Booster pump discharge NRVs
SLF but 40 years operating pumps. Interesting comment on the discharge NRV.
When a Booster pump discharge NRV fails in service and the pump is stopped for operational reasons, what stops the flow of fuel from another booster pump in service from discharging its fuel back through the defective pump NRV and into the fuel tank? (Are there motorised discharge valves on each booster pump which close automatically when a booster pump stops?) In the oil busines this is know as pump cross circulation. It has happened many times and action similar to what you have mentioned taken to minimise the risk. In systems where a failure of this nature would be a life or process threatening event, two NRVs of different manufacture are mounted in series on the critical pump discharge.This cross circulation would also result in a very significant fall in the booster pump header discharge pressure.
Obviously if all booster pumps are running whilst A/C in flight, there would be no backflow and no prospect of fuel starvation to the HP pump from this NRV failure mode.