Welding Inspection & Metallurgy API ICP Self Study Notes

11.2.2 Flow Rates
Under most conditions, it is desirable to maintain some product flow inside of
any material being welded. This helps to dissipate the heat and to limit the
metal temperature during the welding operation, thereby reducing the risk of
burnthrough.
Liquid flow rates in piping shall be between 1.3 ft/ sec. and 4.0 ft/sec. (0.4
m/sec. and 1.3 m/sec.).
Faster liquid flow rates may cool the weld area too quickly and thereby cause
hard zones that are susceptible to weld cracking or low toughness properties
in the weldment. Because this is not a problem when the pipe contains gases,
there is no need to specify a maximum velocity.