Non-return valves (NRV) are generally needed when operating centrifugal pumps in parallel to prevent reverse flow through non-running pumps. I don't believe their use in mandated, however, and their use might be avoided under certain conditions, namely:
1. If the flow diverted through the idle pump(s) does not compromise the required performance of the fluid system. This may rarely be true for critical heat transfer fluid systems like boiler feed or reactor coolant systems although it might be accomodated in initial design so as to avoid non-return valve problems like waterhammer piping loading and NR valve failures usually caused by fatigue of movable components.
2. The idle pumps can withstand the reverse flow through the pump that also drives the motor shaft in reverse. If the bearings can operate in both rotational directions and the reverse flowrates are relatively low, the rotor "runaway" speeds might be no higher than forward rotation. This is usually true for low specific speed centrifugal pumps but not for axial flow pumps of high specific speed. If the runaway speeds are too high, then anti-rotation pawls can be used on the motor shaft to prevent reverse rotation.