Pressure Tank Size Calculator
ANA›Life Services Authority›National Calculator Authority›Pressure Tank Size Calculator
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Pressure Tank Size Calculator
Calculate the required pressure tank (accumulator tank) size for a water pump system based on your flow rate, cut-in/cut-out pressures, and desired pump cycle time.
Peak Flow Rate (GPM)
Maximum flow demand in gallons per minute
Cut-In Pressure (PSI)
Pressure at which the pump turns ON (typically 30 or 40 PSI)
Cut-Out Pressure (PSI)
Pressure at which the pump turns OFF (typically 50 or 60 PSI)
Minimum Pump Cycle Time (minutes)
Minimum time between pump starts (1–2 min recommended to protect motor)
Tank Pre-Charge Pressure (PSI)
Air pre-charge pressure (typically 2 PSI below cut-in pressure)
Calculate
function preCalc() { var resultDiv = document.getElementById('pre-result'); resultDiv.style.display = 'none'; resultDiv.innerHTML = '';
var Q = parseFloat(document.getElementById('pre-flow-rate').value); var P1 = parseFloat(document.getElementById('pre-cut-in').value); var P2 = parseFloat(document.getElementById('pre-cut-out').value); var T = parseFloat(document.getElementById('pre-cycle-time').value); var Pp = parseFloat(document.getElementById('pre-precharge').value);
// --- Validation --- var errors = []; if (isNaN(Q) || Q = P1) errors.push("Pre-Charge Pressure must be less than Cut-In Pressure.");
if (errors.length > 0) { resultDiv.style.display = 'block'; resultDiv.innerHTML = '' + errors.map(function(e){ return ''; }).join('') + ''; return; }
// --- Core Calculations --- // Convert PSI to absolute pressure (PSIA) by adding 14.7 var P1a = P1 + 14.7; // cut-in absolute var P2a = P2 + 14.7; // cut-out absolute var Ppa = Pp + 14.7; // pre-charge absolute
// Drawdown volume (gallons): water delivered per pump cycle // Vd = Q * T / 4 (worst case: pump on half the cycle, off half) // Industry standard formula: Vd = Q * T / 4 var Vd = (Q * T) / 4.0;
// Total tank volume using Boyle's Law acceptance factor: // Acceptance factor (AF) = 1 - (P1a / P2a) * (Ppa / P1a) // = 1 - Ppa / P2a // But the standard formula accounts for pre-charge: // AF = (P2a - P1a) / P2a * (Ppa / P1a) ... various forms exist. // Most widely used (Amtrol / industry standard): // Vt = Vd * P2a / (P2a - P1a) ... simplified (no pre-charge correction) // Full form with pre-charge: // Vt = Vd / [ (P2a - P1a) / P2a - (Ppa / P1a) * (P2a - P1a) / P2a ] // Simplified accepted industry formula: // Vt = Vd * (P1a * P2a) / ( Ppa * (P2a - P1a) ) var Vt = Vd * (P1a * P2a) / (Ppa * (P2a - P1a));
// Acceptance volume (actual usable water storage) var Va = Vd;
// Acceptance ratio var AR = (Va / Vt) * 100;
// Pressure differential var dP = P2 - P1;
// Recommend next standard tank size (common sizes in gallons) var standardSizes = [2, 4, 6, 8, 10, 14, 20, 32, 44, 62, 80, 100, 119, 144, 176, 220, 260, 315, 400, 500, 650, 800, 1000]; var recommendedSize = null; for (var i = 0; i = Vt) { recommendedSize = standardSizes[i]; break; } } if (recommendedSize === null) recommendedSize = Math.ceil(Vt / 100) * 100 + " (custom)";
// --- Display Results --- resultDiv.style.display = 'block'; resultDiv.innerHTML = '### Results ' + '' + 'ParameterValue' + 'Required Drawdown Volume' + Vd.toFixed(2) + ' gal' + 'Minimum Tank Volume (calculated)' + Vt.toFixed(2) + ' gal' + 'Recommended Standard Tank Size' + recommendedSize + ' gal' + 'Acceptance Ratio' + AR.toFixed(1) + '%' + 'Pressure Differential (ΔP)' + dP.toFixed(0) + ' PSI' + 'Cut-In Absolute Pressure' + P1a.toFixed(1) + ' PSIA' + 'Cut-Out Absolute Pressure' + P2a.toFixed(1) + ' PSIA' + 'Pre-Charge Absolute Pressure' + Ppa.toFixed(1) + ' PSIA' + '' + 'Note: Always select the next larger standard tank size above the calculated minimum. A larger tank reduces pump cycling and extends motor life.
'; }
#### Formulas Used
1. Drawdown Volume (Vd):
Vd = (Q × T) / 4
2. Total Tank Volume (Vt) — Boyle's Law with Pre-Charge:
Vt = Vd × (P1a × P2a) / (Ppa × (P2a − P1a))
- P1a = Cut-in pressure + 14.7 (PSIA)
- P2a = Cut-out pressure + 14.7 (PSIA)
- Ppa = Pre-charge pressure + 14.7 (PSIA)
This formula applies Boyle's Law (P1V1 = P2V2) to the air bladder/diaphragm inside the pressure tank to determine the total tank volume needed to deliver the required drawdown volume.
#### Assumptions & References
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