Backflow Preventer Pressure Loss Calculator
ANA›Life Services Authority›National Calculator Authority›Backflow Preventer Pressure Loss Calculator
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Backflow Preventer Pressure Loss Calculator
Calculate the pressure loss (head loss) across a backflow preventer device based on flow rate, pipe size, and device type using the resistance coefficient (K-value) method.
Flow Rate (GPM)
Pipe Diameter (inches)
1/2 inch 3/4 inch 1 inch 1-1/4 inch 1-1/2 inch 2 inch 2-1/2 inch 3 inch 4 inch 6 inch
Backflow Preventer Type
Reduced Pressure Zone (RPZ) Assembly Double Check Valve Assembly (DCVA) Pressure Vacuum Breaker (PVB) Spill-Resistant Vacuum Breaker (SVB) Air Gap (reference only)
Fluid Temperature (°F)
Pressure Loss Units
PSI Feet of Head kPa
Calculate Results will appear here.
function bacCalc() { var resultDiv = document.getElementById('bac-result');
// --- Inputs --- var Q_gpm = parseFloat(document.getElementById('bac-flow-rate').value); var D_in = parseFloat(document.getElementById('bac-pipe-diameter').value); var type = document.getElementById('bac-device-type').value; var T_f = parseFloat(document.getElementById('bac-fluid-temp').value); var units = document.getElementById('bac-units').value;
// --- Validation --- if (isNaN(Q_gpm) || Q_gpm ⚠ Please enter a valid positive flow rate (GPM).'; return; } if (isNaN(T_f) || T_f 212) { resultDiv.innerHTML = '⚠ Temperature must be between 32°F and 212°F.'; return; } if (type === 'ag') { resultDiv.innerHTML = 'ℹ An Air Gap provides complete physical separation. Pressure loss is not applicable — all pressure is lost downstream.'; return; }
// --- K-values (resistance coefficients) by device type and nominal pipe size --- // Sources: AWWA M14, USC Foundation for Cross-Connection Control, // manufacturer data (Watts, Febco, Wilkins) — conservative mid-range values. // K is defined such that: h_L (ft) = K * V² / (2g) var kTable = { // 0.5 0.75 1.0 1.25 1.5 2.0 2.5 3.0 4.0 6.0 rp: [14.0, 12.5, 11.0, 10.0, 9.5, 8.5, 8.0, 7.5, 7.0, 6.5], dc: [ 7.0, 6.5, 6.0, 5.5, 5.0, 4.5, 4.2, 4.0, 3.8, 3.5], pvb: [10.0, 9.0, 8.5, 8.0, 7.5, 7.0, 6.5, 6.0, 5.5, 5.0], svb: [ 8.5, 8.0, 7.5, 7.0, 6.5, 6.0, 5.5, 5.0, 4.5, 4.0] }; var pipeSizes = [0.5, 0.75, 1.0, 1.25, 1.5, 2.0, 2.5, 3.0, 4.0, 6.0];
var idx = pipeSizes.indexOf(D_in); if (idx === -1) { resultDiv.innerHTML = '⚠ Pipe diameter not found in lookup table.'; return; } var K = kTable[type][idx];
// --- Water density at temperature (lb/ft³) using polynomial fit --- // Valid 32–212°F. Reference: Engineering Toolbox / CRC Handbook. var T_c = (T_f - 32) * 5 / 9; // convert to Celsius var rho_kgm3 = 999.842 - 0.0623 * T_c - 0.003821 * T_c * T_c + 0.000004 * T_c * T_c * T_c; var rho_lbft3 = rho_kgm3 * 0.062428; // kg/m³ → lb/ft³
// --- Flow velocity in pipe --- // A = π/4 * D² (D in feet) var D_ft = D_in / 12.0; var A_ft2 = Math.PI / 4.0 * D_ft * D_ft;
// Q in ft³/s: 1 GPM = 0.002228 ft³/s var Q_ft3s = Q_gpm * 0.002228; var V_fps = Q_ft3s / A_ft2; // velocity in ft/s
// --- Head loss (Darcy-Weisbach with K) --- // h_L = K * V² / (2 * g) [feet of head] var g = 32.174; // ft/s² var hL_ft = K * (V_fps * V_fps) / (2.0 * g);
// --- Convert head loss to pressure loss --- // ΔP (psi) = ρ (lb/ft³) * h_L (ft) / 144 var dP_psi = rho_lbft3 * hL_ft / 144.0; var dP_kpa = dP_psi * 6.89476;
// --- Velocity check warning --- var velWarning = ''; if (V_fps > 8.0) { velWarning = '⚠ Velocity (' + V_fps.toFixed(2) + ' ft/s) exceeds 8 ft/s. Consider a larger pipe size to reduce noise, erosion, and pressure loss.
'; } if (V_fps ' + 'ParameterValue' + 'Device Type' + typeLabels[type] + '' + 'Pipe Diameter' + D_in + ' in' + 'Flow Rate' + Q_gpm.toFixed(2) + ' GPM' + 'Flow Velocity' + V_fps.toFixed(3) + ' ft/s' + 'Water Density @ ' + T_f + '°F' + rho_lbft3.toFixed(3) + ' lb/ft³' + 'Resistance Coefficient (K)' + K.toFixed(1) + '' + 'Pressure Loss' + primaryValue + ' ' + primaryUnit + '' + 'Pressure Loss (PSI)' + dP_psi.toFixed(3) + ' PSI' + 'Pressure Loss (ft Head)' + hL_ft.toFixed(3) + ' ft' + 'Pressure Loss (kPa)' + dP_kpa.toFixed(2) + ' kPa' + '' + velWarning; }
#### Formulas Used
1. Flow Velocity:
V = Q / A where A = (π/4) × D²
Q = volumetric flow rate (ft³/s), D = pipe inner diameter (ft)
2. Head Loss (K-Value / Resistance Coefficient Method):
hL = K × V² / (2g)
K = resistance coefficient (dimensionless, device- and size-specific), g = 32.174 ft/s²
3. Pressure Loss:
ΔP (psi) = ρ × hL / 144
ρ = water density (lb/ft³) at the given temperature
4. Unit Conversions:
1 GPM = 0.002228 ft³/s | 1 PSI = 2.3077 ft of water | 1 PSI = 6.89476 kPa
#### Assumptions & References
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