Diaphragm Pump Sizing Calculator
To size an air-operated diaphragm (AODD) pump, turn your system into one duty point: flow and the discharge pressure needed at that flow. Then find the lowest inlet air pressure whose curve passes above that point, keep the duty out of the last ~15% of that curve, and check suction lift, temperature and plant air. This calculator does that on the published curve for the FTI Air FT30P, a 3-inch polypropylene AODD rated to 300 gpm.
Enter your duty
Email me the worksheet and this result
You get the 3-page AODD Duty-Point & Air-Supply Sizing Worksheet (inputs checklist, the FT30P curve, air-supply and suction checks, materials temperature table). The duty you entered goes to James Riggins, so a spec review can start from your numbers instead of a blank form.
How the diaphragm pump sizing calculator works
Step 1: required discharge pressure. Head in feet is converted to pressure with the fluid's specific gravity, then any pressure needed at the end point is added:
Pd is the discharge pressure in psi, H in feet, SG the specific gravity. An AODD's curve is really a pressure curve: the diaphragm pushes the liquid with the air behind it, so a heavier liquid needs more pressure for the same rise.
Step 2: minimum inlet air pressure. The FT30P curve gives discharge head on water at six air pressures, 20 to 120 psi. Converted to pressure (P = H ÷ 2.31), the calculator reads each curve at your flow and interpolates between the two curves that bracket Pd. It never extrapolates past the end of a published curve.
Step 3: curve position. Flow as a share of where the selected curve ends:
Below about 85% is comfortable. Above that the pump is at the right end of its curve, where air use, diaphragm flexing and exhaust icing all climb. The fix is more air, a larger pump, or two pumps sharing the flow.
Step 4: suction, temperature and air. The FT30 manual lists 28 ft of wet suction lift and 20.1 ft dry (19.8 ft with PTFE diaphragms). The polypropylene housing is published for 32–158°F, and each elastomer has its own range. Air use comes from FTI Air's own air-consumption lines on the same chart (30 to 180 SCFM), interpolated at your duty point. Outside those lines the calculator says so instead of guessing.
FT30P curve summary (water, rubber/TPE diaphragms)
| Inlet air | Shut-off head | Curve ends at |
|---|---|---|
| 120 psi | 277 ft (120 psi) | 300 gpm at 21 ft |
| 100 psi | 231 ft (100 psi) | 280 gpm at 20 ft |
| 80 psi | 185 ft (80 psi) | 258 gpm at 17 ft |
| 60 psi | 139 ft (60 psi) | 230 gpm at 15 ft |
| 40 psi | 92 ft (40 psi) | 203 gpm at 14 ft |
| 20 psi | 46 ft (20 psi) | 152 gpm at 10 ft |
Source: FTI Air FT30 plastic pump manual, P/N 212602 Rev. 3, p.9. Full curve and specs on the FT30P 3-inch diaphragm pump page.
The compressor is half the pump
An AODD only makes the pressure its air supply can hold at full stroke rate. If the regulator gauge reads 90 psi at rest but drops to 60 psi when the pump runs, the pump is on the 60 psi curve. Undersized air lines, long hose runs and a shared header are the usual reasons.
Before you buy a pump, check the pressure at the pump inlet while it runs, the line size to the pump (the FT30P inlet is 3/4 in FNPT), and the compressor's free capacity at that pressure. If the pump already stalls or slows down, start with diaphragm pump troubleshooting.
Diaphragm pump sizing questions
How do you size an air-operated diaphragm pump?
Work out the flow you need and the discharge pressure the pump must make at that flow (static rise plus friction plus any end-point pressure, corrected for specific gravity). Then find the inlet air pressure whose performance curve passes above that point, check that it sits away from the far right end of the curve, and confirm suction lift, temperature limits and the air supply.
How much pressure can a diaphragm pump produce?
An AODD pump can make roughly as much discharge pressure as the air pressure feeding it, close to a 1:1 ratio at zero flow. The FTI Air FT30P is rated for 120 psi maximum inlet air, so its published curve reaches about 277 ft of water at shut-off and falls as flow rises.
Why is maximum flow not the sizing number for an AODD pump?
Maximum flow is where the highest air-pressure curve ends, usually at very low head. Real systems need pressure, so the pump runs further left on the curve. Selecting at the far right end means high air use, faster diaphragm wear and more exhaust icing.
How much compressed air does a 3-inch diaphragm pump need?
It depends heavily on the duty point. On the FTI Air FT30P's published chart, air use runs from about 30 SCFM at low flow to more than 180 SCFM near full flow; on the 100 psi curve it passes 120 SCFM at about 89 gpm and 180 SCFM at about 197 gpm. The calculator reads it at your flow and head. Keep a filter-regulator at the pump and confirm the compressor's free capacity at that pressure.
Is the calculator result a specification?
No. It is a Draft Proposal Worksheet based on published water-curve data. Final selection, materials and air supply are confirmed by LibertyCES with the factory, and by the engineer of record where one is involved.
Draft Proposal Worksheet, not a procurement-ready specification. Results use published water-curve data for one pump model and simple hydraulics. Final pump selection, elastomers, air supply and installation must be confirmed by LibertyCES with the manufacturer and, where applicable, by the project's engineer of record.
Want James to check the duty, the chemistry and the air?
Send the result with your fluid, concentration and temperature. You'll get a pump size, diaphragm, ball and seat recommendation, and the air supply it needs — no charge.
All AODD pumps · Chemical transfer pumps · Browse all guides & downloads