During hydrostatic tests of piping, why is pressure relieved before opening test connections?

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Relieving pressure before opening test connections is about safety from stored energy. When piping is filled and pressurized for a hydrostatic test, the fluid and the system components hold energy. Opening a connection while still under pressure can release that energy suddenly, sending a high-velocity jet of liquid and potentially causing injuries, equipment damage, or movement of the piping. By venting the pressure first through proper drains or relief lines, the energy is dissipated safely and the connections can be opened without the risk of a dangerous release. This is why the best practice is to relieve pressure before opening test connections.