wrote in message
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SNIP

If you put a pressure gauge one foot away from the end of a
water supply that is plugged you will see 50 psi. Gradually open
a gate valve to create flow and what was once 50psi will start to
drop immediately. Once the flow is completely unrestricted you will
see 0psi on the same gauge. At that time you will have your
maximum free flow rate.

If you put a pump inlet at that same water supply, and that pump can
develop a vacuum, you should be able to draw slightly more flow, but
your gauge will be below atmospheric if this occurs.

The pump manufacturer suggest to have a free flow supply of 5gpm
for their 3.9GPM pump. This means they don't want negative pressures
at their pumps inlet. Cavitation can result. Some pumps have ceramic
plungers that wear great but are easy targets for cavitation damage.

As you have stated when the relief valve is activated the actual
flow at the inlet is reduced so that a less than adequate supply may
suddenly become adequate, however is your relief valve always
activated? It is not, and hence the manufacturers recommendations.

So what are you disagreeing with?? Pump manufacturers don't want to run
their pumps under conditions that will result in cavitation-very bad, will
ruin a pump. In most cases they will specify the inlet conditions (pressure
& temperature) under which they (pumps) should be operated. Specifying a
free flow supply is just a loose way of defining the pump inlet
requirements.
Don't understand your comment-"when the relief valve is activated the actual
flow at the inlet is reduced so that a less than adequate supply may
suddenly become adequate".
When you are on the relief valve just about all the flow is being
recalculated via the valve. Almost nothing gets out so in the end, an
adequate supply drops to basically no supply. In answer to your
question--No, relief valves are not normally activated but be glad when they
do--dead ending a positive displacement pump, without relief, will destroy
it.
MLD