Pressure Reducing Valve Calculation Example Procedure to Calculate the Pressure Values required to factory set a Pressure Reducing Valve. 1) First calculate the maximum static pressure available on the supply side of the PRV at all levels. Calculate the Maximum Static Pressure at the Pump: Add the City Static Pressure to the Fire Pump Maximum Churn Pressure. The City Test and Fire Pump are at the same elevation so an elevation correction is not required. Maximum Static = 90 PSI + (150 psi x 1.18) = 90 + 177 = 267 PSI at the pump elevation. The Maximum Static Pressure at each PRV equals the Maximum Static Pressure at the Pump reduced by the elevation above the pump. The Fire Sprinkler Outlet on the 10th floor is 110 feet above the Fire Pump. Maximum Static = 267 PSI - (110 Ft x.4331 PSI/Ft) = 219.36 PSI The Hose Outlet on the 10th floor is 105 feet above the Fire Pump. Maximum Static = 267 PSI - (105 Ft x.4331 PSI/Ft) = 221.52 PSI Subtract the elevation pressure to determine the Maximum Static Pressure at each Outlet. See the chart below. 2) Then calculate the demand flow and pressure required on the system side of each PRV. a) Standpipe Top hose outlet demands a flow of 250 GPM and Minimum Required Pressure of 100 PSI at the PRV outlet plus 250 GPM at the next outlet. b) Calculate the sprinkler system demand. Use a flow of 225 GPM at 110 PSI at the PRV outlet - typical each level plus 100 GPM Inside Hose allowance. 3) Next calculate the maximum residual pressure available at the demand flow on the supply side of each PRV at all levels. It will be easier to calculate the pressures for all Hose Outlets first and then the pressures for the sprinkler system outlets. For this calculation all Safety Margins are taken on the system side of the PRV and the remainder of the Standpipe and Supply Pipe will flow at the maximum pressure that the combined City and Fire Pump can provide. Calculate from the top Sprinkler System or Hose Outlets using the demand flow for each to the city supply leaving NO Safety Margin. (To find the maximum start residual pressure add the Safety Margin Pressure to your Start pressure.)
This is the Initial Input Screen used to calculate the maximum residual pressure available at the demand flow on the supply side of each PRV at all levels. Start with the Demand Flow (225 GPM) and Pressure (110 PSI + 10 PSI Safety). (Any start pressure will do to begin with as we will be adding the safety margin back in and recalculating.) Initial calculation is 83.8 PSI below curve. Add this to the start pressure (120 + 83.8) and recalculate. Adjust the start pressure until the safety margin is equal to or very close to 0.0 PSI. Note: A separate calculation is required to find the maximum pressure for the first floor Node FS1 as this is not an outlet along the hydraulic path we are currently calculating.
General Calculation Program Job - Pressure Reducing Valve Calculation Example Date - 03/21/12 File - F:\HES\HydraCALC\Ver50\Data\Jobs\Pressure Reducing Valve Calculation Example.wx1Time - 09:25:02 AREA CALCULATED - HEAD FLOW SUMMARY Actual Req. Delta Actual Req. Delta ID K-Factor Flow Flow Flow. Press. Press. Press PRV n/a 225.00 225.00 0.00 203.784 Pt 203.784 0.000 H10 n/a 100.00 100.00 0.00 208.174 Pt 0.000 208.174 Total K Factors.... 0 Total Fixed Flows... 2 Sum Actual Flow.... 325.00 Sum Required Flow... 325.00 Sum Delta Flow.... 0.00 Max Delta Flow.... 0.00 Max Delta Pressure.. 208.174 AREA CALCULATED - COMPLETE SUMMARY Start Finish Diam. Start Normal Pf Elev/Fixed Flow Vel Point Point Pres. Pres. Pres. PRV <- FS10 2.469 203.784 2.216 0.000 225.00 15.08 FS10 <- H10 6.357 206.000 0.009 2.165 225.00 2.27 H10 <- FS9 6.357 208.174 0.024 3.032 325.00 3.29 FS9 <- H9 6.357 211.230 0.017 2.165 325.00 3.29 H9 <- FS8 6.357 213.413 0.024 3.032 325.00 3.29 FS8 <- H8 6.357 216.469 0.017 2.165 325.00 3.29 H8 <- FS7 6.357 218.652 0.024 3.032 325.00 3.29 FS7 <- H7 6.357 221.709 0.017 2.165 325.00 3.29 H7 <- FS6 6.357 223.892 0.024 3.032 325.00 3.29 FS6 <- H6 6.357 226.948 0.017 3.032 325.00 3.29 H6 <- FS5 6.357 229.997 0.024 2.165 325.00 3.29 FS5 <- H5 6.357 232.187 0.017 2.165 325.00 3.29 H5 <- FS4 6.357 234.370 0.024 3.032 325.00 3.29 FS4 <- H4 6.357 237.426 0.017 2.165 325.00 3.29 H4 <- FS3 6.357 239.609 0.024 3.032 325.00 3.29 FS3 <- H3 6.357 242.665 0.017 2.165 325.00 3.29 H3 <- FS2 6.357 244.848 0.024 3.032 325.00 3.29 FS2 <- H2 6.357 247.905 0.017 2.165 325.00 3.29 H2 <- SP1 6.357 250.088 0.151 2.382 325.00 3.29 SP1 <- TOR 6.357 252.621 0.184 0.000 325.00 3.29 TOR <- BOR 6.357 252.805 0.210 4.681 325.00 3.29 BOR <- PMPO 6.357 257.696 0.171 0.000 325.00 3.29 System Demand Pressure... 257.868 Safety Margin................ 0.016 Press. @ Pump Outlet 257.883 Press. From Pump Curve ( PMPI - PMPO )... -170.144 Press. @ Pump Inlet 87.739 PMPI <- UG1 6.357 87.739 0.103 0.433 325.00 3.29 UG1 <- TEST 6.160 88.275 0.820-0.433 325.00 3.50 Total Flow..................... 325.000 Pipe with highest velocity: PRV - FS10 (15.078) Water Supply Summary @ Point TEST Total Flow From Water Supply Curve........ 325.000 Pressure From Water Supply Curve......... 88.662 System Pressure Demand At Supply Curve...... 88.646 Safety Margin at Supply Curve......... 0.016
4) Fill in the Specification Chart with the required hydraulic information for the manufacturer. The maximum residual pressure available at the Fire Sprinkler system demand at the 10 th Floor PRV is shown below. The maximum residual pressure available at the Fire Sprinkler system PRV for all the other floors can be quickly found by subtracting the friction loss of the 2 ½ pipe from the pressure available at each Fire Sprinkler system node along the standpipe. Examples - (At node FS9 211.23-2.216 = 209.01 PSI) (At node FS2 247.905-2.216 = 245.69 PSI) All the maximum residual pressures can now be supplied to the PRV manufacturer. Shown are values for the Fire Sprinkler System for the 10 th and 2 nd floors. Note: The actual required pressure on the system side of the PRV must include the safety margin desired. Therefore, the minimum pressure required is 120 PSI = 110 PSI as calculated + (10 PSI Safety). The Fire Sprinkler System Demand Flow is 225 GPM with a required Pressure of 120 PSI.
Repeat the calculation to determine the maximum residual pressure available at the Hose PRV for all the floors. Initial Input Screen used to calculate the maximum residual pressure available at the hose flow on the supply side of each PRV at all levels. The Standpipe Top hose outlet demands a flow of 250 GPM and Minimum Pressure of 100 PSI at the PRV outlet plus 250 GPM at the next outlet. (Any start pressure will do to begin with as we will be adding the safety margin back in and recalculating until there is no safety margin remaining.)
General Calculation Program Job - Pressure Reducing Valve Calculation Example Date - 03/21/12 File - F:\HES\HydraCALC\Ver50\Data\Jobs\Pressure Reducing Valve Calculation Example.wx1Time - 10:35:40 AREA CALCULATED - HEAD FLOW SUMMARY Actual Req. Delta Actual Req. Delta ID K-Factor Flow Flow Flow. Press. Press. Press PRV n/a 250.00 250.00 0.00 181.380 Pt 181.380 0.000 H9 n/a 250.00 250.00 0.00 189.296 Pt 1.000 188.296 Total K Factors.... 0 Total Fixed Flows... 2 Sum Actual Flow.... 500.00 Sum Required Flow... 500.00 Sum Delta Flow.... 0.00 Max Delta Flow.... 0.00 Max Delta Pressure.. 188.296 AREA CALCULATED - COMPLETE SUMMARY Start Finish Diam. Start Normal Pf Elev/Fixed Flow Vel Point Point Pres. Pres. Pres. PRV <- H10 2.469 181.380 2.693 0.000 250.00 16.75 H10 <- FS9 6.357 184.073 0.015 3.032 250.00 2.53 FS9 <- H9 6.357 187.119 0.011 2.165 250.00 2.53 H9 <- FS8 6.357 189.296 0.054 3.032 500.00 5.05 FS8 <- H8 6.357 192.382 0.039 2.165 500.00 5.05 H8 <- FS7 6.357 194.586 0.054 3.032 500.00 5.05 FS7 <- H7 6.357 197.672 0.039 2.165 500.00 5.05 H7 <- FS6 6.357 199.876 0.054 3.032 500.00 5.05 FS6 <- H6 6.357 202.962 0.039 3.032 500.00 5.05 H6 <- FS5 6.357 206.033 0.054 2.165 500.00 5.05 FS5 <- H5 6.357 208.253 0.039 2.165 500.00 5.05 H5 <- FS4 6.357 210.457 0.054 3.032 500.00 5.05 FS4 <- H4 6.357 213.543 0.039 2.165 500.00 5.05 H4 <- FS3 6.357 215.747 0.054 3.032 500.00 5.05 FS3 <- H3 6.357 218.833 0.039 2.165 500.00 5.05 H3 <- FS2 6.357 221.038 0.054 3.032 500.00 5.05 FS2 <- H2 6.357 224.124 0.039 2.165 500.00 5.05 H2 <- SP1 6.357 226.328 0.335 2.382 500.00 5.05 SP1 <- TOR 6.357 229.045 0.408 0.000 500.00 5.05 TOR <- BOR 6.357 229.454 0.466 4.681 500.00 5.05 BOR <- PMPO 6.357 234.601 0.380 0.000 500.00 5.05 System Demand Pressure... 234.981 Safety Margin................ 0.002 Press. @ Pump Outlet 234.983 Press. From Pump Curve ( PMPI - PMPO )... -150.000 Press. @ Pump Inlet 84.983 PMPI <- UG1 6.357 84.983 0.228 0.433 500.00 5.05 UG1 <- TEST 6.160 85.644 1.819-0.433 500.00 5.38 Total Flow..................... 500.000 Pipe with highest velocity: PRV - H10 (16.753) Water Supply Summary @ Point TEST Total Flow From Water Supply Curve........ 500.000 Pressure From Water Supply Curve......... 87.030 System Pressure Demand At Supply Curve...... 87.028 Safety Margin at Supply Curve......... 0.002
The maximum residual pressure available at the Standpipe demand at the 10th Floor Hose PRV is shown below. The maximum residual pressure available at the Hose PRV for all the other floors can be quickly found by subtracting the friction loss of the 2 ½ pipe from the pressure available at each Standpipe Hose node along the standpipe. Examples - (At node FS9 187.119-2.693 = 184.43 PSI) (At node H2 226.328-2.693 = 223.64 PSI) All the maximum residual pressures can now be supplied to the PRV manufacturer.
Below is an example of a field adjustable PRV made by Elkhart Brass Fire Fighting Equipment. Follow the manufacturer s guide for proper field adjustment of the pressure reducing valves. Similar hydraulic pressure information is required in order to factory set or to field adjust a PRV.