But if it's cavitating it's often not really the 'pump' but rather the volatile liquid and the restriction in the tubing/piping. When the liquid is hot it offgases a lot easier than when it's cool (more volatile). If the tubing/piping isn't right then the suction of the pump will pull gas out of solution and create bubbles and the pump cavitates. I see it all the time at work where we pump extremely volatile solvents through hot tubing that's been sitting out in the sun (we insulate it but it still gets pretty warm).
You said that the pump will prime and pump fine but then loses prime. So it appears the pump is up to the task but the cavitation is killing the whole thing.
+50,000 You may be able to pump near boiling liquids, as Bobby apparently does, under the right conditions. OTOH, when conditions are right for even a slight amount of cavitation to occur, it will and it will likely interfere with the pump possibly causing it to lose prime.
1. Maintain as much positive suction head as possible
2. Avoid using reducers on the suction side of the pump
3. Use the shortest hose that is practical, especially on the suction side
4. Use the largest hose diameter possible (within reason, of course). This is very important on the suction side.
5. Use hoses (AND FITTINGS) with an inside diameter of no less than the ID of the inlet port on the pump (ie. typically 1/2" for the common March pumps). This includes the kettle DIP tube, etc. Barbed adapters and QD's often have a smaller inside diameter and can cause a restriction.
6. Do not use a flow control valve on the suction side of the pump. It should be used on the output side.
7. Minimize the number of elbows etc, if used, on both the suction side and the output side of the pump.
8. Be sure the pump has sufficient power to supply the desired flow rate at the operational head pressure of your system. Factor in all of the system losses when looking at this.
You can easily test your system using cold water in order to determine if it's a cavitation problem. If it works with cold water, but not with very hot water, you will know right away.
I've learned most of the above the hard way. I no longer have priming or cavitation problems. None and never. I can't help it if I'm lucky.