How common is the practise of drilling holes in pump check valves?
Check valve bybass orifices are commonly used in pressurized, high temperature fluid systems where multiple pump branches are employed and check valves are used to prevent or limit reverse flow into idled (standby pump?) branches. Pressurized thermal shock damage to piping or components is the concern if the fluid temperature in the idled branch departs too much from that in the operating pump branches (a 50 degree F differential may be a maximum allowed in pressurized water nuclear power coolant systems). I would not expect check valve bypasses to be necessary or critical for non-pressurized, low temperature, non-paralled fluid pumping systems. A bonus of temperature equalizing check valve bypasses in parallel pumping systems is some mitigation of the magnitude of check valve slam pressure surges in the system because the bypass orifice "bleeds off" the pressure as the check valve disk impacts the seat at high closing velocities. In sizing check valve seat orificies for fluid temperature equalization and/or pressure surge relief, the maximum flow allowed through the bypass should not be high enough to "breakaway" an idled pump and cause it to rotate in reverse direction under reverse flow conditions. A check valve seat bypass orifice establishes a maximum system resistance flow condition above shutoff (zero) flow which may or may not be sufficient to limit fluid temperature rise during startup of an idled pump with its check valve temporarily closed by the head of the running pumps. This may add a third consideration to check valve bypass orifice sizing besides fluid temperature equalization and pressure surge suppression.

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