I'm trying to calculate the orifice diameter for an application where the purpose is to dampen the high pressure spikes of a fuel pump.  The pressure pulsations damage a pressure sensor, which requires the dampened lower pressure.  The flow rate is 0 because the orifice leads to a blind spot (the sensor itself). The target DP is around 500 psi.  

All I have is:

D= 0.23 * sq(Flow rate/sq(dP))

Is there a better equation where the flow rate is not a factor? 

If you are trying to "average" pulses going to the sensor, a needle valve can be used.  For this to work best, there needs to be an expandable volume between needle valve and sensor.  sometimes the sensor (pressure gauge) provides enoght for most applications.  If not, adding an acummulator makes a very nice "filter".  Using an electrical anologe, the neddle valve is a trim resistor and the accumulator the capacitor.  Like with any filter, this will add a time lag between sensor and process pressure.  The more filtering, the more time lag.

If you want to also reduce the pressure as seen by the sensor, that is a different problem.  This will require flow.  Thus the sensor could be "teed" between two orifices.  Again using electrical anologe, the area of ofices is used as resitors in a "voltage divider".

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