Today we are talking about centrifugal pump curves. Centrifugal pump curves are derived from the affinity laws, a topic we are not going to address today. In the real sanitary process world, you do not need to understand the affinity laws(although it is probable not a bad thing to know). But it does help if you can read and understand a pump curve when trying to specify a pump.
Every centrifugal pump manufacturer, including sanitary centrifugal pump manufacturers publishes curves for their pumps. They pretty much all use the same type of format. Some provide more details than others. Many sanitary centrifugal pump manufacturers shown some very basic curves in their sales literature but have more detailed curves upon request.
The curve we are using as an example was generated in Pumpflo, a software package that we use to do our own pump sizing. Pumpflo allows you to compare multiple models and manufacturers in one spot. This curve was generated by searching for a Waukesha 200 Series sanitary series centrifugal pump to pump water 130gpm at 140ft of discharge head. We chose to generate the curve for the Model 20655HV because of the models available, Pumpflo showed this model to have the highest efficiency.
Let’s examine the curve
- The curve has a vertical axis that shows pump output pressure (feet) (#1) and a horizontal axis show flow (gpm) (#2)
- The yellow area represents the performance envelope the pump will operate in with the various impeller sizes available.
- The 3 black parallel lines (#3) represent the pumps performance with the largest impeller size available, the smallest available and the line in the middle represents the performance with the impeller size chosen for this specific application (5.625”).
- The dotted brown lines moving from the upper left to the lower right of the curve (#4)shown the pumps horsepower requirements at various duty points.
- The green lines (#5) represent the pumps percent efficiency at various duty points.
- Finally the blue line (#6) on the bottom show the net positive suction requirements for this pump with the 5.625” impeller.
The box below the curve also gives us a lot of information
- At the specified duty point the pump requires 7.75 horsepower
- The non-overloading horsepower requirement of the pump with a 5.625” impeller is 15.
- The NPSHr at the duty point is 4.25ft
We have a bunch of data. What now? Here is how we use this to size sanitary centrifugal pumps. Let’s start with why we chose this pump. Waukesha has several other models of sanitary centrifugal pumps that would meet the duty. Here is why we chose this one.
- It has the highest efficiency of all of the other pumps we looked at with the duty point still to the left of the highest point of the efficiency curve. As a rule of thumb, you always want to stay to the left of the highest efficiency point on the curve.
- This pump meets the duty point with an impeller size that is in the midpoint of the range. We could have chosen a smaller, slightly less expensive pump. But to meet the duty point it required the largest size impeller available for that model. If that pump is installed out in the field and it is determined that more flow and or pressure is needed you would have to replace the pump. Using a pump with an impeller sized in the mid-range allows you to go to a larger impeller if needed at some point without replacing the pump.
- At the duty point, this pump requires only 4.25ft of suction head. That is pretty low.
So we decide we like this pump and want to go with it. What else do we need? First we have to decide on the motor. The curve tells us we need 7.75hp at the duty point. The smallest size motor commercially available to meet that duty would be 10hp. That should work fine as long as the pump runs right around the duty point . But if you want to ensure never overloading the motor a 15hp motor is a better bet. Horsepower is pretty cheap so going with 15hp might be the smart thing to do. Also at the same duty point a 10hp or 15ph pump will consume the same amount of energy.
What else do you need? If this is a sanitary pump, you still have to decide on seals, elastomers, surface finish, casing drains and motor rating. These are issues for future posts. Hopefully this post brought a little clarity to pump curves for sanitary centrifugal pumps. This is a high level view on the subject. There a many other nuances within the curves we may address in the future. If you have any questions or need assistance as always, contact us via our website or call us at 800-800-8464.