PRV calcs are a multi-step, iterative process. Also, a major pain in the neck at the best of times.
I'll preface this by stating it has been years since I switched from Autosprink to Revit - so if an easier method has been invented by MepCad in that time period - sorry for wasting time with the long post. The below procedure is for calculating loss through a pilot-operated valve.
Step 1 - Lay out your system normally, with the PRV in the correct location. Make sure to place a hydraulic node at the inlet of the PRV.
Step 2 - Run your calc. There will be a large safety factor, because the pump is oversized, and the PRV is not currently doing anything about it. Record the safety factor.
Step 3 - Review the calc, and make sure the system demand at the PRV node does not exceed what you expect to get out of the PRV (~155 psi, generally).
Step 4 - Record the calculated demand pressure at the PRV node.
Step 5 - Determine the available residual inlet pressure. This is done by adding the safety factor determined in Step 2 to the system demand at the PRV node determined in Step 3. This represents the pressure that will be available on the inlet (high pressure side) of the PRV when flowing at system demand.
Step 6 - Determine the pressure loss at the PRV by subtracting the outlet pressure (i.e whatever pressure you want to set the pilot at, say 165 psi) from the available residual inlet pressure from Step 5.
Step 7 - Determine the friction loss that results from water flowing through the body of the open PRV. The PRV datasheet should have a chart indicating pressure loss at various flow rates. See the bottom of Page 2 as an example:
Step 8 - Add the pressure losses determined in Steps 6 & 7 together.
Step 9 - Enter the total friction loss from Step 8 into the PRV properties as a fixed loss (same as you would for a backflow preventer).
Step 10 - Re-run the calculation and determine that there is still an adequate safety factor.
As noted, the above procedure is for determining losses through a pilot-operated valve that have a constant downstream pressure. Calculations for direct-acting valves are similar, but slightly different.
*** Important Notes
Never forget Step 7. Figuring loss through the body of the valve is almost always missed, and causes major issues during commissioning, when you expect say, 165 psi at system flow but can only get 155 psi out of the valve.
Calculating different systems with different flow rates will result in different available inlet pressures. All the steps above will need to be repeated for each calculation to accurately determine pressure loss through the valve for those specific system flow rates. It is horribly tedious at first, but once you've done it a couple times, you'll know exactly which numbers you're looking for and how to quickly find them.
