The 300 GPM Myth: How to Spec a 3-Inch Air Operated Double Diaphragm Pump

Show notes

An air operated double diaphragm (AODD) pump moves fluid with compressed air and two diaphragms joined by a shaft: air pushes one diaphragm out while the shaft pulls the other in, and check balls keep the flow one-way. Because it can only push as hard as the air behind it, its rating is a family of curves, one per air pressure, not a single point. This episode works the FTI Air FT30P, Finish Thompson's 3-inch glass-filled polypropylene AODD, from the spec sheet to the specification.

The worked example

Interpolated by LibertyCES from FTI Air's published FT30P curve (FT30 manual P/N 212602 Rev. 3, p.9; water at ambient temperature, rubber/TPE diaphragms). Run your own duty in the free diaphragm pump sizing calculator.

Numbers from the episode

• 300 gpm is the end of the 120 psi air curve at about 21 ft of head; at 100 psi air the curve ends near 280 gpm.
• At zero flow, discharge pressure equals air pressure: 120 psi × 2.31 ≈ 277 ft.
• Air use runs from about 30 to 180+ SCFM; FTI's published point is 115 SCFM at 100 psi.
• Glass-filled polypropylene (FTI's own column) is excellent for HCl to 37%, sulfuric to 60%, phosphoric to 85%, NaOH/KOH to 50%, ferric, alum, lime, plating solutions and sewage, and not recommended for sodium hypochlorite, sulfuric 70%+, chromic, nitric and hydrofluoric acid. See the AODD chemical compatibility chart.
• Housing temperature range 32–158°F; suction lift 28 ft wet, 20.1 ft dry (rubber diaphragms); solids to ½ in.
• Graco Husky 3300 polypropylene: 280 gpm at 100 psi (its maximum air), ½-in solids, about 200 lb vs the FT30P's ~140 lb.

Related: how an air operated double diaphragm pump works (6-minute video) · diaphragm pump troubleshooting.

Episode 3 transcript

Timestamps are aligned to the audio word by word and jump the player to that moment.

0:00 · The number on the spec sheet

[0:00]This pump is rated at 300 gallons per minute. Most people who buy it will never see that number. That's not a knock on the pump. It's how every air operated double diaphragm pump is rated, and it's the most misread number in chemical transfer. I'm Marcus Holt, and this is the Liberty Specs Podcast, where we take one equipment decision and work it all the way down to the spec.

[0:23]Today: the FTI Air FT30P, Finish Thompson's three-inch plastic diaphragm pump. How it works, how to read its curve so you size it right the first time, what it costs you in compressed air, which chemicals it handles and which ones it doesn't, and how it stacks up against the pump most engineers already know.

0:47 · Meet the FT30P

[0:47]Finish Thompson launched the FT30P in March 2026 under its FTI Air brand. It's a three-inch, non-metallic, air operated double diaphragm pump. The wetted housing is glass-filled polypropylene. The three-inch flanged ports sit in the center of the pump, and their position is adjustable, which makes it easier to drop into existing piping. It weighs about 140 pounds, stands just under 44 inches tall, and carries a five-year warranty.

[1:17]On paper: up to 300 gallons per minute, up to 120 psi of air supply and 120 psi of outlet pressure, solids up to half an inch, and 28 feet of wet suction lift. Every one of those numbers is real. But two of them only make sense once you understand how this kind of pump actually moves fluid.

1:39 · How the stroke works

[1:39]So let's build it from the inside. An air operated double diaphragm pump has two fluid chambers, one on each side. Each chamber is closed off by a flexible diaphragm, and the two diaphragms are joined by a single shaft that runs through the center section. In that center section sits the air valve. Compressed air goes behind one diaphragm and pushes it outward.

[2:02]That squeezes the fluid in that chamber, and it leaves through the top check valve, into the discharge manifold. Because the two diaphragms share a shaft, the other diaphragm gets pulled inward at the same time. That lowers the pressure in the opposite chamber, and fluid is drawn up through the bottom check valve from the suction manifold.

[2:23]At the end of the stroke, the air valve shifts. Air goes to the other side, and everything reverses. The check balls and seats are what make it a pump instead of a bellows: they open and close every stroke, so fluid only ever moves from suction to discharge. Two consequences fall right out of that design.

[2:43]First, there's no motor to overload. The pump can only push as hard as the air behind it. Second, the flow pulses, one pulse per stroke, which matters later when we talk about installation.

2:55 · The air valve and why diaphragm pumps stall

[2:55]The air valve is where most diaphragm pump trouble actually starts. On many pumps, the main air spool is shifted by a pilot valve, or by pilot pins that the diaphragm shaft bumps at the end of each stroke. Those small parts wear. Dirty plant air gums them up. And as compressed air expands through the exhaust, it gets cold, cold enough to form ice in a humid plant.

[3:21]Any of those can leave the spool stuck halfway, and the pump stops at the end of a stroke. That's a stall. On the FT30P, the air valve body is stainless steel, and it needs no separate pilot valve. Finish Thompson's launch release calls the design stall-resistant, with fewer parts in the air system. That's a real design choice, and it removes part of that failure path.

[3:48]It doesn't remove the need for clean, dry, regulated air. Feed any diaphragm pump wet, dirty air at a starved pressure, and it will tell you about it.

4:00 · Reading the curve, with a real duty point

[4:00]Now, the 300 gallons. A centrifugal pump has one curve. An air operated diaphragm pump has a family of curves, one for each air pressure. FTI publishes six for the FT30P, from 20 to 120 psi of inlet air. Here's the rule that makes the whole chart readable: at zero flow, the discharge pressure equals the air pressure.

[4:24]So on 120 psi of air, the pump can hold about 277 feet of head, which is just 120 times 2.31. As flow rises, the head it can deliver falls. The 300 gallon headline is where the 120 psi curve ends, at roughly 21 feet of head. That's almost no system at all. On 100 psi of air, which is closer to what most plants actually have, the curve ends near 280 gallons per minute.

[4:56]So here's how you size it. Take your real duty: the flow you need, and the total discharge head, meaning the elevation you're lifting to, plus friction in the pipe, plus any pressure at the end of the line. Let's use a real example: 150 gallons per minute against 80 feet of total head. Go to 150 gallons on the chart.

[5:18]The 60 psi curve only makes about 73 feet there, so it can't do the job. The 80 psi curve makes about 100 feet, so it can. Interpolate between them, and this duty needs about 65 psi of air, measured at the pump inlet while it's running, not at the compressor. That's your minimum. If your header pressure sags to 60 at the pump during the day shift, this pump will slow down and eventually stall, and nothing is wrong with the pump.

5:49 · The air bill

[5:49]Which brings us to the part that never shows up on the purchase order: compressed air. The FT30P curve has a second set of lines on it, dotted ones, and they're air consumption in standard cubic feet per minute. They run from about 30 SCFM at low flow to more than 180 SCFM near full flow.

[6:09]FTI publishes one fixed reference point: 115 SCFM at 100 psi. Go back to our example, 150 gallons against 80 feet. That point sits between the 90 and the 120 SCFM lines, so you're spending a little over 100 standard cubic feet per minute of compressed air, every minute the pump runs. That's the operating cost of an air operated pump, and it's why where you land on the curve matters as much as whether you land on it.

[6:38]Out at the right-hand end of the curve, every additional gallon costs more air. The diaphragms flex harder and wear faster. And the exhaust gets colder, so icing gets worse. Our rule at LibertyCES: don't select a diaphragm pump in the last stretch of its curve. If your duty only works at 110 or 120 psi near the end of the line, go up a size, or run two pumps.

7:05 · The chemistry trap

[7:05]Now the part that catches experienced people: chemical compatibility. You'd expect a polypropylene pump to follow the polypropylene column in a compatibility chart. The FT30P's housing isn't plain polypropylene. It's glass-filled polypropylene, and FTI's own chemical resistance guide gives glass-filled polypropylene its own column, labeled for the three-inch pump. It doesn't rate the same as plain polypropylene.

[7:32]Glass-filled polypropylene is rated excellent for hydrochloric acid up to 37 percent, sulfuric acid up to 60 percent, phosphoric acid up to 85 percent, sodium and potassium hydroxide up to 50 percent, ferric chloride and ferric sulfate, alum, lime, copper, nickel and zinc plating solutions, and sewage. Chrome plating solutions are rated fair. And it is not recommended for sodium hypochlorite, at five and twelve and a half percent, sulfuric acid at 70 percent and stronger, chromic acid, nitric acid, or hydrofluoric acid.

[8:10]That hypochlorite line matters, because bleach is one of the most common chemicals in water treatment. For bleach, use a PVDF-bodied diaphragm pump. The housing is only half the decision. The diaphragms, the check balls and the O-rings touch the fluid too. The FT30P offers eight diaphragm materials: Neoprene, Santoprene, FKM, EPDM, PTFE, Buna, Hytrel and polyurethane, with check balls in Neoprene, Buna, EPDM, FKM, Santoprene, PTFE or 316 stainless, and seats in polypropylene or PVDF.

[8:49]With that many options, the elastomer is usually the deciding part. Temperature caps all of it. The polypropylene housing is published at 32 to 158 degrees Fahrenheit, no matter which elastomer you pick, and polyurethane tops out at 150. And one rule from the manual that isn't a judgment call: never run flammable or combustible fluids through a polypropylene pump.

9:15 · The suction side and solids

[9:15]On the suction side, the manual lists 28 feet of wet suction lift, and 20.1 feet of dry lift with rubber diaphragms, 19.8 with PTFE. That covers most sumps, pits and totes where a centrifugal pump would need a flooded suction. One installation detail from the manual is easy to miss: mount the pump vertically, or it may not prime properly.

[9:42]As for solids, the FT30P passes particles up to half an inch, 12.7 millimeters. That's gravel-size material in wastewater, sludge and slurry. It's a rating for particle size, not a promise about abrasion. Abrasive slurry still wears diaphragms, balls and seats, so match the elastomer and the seat material to the solids, and expect those parts to be your maintenance kit.

10:10 · A fair comparison: FT30P versus Graco Husky 3300

[10:10]The pump most engineers compare this to is the Graco Husky 3300 in polypropylene, so let's do it fairly. Graco rates the polypropylene Husky 3300 at 280 gallons per minute with standard diaphragms, on 100 psi of air, which is that model's maximum air pressure. On the same 100 psi of air, the FT30P curve also ends near 280.

[10:37]So at equal air, they're in the same place. The FT30P's 300 gallon headline comes from its 120 psi rating, which the polypropylene Husky isn't rated for. If anyone tells you 300 beats 280, ask them, at what air pressure. Both pumps pass half-inch solids. The clearer differences are weight, about 140 pounds for the FT30P against 200 for the Husky, and the extra 20 psi of air headroom, which matters when your duty needs more head than 100 psi of air can deliver.

11:12 · Installing it right

[11:12]A few installation points, because most diaphragm pump problems are installation problems. Give it clean, dry, regulated air: a filter-regulator and a gauge right at the three-quarter-inch air inlet, and an air line big enough to hold pressure at full stroke rate. The exhaust is one inch. Use a muffler, the pump runs at about 87 decibels, and on hazardous fluids, pipe the exhaust away so a ruptured diaphragm can't vent product into the room.

[11:42]The flanges are three-inch ANSI, DIN or ISO. Support the piping so its weight doesn't load the plastic manifolds. Add a pulsation dampener to protect valves, meters and instruments downstream from that stroke-by-stroke pulse. And on corrosive service, add diaphragm-failure detection, so a leak shuts the pump down instead of running chemical into the air side.

12:10 · The spec, not the catalog

[12:10]So here's the specification, start to finish. Put your real duty point, flow and total discharge head, on the curve, and read the minimum air pressure at the pump. Check that your plant can hold that pressure while the pump runs, and look at the air consumption at that point. Stay out of the last stretch of the curve.

[12:30]Check the housing and every elastomer against the actual chemical, concentration and temperature, and remember that glass-filled polypropylene has its own column. Confirm the suction conditions. Then install it with clean air, supported piping, a dampener and leak detection. None of that is in the 300 gallon headline. That's the difference between a specification and a catalog purchase.

[12:56]If you want help with the first step, LibertyCES built a free diaphragm pump sizing calculator. You enter your flow, discharge pressure and plant air, and it places your duty on the FT30P curve, estimates the air use, flags suction-lift and right-of-curve problems, and emails you a worksheet for a spec review. The link is in the description, along with the full transcript of this episode.

[13:22]LibertyCES is an authorized Finish Thompson distributor serving California, Arizona, Nevada and New Mexico. I'm Marcus Holt. Thanks for listening to the Liberty Specs Podcast.

Frequently asked questions