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What is an air diaphragm pump?

AODD pumps are designed to transfer highly abrasive or viscous media. They are reliable and generally easy to maintain due to the use of a design that using simple classic diaphragm technology.  Due to their reliability and easy maintainability,these pups are used in wide variety of industrial applications including Food Processing, Chemical Transfer, Oil and Gas, Paint, Acid Transfer Metering, Abrasive/Corrosive/Viscous Materials, Low-Density Powders, Large Solids etc.

Working of AODD Pump?

An air operated double diaphragm pumps is a type of positive displacement pump that utilizes compressed air to provide the power. The compressed air shifts from one chamber to the next, connected by a linked shaft to enable the chambers to move in unison. This creates a strong back-and-forth motion to force the liquid through the pump. As liquid gets forced out of one chamber into the discharge piping, the other chamber is being filled simultaneously with more liquid. The reciprocating diaphragms in the pump keep liquid continuously pulsating from one chamber to another.

 

 

how air operated diaphragm pumps work
FIGURE 1 OPERATION OF AODD PUMP 1

1 https://www.rodem.com/sites/default/files/literature/AODD-Sanitary-Standard-pump.pdf

Components of AODD Pump

The main components of AODD are:

  1. Diaphragms
    The AODD pump uses two diaphragms. The diaphragm provides separation between two air chambers and controls the suction and discharge from one chamber to the other.
  2. Suction and Discharge Manifolds
    When the compressed air fills the chamber, the diaphragm expands and pushes the fluid into the discharge manifold.
  3. Suction and Discharge Valves
    As the discharge fluid is pulled into the manifold, a vacuum is created. This pulls the fluid past the suction valve and into the pump chamber. Once the chamber has reached its maximum capacity, the whole process reverses and a repetitive pumping action is created.
  4. Liquid and Air Chambers
    Liquid and air chambers draw in, hold and then release the process liquid and compressed air respectively during the pumping action.

Considerations in Selecting AODD Pumps

The temperature of the material to be pumped

If the material to be pumped is at variance from ambient temperature then pumps made of a material compatible to the temperature range need to be used. In general, the temperature range of pumps is as follows:

 

TABLE 1 PUMP TYPES FOR VARIOUS TEMPERATURE RANGES

Pump Type Temperature range
Polypropylene pump 32° to 150° F (0°C to 66°C)
Nylon pump 0° to 150° F (-18°C to 66°C)
PVDF, Aluminum or 316 Stainless Steel 0° to 200° F (-18°C to 93°C)

 

Material to be pumped

The Type, Composition & Nature of the fluid is one of the prime consideration in selecting the pump. This is necessary to prevent corrosion of the pump and its smooth operation and long life.

Nature of fluid

  • Corrosive fluids: Corrosive fluids like sulfuric acid, Nitric acid, Sodium hydroxide etc. requires wetted parts and diaphragm that are not reactive to these liquids.
  • Abrasive Fluids: Abrasive fluids like paints, inks, ceramic slurry etc. which contains solid particles that can damage the contact material by scraping or wearing.
  • Toxic Fluids: Toxic fluids like lead, mercury etc. are poisonous. The seal-less and leak free design of Diaphragm pumps make them suitable for such liquids.
  • Flammable Fluids: Flammable chemicals like acetone, benzene, petroleum etc. that easily catches fire need to be handled carefully. Since AODD pumps are air driven, it makesthe pump intrinsically safe and suitable for flammable liquids. However, the pumpshould be completely grounded to take care of static charge generation by some liquids.
  • Shear Sensitive Liquids: The characteristics of some products such as tomato ketchup, emulsions, curd, paste, etc. change when they are churned. The lower speed of Diaphragm pumps makesthem ideal for such liquids.
  • Vapour Pressure: Liquids such as diesel, hydrocarbons have low vapourpressure and require low speed to deliver the requiredflow making AODD pumps suitable for these applications.

The chemical compatibility of the wetted components (housing, diaphragms, balls & seats) with that of material being pumped need to be ascertained to ensure that the pump is capable of handling the fluid in question. For this purpose,the Chemical Compatibility chart of the manufacturer which provide the compatibility of liquid to be pumped and the pump construction material must be referred.

If the pump is required to handle any solids in suspension then consideration needs to be taken with regards to the max size of solids the pump is capable of passing.

If the material to be pumped is corrosive in nature or has some unique characteristics then, for the correct pump material one must refer to Solvents can corrode parts made from aluminum or galvanized steel. Only stainless steel or PVDF pumps should be used to pump solvents.

Table 2 DIAPHRAGM MATERIAL COMPATIBILITY CHART FOR FREQUENTLY USED MATERIALS 2

Diaphragm Material Remarks Temp range Recommended for
Nitrile (Buna-N) Rubber type. General purpose, oil resistant.

Moderate flex life and abrasion resistance

-230C to 880C
-100F to 1900F
Hydrocarbons (fuels, oils), water, hydraulic fluids, solvents, food industry
FKM (VitonTM) Rubber type.
Moderate flex life and abrasion resistance
-400C to 1770C
-400F to 3500F
Extremely good for high temperature, aggressive chemicals (strong chlorinated hydrocarbons), animal and vegetable oils
Neoprene Rubber type.

All purpose, oil resistant.

Good flex life and abrasion resistance

-230C to 930C

-100F to 2000F

Water, Non-aggressive chemicals, water basedslurries
EPDM Rubber type. Good flex life and moderate abrasion resistance -400C to 1380C

-400F to 2800F

Good water resistance and diluted chemicals, foodindustry
SantopreneTM Injection molded thermoplastic type. Long flex life & excellent abrasion resistance -400C to 1350C

-400F to 2700F

Acidic & Caustic fluids (sulfuric acid, HCl), Water based slurries, Chemicals
Urethane Injection molded thermoplastic type. Good flex life & abrasion resistance 00C to 660C

-320F to 1500F

Water, wastewater, sea water
Polytetrafluoro-ethylene(PTFE) Thermoplastic type. Long flex life & moderate abrasion resistance. Back up diaphragm is required as PTFE is non-elastic -370C to 1040C

-350F to 2200F

Chemically inert, virtually impervious. Paints, Inks, Coatings, Solvents, Aromatics,  Ketones, Acetates, Esters, food/ pharma industry

2 http://madhavfluid.com/entries/general/how-to-select-diaphragms-for-an-aodd-pump-

The viscosity of the material to be pumped
The viscosity of the liquid is resistance to the flow and cause frictional loss. The viscosity of the material to be pumped determines its sizing. The viscosity of the liquid plays the important role in deciding the flow rate selection.

Normal Liquids
If the fluid is not a viscous fluid, such as water or a solvent, then review pump flow rates below and determine the correct pump size. For optimal efficiency, the pump should operate at 50% to 60% of the maximum flow rate. Slowing the speed of the pump is always good and extends cycle life. Slowing the speed of the pump will also prevent abrasive wear.

Viscous Liquids
If the fluid is viscous, use the viscosity conversion chart shown below to calculate a percent reduction in flow rate.
Remember any material that can be poured can also be pumped. However, key successful pumping is to keep suction line much larger than the pump inlet and Position the pump as close to (or below) the level of the fluid as possible.

 

diaphragm pump viscosity chart
FIGURE 2 VISCOSITY CORRECTION GRAPH 3

Pressure

The pressure conditions at the suction pressure (Inlet) & discharge pressure (outlet) determine pump size. The manufacturer’sdata must be referred for this. It is normally recommended that suction and discharge ports must of the same size or 1 size smaller than the installation pipework. A smaller suction inlet than the pumpport may starve the pump.

Flow Rate

The flow rate along with pressure requirements together determine the sizing of the pump. This requires referring to “pump curve” graph to help determine the relationship between flow and pressure. The AODD creates a pulsating flow and o=if the material to be pumped requires a constant flow then lowering the pump speed and use of pulsating dampener need to be considered for better results.

3 https://www.pumpsandsystems.com/proactively-seeking-viscosity-correction

 

aoddp performance curve chart
FIGURE 3 4

Standards Compliance

The last and important part is to check if the pump meets the FDA requirement and compliant withother standards such as hygienicy standards for food processing.

Sanitary standards

Applications in the food & beverage and pharmaceutical industry must utilize diaphragms made of a material(FDA approved elastomers)that comply with industry regulations and compliance.

Other points

  • Submerging the pump: The pump can be submergedonly ifthe fluid is compatible with the pump housing and fasteners and if you pipe the exhaust above the level of the fluid.
  • Mixed Fluids: In case of mixed fluids you must ask the manufacturer aboutthe suitabilityof the pump material.
  • Environment: The pumps’locational environment must be considered while selecting the pump. For example, Polypropylene is not resistant to UV rays and is limited to use in temperatures over 32°F, hence it can not be used for outdoor applications.

4 https://all-flo.com/metal-diaphragm-pumps/2-inch-a200/

Comparison of AODD pumps of various manufacturers

Here we compare the 2″ metal pumpspecifications of various manufacturers.

Performance Specification All-Flo A200 Yamada™ NDP505 Wilden™ P86 Wilden™ PS8007
Maximum Flow (GPM) 190 164 156 186
Max. Dry Lift (ft-H20) 24 19 22 21
Max. Operating Pressure 120 100 125 125
Bolted or Clamped Design Bolted Bolted Clamped Bolted
Number of Bolts in Wet End 28 28 N/A 60
Number of Parts In Air Valve 28 80 29 32
Average Energy Efficiency +67% Baseline Baseline Baseline

 

WETTED PUMP MATERIALS8

PTFE

PTFE or polytetrafluoroethylene is a thermoplastic polymer that is inert to most chemicals. Has average abrasion resistance. Temperature range from40°F to 220°F (4 to 104°C). PTFE is an FDA accepted material.

Polyethylene

Polyethylene is a thermoplastic polymer with superior chemical resistance. It has excellent abrasion resistance. Temperature range 32 to 158°F (0 to 70°C).

Polypropylene

Polypropylene is a good, general-purposeplastic material used in a wide variety of pumping applications. All-Flo does not use glass fillers in the polypropylene. (Hydrofluoric acid and similar chemicals will attack glass fillers.) Temperature range 32 to 180°F (0 to 82°C). Natural polypropyleneneeds to be protected from direct sunlight. Natural polypropylene will allow light to pass through to the pumped material. (Not suitable for UV sensitive paints or inks)

PVDF

Polyvinylidene fluoride or PVDF is a specialty plastic material in the fluoropolymer family and is used generally in applications requiring the highest purity, strength, resistance to solvents, acids, bases, andlow smoke generation during a fire event. Temperature range from0 to 250°F (-18 to 181°C). PVDF is an FDA accepted material.

5 https://www.yamadapump.com/ndp-50-series/

6 http://www.wilden-pumps.com/flyers/p8m.pdf

7 https://www.psgdover.com/de/2012-05-25-22-45-50/pro-flo-shift/ps800-51-mm-2-advanced-metal-pump

8 https://all-flo.com/metal-diaphragm-pumps/2-inch-a200/

 

Conductive Nylon

Conductive nylon is nylon with stainless steel fillers to allow the pump to be groundable.

Conductive nylon is used for pumping solvents that have the potential to cause an explosion from an electrical spark from static electricity. Temperature range from 0°F to 150°F (-18 to 66°C).

Aluminum

Aluminum is a lightweightmetal used for many noncorrosive chemicals, oils, andsolvents. Do not use aluminum when pumping halogenated solvents. Temperature limit of 212°F (100°C).

316 Stainless Steel

316 stainless steel (ANSI CF-8M) is used on moderately corrosive liquids and halogenated solvents and has excellent abrasion resistance. Temperature limit of 212°F (100°C).

Electro polished & Passivated 316 Stainless Steel

Electro polishing or electrochemical polishing is a process that removes material from a metallic surface. Passivation is the formation of a hard non-reactive surface film that inhibits further corrosion by dipping the stainless steel in a nitric acid solution. Electro polished and passivated 316 SS pumps are normally used on food applications where FDA approved materials are required. Temperature limit of 212°F (100°C).

WETTED ELASTOMERS USED IN AODD9

Buna-N (Nitrile)

Nitrile is a general purpose elastomer used on water and most oils. Temperature range

10°F to 180°F (-12 to 82°C).

Geolast®

Geolast® is an injection molded thermoplastic material with characteristics similar to Nitrile. Has excellent abrasion resistance. Temperature range 10°F to 180°F (-12 to 82°C).

EPDM

EPDM is a general purpose elastomer with good resistance to many acids and basis. Temperature range -40°F to 280°F (-40 to 138°C).

Santoprene®

Santoprene® is an injection molded thermoplastic material with characteristics similar to EPDM. Has excellent abrasion resistance. Temperature range -40°F to 225°F (-40 to 107°C).

Viton®

Viton® is an elastomer with good corrosion resistance to a wide variety of chemicals. Temperature range -40°F to 350°F (-40 to 177°C).

PTFE

PTFE or polytetrafluoroethylene is a thermoplastic polymer that is inert to most chemicals. Has average abrasion resistance. Temperature range from40°F to 220°F (4 to 104°C). PTFE is an FDA accepted material.

FDA Nitrile, FDA EPDM, FDA Viton® , FDA Santoprene® and FDA Hytrel® are FDA accepted materials

9https://all-flo.com/wp-content/uploads/2018/03/All-Flo-Product-Catalog.pdf

 

FLUID VISCOSITY/DENSITY GUIDE

Various units of measurement for viscosity are used, below are some simple and useful conversions:

cSt (Centistokes) or mm²/s x 4.62 = SSU (Seybolt Seconds Universal)

cPs (Centipoise) or mPa = cSt / S.G (Density)

º Engler / 0.132 = cSt

 

Table 3Viscosity of various fluids10

 

 

Fluid TempºC ViscositycSt Density kg / Litre Fluid TempºC ViscositycSt Density kg / Litre
Acetaldehyde 20 0.295 0.788 Machine Oil – Light 20 47 0.9
Acetic Acid 20 1.232 1.048 Machine Oil – Medium 20 850 0.94
Acetic Acid Anhydride 20 0.88 1.084 Mayonnaise 20 2 N/A
Acetone 20 0.41 0.79 Mercury 20 0.119 13.57
Allyl Alcohol 20 1.603 0.852 Methyl Acetate 20 0.44 0.959
Allyl Alcohol 30 1.36 0.848 Methyl Acetate 40 0.35 0.916
Allyl Chloride 20 0.354 0.94 Methyl Alcohol 0 1.04 0.81
Aluminium Chloride (5% Sol) 20 3.54 1.03 Methyl Alcohol 10 0.855 0.801
Aluminium Nitrate (10% Sol) 20 4.54 1.051 Methyl Alcohol 20 0.745 0.792
Aluminium Sulphate (10% Sol) 20 1.34 1.115 Methyl Glycol 20 1.6 0.975
Amyl Acetate 20 4.34 0.885 Methylene Chloride 20 0.9 1.326
Aniline 10 6.4 1.03 Milk 20 1.13 1.035
Aniline 20 4.37 1.021 Molasses 80 Bx 20 10 N/A
Beer 20 1.8 0.996 Molasses 83 Bx 20 50 N/A
Benzene 20 0.744 0.879 Molasses 85 Bx 20 100 N/A
Benzene 30 0.65 0.868 Nitro Benzene 20 1.67 1.203
Benzene 40 0.58 0.858 Nonane 0 1.35 0.733
Benzene 50 0.54 0.847 Nonane 10 1.15 0.725
Benzyl Alcohol 20 5.52 1.045 Nonane 20 1 0.717
Bromine 20 0.34 3.12 Octane 0 1.05 0.719
Butyl Acetate 20 0.832 0.885 Octane 10 0.935 0.711
Butyl Alcohol 20 3.64 0.81 Octane 20 0.805 0.702
Butyl Alcohol 30 2.85 0.803 Octane 40 0.64 0.685
Butyric Acid n 0 2.35 0.977 Oil, SAE 10W-30 20 130 0.875
Butyric Acid n 20 1.61 0.957 Oil, SAE 10W 20 115 0.87
Calcium Chloride (25% Sol) 20 3.9 1.227 Oil, SAE 20W-20 20 200 0.885
Carbolic Acid 20 11.3 1.078 Oil, SAE 40 20 900 0.9
Carbolic Acid 30 9.7 1.069 Oil, SAE 50 20 950 0.902
Carbolic Acid 40 7.95 1.059 Olive Oil 20 91.5 0.91
Carbon Disulphate 0 0.33 1.292 Palm Oil 20 130 N/A
Carbon Disulphate 10 0.316 1.277 Paraffin Oil 20 2.4 0.804
Carbon Disulphate 20 0.298 1.262 Paraffin Oil 30 1.85 0.78
Carbon Tetrachloride 20 0.612 1.595 Pentane 0 0.44 0.646
Carbon Tetrachloride 30 0.525 1.525 Pentane 10 0.39 0.636
Castor Oil 20 1017 0.96 Pentane 20 0.36 0.626
Castor Oil 40 315 0.95 Phenol 20 11.3 1.078
Castor Oil 60 115 0.94 Phenol 40 7.95 1.059
Caustic Soda, 50% 20 45 N/A Polyester Resin 20 3 N/A
China Wood Oil 20 308 0.933 Polymer Solution 20 20 N/A
Chloroform 20 0.38 1.489 Potassium Hydroxide 20 67 N/A
Chloroform 40 0.37 1.452 Printing Ink & Colours 20 550-2200 N/A
Chloroform 60 0.35 1.415 Propanol 20 2.8 0.804
Cod Liver Oil 40 35 N/A Propanol 40 1.7 0.786
Cotton Seed Oil 20 76 0.926 Propanol 50 1.4 0.777
Cream, 30-50% Fat 20 11-115 N/A Propionic Acid 20 1.13 0.99
Cyclohexanol 20 71 0.952 Propylene Glycol 20 54 1.038
Cyclohexanone 20 4.9 0.952 Rapeseed Oil 20 178 0.92
Cylinder Oil 20 50000 0.94 Sea Water 0 1.774 1.028
Dioxan 20 2 1.03 Sea Water 10 1.346 1.028
Ethyl Acetate 20 0.51 0.905 Sea Water 20 1.044 1.025
Ethyl Alcohol 20 1.51 0.772 Sea Water 40 0.659 1.019
Ethyl Alcohol 40 1.16 0.737 Sea Water 60 0.442 1.01
Ethyl Glycol 20 2.3 0.93 Sea Water 80 0.311 0.998
Ethylene Glycol 20 18 1.112 Shampoo 20 3 N/A
Ethylene Glycol 30 16.5 1.104 Sodium Chloride (25% Sol) 20 2.4 1.19
Formic Acid 20 1.5 1.22 Sodium Hydroxide (20% Sol) 20 4 1.226
Formic Acid 30 1.38 1.208 Sodium Hydroxide (30% Sol) 20 10 1.33
Fuel Oil, Extra Light 20 6 0.85 Soya Bean Oil 20 75 0.926
Fuel Oil, Medium 20 520 0.99 Sulphuric Acid 20 14.6 1.839
Fuel Oil, Heavy 20 8000 0.99 Tetrachloroethane 20 1.1 1.593
Furfural 20 1.45 1.16 Tetrachloroethane 20 0.95 1.621
Gear Oil 20 3000 0.905 Toluene 20 0.68 0.867
Gelatine 45 1.2 N/A Toluene 40 0.55 0.849
Glycerine 20 1183 1.261 Toluene 60 0.46 0.831
Glycol 20 20 N/A Tomato Ketchup 30 1 N/A
Heptane 0 0.74 0.702 Transformer Oil 20 30 0.95
Heptane 10 0.66 0.692 Trichloroethylene 20 0.95 1.463
Heptane 20 0.6 0.682 Water 0 1.788 1
Heptane 40 0.51 0.661 Water 10 1.307 1
Hexane 0 0.62 0.678 Water 20 1.002 0.998
Hexane 10 0.57 0.668 Water 40 0.662 0.992
Hexane 20 0.51 0.658 Water 60 0.475 0.983
Hexane 40 0.4 0.639 Water 80 0.365 0.972
Honey 40 2 N/A Water 100 0.295 0.958
Kerosene 20 2.4 0.804 Xylene-O 20 0.93 0.864
Kerosene 30 1.85 0.78 Xylene-O 40 0.74 0.847
Linseed Oil 20 47 0.92 Yoghurt 40 150 N/A

10https://www.tapflopumps.co.uk/fluid-viscosity–density-guide