PD Pump & Motor Sizing- Sizing for High Turndown Applications

At Triplex Sales, we size a lot of PD pumps. Many of these applications are pretty straightforward- the customer is running at a pretty constant duty speed, without too much turndown. For these applications, we get some basic rheologic data, find out how much they’re pumping, how far they’re pumping, and how they’re going to clean. We plug those parameters into a sizing program, it spits out a pump size and brake horsepower (BHP) and we’re all set. For some applications, however, specifically high turndown applications, there are a few other things we need to consider in more detail, such as the amount of turndown, motor horsepower, and derating. So how do we do that? Let’s work through an example.

Let’s take an application where we’re pumping an emulsion with a viscosity of say 1,000 centipoise at 35 gallons per minute. We’ve calculated our friction loss and know that we have 200 psi of downstream line pressure. Seems simple enough. But in this application, we want to use the PD pump as our CIP pump as well. For CIP, we’ll need to run at about 120 GPM with a discharge pressure of 30 PSIG. So turndown (or really turn up) of about 3.5 times. When we punch this into our sizing program, we get something like this:

So first glance, we see that our highest duty BHP is 5.2 HP and we’ll need to spin the pump at about 525 RPM to hit our CIP flow rate. We can’t get a 6 HP gearmotor, so we should just use a 7.5 HP gearmotor with a 550 RPM output speed, inverter rated for 10:1 CT turndown, wire in the VFD, and call it a day, right?

Not so fast. Remember, positive displacement pumping applications are considered “constant torque” loads. With a constant torque load, torque is not a function of speed. As the speed changes, the torque required remains constant, but the horsepower changes linearly with speed. This is shown the graph below:

What this means is that our 7.5 HP 525 RPM gearmotor, when running at 138 RPM, is only going to give us about 2 HP. The is not enough horsepower to pump our product! Additionally, when we check the torque output of a 7.5 HP 525 RPM gearmotor, we see that we’re only going to get 833 inch pounds of torque which is not nearly enough for our duty! And if we selected a 7.5 HP 138 RPM gearmotor, we’d have to run at ~230 Hz to hit our CIP flow rate. No thanks.

So what motor size should we select to handle this? Typically, we’ll try to split the difference between the duties so we can handle the low end and then still have enough speed to handle CIP. For this application, we would select a 15 HP 350 RPM gearmotor. With this combination, we’ll have to run it at about 25 Hz for our process, but we’ll still have about 6 HP and an output torque of 2586 in lbs. So we’re set there. For CIP, we’ll have to crank the pump back up and run at 90 Hz to get our 120 GPM. This is not an issue for a gearmotor being driven with a VFD.

So the next time you’re sizing a PD pump for a high turndown application, remember that as you slow a gearmotor down, you still have torque, but “lose” horsepower, so always double check that you have enough torque and horsepower to run at the low end of the application. And as always, if you have any questions about sizing your PD pump, please contact a Triplex Sales Engineer today!

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