Why must valves be installed in the open position when welded?
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When a welded gate is installed by welding, the ends are heated during the welding process, which causes the valve body to expand. If welding is carried out without first opening the valve, waiting for it to cool down may result in leakage at the seal. Tungsten inert gas welding is recommended for butt welds. When using flange connections, the valve should be kept closed to prevent debris from entering the pipe and contacting the valve seat
What you mean is: when manufacturing valves by welding, they need to be in the open position before welding.
He was referring to the situation where some valves do not have flanges at either end and are welded directly.