I'm trouble shooting an instesting problum. Numerious 3" 300# ball valves (floating design) hold fine at high pressure (600 psi) but leak at low pressure (aprox 15psi head pressure) when pumps are off.Water service on pump discharge. Is this a common trait for floating ball valves?

What do you mean by "leak".  The two normal answers are "the valve leaks through" or "the valve-stem packing leaks to atmosphere".

I'm going to stick my neck out here and assume you mean the former.  A floating ball valve seals by the ball shifting onto the downstream seat.  The more dP, the harder the ball is pressed into the seat.  When dP is very low, the ball is not held against the seat as hard (or at all) and the clearance between the ball and seat can allow leakage.  If this is intolerable to your process I would replace the floating balls with a trunnion ball valve where the seats are held against the ball by springs.

Actually, low pressures are a problem for all pressure- energized seals, particularly if they are dynamic, more particularly if the surface speed is low ... i.e. in ball valves... and to add to above answer, seat leakage of floating ball valves may start at low pressure, but might increase over time and after a time also include leakage at increasingly higher pressure.

A thorough analysis will include a large number of parameters, in fact all normal when selecting a valvetype, plus some 'not normal'.

Among others:
Are the valves built for and with a quality suitable for the service?
Expected lifetime?
Operation mode, cycles?
Fluid within given parameters for the valve?
Clean fluid or particles or unexpected particles and additions?
Seal quality, materials?
Maintenance done according to manufacturer specification?
Abrasive or sticking fluid particles when drying?
etc, etc,....

Solution may be given by answers to the analysis, but might also be to change to trunnion mounted ball valves, or to another type  of valve construction entirely.

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