After a small tweak, I was able to get my idea of RIMS+loop to work basically as I envisioned it in a water test. A ball valve controlling the loop flow rate more linearly (compared to the butterfly valve used in my first revision) proved to be essential.
While I was setting everything up for my test [and preparing myself for failure], I figured even if only a small amount of flow was looped in the design, that would offer some meaningful safety for use of an inline heater – and surely that was possible.
Using two ball valves, I am able to dial in a RIMS loop recirculation rate ranging from 1/2 to 3/4 of the overflow (for example, 12 litres/min flow rate through the RIMS tube, with 9 litres/min recirculating back through the pump immediately, and the flow rate in/out of the mash tun consisting of the remaining 3 litres/min. I keep the pump frequency around 50%. Centrifugal pump RPM and flow rates have roughly a quadratic relationship, so this is may be more of a reduction in actual flow rate than you might think (N.B. I don’t actually have a flow meter, unfortunately. Sanitary flow meters are super expensive – even without taking into account the ability to handle some suspended solids).
With the water test a success. I should probably brew with it. For the test brew, I won’t worry about making the plumbing look perfect and will focus on functionality. If the real world performance impresses, I will consider implementing the design more permanently (which I figure may require one custom Tri Clamp tee to pull off). I can’t brew with the set up shown here exactly as I removed a temperature probe from my RIMS tube to run the tests. Yes that’s my old stand. Good think I didn’t demolish it yet.