How an Air Operated Double Diaphragm Pump Works
An air operated double diaphragm (AODD) pump moves fluid with compressed air and two diaphragms joined by a shaft. Air pushes one diaphragm out, forcing fluid through the discharge, while the shaft pulls the other diaphragm in and draws fluid into the opposite chamber. The air valve then shifts and the stroke reverses. Check balls keep the flow one-way, and the pump's discharge pressure tops out at about the air pressure feeding it.
How an Air Operated Double Diaphragm Pump Works (3-Inch FT30P) · 6:19 · LibertyCES. Narration is an AI voice; installed scenes are photoreal renders. Specs from the FTI Air FT30 manual (P/N 212602 Rev. 3) and FT30P flyer. Watch on YouTube →
How an AODD pump moves fluid, step by step
- Two chambers, one shaft. Each side of the pump has a fluid chamber closed off by a diaphragm. A shaft through the center section ties the two diaphragms together.
- Air pushes one side. The air valve sends compressed air behind one diaphragm. It moves outward and pushes fluid out of that chamber through the top (discharge) check ball.
- The shaft pulls the other side. The same shaft pulls the opposite diaphragm inward, which lowers the pressure in that chamber and draws fluid up through the bottom (suction) check ball.
- The air valve shifts. At the end of the stroke, air switches to the other diaphragm and the motion reverses. The check balls seat and unseat so fluid only ever moves from suction to discharge.
The air valve, and why AODD pumps stall
The air valve is the part that decides whether an AODD keeps cycling. It routes air to one chamber and exhausts the other, then shifts at the end of each stroke. On many pumps a pilot valve or pilot pins move the main spool, and worn pilot parts, dirty plant air or exhaust ice can leave it stuck mid-shift.
The FTI Air FT30P uses a stainless-steel air valve with no separate pilot valve; Finish Thompson calls it stall-resistant. Every AODD still needs clean, dry, regulated air at the inlet. If a pump stops at the end of a stroke, start with the diaphragm pump troubleshooting guide.
Reading an AODD curve: what "300 gpm" means
An AODD pump has a family of curves, one per air pressure, not a single rating point. At zero flow, discharge pressure equals air pressure, so the FT30P at 120 psi air makes about 277 ft of head. As flow climbs, head falls. The published 300 gpm is where the 120 psi curve ends at about 21 ft of head; at 100 psi air the curve ends near 280 gpm.
Air consumption is the operating cost
The dotted lines on the curve are air use: about 30 SCFM at low flow, more than 180 SCFM near full flow, with 115 SCFM at 100 psi as FTI's published reference point. Out at the right end of the curve every gallon costs more air, the diaphragms flex harder and exhaust icing gets worse. Select in the middle of the curve, or run two pumps, and make sure the plant air can hold pressure at full stroke.
Put your flow, head and plant air into the free diaphragm pump sizing calculator → It places your duty on the FT30P curve, estimates air use and emails a PDF worksheet.
Where an air operated diaphragm pump fits
AODD pumps earn their place where solids, suction lift, changing duty or harsh chemistry make a centrifugal pump awkward: wastewater sump and sludge-pit transfer, ferric, alum and caustic transfer on water-treatment skids, plating bath transfer and waste-treatment feed, tank and tote unloading, and lime slurry. Chemical fit starts with the housing and the elastomer. The FT30P's glass-filled polypropylene is rated excellent for hydrochloric acid to 37%, sulfuric to 60% and caustic to 50%, and not recommended for sodium hypochlorite, sulfuric 70% and stronger, chromic, nitric or hydrofluoric acid. Check any chemical in the AODD diaphragm chemical compatibility chart, and see full specs on the FT30P page.
Electric double diaphragm pumps: the same stroke, driven by a motor
An electric double diaphragm pump keeps the two diaphragms and check valves and replaces the air valve with a motor and mechanical drive. Graco's QUANTM i230 is the 3-inch example: 230 gpm free flow, 100 psi, and it still stalls against a closed valve and runs dry. The trade is power: it needs 380–480 V three-phase instead of an air line, and it is aluminum only and non-hazardous only.
Frequently asked questions
How does an air operated double diaphragm pump work?
Two diaphragms sit in two fluid chambers and are joined by a shaft. Compressed air pushes one diaphragm outward, forcing fluid out of that chamber through the discharge check valve, while the shaft pulls the other diaphragm inward and draws fluid into the opposite chamber through the suction check valve. The air valve then shifts air to the other side and the stroke reverses. Check balls and seats keep the flow moving one way.
What does the air valve do in an AODD pump?
It sends compressed air to one diaphragm chamber at a time and exhausts the other, switching sides at the end of every stroke. On many AODD pumps a pilot valve shifts the main air spool; the FTI Air FT30P uses a stainless-steel air valve that needs no separate pilot valve, which Finish Thompson describes as stall-resistant.
How much pressure can an air operated diaphragm pump make?
Roughly the air pressure feeding it. At zero flow the discharge pressure equals the inlet air pressure, so the FT30P at 120 psi air makes about 277 ft of head (120 psi × 2.31). As flow rises, discharge head falls along that air pressure curve.
What does a "300 gpm" rating mean on a diaphragm pump?
It is the end of the highest air-pressure curve, not the flow you will see in a real system. For the FT30P, 300 gpm is where the 120 psi air curve ends at about 21 ft of head; at 100 psi air the curve ends near 280 gpm. Read your required flow and head against the curve to find the air pressure the duty needs.
How much compressed air does a diaphragm pump use?
It depends on where the pump runs on its curve. The FT30P chart shows air use from about 30 SCFM at low flow to more than 180 SCFM near full flow; FTI publishes 115 SCFM at 100 psi as one reference point. Selecting near the right end of the curve costs the most air and makes exhaust icing worse.
Are air operated diaphragm pumps self-priming?
AODD pumps lift fluid on their own up to their dry suction-lift rating. The FT30P manual lists 20.1 ft of dry lift with rubber diaphragms (19.8 ft with PTFE) and 28 ft wet, and says to install the pump vertically or it may not prime properly.
Can you pump bleach with a polypropylene diaphragm pump?
Not with the FT30P. FTI's chemical resistance guide rates its glass-filled polypropylene housing not recommended for sodium hypochlorite at 5% and 12.5%. Use a PVDF-bodied diaphragm pump for bleach, and never run flammable or combustible fluids through any polypropylene pump.
Full video transcript
0:00 · Hook
This is a three-inch air operated double diaphragm pump. It moves up to 300 gallons per minute, passes half-inch solids, and its wetted housing is glass-filled polypropylene. It's the FTI Air FT30P from Finish Thompson. In the next few minutes: how it works, how to read its curve honestly, which chemicals it handles, and where it earns its place.
0:25 · What it is
Finish Thompson launched the FT30P in March 2026 as FTI Air's three-inch non-metallic pump. The three-inch flanged ports sit in the center of the pump, and their position is adjustable, so they can line up with existing piping. It weighs about 140 pounds, stands just under 44 inches tall, and carries a five-year warranty.
0:50 · How an AODD pump works
Here's how an air operated double diaphragm pump works. Two diaphragms sit in two fluid chambers, joined by a shaft. Compressed air pushes one diaphragm outward, forcing fluid from that chamber out the discharge. At the same time, the shaft pulls the other diaphragm inward, drawing fluid into the opposite chamber. Then the air valve shifts, and the stroke reverses. Check balls and seats on each side keep the fluid moving in one direction only. On the FT30P, the air valve is stainless steel, and it needs no separate pilot valve. Finish Thompson calls it stall-resistant, with fewer parts in the air system.
1:34 · Specs that matter
The numbers that matter. Maximum flow: 300 gallons per minute. Maximum air supply: 120 psi, and maximum outlet pressure is also 120 psi. Solids: up to half an inch. Suction lift: 28 feet wet, and about 20 feet dry with rubber diaphragms. It runs at about 87 decibels, so plan for a muffler or piped exhaust.
1:58 · Reading the performance curve
Now the part most spec sheets skip. An AODD pump doesn't have one rating point. It has a family of curves, one for each air pressure. At zero flow, discharge pressure equals air pressure, so at 120 psi of air, the pump makes about 277 feet of head. The 300 gallon figure is where the 120 psi curve ends, at roughly 21 feet of head. That's not what you'll see against a real system. At 100 psi of air, the curve ends near 280 gallons per minute. The dotted lines are air consumption, from about 30 to more than 180 standard cubic feet per minute. That's the real operating cost. Our rule: don't select at the far right end of the curve. Every gallon costs more air out there, and exhaust icing gets worse.
2:47 · Chemical compatibility
Chemical fit starts with the housing. The FT30P is glass-filled polypropylene, and FTI's chemical guide gives that material its own column. It's rated excellent for hydrochloric acid to 37 percent, sulfuric acid to 60 percent, sodium hydroxide to 50 percent, ferric chloride, alum, lime, most plating solutions, and sewage. It is not recommended for sodium hypochlorite, sulfuric acid at 70 percent or stronger, chromic acid, nitric acid, or hydrofluoric acid. For bleach, use a PVDF diaphragm pump instead. Then choose the elastomer. The diaphragms, balls and O-rings touch the fluid too, and with eight diaphragm materials, the elastomer is usually the deciding part. One hard rule from the manual: never run flammable or combustible fluids through a polypropylene pump.
3:43 · Temperature
Temperature caps the whole pump. The polypropylene housing is published at 32 to 158 degrees Fahrenheit, no matter which elastomer you choose. Inside that window, EPDM, FKM and Santoprene handle heat best, while polyurethane tops out at 150.
4:02 · Where it fits
So where does a three-inch plastic AODD earn its place? Wastewater: sump and sludge-pit transfer, where 28 feet of wet suction lift and half-inch solids matter. Water treatment: moving ferric, alum and caustic on chemical skids. Plating and surface finishing: bath transfer and waste-treatment feed, with chromic baths the exception. Bulk chemical storage: unloading and transferring from tanks and totes. And mining: lime slurry and dirty water, where solids handling matters more than efficiency.
4:38 · FT30P vs Graco Husky 3300
How does it compare with the Graco Husky 3300 in polypropylene? At the same 100 psi of air, both curves end near 280 gallons per minute. The FT30P's 300 gallon headline comes from its 120 psi rating, which the polypropylene Husky isn't rated for. Both pass half-inch solids. The clearer differences are weight, about 140 pounds versus 200, and that extra 20 psi of air headroom.
5:09 · Installation
Installing one? Feed it clean, dry, regulated air: a filter-regulator and gauge at the three-quarter-inch inlet, with an air line big enough to hold pressure at full stroke. Mount the pump vertically, or it may not prime properly. Support the piping so it doesn't load the plastic manifolds. On corrosive service, use diaphragm-failure detection, and pipe the exhaust away, so a ruptured diaphragm can't vent product into the room.
5:40 · Size it before you buy
Before you buy, check the pump against your actual duty. LibertyCES built a free diaphragm pump sizing calculator. Enter your flow, discharge pressure and available plant air, and it places your duty on the FT30P curve, estimates air use, and flags suction-lift and right-of-curve problems. Then it emails you a PDF worksheet for spec review. The link is in the description. LibertyCES is an authorized Finish Thompson distributor serving California, Arizona, Nevada and New Mexico.
Size it right before you buy
Enter flow, head, SG and plant air. See the air pressure and SCFM your duty needs on a real FTI Air FT30P curve, then get the worksheet by email.
The Finish Thompson MSDB pump-review checklist: carbon-bushing run-dry limits, NPSH margin and the suction-piping rules that decide pump life.
Send James your spec
Fluid, concentration, temperature, flow or volume, and where it goes. James replies with a real answer, not a catalog page.
Reviewed by James Riggins, 30+ years specifying water and chemical process equipment · Updated