FAQ why select a Diaphragm Pump, general uses and applications

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WHY SELECT A DIAPHRAGM PUMP?

Diaphragm pumps are one of the oldest pumping techniques used by man. We have progressed from animal skin diaphragms through mechanically driven single diaphragms, to air driven double diaphragm pumps.
In the past 50 years, design innovations have made the air driven double diaphragm pump one of the most reliable, versatile broad application pumps available.

Features of air driven double diaphragm pumps.

1) Pumping chambers and the material being pumped are not in contact with any close fitting rotary or sliding seals. This makes double diaphragm pumps ideal to be used with abrasives, slurries or even run dry.
2) Capacities are infinitely variable within the pumps range. No need to use variable speed motors or variable drives.
3) Inherently pressure balanced diaphragms, always balance air pressure against the fluid being pumped. The diaphragm acts as a membrane that separates fluid and air however, the diaphragm is not stressed as are membranes of mechanically driven pumps.
4) Although the pumps deliver large volumes at intermediate pressures, they can also develop pressures up to 300 psi when substantial pressures are necessary. 
5) Air driven double diaphragm pumps can be run dry indefinitely without damage, because there are no rotary seals or packing glands that need lubrication from the pumped medium (althought mechanical life of the diaphragms is affected).
6) The pump discharge may be shut off at anytime and left off for an indefinite period with no damage to the pump or power consumption. Compressed air is only consumed when the pump is moving fluid.
7) Pump discharge pressure normally can be no higher than air pressure, therefore fluid pressure relief valves or other pressure control devices for the fluid are not usually needed.
8) Self priming with suction lifts up to 20 feet or better.
9) The pump can be totally submerged in the fluid in many cases, even corrosive fluids.
10) Can pump extremely high viscosity fluids or slurries.
11) No electrical motors or controls to cause fire or explosion hazards.
12) Easy to install and portable.
13) A wide selection of materials of construction to handle a wide variety of fluids from water soluble to corrosives and organic solvents.

Based on the above characteristics here are some of the applications for air driven double diaphragm pumps.
PACKAGING INDUSTRY
Glue dispensing, paint dispensing, ink dispensing.
PAINT MANUFACTURING
Drum transfer, tank level controls, tank filling, pigment transfer, solvent handling.  
SEWAGE
Sludge handling, chemical transfer or metering.
PLATING INDUSTRY
Drum transfer of chemicals and cleaners, tank transfer.
CERAMIC AND TILE INDUSTRIES
Handling slips, glazes and mud of all viscosities.  
AUTOMOTIVE SERVICE
Oil and solvent drum transfer, degreasing fluids, antifreeze mixing, dispensing fluids.
CONSTRUCTION
Dewatering of muds, plaster or grout transfer.
CHEMICAL PROCESSING
Handling abrasive and corrosive fluids, tank cleaning, drum transfer.

This short list gives just a small sampling of the applications for air driven double diaphragm pumps. The limitation of applications are subject to your ability to be innovative.

SOME COMMON QUESTIONS ABOUT DIAPHRAGM PUMPS

• What is the heaviest most viscous material that can be pumped?

If the material can pour it can be pumped. However, as the material viscosity increases to a consistency similar to mayonnaise the suction developed by the pump can pull an air hole through the suction hose from the supply tank. The pump will sound like it is stoking too fast and no fluid will come out of the discharge. A close inspection of the supply tank will show that the fluid does not back fill on itself to keep the suction hose supplied with fluid. The most common fix is to put a follower plate on top of the fluid in the supply tank to prevent the air hole or worm hole' from forming. Probably the easiest way to handle very viscous fluids on the suction end of the pump is to have the fluid packaged in a bag that will totally collapse as the fluid is being drawn out of the bag. Then "worm holes" cannot form. The more viscous the fluid being pumped, the slower the pump will stroke. Typical examples are Tomato Paste , Peanut Butter, Concrete & Pie Fillings

• What is cavitation and how do I stop it from happening?

A double diaphragm works efficiently because air pressure on the backside of one diaphragm pumps fluid out of one chamber while a rod connected to the other diaphragm pulls a vacuum in the other chamber drawing fluid into the chamber for the next pumping stroke. Fluid is pumped Out of the pump driven by compressed air pressure. Fluid is drawn into the pump by vacuum which can be no higher than atmospheric pressure or around 13 psi. You can therefore see that it's much easier to pump out of a chamber than to suck fluid into a chamber. It is easily possible to make the pump stoke so fast that the fluid can not be pulled or sucked into the chamber as fast as the diaphragm is withdrawing or creating a vacuum. When this occurs, a vacuum hole or cavity will occur in the fluid in the suction chamber. Thus the word cavitation. The pump will abruptly increase speed with no increase in discharged fluid and will generally sound erratic. This condition can be eliminated by slowing the cycling rate of the pump with a throttle valve. By using an air supply throttling valve you can eliminate cavitation by throttling air supply, no matter what the fluid and no matter what the pumping conditions.

• Is there any advantage to "oil-less" vs. lubricated air valve?

A "oil-less" valve generally has its moving member made of Teflon® or coated with Teflon®. Teflon® is a very slippery material but it is also very soft. If any small contaminants come through the air supply or were in the supply hoses, then the contaminants can act as cutting edges to wear through the Teflon®. Everyone is familiar with Teflon® coated frying pans. It seems that no matter how carefully the pan surface is treated, it's just a matter of days before the surface is scratched. A lubricant acts not only to make the moving parts slide easily, but also to catch contaminants and flush them through the system with a minimum of damage. That's why you change oil in your car, to get rid of the contaminants in the oil on a regular basis.

The greater issue in valve selection is the valve performs throughout the full range of conditions that the pump is expected to perform. For example, will the pump cycle at a rate necessary for the pump to deliver its full rated output? This might be as low as one stroke every several minutes or even discharge shut off completely for hours with the pump under pressure. Upon reopening the discharge, the pump should begin pumping with no interruption. Compressed air preparation is as important to long term trouble free operation as using the right kind of oil for your car and changing it on a periodic basis. Some people routinely get 150 ,000 miles Out of a car, others are having problems at 30,000 miles. Proper care and maintenance is usually the difference. Proper air preparation amounts to using a filter, regulator, lubricator in the air supply line. Use a good grade of SAK 10 wt. oil or lighter. Do not use a multiviscosity motor oil. The oil should be fed at a rate of one drop every 100 SCFM. That would be one drop per 5 minutes if operating at maximum flow at maximum air pressure. The filter should remove dirt as well as water from the supply air. However, the filter should be emptied or left open to bleed the trapped water out of the system. Too high viscosity oil or water mixed in with the oil will cause the valve to shift slowly or irratically. A quick check of the lubrication quality would be to remove the muffler, wipe a cloth on the inside, you should have a thin light film of oil. If the oil is heavy, gummy or milky colored it is probably to high viscosity or contains water. If the air is contaminated with dirt, the dirt will also show up in the muffler.

• Do plastic pumps always leak?

No, however they must be inspected and retightened more often then a metallic pump. Plastic materials by definition, flow or deform whenever they are put under Stress. This deformation happens even faster at higher temperatures. This is best illustrated by clamping two plastic parts together with bolts and tightening the bolts to, for example, 50 inch pounds torque. In 48 hours it is likely that the clamping torque would have decayed to about 35 inch pounds. If the same part were retightened to 50 inch pounds torque then looked at again in 48 hours it would again have decayed to about 35 inch pounds. This process can be repeated again and again until the plastic has completely flowed away from under the bolts. With proper design, selection of materials and installation, this cold flow or creep will not impair the function of the pump, but does require some added maintenance especially at high pressures and high operating temperatures. Bolts, nuts, ring clamps and press fits are all subject to this kind of cold flow or creep. Some air leakage may occur around the valve assembly or Out the exhaust port when the pump is pressurized but with discharge closed. This would be air bypassing the shifting members. That leakage should total less than 0.15 SCFM.

• Should I use an air pressure control or a throttling valve to adjust pump speed?

By all means use the throttling valve to control pump stroking speed. The air pressure control should be used to provide adequate pressure to force the fluid to flow from the discharge through all the discharge piping, always with some pressure to spare in case the material becomes more viscous or more resistance is developed in the discharge piping. For most transfer operations, pressures in the range of 30 to 50 psi are typical. If you are pumping through a complex of pipes in the discharge line and pumping up to the third floor, then 70 or 80 psi or higher may be required. Once the pressure is established at a very slow stroking speed, then the throttling valve can be opened until the pump cavitates, that is fluid is not able to be sucked into the suction port fast enough to keep up with the pump stroking speed. The pump will sound erratic with many false strokes. Close the throttling valve until the pump strokes uniformly and smoothly.

• How can I get a uniform non-pulsing discharge?

Buy a pulsation dampener or if this is unsuitable:- At the pump discharge, add a flexible hose with internal diameter three or four times the pump discharge size. The length of the hose should be 8 to 10 feet. Then reduce pipe size to suit the application. A throttling valve may need to be placed in the discharge line at this point. The hose should be no stronger than a 300~ safety factor over the pump pressure. Additional flow control valves may be used further downstream as the application requires. The flexible hose will act as a capacitor to store the pressure from the pump while the valve is shifting directions from one chamber to the other.   Adapted from an article at http://www.uvcuring.com/