Welding Inspection & Metallurgy API ICP Self Study Notes

For making attachment welds to equipment containing a large quantity of
liquid such as a storage tank 36 in. (0.9 m) below the liquid/vapor line, normal
circulation will effectively cool the weld area.
Welding on a line under no-flow conditions or intermittent flow conditions, e.g.,
a flare line, shall not be attempted unless it has been confirmed that no
explosive or flammable mixture will be generated during the welding
operation. In this respect, it shall be confirmed that no ingress of oxygen in
the line is possible. In cases where this requirement cannot be met, inert gas
or nitrogen purging is recommended.