Irrigation System Pressure Loss Calculator
ANA›Life Services Authority›National Calculator Authority›Irrigation System Pressure Loss Calculator
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Irrigation System Pressure Loss Calculator
Calculate total pressure loss in an irrigation system using the Hazen-Williams equation, including pipe friction losses, elevation head, and minor losses from fittings.
Flow Rate (GPM)
Pipe Inside Diameter (inches)
Pipe Length (feet)
Hazen-Williams C Coefficient
150 — PVC / Plastic (new) 140 — Fiberglass / Smooth Lined 130 — Copper / Brass 120 — Galvanized Steel (new) 100 — Cast Iron (old) 80 — Concrete (rough)
Elevation Change (feet) (positive = uphill, negative = downhill)
Fittings & Minor Losses Factor (% of pipe friction loss, e.g. 10 = 10%)
Calculate Pressure Loss
function irrCalc() { var resultDiv = document.getElementById('irrResult');
// --- Read inputs --- var Q = parseFloat(document.getElementById('irrFlowRate').value); var D = parseFloat(document.getElementById('irrPipeDiameter').value); var L = parseFloat(document.getElementById('irrPipeLength').value); var C = parseFloat(document.getElementById('irrHWC').value); var elev = parseFloat(document.getElementById('irrElevation').value); var fittPct = parseFloat(document.getElementById('irrFittingsFactor').value);
// --- Validation --- var errors = []; if (isNaN(Q) || Q 200) errors.push("Fittings factor must be between 0 and 200%.");
if (errors.length > 0) { resultDiv.style.display = 'block'; resultDiv.className = 'calc-result calc-error'; resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }
// --- Hazen-Williams Friction Loss --- // Formula: h_f (ft per 100 ft) = 0.2083 * (100/C)^1.852 * Q^1.852 / D^4.8704 // Total friction head loss (ft): H_f = h_f * L / 100 var hf_per100 = 0.2083 * Math.pow(100 / C, 1.852) * Math.pow(Q, 1.852) / Math.pow(D, 4.8704); var H_friction = hf_per100 * L / 100; // feet of head
// --- Minor / Fittings Loss --- var H_fittings = H_friction * (fittPct / 100);
// --- Elevation Head --- // Positive elevation = pressure loss (pumping uphill) var H_elevation = elev; // feet of head (signed)
// --- Total Head Loss --- var H_total = H_friction + H_fittings + H_elevation;
// --- Convert feet of head to PSI (1 ft H2O = 0.4335 PSI) --- var psi_friction = H_friction * 0.4335; var psi_fittings = H_fittings * 0.4335; var psi_elevation = H_elevation * 0.4335; var psi_total = H_total * 0.4335;
// --- Velocity (ft/s) for reference --- // A = pi/4 * D^2 (in^2) -> convert to ft^2: / 144 // Q in GPM -> ft^3/s: / 448.83 var A_ft2 = Math.PI / 4 * Math.pow(D / 12, 2); var Q_cfs = Q / 448.83; var velocity = Q_cfs / A_ft2;
// --- Format helper --- function fmt(v, d) { return isNaN(v) ? 'N/A' : v.toFixed(d !== undefined ? d : 2); }
var elevSign = elev >= 0 ? '+' : '';
resultDiv.style.display = 'block'; resultDiv.className = 'calc-result calc-success'; resultDiv.innerHTML = '### Results ' + '' + 'Component' + 'Head Loss (ft)' + 'Pressure Loss (PSI)' + 'Pipe Friction (Hazen-Williams)' + '' + fmt(H_friction) + ' ft' + '' + fmt(psi_friction) + ' PSI' + 'Fittings & Minor Losses (' + fittPct + '%)' + '' + fmt(H_fittings) + ' ft' + '' + fmt(psi_fittings) + ' PSI' + 'Elevation Head (' + elevSign + elev + ' ft)' + '' + elevSign + fmt(H_elevation) + ' ft' + '' + elevSign + fmt(psi_elevation) + ' PSI' + 'TOTAL Pressure Loss' + '' + fmt(H_total) + ' ft' + '' + fmt(psi_total) + ' PSI' + '' + 'Flow velocity in pipe: ' + fmt(velocity, 2) + ' ft/s' + (velocity > 5 ? ' ⚠ Velocity exceeds recommended 5 ft/s — consider larger pipe.' : ' ✔ Within recommended range.') + '
' + 'Friction loss rate: ' + fmt(hf_per100, 3) + ' ft per 100 ft of pipe (' + fmt(hf_per100 * 0.4335, 3) + ' PSI/100 ft)
'; }
#### Formula
Hazen-Williams Friction Loss (ft per 100 ft of pipe):
hf = 0.2083 × (100/C)1.852 × Q1.852 / D4.8704
Where: Q = flow rate (GPM), D = inside pipe diameter (inches), C = Hazen-Williams roughness coefficient
Total Friction Head Loss: Hf = hf × L / 100 (feet)
Fittings Loss: Hfit = Hf × (fittings factor / 100)
Elevation Head: Helev = elevation change (ft) — positive = uphill (loss), negative = downhill (gain)
Total Head Loss: Htotal = Hf + Hfit + Helev
Pressure (PSI): PSI = Htotal × 0.4335 (1 ft of water = 0.4335 PSI)
#### Assumptions & References
- Uses the Hazen-Williams equation, the industry standard for irrigation and water distribution design (ASABE, NRCS).
- Assumes full-flow, turbulent conditions in a circular pipe — valid for typical irrigation velocities (1–5 ft/s).
- Water temperature assumed at 60°F (15.6°C), standard for irrigation design.
- Recommended maximum pipe velocity: 5 ft/s for mainlines, 7 ft/s for laterals (Irrigation Association guidelines).
- Reference: Hazen, A. & Williams, G.S. (1920). Hydraulic Tables. NRCS National Engineering Handbook, Part 623.
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