MAIN PRV'S - AUTOSPRINK

[FireSprinklerDesignerTX]

I'm trying to figure out how to accurately input the PRV information in Autosprink and have hit a roadblock as far as adjusting the pressures, etc. I honestly don't have much experience in this and I am trying to learn this. I've designed several pumps but never one with a main pressure reducing valve after the discharge of the fire pump. System details are below

ESFR System - K16.8 ESFR's Operating at 52 PSI Each
Fire Pump is a 1500 GPM @ 85 PSI
City Water Pressure is 118 PSI Static, 94 PSI Residual, Flowing 2123.20 GPM
This is an existing building that we are modifying

Basically just looking on how I need to input the PRV valve and Fire Pump in Autosprink to be able to accurately calculate.

Thank you guys in advance!

 

[skdesigner]

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:

https://www.zurn.com/media-library/web_documents/pdfs/specsheets/acv-zw209fp-pdf-(1)

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.

 

[skdesigner]

I suppose I should point out, before someone else jumps on it - NFPA 20 doesn't permit the use of pressure regulating valves to maintain system pressures at or below the pressure rating of downstream components (20-2016 4.7.7.1 and .2). NFPA 20's jurisdiction ends at the outlet side of the discharge control valve. Since NFPA 13 does not have the same restrictions, that valve just needs to be installed after the fire pump discharge control valve in order to be compliant.

 

[FireSprinklerDesignerTX]

skdesigner - Thank you so much for that breakdown and I really appreciate the time you spend putting this together!