Diaphragm Pump
Diaphragm Pump
A diaphragm pump is a type of positive displacement pump that uses two flexible and reciprocating diaphragms to draw in and expel the liquid through the pump. They are classified under the positive displacement pumps because the flowrate is not greatly affected by the pressure head against which the pump is working at the pump speed.
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What is a diaphragm pump?
A diaphragm pump is a type of positive displacement pump that uses two flexible and reciprocating diaphragms to draw in and expel the liquid through the pump. They are classified under the positive displacement pumps because the flowrate is not greatly affected by the pressure head against which the pump is working at the pump speed.
Fig: Diaphragm pump and its main components (image credits: mechanicalboost.com)
Diaphragm pumps are used to transfer a wide range of liquids including liquid containing large sediments and liquids of a low, medium, or high viscosity. These pumps may also be designed to handle numerous aggressive chemicals in many industrial applications.
Depending on the type, a diaphragm pump can be powered by electricity or compressed air. Diaphragm pumps are manufactured from a variety of materials including plastics and metals such as stainless steel, aluminum. The diaphragm pump manufacturers decide on the type of material to use depending on the pump applications.
How does a diaphragm pump work?
The working principle of the double diaphragm pump is based on the air displacement principle where the diaphragms act as a separation wall between the air and the liquid. The image below will be used to describe the working principle of a diaphragm pump.
Fig: The working of a diaphragm pump (image credits: mechanicalboost.com
The two diaphragms (A and B) are connected by a single shaft allowing them to move in unison. An air valve is located at the center of the diaphragms and it directs compressed air to the back of the diaphragms. The liquid chambers are alternatively filled and emptied by fluid that is drawn through a common inlet and discharged through a single outlet. When the air valve directs the compressed air to the back of diaphragm A, chamber B begins its suction stroke, which starts as the diaphragm B is moved towards the center of the pump thereby creating a vacuum in chamber B. Atmospheric pressure then forces fluid into the chamber B past the inlet ball valve. At the same time, the pressurized diaphragm A is discharging and when it reaches the limit of its discharge stroke, the air valve redirects the pressurized air to the back of diaphragm B. This begins the discharge stroke of diaphragm B and the hydraulic forces developed inside chamber B force the inlet ball back to its seat and the discharge ball out from its seat. This condition allows the fluid to flow through the pump discharge. The same process occurs in the opposite chamber constituting one full cycle.
Characteristics of diaphragm pump
- Discharge fluid with pulses
- Check valve sealing is very important
- The pumping fluid is separated from sentive pump components
- Excellent idling properties
- self-priming capabilities
- Applicable for flammable, corrosive, abrasive liquids
- Diaphragm pumps are displacement pumps
- Good suction lift capabilities
Diaphragm pump types
The types of diaphragm pumps can be categorized based on the construction materials and the source of the driving force.
By material
- stainless steel diaphragm pump
The components of this type of diaphragms pump are constructed from stainless steel. Stainless steel diaphragm pumps are extensively used to transfer liquid in wet commercial applications. Most stainless-steel pumps turn on compressed air. They are suitable for general pump transfer, rough metering dewatering, low shearing transfer, water or chemical recirculation, dispensing, spraying, filling, and fuel evacuation. Manufacturers of stainless-steel dry pumps also provided a dry-running design model of the pumps, which are portable and intrinsically safe. These pumps have numerous desirable characteristics such as resistance to overheating and applications versatility. Generally stainless-steel diaphragms pumps can pump a wide range of liquids, from acids, caustics, and solvents, to high-viscosity shear-sensitive liquids including paints, inks, and adhesives. Also suitable for abrasives and slurries.
- Plastic diaphragm pump
These are diaphragm pumps with are plastic construction. They are most suitable for handling chemicals like acids and bleach. However, it may not be able to handle various chemicals such as styrene and certain resins. One attractive fact about plastic diaphragm pumps is that they do not leak. However, these pumps require regular inspection and maintenance compared to their stainless-steel counterparts. The operation of a plastic diaphragm pump can be affected by high pressure and high temperature.
- Aluminum diaphragm pump
This type of diaphragm pump is mainly constructed from aluminum material. Most of these pumps are motor operated and they deliver optimum air efficiency. Aluminum diaphragm pumps are capable of handling tough chemicals. Their bolted construction also
prevents fluid leakage; hence they are suitable for handling leak-free chemicals. The pumps also have easily accessible ball valves and fluid caps which helps with fast maintenance and repair of the pump.
By driver
- air diaphragm pump
An air-operated diaphragm pump is one of the most common types of diaphragm pumps. The pump is mainly an operation of the pump is based on compressed air, which provides the driving force required to deform the pump diaphragms to facilitate suction and discharge strokes of the pump. An air-operated diaphragm pump contains two liquid chambers with two diaphragms; hence it is also known as an air-operated double-diaphragm (AODD) pump. In addition, the pump comprises both discharge and suction check valves in each pumping chamber. An air distribution system is installed inside the pump to help with moving the air supply from one chamber to another. The systematic control of the flow of compressed air from one chamber to the second chamber leads to one chamber discharging all of its liquid into the expulsion piping while the other chamber fills with the liquid. A major challenge with the air-operated diaphragm is pulsation in the discharge flow, which necessitates the installation of dampeners within the discharge pipe network.
- electric diaphragm pump
An electric diaphragm pump is powered using electricity. They are often small-motor driven pumps and can produce a mechanical force of between 60 and 100 PSI. The electric diaphragm pump has two different styles. The first style is called the primary style and it contains fixed force switches to regularly and accurately switch the pump ON and OFF depending on the force settings. The second style is known as the bypass style. In the bypass style, the pump operation depends on whether the outlet valve is closed or open, but not on the force settings. If the outlet valve is open, the pump will continue to operate regardless of the magnitude of force applied to the system. If the outlet valve is closed, the pump’s internal flow stops.
Advantages of diaphragm pump
- Extremely efficient because it can work on a wide range of liquids from low viscosity to highly viscous
- large displacement volume
- Diaphragm pump has self-priming capabilities
- can operate over wide range flow rate and pressure
- Long service life
- Diaphragm pump has high energy efficiency; the forward hit of the pump applies an extra force
- Suitable for transferring corrosive chemicals that would be hazardous to the environment
- Can help to eliminate standing forces within flow line
- Portable and easy to install: can be moved to different locations easily
- Easy to maintain
Disadvantages of diaphragm pump
- They don’t deliver high pressure during operation
- Diaphragm pumps have low speed
- Powerful operation requires a large volume of compressed air to raise 100Psi
- Diaphragm pumps are not very energy efficient
- produces a pulsating discharge
Pulses in the pump discharge are caused by the cyclic action of the pump membranes, which accelerates the fluid during the compression cycle and slows it down during the suction cycle. A pulsating pump discharge is undesirable because it results in strong vibration within the pump discharge line which can loosen the pump connections and lead to leakage. Pulsing can be solved using two main techniques. The first technique involves installing a dampener at the top of the pump to decrease pulsing. The other technique involves the use of two cylinders with one of them in the compression phase and the other one in the suction phase. However, the latter technique requires smothering out at the pump outlet.
- The pump operation can be noisy
Noisy operation is most common with air-operated diaphragm pumps. The noise comes from the venting of air valves and vibration due to the pulsing action of the pump membranes. To reduce the noise level, one can install a muffler on the air vent line.
Diaphragm pump applications
Diaphragms pumps are widely used in the chemical industries to move a variety of liquids including the following:
- corrosive chemicals e.g., sulphuric acid
- Abrasive slurry and dirty water
- Highly viscous and sticky fluids
- Smaller solids
- Oil, creams, and gels
- Volatile solvents
- pharmaceutical products
- sensitive foodstuffs
Diaphragm pumps are also applied in the following industrial process
- chemical mixing
- paint filtration
- powder coating
- supply or transfer of air spray
- printing and ink
- food and beverage processing
Maintenance of diaphragm pump
Pump maintenance is an essential activity to help to avoid costly repairs and production delays. Diaphragm pumps are easy to maintain compared to the other types of pumps, which are made up of many mechanical and electrical components. However, effective maintenance of the diaphragm pump requires an understanding of the 4 main maintenance areas.
- Inlet and outlets-The pumps inlets and outlets also called the suction and discharge ports should be consistently inspected and cleaned to prevent the accumulation of debris or rocks, which can block the pump flow.
- Air quality -air quality and moisture content affect the operation of an air-operated diaphragm pump. Maintaining the best air quality can help extend the service life of the pump. The air filters should be regularly checked and cleaned to prevent them from clogging with poor quality air. The moisture content in the air is monitored to enhance proper pump flow. Also, the air pressure should be regulated to ensure it is within the operation limit of the pump. Diaphragm pump manufacturers usually provide a manual to help to guide on the pressure limits and pump flow rates.
- Elastic components-Basically, this refers to the pump membranes. The membrane should be examined every two months to prevent uneven wear.
- Torque settings- Too high torque settings can compress and damage the pump, particularly plastic diaphragm pumps. The torque on the pump’s bolts should be regularly checked and maintained within the recommended range by the pump manufacturer.
Diaphragm pump troubleshooting
Pump cycles at a stall or fails to hold pressure at a stall
- Worn check valve balls, seats, or O-rings (Replace valve balls, seats, or O-rings)
Diaphragm pump not cycling or cycles and then stops
- Valve ball severely worn-out (replace ball and seat)
- Ball valve wedged into seat due to excessive pressure (install pressure relief valve)
- Dispensing valve is clogged (Reduce pressure and clear the valves)
Erratic diaphragm pump operation
- clogged suction line (inspect and clean the suction line)
- Ball valve leaking or sticky (clean or replace the valves)
- Ruptured diaphragm (Replace the diaphragm)
- Restricted exhaust (remove restriction)
- Damaged pilot valves, air valves, gaskets (inspect, and replace damaged components)
- Erratic air supply
- Icing in the exhaust muffler (use drier air supply)
Diaphragm pump cycles but does not prime
- Pump speed too high leading to cavitation before priming (reduce the inlet pressure of air)
- Check valves balls severely worn out (replace the balls)
- clogged inlet or outlet (clean)
- lost inlet fittings or manifolds (Tighten)
premature diaphragm failure
- Cavitation (Enlarge pipe diameter on the suction side of the pump or move closer to the product)
- Excessive pressure on the suction side (raise the pump or place it on top of the tank to reduce pressure
- Incorrect diaphragm plates or plates installed backward (consult manufacturers guide on correct installation)
- Startup with full pressure (Diaphragm pump manufacturers recommend starting the pump at low pressure)
Unbalanced cycling
- Excessive suction lift (fill the chambers with liquid to prime the pump)
- pumped fluid in the exhaust air muffler
- Air leakage on the suction side
- Obstructed check valves (dislodge the obstruction in the check valve pocket)
- Entrained air or vapor lock in the chamber (use chamber vent plugs to purge chambers)
Air bubble in the fluid
- Loose suction line (tighten)
- loose fluid side diaphragm plate
Diaphragm pump air valve freezing
- Excessive moisture in compressed (use filter to remove moisture from compressed air; install hot air generator or a drier for the compressed air)
Excessive pump exhaust
- Worn air valve block, O-rings, plate, pilot block, u-cups, or pilot pin O-rings (inspect and replace if necessary).
- Worn out shaft seals (replace)
Summary
This article provides detailed information about diaphragm pumps. These pumps use two flexible membranes to draw in and pump liquids through the pump. They are a great way to move large volumes of liquids easily and efficiently. Diaphragm pumps are available in different designs which increase their usage for different industrial purposes.
These pumps are classified based on the construction material and the type of drive used for a particular pump. Based on the type of materials the diaphragm pumps have 3 main types: stainless steel, plastic, and aluminum diaphragm pumps. Concerning the source of drive force, the diaphragm pump can be classified as air operated and electric operated diaphragm pump. Generally, diaphragm pumps are extensively used in the chemical industry to move a variety of liquids including corrosive chemicals, abrasive slurry, and volatile liquids. They are also used in chemical mixing, powder coating, paint filtration, and food and beverage processing.
A diaphragm pump offers many benefits including large displacement volume, self-priming capabilities, high energy efficiency, and long service life. A major downside of diaphragm pumps in applications is the production of pulsating discharge Although the pulses can be significantly reduced by installing a pulse dampener, it leads to an added cost on the pump. Also, most of the pumps are low-speed pumps and they deliver liquid at low pressure. one of the common troubleshooting challenges for the diaphragm pump is excessive pump exhaust. This problem is caused by worn-out air valve blocks, worn-out shaft seals. Other issues include freezing in the pump air valve, irregular pump cycling, premature diaphragm failure, pump cycling at a stall, or failing to hold pressure at a stall. This article also clearly highlights the possible cause of these issues at the possible solutions.