Question About Pumps in Series

Tank level too low to prime Pump1. Pump2 primes Pump1. So far OK.

Pump1 turns on, and both for recirculation and taking some load off Pump2. Probably OK thanks to recirculation making up flow difference.

Pump2 turns off. This is where things get a little hairy since Pump1 might actually end up driving Pump2 and Pump2 may not handle that. solution is a bypass of pump2 with a check valve.

Tank level rises and reaches point where almost pure recirc dominates if head from Reservoir is insufficient. To raise level higher, turn on Pump2. This point is the crux of your design if you need Pump1 to fill the Tank without a shutoff at the recirc intake.

Pump1 turns off. Tank might flow (slowly?) into reservoir through pump2 depending on relative heights. To prevent, add valve between reservoir and connection to pump1 system. Could be a shutoff to turn tank into a pure recirc situation or a check to just automatically prevent backflow.

In conclusion, I'd add a bypass to Pump2 and two shutoffs - one at each pump1 inlet to control where it draws from.

Will a priming pump push past the circulation pump if I leave the priming pump on (if both are on, if just the one is on)?

If the flow rates of both pumps are closely matched, the circulation pump will pump the medium delivered by the priming pump. If the circulation pump is turned off, the medium will be pumped into the vessel through the outlet - the pump will pose a far higher resistance than a short piece of pipe, unless there's a check valve somewhere.

When the flow rate of the circulation pump is higher, you will pump the fresh medium to the top of the tank + some from the tank, if the flow rate of the circualtion pump is lower part of the fresh medium will enter the tank through the bottom outlet (unless there's a check valve)

Will the circulation pump pull through the priming pump if the priming pump is off?

Possible, but not likely, as the flow resistance through the priming pump will be far higher than through the outlet pipe.

I think the way this will work is I use the priming pump until the line is filled with new fluid then turn it off and use the circulation pump to fill the rest of the tank.

I find this dubious. The line will always be filled, there will be no vacuum.

I am worried that if one of two pumps in series is off, flow will be stopped altogether.

Unless there's something we miss that should be no problem.

Are there any other possible problems I should consider when running pumps in series?

In general, I would be concerned about total pressure. In this case with the two open vessels (or are these pressure vessels?) this should be no issue.

You should check the documentation of your pumps or talk to the vendor or service person about behavior when tunred off. Centrifugal pumps allow some medium to pass when not pumping, how much or how much resistance they pose depends entirely on the pump. PD pumps can block the flow as well as a valve, or be driven by the flow, this depends entirely on the pump.

Edit to add:

Both pmps centrifugal, there's a switch valve that turns off the line from the outlet to the entrance where Pump 2 enters the line.

We can't know for sure without seeing the performance curves of the pumps and a system curve. I would size Pump 2 so that head at the inlet of pump 1 is about the same total head when pump 1 is recirculating, for the flowrate that pump 1 recirculates. Then you should find a stable point of operation for both pumps.

If pump 2 is undersized, pump 1 will cavitate, if pump 2 is oversized you will oeprate off it's BEP, waste energy and have a shorter pump lifespan.

However, you could simplify the setup by putting the line from pump 2 somehwere on the pressure side of pump 1, and have a check valve or switch valve between.