the reasons given are lack of lubrication causing wear, overheating, and needlessly having electricity supplying a component in a fuel vapour rich container that could be a problem during a crash, the last is much the same reason not to have prime pumps on in flight once they have got the engines started.
Prime pumps are there in case a check valve has allowed fuel to drain back into the Tank and thus allowing air into the fuel line. The function of the Prime Pump is to provide a Head of Fuel to the Engine Driven Fuel Pump. Once the Engine is running...the Engine Driven Pump can pull the Fuel to the Engine.
The Transfer Pumps are fuel lubricated are they not?
The Transfer Pumps have a thermal switch on them that will switch them off if they begin to over heat....don't they?
So if left on and the residual fuel is insufficient to properly lubricate the pump....and it begins to heat up.....at some point the Thermal Switch closes and shuts off the pump....end of that problem....but you would get a Cockpit indication of a Pump Failure.
Am I right about the Thermal Switch?
How long can you fly once the Transfer Pumps cease to pull Fuel from the Main Tank and you are left with only the contents of the Supply Tanks?
If 20 Minutes is the advertised limit to run the Pumps without any fuel passing through them.....why is leaving them ON a problem?