Diaphragm Pump Technical Guide

Selection

How to select an air operated double diaphragm pump

Selecting the right diaphragm pump starts with knowing the duty. Before looking at models, gather this data:

1. Fluid and its concentration.
2. Operating temperature.
3. Fluid viscosity.
4. Solids: maximum size and percentage.
5. Required flow rate.
6. Discharge pressure: head and pipe length.
7. Available air: compressor pressure and flow.
8. Environment: explosive atmosphere or outdoor installation.

Port size and flow rate

AODD pumps are typically built with 1/2″ to 3″ ports. Port size alone does not define flow rate: the duty point is found on the pump curve. A common practice is to select a pump that runs between one third and one half of its maximum flow. Diaphragms last longer and air consumption drops.

Air and discharge pressure

In a standard pump, discharge pressure cannot exceed air supply pressure. For higher discharge pressures, such as filter press feeding, high pressure diaphragm pumps multiply the air pressure.

Materials and valves

The pump body can be metal (aluminum or stainless steel) or plastic (polypropylene or PVDF), depending on the fluid. Diaphragms, balls and seats are selected using the chemical compatibility chart. For fluids with large solids, a flap valve is better than a ball valve.

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Selection

Diaphragm pump chemical compatibility: materials chart

A diaphragm pump has three groups of wetted parts: the body, the diaphragms, and the balls and seats. All three must be compatible with the pumped fluid.

Body materials

Diaphragm, ball and seat materials

These charts are a guide only: concentration and temperature change compatibility. For critical services, the Skylink technical team confirms materials against the product safety data sheet.

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Selection

Ball valve or flap valve: which one to choose

Check valves open and close the fluid path on every pump stroke. Their design determines which solids the pump can handle.

Ball valve: a sphere seating on a ring. It seals reliably and is the standard choice for clean, viscous and fine-solids fluids.

Flap valve: a hinged flap that opens almost the full bore. It passes solids close to the port diameter, plus fibers and rags that would jam a ball.

For these services, Skylink offers flap valve diaphragm pumps for solids.

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Selection

Air operated or electric diaphragm pump

Both pump the same way, with two diaphragms and check valves. The difference is the drive.

Air operated pump (AODD)

Runs on compressed air, with no electricity at the pump. If the discharge is closed, it stalls without damage. With the proper certification it can be used in explosive atmospheres. Its weak point is energy cost: compressed air is expensive to produce.

Electric pump (EODD)

A motor and gearbox drive the diaphragms mechanically. It uses far less energy and needs no compressor. With a variable frequency drive, flow is controlled precisely. It generally needs overpressure protection if the discharge can be closed.

See Skylink electric diaphragm pumps.

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Comparisons

Diaphragm pump vs centrifugal pump

The centrifugal pump is the most common in industry, but it does not suit every job. The diaphragm pump wins when the fluid is difficult.

Rule of thumb: for clean water and large flows in continuous duty, choose centrifugal. For chemicals, sludge, viscous fluids, solids or dry-running risk, choose diaphragm.

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Comparisons

Diaphragm pump vs peristaltic pump

Both are positive displacement, self-priming and tolerate dry running. They differ in cost, maintenance and accuracy.

The peristaltic pump suits accurate dosing or very abrasive slurries. The diaphragm pump is usually more economical for general transfer.

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Comparisons

Diaphragm pump vs progressive cavity pump

The progressive cavity pump, also called a helical rotor or eccentric screw pump, uses a helical metal rotor turning inside an elastomer stator. Both are positive displacement and used for sludge and viscous fluids, but they work differently.

The progressive cavity pump suits dosing sludge or polymer at a constant rate, or feeding equipment that needs pulse-free flow. The diaphragm pump suits services with dry-running risk, portable use, or a lower purchase and maintenance cost.

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Operation

How an air operated double diaphragm pump works

Main parts

• Two fluid chambers, each with a diaphragm.
• A shaft connecting both diaphragms.
• An air valve in the center section that switches air between the diaphragms.
• Four check valves: two suction and two discharge.
• Suction and discharge manifolds.

Pumping cycle

1. Air enters behind diaphragm A and pushes it: chamber A discharges the fluid.
2. The shaft pulls diaphragm B toward the center, creating a vacuum in chamber B, which draws fluid in.
3. At the end of the stroke, the air valve reverses the air flow and the process repeats on the other side.

Why it has these advantages

It is self-priming because it creates vacuum even with an empty suction line. It can run dry because it has no seals that fail without liquid. If the discharge is closed, pressures balance and it stalls without damage. Flow is pulsating; a pulsation dampener smooths it.

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Operation

Diaphragm pump curve and air consumption calculation

A diaphragm pump curve shows flow rate on the horizontal axis and discharge pressure on the vertical axis. It includes one line per air pressure and air consumption curves.

Step-by-step sizing

1. Calculate the discharge pressure: static head plus pipe friction losses.
2. Locate the flow and pressure on the curve: this is the duty point.
3. Read the required air pressure from the line passing through that point.
4. Read the air consumption from the consumption curve, in Nm³/h or SCFM.
5. Size the compressor with a margin, typically 20–30% extra, adding other plant air demands.

Illustrative example: if the curve shows 40 Nm³/h at the duty point, choose a compressor that delivers at least 50 Nm³/h available for the pump.

High viscosity and high suction lift reduce actual flow compared to the curve, which is measured with water.

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Maintenance

Diaphragm pump troubleshooting: common problems and solutions

Most of these problems are solved with wear parts and proper compressed air treatment.

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Maintenance

Diaphragm pump spare parts: diaphragms, balls and seats

Wear parts

• Diaphragms.
• Balls or flaps, and seats.
• Gaskets and O-rings.
• Air valve kit.
• Muffler.

What service life depends on

Fluid abrasiveness, temperature, chemical compatibility, air pressure and cycles per minute. An oversized pump running slowly lasts longer.

Signs it is time to replace

Drop in flow, fluid in the exhaust, erratic cycling or leaks at the joints.

Recommendations

• Keep a spare parts kit in stock for each critical pump.
• Replace diaphragms, balls and seats together.
• Schedule preventive maintenance by operating hours.

How to order Skylink spare parts

Send the full model code from the pump nameplate, or a photo of it, and the materials. Skylink Pumps supplies genuine spare parts and accessories: diaphragm, ball, seat and air valve kits.

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Applications

Diaphragm pumps by industry

Each configuration is adjusted to the specific case. To combine materials, see the chemical compatibility chart.

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Applications

How to select a metering pump

A metering pump injects a small, accurate flow of chemical. To select one you need:

• Maximum flow, in L/h.
• Injection point pressure.
• Chemical and concentration.
• Control type: manual, pulse or 4–20 mA signal.

Types

Solenoid (electromagnetic): simple and economical, for low flows. Motor-driven diaphragm: for higher flows and accuracy; adjusted by stroke length and speed.

Materials and accessories

PVDF, PP or stainless pump head depending on the chemical, with a PTFE diaphragm. Add an injection valve, back-pressure valve, relief valve and calibration column. For hypochlorite, which releases gas, a self-degassing head is recommended.

Common practice: select the pump so the normal dose falls between 30% and 70% of its capacity. See Skylink metering pumps.

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Diaphragm pump frequently asked questions

What is an air operated double diaphragm pump?

It is a positive displacement pump that uses compressed air to move two diaphragms. It transfers corrosive, viscous or solids-laden liquids without mechanical seals or an electric motor.

Can a diaphragm pump run dry?

Yes. It can run dry without damage and is self-priming, unlike most centrifugal pumps.

What air pressure does a diaphragm pump need?

The required air pressure depends on the duty point and is read from each model’s curve.

Which diaphragm material is best for aggressive chemicals?

PTFE has the highest chemical resistance. For less aggressive fluids, Santoprene offers longer life at a lower cost.

How long does a diaphragm last?

It depends on the material, fluid abrasiveness, temperature and cycles per minute. A properly sized pump running well below its maximum flow extends diaphragm life.

Where can I get spare parts for Skylink pumps?

Skylink Pumps supplies genuine kits of diaphragms, balls, seats and air valves. Just send the pump model or a photo of its nameplate.

Does Skylink help with pump selection?

Yes. With the fluid, flow, pressure and temperature, the technical team recommends the model and materials.

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