I have a small open hydraulic system and I simply need to circulate hot (85C) water from the bottom of a reservoir back into the top at ~4GPM. At its lowest point the system has only 3ft of static head. The problem is that all the pumps I've found so far can't run without an inlet pressure (NPSHA) of 9ft because that would cause the water to flash inside the pump and the cavitation would damage the pump. I can't pressurize the system and I have no place to lower the pump to increase the NPSHA. Will closing the discharge valve after the pump lower the NPSHR and if so would it be enough to be able to safely run the pump? Pump in question is Grundfos UPS15-58FC.
The spec sheet doesn't specifically say, but it is likely that these minimum values are the rating for the top speed of the pump. If you application can handle the lower flow, use the lower speed setting of the pump, which results in lower impeller speed which reduces NPSH requirements. Hard to extrapolate that data to know what to expect for numbers, but it will be less.
These pumps are designed for pressurized closed loop hydronic heating systems, so not much design effort goes into reducing the NPSH.
Specifically to your question; Throttling the discharge can help in some specific situations because it reduces the flow in the plumbing between the reservoir and the pump (generally a band-aid on a poorly engineered system). In your case however, where you are designing it new, I would recommend just using a large diameter pipe (say 2" or larger) between the reservoir and your pump to reduce these losses to nearly zero (then there is no reason to throttle the pump). The spec sheet is assuming no loss in your plumbing, so this configuration will not reduce your NPSH requirements.
This 250 Series 1/3 HP Liberty sump pump (or different high temperature sump pump) might be a better fit for your application. Roughly the same price with a bit more flow. Probably not quite as long of service life, but sump pumps are actually build pretty robust since they typically have lots of start-stop cycles. I would certainty set it up to run continuously in your application for longer life. The spec sheet does not specify the NPSH, but the large intake, larger rotor diameter, especially the ~12inch float, indicate to me that it should preform well in a 3ft deep reservoir.