Back in December of 2021, we kicked off a series of articles on this blog we called “Sanitary Pump 101”. Since then, our business has grown a lot and we’ve learned a lot more working with our customers to help them chose the right pump for their high purity process application. So we’re circling back today to kick off our “Sanitary Pump 102” series and revisit what PD pump and centrifugal pumps are as well as talk about where another pump we are increasingly specifying- Air & Electrically operated double diaphragm pumps- fit in. So let’s get started.
Centrifugal Pumps- What are they again?
As we talked about last time, a centrifugal pump uses a rotating impeller and a volute to impart centrifugal force into our process fluid and force the fluid from the inlet outward through the casing discharge with increased velocity and speed.
Centrifugal pumps operate based on the affinity laws, meaning they displace a variable amount of fluid depending on the suction and discharge head in the system.
Centrifugal pumps do a great job of moving a high volume of fluid with relatively low power requirements.
They are our lowest cost pump per volume displaced and are ideally suited for thin, clean liquids. They are not great for high viscosity fluids or exceptionally high discharge heads.
And what’s a PD pump again?
PD pumps are a big, broad category. PD’s include several pumping technologies, including rotary lobe, twin screw, quaternary diaphragm, gear, plunger, progressive cavity, and our favorite- external circumferential piston. All PD pumps are defined by their ability to move a specific volume of fluid per revolution generally regardless of discharge head.
PD pumps accomplish this using the same basic premise- there is an expanding cavity at the pump inlet that creates a low pressure area to draw fluid in, and as the motor turns, the cavities collapse, pressurizing the fluid and forcing it out the discharge.
PD pumps are great for metering and dosing applications, as well as for handling high viscosity fluids. They are, generally speaking, however, the most expensive type of pump per volume of process fluid displaced we specify and typically take up the most space. Typically, they can’t be run dry and you certainly don’t want to deadhead them.
And AODD/EODDs?
Finally, we have AODDs and EODDs. Air operated double diaphragm pumps use compressed air to direct flow towards one of two diaphragms while simultaneously exhausting air from the opposite diaphragm chamber. This reciprocating action pressurizes the discharge chamber and creates a vacuum on the suction chamber which actuates a series of product inlet and outlet check valves.
AODDs have no close fitting or rotating parts, which allows them to handle a variety of products with high solids contents. Not only do they create excellent suction lift, they can dry prime, run dry indefinitely, and will also stall under pressure.
As far as performance characteristics go, they fall somewhere in between a centrifugal and a PD. Flow is somewhat constant independent of discharge head, though you may need to increase your air supply.
And while these pump have traditionally required compressed air, which takes a considerable amount of electricity to produce, Graco’s new Quantm electrically operated double diaphragm pump delivers the same performance characteristics engineers and operators love at a fraction of the operating cost.
So which should you use?
So here we are- the thrilling conclusion- and I’m sure you’re wondering which type of pump you should use. Well, if you’ve been following along, it should be pretty clear that the answer is “it depends”. Looking to moving a lot of water for a relatively low price? Go with a centrifugal. Trying to pump meat paste at discharge pressures up to 350 PSI? That’s a slam dunk PD pump application. And for simple, medium duty transfer applications, it’s hard to beat the simplicity and price of an air or electrically operated double diaphragm pump.
Still have questions about which type of pump is best for your application? Please contact a Triplex Sales Engineer today!
