Calculating the velocity of a pressure vessel's end-cap after a girth weld failure involves understanding hydrostatic pressure, which limits kinetic energy transfer to the end-cap. Once the weld fails, the water will not propel the end-cap further but will instead flow downwards. High flow rates can create hazardous conditions, making the water flow as dangerous as the end-cap itself. The maximum horizontal speed of the end-cap is influenced by the flow rate and vessel diameter. Prior to complete weld failure, water jets from seams or flange bolts may pose significant risks.