Diaphragm pumps are positive displacement pump which move gases, liquids, or gas-liquid mixtures via a reciprocating diaphragm. They are highly reliable because they do not include internal parts that rub against each other. They also contain no sealing or lubricating oils within the pumping head, meaning there is no chance of oil vapor leakage or contamination of the handled media.

Pump Composition

Simple diaphragm pumps consist of a diaphragm, displacement chamber, two valves, and a driving mechanism. The diaphragm is made of a flexible material compatible with the pumped media. It is sealed in place between the side of the displacement chamber and an attached flange. The chamber's volume is slightly greater than what the diaphragm can displace. The valves are typically spring-loaded ball valves or flapper valves made of the same material as the diaphragm, and they function to admit the fluid in and out of the chamber. The driving mechanism is what flexes the diaphragm - in the diagram below, the drive mechanism is a spring activated by a solenoid.

Diagram of solenoid diaphragm pump. Image Credit: Honda-varadero-uk.org

Pump Operation

Diaphragm pumps are positive displacement pumps, meaning they use contracting and expanding cavities to move fluids. For more information about this category of pumps, visit the Positive Displacement Pump Selection Guide page on IEEE GlobalSpec.

Diaphragm pumps work by flexing the diaphragm out of the displacement chamber. When the diaphragm moves out, the volume of the pump chamber increases and causes the pressure within the chamber to decrease and draw in fluid. The inward stroke has the opposite effect, decreasing the volume and increasing the pressure of the chamber to move out fluid. This operation is very similar to the draw in, push out, concept of human breathing.

The motion of the diaphragm looks like this:

Image Credit: animatedsoftware.com

This video provides further demonstration:

Video Credit: PrecisionPlus / CC BY-SA 4.0

Types

Diaphragm pumps can be classified based on their drive mechanism and whether they are single or double acting.

Drive Mechanisms

The diaphragm in diaphragm pumps can be driven by a number of different mechanisms.

• Mechanical - These pumps are operated using a simple and robust reciprocating mechanical linkage directly attached to the diaphragm. This is typically a crank or gear mechanism which converts motor rotation into reciprocating motion of the linkage (spring, piston, plunger) attached to the diaphragm. Flow is adjusted by changing the stroke length or the pump speed. These pumps have similar characteristics to piston pumps and plunger pumps.
• Hydraulic - These pumps implement an intermediate hydraulic fluid on the opposing side of the diaphragm. The fluid is pressurized by a plunger or piston in order to flex the diaphragm. These operate similarly to mechanical pumps, but the hydraulic fluid contacts the diaphragm rather than a mechanical linkage. Flow is varied by adjusting pump speed or the amount of contained hydraulic fluid.
• Solenoid - These pumps have an electric motor that controls a solenoid magnet. When activated, the solenoid creates a magnetic force which interacts with a ferro-magnetic metal part on the diaphragm, causing it to flex. Flow is adjusted by changing the pump speed.
• Air - These pumps are double-acting (double diaphragm) pumps which use compressed air to drive two separate diaphragms alternatively. The air flow is alternated between diaphragms via a shuttle valve. Flow is changed by adjusting the air pressure supplied to the pump.

Single or Double Action

The action of a diaphragm pump refers to how many diaphragms and sets of valves the pump incorporates.

• Single acting pumps incorporate one diaphragm and one set of valves, and are characteristic of mechanical drive pumps.
• Double acting pumps, also known as double diaphragm pumps, incorporate two diaphragms and two sets of valves. Air-operated diaphragm pumps are characteristically double-acting. For more information on specifically selecting double diaphragm pumps, visit the Double Diaphragm Pumps Selection Guide on IEEE GlobalSpec.

Design

Some manufacturers design pumps to the specific needs of a customer's application. In these cases, it is especially important for the buyer to understand how the pump design affects performance.

Pump Stroke

The pump stroke length is the principal factor defining a pump's capacity and pressure ratings. Higher offset (longer stroke) produces higher flow rates and greater pressure/suction per stroke.

Diaphragm

The diaphragm is the main determinant of a diaphragm pump's performance, and must be sized to coincide with the desired pump stroke. Three different characteristics define a diaphragm's design.

Durometer - Measures the stiffness of the diaphragm elastomer. Higher durometer means a harder diaphragm which can generate higher pressures and flows. Lower durometer diaphragms product are rated for lower pressures and require less energy to operate. 

Image Credit: Design World - KNF Neuberger Inc.

Materials - The flexible material of the diaphragm. The industry standard is EPDM rubber (ethylene propylene diene monomer). Other materials include PTFE (polytetrafluoroethylene), rubber, and other plastics and elastomers.  Material affects the diaphragms lifespan, stiffness (durometer), and resistance to chemical or environmental corrosion.

System

The design of the corresponding valves and the motor which powers the pump also are important factors to consider.

• Valves - Parameters include shape, materials (compatible with the pumped fluid), and durometer (affects flow and noise level).
• Motor - The motor must be compatible with the energy requirements of the pump. In addition to meeting the pump's needs, the RPM and torque of the motor affect the pump's stalling pressure, energy efficiency, and noise level.

Materials

Pumps are typically designed with a number of different materials. The base materials, which constitute the parts of the pump exposed to the pumped media and the outside environment, are the most important to consider. Fluid characteristics, pressure ratings, and operating environment factors should be considered when selecting these materials (Images Credit: Direct Industry). 

• Cast iron provides high tensile strength, durability, and abrasion resistance corresponding to high pressure ratings.

• Plastics are inexpensive and provide extensive resistance to corrosion and chemical attack.

• Steel and stainless steel alloys provide protection against chemical and rust corrosion and have higher tensile strengths than plastics, corresponding to higher pressure ratings.

Other materials used in pump construction include:

• Aluminum
• Brass
• Bronze
• Ceramics
• Nickel-alloy

The Pump Features page on IEEE GlobalSpec provides more information on pump material characteristics and other features.

Specifications

The primary specifications to consider when selecting diaphragm pumps are flow rate, pressure, horsepower, power rating, and operating temperature. Information on interpreting these specifications can be found on IEEE GlobalSpec's Pump Flow page.

Design tip: Diaphragm life can be significantly prolonged by maintaining lower diaphragm temperatures. Pumps designed with additional fan cooling, especially those directed over the pumping head surface, can reduce unnecessary heat exposure and buildup.

Media Type

Selecting the right pump requires an understanding of the properties of the liquid in the addressed system. These properties include viscosity and consistency.

Viscosity is a measure of the thickness of a liquid. Viscous fluids like sludges generate higher systems pressures and require more pumping power to move through the system. Diaphragm pumps are designed to handle both viscous and thin liquids.

Consistency is the material makeup of the liquid solution in terms of chemicals and undissolved solids. Positive displacement pumps like diaphragm pumps are generally better suited for handling these solids, but dynamic pumps which are designed correctly (i.e. with certain impeller blades) can handle them as well. Solutions with corrosive chemicals should be handled by pumps with materials and parts designed to withstand corrosion.

Applications

Diaphragm pumps are commonly called "mud hogs" and "mud suckers" because of their use in pumping slurries and wastewater in shallow depths. They are capable of handling all sorts of aggressive media including gases and gas/liquid mixtures, and can achieve very high pressures. They should not be used to pump dangerous or toxic gases, since diaphragm pumps are not hermetically sealed.

Larger models of this pump type are used to move heavy sludge and debris-filled wastes from trenches and catch basins, applications where centrifugal pumps perform poorly due to high discharge volumes and low water levels which would cause them to lose their prime.

Smaller models are typically used in chemical metering or dosing applications where very constant and precise amounts of liquid delivery are required.

References

KNF Pumps & Systems - Diaphragm Pumps

Pump Scout - Diaphragm Pumps

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