Reduced Pressure Zone (RPZ) Valve Sizing Calculator

ANA›Life Services Authority›National Calculator Authority›Reduced Pressure Zone (RPZ) Valve Sizing Calculator

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Reduced Pressure Zone (RPZ) Valve Sizing Calculator

Determine the appropriate RPZ valve size based on design flow rate, inlet pressure, and system pressure-loss constraints. The calculator uses the valve flow coefficient (Cv) method and standard RPZ pressure-drop characteristics.

Design Flow Rate (GPM)

Inlet Pressure (psi)

Maximum Allowable Pressure Loss Through Valve (psi)

Fluid Specific Gravity (dimensionless, water = 1.0)

Calculate

Results will appear here.

function redCalc() { const resultDiv = document.getElementById('red-result');

const Q = parseFloat(document.getElementById('red-flow').value); const Pin = parseFloat(document.getElementById('red-inlet-pressure').value); const dPmax = parseFloat(document.getElementById('red-max-loss').value); const SG = parseFloat(document.getElementById('red-sg').value);

// --- Input validation --- if (isNaN(Q) || Q Please enter a valid positive flow rate (GPM).'; return; } if (isNaN(Pin) || Pin Please enter a valid positive inlet pressure (psi).'; return; } if (isNaN(dPmax) || dPmax Please enter a valid positive maximum allowable pressure loss (psi).'; return; } if (isNaN(SG) || SG 2.0) { resultDiv.innerHTML = 'Specific gravity must be between 0.5 and 2.0.'; return; } if (dPmax >= Pin) { resultDiv.innerHTML = 'Maximum allowable pressure loss must be less than inlet pressure.'; return; }

// --- RPZ inherent pressure drop --- // A standard RPZ assembly has a fixed internal pressure drop of ~10–12 psi // (first check valve ~5 psi + differential relief valve ~2 psi + second check valve ~3 psi). // This calculator uses 10 psi as the baseline inherent drop per ASSE 1013 / USC Model Plumbing Code guidance. const dP_inherent = 10.0; // psi, fixed RPZ assembly drop

if (dPmax RPZ assemblies have a minimum inherent pressure drop of ~10 psi. Increase the allowable pressure loss.'; return; }

// --- Required Cv calculation --- // Cv = Q * sqrt(SG / dP) [standard valve sizing equation, ISA/ANSI S75.01] // We size so that the TOTAL valve dP (inherent + dynamic) = Cv_required AND maxGPM >= Q let selected = null; for (let v of rpzSizes) { if (v.Cv >= Cv_required && v.maxGPM >= Q) { selected = v; break; } }

// --- Actual pressure drop through selected valve --- // dP_total = dP_inherent + (Q / Cv_selected)^2 * SG let dP_actual = null; let dP_dynamic_actual = null; let outlet_pressure = null;

if (selected) { dP_dynamic_actual = Math.pow(Q / selected.Cv, 2) * SG; dP_actual = dP_inherent + dP_dynamic_actual; outlet_pressure = Pin - dP_actual; }

// --- Velocity check (informational) --- // Pipe velocity at valve inlet using nominal pipe ID approximations (Schedule 40) const pipeData = { '3/4"': { id: 0.824 }, '1"': { id: 1.049 }, '1-1/4"': { id: 1.380 }, '1-1/2"': { id: 1.610 }, '2"': { id: 2.067 }, '2-1/2"': { id: 2.469 }, '3"': { id: 3.068 }, '4"': { id: 4.026 }, '6"': { id: 6.065 }, '8"': { id: 7.981 }, '10"': { id: 10.020} };

let velocity = null; if (selected && pipeData[selected.size]) { const id_in = pipeData[selected.size].id; // inches const area_ft2 = Math.PI * Math.pow(id_in / 24, 2); // ft² (id/2 in feet = id/24) // Q in GPM → ft³/s: Q / 448.831 velocity = (Q / 448.831) / area_ft2; // ft/s }

// --- Build output --- let html = '';

if (!selected) {
html += 'No standard RPZ valve size found for the given conditions. Consider splitting the flow into parallel assemblies or relaxing the pressure-loss constraint.';
} else {
const velWarning = (velocity !== null && velocity > 8)
? ' ⚠ Velocity exceeds 8 ft/s — consider next larger size.'
'';
const velOK = (velocity !== null && velocity ✔ Within recommended range.'
'';

html += `

ParameterValue Design Flow Rate${Q.toFixed(1)} GPM Inlet Pressure${Pin.toFixed(1)} psi Specific Gravity${SG.toFixed(3)} Max Allowable Pressure Loss${dPmax.toFixed(1)} psi RPZ Inherent Pressure Drop${dP_inherent.toFixed(1)} psi Available Dynamic Pressure Drop${dP_dynamic.toFixed(2)} psi Required Minimum Cv${Cv_required.toFixed(2)} Recommended RPZ Valve Size${selected.size} Selected Valve Cv${selected.Cv.toFixed(1)} Actual Dynamic Pressure Drop${dP_dynamic_actual.toFixed(2)} psi Total Actual Pressure Drop${dP_actual.toFixed(2)} psi Estimated Outlet Pressure${outlet_pressure.toFixed(1)} psi Pipe Velocity at Valve${velocity !== null ? velocity.toFixed(2) + ' ft/s' : 'N/A'}${velWarning}${velOK} Valve Max Rated Flow${selected.maxGPM} GPM `; }

resultDiv.innerHTML = html; }

#### Formulas Used

1. Required Flow Coefficient (Cv)

Based on the ISA/ANSI S75.01 liquid valve sizing equation:

Cv_required = Q × √(SG / ΔP_dynamic) Where:

2. RPZ Inherent Pressure Drop

ΔP_inherent ≈ 10 psi Standard RPZ assemblies (ASSE 1013) have a fixed internal pressure drop of approximately 10 psi regardless of flow, comprising: 1st check valve (~5 psi), differential pressure relief valve (~2 psi), and 2nd check valve (~3 psi).

3. Total Actual Pressure Drop

ΔP_total = ΔP_inherent + (Q / Cv_selected)² × SG

4. Pipe Velocity

V = Q / A [ft/s] A = π × (ID/2)² [ft²] Q converted: GPM ÷ 448.831 = ft³/s Recommended maximum velocity through an RPZ assembly: 8 ft/s.

#### Assumptions & References

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