Sizing Pumps for a Range of Viscosities- Things to Consider

Customers from around the world seek out Triplex for our expertise sizing and applying sanitary positive displacement pumps for their applications. We see plenty of tough applications, but some of the toughest applications customers come to us for is sizing positive displacement pumps for a range of viscosities. Maybe you’re pumping mayonnaise and CIP solution or you’re a contract manufacturer running a variety of products and you want to make sure you’re covered. Whatever your application, here are the things you want to consider when sizing a PD pump for a range of viscosities.

Suction Conditions

Whether or not you have a range of viscosities, we always want to double check the pump suction conditions. A pump can’t pump what isn’t there. When we’re pumping a range of products though, we want to consider each one and make sure we’re able to get them to the inlet. Just because CIP can flow into the pump, doesn’t mean a meat slurry can. So when you’re handling thick products, always double check you can get them to the inlet and are able to satisfy the pumps net positive suction head requirements (NPSHr).


Is this one too obvious? Maybe. But let’s do a deeper dive. When sizing a pump, especially one pump for multiple viscosities, we absolutely need to have good viscosity data. Not only on the high end, but also the low. Why? Well, if we’re running say, chocolate, and we use an undersized rotor and then switch over to a lower viscosity product, we could see significant volumetric inefficiency, otherwise known as slip. If we know slip is going to be an issue, we’ll need to make sure we compensate for that by running the gear unit at a high output speed. Which is a nice tie into our next consideration.


Why is speed important? Well, ultimately, we need those pistons expanding and contracting to push product. And we need to know what output speeds we’ll need to hit our desired flowrates. Commonly, we’ll see a customer application where we’re running a high viscosity product with duties of say 35 RPM and 10 GPM and a low viscosity product with a duty of 120 GPM and 400 RPM. If we only have a 10:1 turndown motor, this is going to be an issue, because we won’t be able to run slow enough to hit high viscosity flow rate. And if we do run it slow that slow, we risk overheating the motor.  Additionally, running at higher speeds typically means we use a lower gear ratio on the reduction unit. What’s the issue with that? Well that brings us to our next topic- torque.


When we consider power to drive our PD pumps, most people think horsepower. But what we really want to look at is torque. While torque and horsepower are related, they are subtly different. Torque, simply put, is the amount of force required to turn a pump shaft, whereas horsepower is work done over a period of time. In PD pump applications, we use a gearmotor to gain a mechanical advantage required to turn the pump shafts.

How does this apply to pump sizing? Well if we use a lower gear ratio to run at a high pump output speed, it’s possible we won’t be able to generate the torque required to run the pump shafts at slower speed with thicker products that require more torque. We always sized pump motors based on torque, not horsepower.


Those are just a few of the things you want to keep in mind when sizing PD pumps for a range of products or viscosities. The next time you have an application where you’re trying to run a Waukesha, Ampco, Fristam, or Alfa Laval sanitary positive displacement pump on multiple products with a range of viscosities, make sure you have good viscosity data, double check the suction conditions, watch your speeds, and make sure you have enough torque. And finally, if you have any questions about sizing your sanitary positive displacement, contact a Triplex Sales Engineer today.

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