Double Diaphragm Pumps Information
Double diaphragm pumps are specific type of diaphragm pump designed to move gases, liquids, or gas-liquid mixtures via two separate reciprocating diaphragms. In addition to the seal-less construction and reliable operation of all diaphragm pumps, double diaphragm pumps provide a more even flow because of their alternating pump mechanism. They are almost always air-driven, meaning they can operate in dangerous environments where the spark from an engine could be hazardous. Double displacement pumps are the most commonly used type of diaphragm pump.
Double diaphragm pumps consist of two diaphragms connected to a piston and contained within two separate displacement chambers, each with an inlet and discharge valve. The diaphragms are made of a flexible material compatible with the pumped media. They are 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 behind double diaphragm pumps is almost always compressed air.
Diagram of double diaphragm pump. Image Credit: Pump Fundamentals
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 Engineering360.
Diaphragm pumps work by flexing a 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. For more information on all types of diaphragm pumps, visit the Diaphragm Pump Selection Guide page on Engineering360.
Double diaphragm pumps utilize two chambers which operate concurrently (one draws in while the other pushes out). The effectiveness of this pump is due to the balanced flow of the media cycling through the dual chambers.
Check out this animation for a visual explanation of how double diaphragm pumps operate. Here is another animation:
Image Credit: Animatedsoftware.com
A variation on the typical double diaphragm pump consists of a working diaphragm with an additional backup diaphragm beneath it. These pumps operate more like single-acting diaphragm pumps, but the backup diaphragm allows hazardous or expensive liquids to be pumped safely and reliably, providing insurance if the working diaphragm fails while in use.
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.
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.
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.
Shape - Determines the flow and pressure level of the pump. Shapes include flat, rolled (molded), and structured types.
Flat - Inexpensive and can provide adequate vacuum separation between chambers, but has a limited stroke/offset length and lower energy efficiency. It also has a lower lifespan since it tends to deform as it adjusts to the stroke.
Rolled/molded - Shaped to factor in the required stroke length, it provides a repeatable displacement with a constant effective pressure area. It has a longer life than the flat type and creates less resistance (higher efficiency) but is more expensive.
Structured - Patented diaphragm design that incorporates a ribbed underside to accommodate a specific load. Provides higher strength, good capacity, and higher efficiency than other types in a small size.
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.
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.
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. Most double displacement pumps are made from plastic and stainless steel, those best suited for corrosive environments and chemicals. 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:
The Pump Features page on Engineering360 provides additional information on materials and other pump features important to selection.
The primary specifications to consider when selecting diaphragm pumps are flow rate, pressure, horsepower, power rating, and operating temperature. These specifications are standard to most pumps, and are described in detail in on Engineering360'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, will reduce unnecessary heat exposure and buildup.
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. Double diaphragm pumps are versatile, and can be designed to handle most levels of fluid viscosity.
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.
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.
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