Air-diaphragm pumps (double diaphragm pumps) are one type of positive displacement pump. Their main applications are to transfer fluid, low pressure spray, and other low-pressure applications requiring less than 120 psig. They do not meet either ANSI or API standards and are most often used because of their profitability and ability to run on compressed air.

Basic Components of Pumps

Figure below shows components of air-diaphragm pumps.

The function of each component is:

Air Chambers, there are two air chambers, one in the right and one in the left. Compressed air flows into or out of the air chambers.
Fluid Housings, there are also two fluid housings, one in the right and one in the left. The fluid being pumped will flow through the fluid housings.
Diaphragms, the diaphragm is a material that will flex with increasing or decreasing air pressure. Diaphragms separate the air chambers and fluid housings in the pump. A common shaft connects the two diaphragms.
Inlet manifold, fluid flows from the fluid container through the inlet manifold either to the right or left fluid housing.
Outlet manifold, fluid flows out of the right or left fluid housing past the check valves, then through the outlet manifold.
Air valve, the air valve directs compressed air to either the right or left air chamber. The air valve direct compressed air through a port to one of the air chambers while the opposite air chamber is directed to the exhaust port.
Check valves, an air-diaphragm pump has four check valves, two inlet check valves, and two outlet check valves. The check valves control the fluid through the fluid housings and manifolds of the pump. Most air-diaphragm pumps use ball type check valves.

Wetted parts of air-diaphragm pump include all parts that come into contact with the pumped liquid. They include, inlet manifold, ball checks and seats, diaphragms, fluid housing, and outlet manifold.

In several application, muffler minimizes the noise of exhaust air. Degree of noise reduction will depend on muffler types.

Working Principle

The driver of air-diaphragm pump is compressed air. Two diaphragms are working simultaneously to prime and push the liquid through the pump system. Valve balls work as check valves to let the liquid thorough in the right direction.

Compressed air flows into the right Air Chamber, causing the right Diaphragm to flex. This

expansion creates a high pressure in the right Fluid Housing equal to the air pressure applied to the pump. The inlet Check Valve (4) of the right Fluid Housing closes, the outlet valve (2) opens, and the fluid is pumped through the Outlet Manifold. The pump shaft moves right, creating a vacuum in the left Fluid Housing. The left inlet Check Valve (3) opens, the left outlet valve (1) closes, and the fluid flows into the left Fluid Housing. See figure below for an illustration of the cycle.

At the end of the pump stroke the air valve switches and compressed air will flow into the left Air Chamber, causing the left Diaphragm to flex. The inlet Check Valve (3) of the left Fluid Housing closes, the outlet valve (1) opens, and the fluid is pumped through the Outlet Manifold. The pump shaft moves left creating a vacuum in the right Fluid Housing. The right outlet valve (2) closes, the right inlet check valve (4) opens, and the fluid flows into the right Fluid Housing. See figure below for illustration.

Applications

Typical applications of air-diaphragm pump are:

Drum transfer for fluids up to 1000 cP
Sampling and testing
Fluid circulation and evacuation
Waste fluid removal
Low viscosity adhesive supply
Consistent low pressure process fluid supply
On-demand batch chemical metering
Circulation of low viscosity inks, detergents, and chemicals

Performance Curve

Performance curve of air-diaphragm pumps may be different from vendor to vendor. The performance curve is usually based on certain condition, for example based on water at 20^oC. Several condition may change the performance, for example viscosity difference and suction lifts.

Figure below is example of performance curve.

A flow of 25 lpm is desired.

Based on figure the discharge head is approximately 25 mWC. It requires an air pressure 4 bar and will consume approximately 0.2 Nm³/min.

Installation

I checked in several vendor brochures that air-diaphragm pump can be installed in flooded system, self-priming, and submerged.

In flooded installation, the piping system is designed with a positive suction head. This is the best way of installation where it is necessary to completely empty all liquid from the container.

In self-priming installation, the pump evacuates an empty suction pump. This is example of liquid transfer from a drum.

In submerged application, it is important to ensure that all components which are in contact with the liquid are chemically compatible. The air exhaust must be led to the atmosphere using a hose.

Figure below shows recommended installation of an air-diaphragm pump.

Recommended installation of air-diaphragm pump

We also need to consider several aspects when installation air-diaphragm pump:

Install appropriate size of inlet air line size and pressure.

We can find appropriate air line size for our pumps in the vendor installation and operation manuals. As a general guideline for air-diaphragm pump, it is best practice to match the air line hose size to the air inlet port size on the air valve.

Inlet air pressure also plays a key role in performance of pump. Because diaphragm pumps operate on 1:1 ratio, meaning the pressure of the inlet air is directly related to the fluid pressure at the outlet of the pump. For example, if the target outlet pressure is 100 psig, the inlet air pressure entering the air valve must be greater or equal to 100 psig.

Torque the pumps in accordance with the vendor recommended rating, especially for plastic pumps and replace all O-rings after service.
Ensure proper tubing and piping size, to reduce risk of cavitation. It is also recommended that air-diaphragm pump be installed with flexible inlet and outlet connection instead of hard plumbed. This is because as pump speed increases, vibration increases. This cause increasing the risk of loosening a hard plumb connection, creating potential for leaking.
If pump is used in self-priming installation, use low pressure air first to prime. Use air regulator to decrease the air pressure entering the air valve and slow down the pump. Once the pump speed has been reduced and the fluid has been provided enough time to enter the pump, then we can increase the air pressure and operate the pump at faster speed.
Clear any fluid line restriction. Restriction can create back pressure that may be negatively affect the pump.

Advantages

The advantages of air-diaphragm pumps are:

Extremely portable and reusable
Easy to install on a cover, pail, wall, or submerged
Operate on compressed air
Can handle entrained air and solids
Excellent for abrasive, low-to-medium viscosity fluids and shear sensitive materials
Simple and relatively dependable
Seal-less, leak-proof design prevents fluid waste, mess, and hazard
Can run dry without damage
Nonsparking
Available in corrosion-resistant materials
Self-priming
Low initial cost

Disadvantages

The disadvantages of air-diaphragm pumps are:

They do not meet normal standards for continuous duty process service
Air supply valve occasionally become plugged
Ball check valves wear and occasionally stick
Limited to relatively low pressure (approximately 93^oC) and pressures (120 psig). The fluid to air rotation relationship of all air-diaphragm pumps is 1:1, meaning if 100 psig of compressed air is applied to pump, theoretically we will only achieved 100 psig of fluid output.
More surge than other pumps. An accumulator will reduce surging in applications where surging is not acceptable
Air consumption in a continuous duty operation may result in higher energy consumption that other pump style
Cannot pump stringy, viscous liquid
Noisy operation

References:

Stewart, Maurice, “Surface Production Operations Pumps and Compressors Volume Four”, Gulf Professional Publishing, 2019.
5 Minute Fix to Troubleshoot a Diaphragm Pump: https://www.mgnewell.com/wp-content/uploads/2017/07/Trouble-shooting-a-diaphragm-pump.pdf
Diaphragm Pumps Tough Technology Gilkes: https://www.pumpfundamentals.com/pumpdatabase2/gilkes-diaphragm.pdf
Double Diaphragm Pumps Concept and Theory: http://wwwd.graco.com/training/concept_and_theory/Double%20Dia.%20Pumps%20v3.pdf
Air operated diaphragm pump Tapflo: https://tapflo.com/en/images/brochures/Diaphragm_Pumps_brochure_EN.pdf

How Air-Diaphragm Pumps Work