From Website: www.warrenrupp.com

There are hundreds of pump types commercially available throughout the world today. Most can be classified as either centrifugal or positive displacement. Warren E. Rupp recognized limitations with a one-design-fits-all approach to solving difficult pumping problems. The SANDPIPER® air-operated, double diaphragm (AODD) (non-positive displacement) pump range offers customers a variety of unique problem solving AODD pump designs.
Today, our core designs include heavy duty ball, heavy duty flap, containment duty, and standard duty configurations, enabling our customers “with more ways than one” to address their pumping needs. There is no other pump type on the market today that is so universally applicable and so responsive to pumping problem fluids. And helping customers make better-informed choices has been a hallmark commitment from Warren Rupp, Inc. for more than 40 years.