For the minimum amount of time that the pump is operating, it would probably be a better to design to omit the relief valve entirely and use a constant recirculation line with an appropriately sized orifice.  This arrangement would require the pump to be sized accordingly which may be a problem if you have already purchased the pump. 
Also, I'm surprised that you would need a multistage orifice block in conjunction with the relief valve.  I would expect high built-up backpressure at the relief valve would reduce the capacity quite a bit requiring a larger valve.  If a constant recirc cannot be accomodated and you wish to control based on a pressure signal, I think that a differential pressure switch and an on-off actuated gate/ball/butterfly valve w/ orifice or modulating control valve would be a better option. The orifice should be designed to pass the minimum stable flow from the pump at the corresponding pump head.  When normally operating and transferring demin to the condenser hotwell, the actual flow through the orifice will be a little less as you noted.

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