If you remove your hard carriage returns from the string then it works.

OrificeFlow::usage = 
  "OrificeFlow[OrificeBore, PipeID, P1, P2, Density, Viscosity, \
OrificeDischargeCoefficient] calculates the flow rate of an \
incompressible fluid through an orifice installed in a pipeline.  \
Uses Eq.6-111 from Perry's Chemical Engineer's Handbook, 7th ed. The \
orifice coefficient is entered as an optional argument with a default \
value of 0.62, which is normally valid if the Reynolds number is > \
20,000.  The Reynolds number is calculated in the module and an error \
message is displayed if it is < 20000. P2 is the recovered downstream \
pressure, which is converted in the module to the vena contracta \
pressure for use in Eq.6-111.\nDefault units: Flow -> \
Gallons/Minutes, OrificeBore, PipeID -> Inches; P1, P2-> \
PoundsForce/Inches^2; Density -> Pounds/Feet^3, Viscosity -> \
Centipoise Input: Numeric value in default units or Quantity in any \
consistent units. Output: Quantity";