Donnier;n1129438 said:
Thanks for the reply. In this case there is no engine compartment and my concern was that with the pumps in parallel, do the check valves fight against each other. In other words, will the fuel flow pressure from one pump act against the check valve or valves of the the other to hinder fuel flow through them?
In your case the pressure and flow will be whatever pressure is delivered by the stronger pump.
The engines diaphragm pump already has internal check valves, so the external one is redundant.
The Electric pump is capable of backflow so the check valve will be needed.
If the Electric pump is the stronger of the two, The diaphragm pump will idle harmlessly without any consequences.
In the event that the diaphragm pump is slightly stronger than the electric pump,
the check valve after the electric pump will never open due to the higher pressure after the valve.
This also means that there might not be any actual fuel flow through the electric pump.
This could lead to problems when there are no pump failure and both pumps are running normally.
The fuel flow through the electric pump is what cools the pump.
With no cooling flow flow the pump will run warmer/hotter than normal.
Another check mark to the Series side of the argument.