounted directly on a clean surface where a roughness of Ra= 3 to 6 microns is recommended. A completely smooth surface lacks friction, which is necessary for a stable mounting of gaskets.
CROSS-TIGHTENING AND OPPOSITE CROSS-TIGHTENING
Analyses show that with certain soft types of gaskets, it is advantageous to use both cross-tightening and reverse-cross-tightening. This procedure ensures that the surface pressure on the individual gasket is evenly distributed.
TIGHTENING OF TORQUE
It is important to use a calibrated torque wrench. For hard gaskets, such as fibre gaskets, e.g., STEFFCA Universal, tighten crosswise in three steps 30-60-100%.
For soft gaskets, such as expanded graphite and PTFE, tighten first and then in the opposite direction in four steps 25-50-75-100%.
Tests carried out by our gasket plate supplier Donit clearly show a more even distribution of the surface pressure by cross-tightening with an inverting (opposite) direction. As a result, the surface pressure on the seal is increased by up to 15% at the weakest point. Of course, the four-step method can also be used on fibre gaskets, but with less effect. See also figure 2.
If a fibre gasket develops a leak, it must not be retightened but replaced, while PTFE, clean graphite gaskets, and spiral wound gaskets can be retightened.
CORRECT TIGHTENING TORQUE
At STEFFCA, we use a calculation program to calculate the correct tightening torque. The torque is calculated based on the M and Y values of the individual gasket materials.
SELECTION OF GASKET MATERIAL
To select the correct gasket material, you must at least know the medium, pressure, and temperature. To calculate the tightening torque correctly, the flange type, bolt quality, and gasket design must also be known.
Thickness is selected based on the flange surface, any misalignments, etc. As a starting point, as thin a gasket as possible is preferred, which must be able to compensate for any misalignments. The thicker the gasket, the greater the torque requirement and the size of the sealing surface with which the medium comes into contact.