Describe how you would tighten up a flange with eight bolts and a gasket in between?
Prior tightening, make sure that the gap between flanges is not too big and matched with proper gasket. If there is a gap of more than 1 mm, consider thicker gasket or re-welding of the flange.
In case of an attempt of removing the flanges gap by overtightening the bolts, the associated pipe mail crack. This often happens with cast iron valves, which flanges are fragile and can't withstand high tension.
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Tighten 1/3 torque
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Repeat tightening 2/3 torque
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Tighten to full torque.
Gradually tighten all of the bolts working diagonally across and around the flange.
