Because the operator manually intervened to clear the alarm and thus reset the control system, the control system did not recognize that the blast freezer evaporator unit contained high-pressure hot gas when it allowed the suction stop valve to open during the system restart. Rather, the control system signaled the suction stop valve and liquid feed valves to simultaneously open in order to return the evaporator to cooling mode operation. This manual bypass of the programmed defrost sequence allowed the low-temperature liquid and hot gas to mix in the same pipe, causing the hot gas void to collapse as it rapidly condensed to a liquid. This created pressure shocks that ruptured the evaporator piping manifold and the low-temperature suction piping on the roof.