WorldWide Drilling Resource

More on NPSHR by Bill Corey Pentair Water Training Institute To recap, in the last article I mentioned NPSHR is built into the pump. It is a function of the pump, inherent losses, losses we really can’t change. Remember, we can change NPSHA, but not R. The en- gineer builds it in. I also want to reiterate it is a function of an individual pump. Typically, a centrifugal pump has the best NPSHR because it has the best control of the water. A self-priming pump has more water to control and therefore most have a higher NPSHR. A jet pump is really a booster pump, and seems to have the best NPSHR because it is two pumps in one. Submersibles, for the most part, have a positive NPSHA because of all the water over them - don't forget the story about the 150-horsepower, 1900-gallon per minute submersible. Cavitation is the result of not understanding NPSHR. This is why I’m spending time on this topic. Cavitation is preventable simply by applying the correct amount of back pressure on the pump, in most cases. As we discussed previously, you can do this by changing the NPSHA - moving the pump closer to the water or setting a submersible lower in the well. Other times, you can just close a valve some and slow the flow of the water down, which gives you backpressure. This picture is of an 18-inch impeller with cavitation damage. If you look at the 6:00, 10:00, and 1:00 positions highlighted, you can see holes through the impeller. We told the customer if he would just slow the flow down by putting a valve on the discharge side, the pump would not cause this damage. He said, “I need that flow. I’d rather pay for a new impeller.” Don’t you love those kinds of customers? In all seriousness, this impeller costs about $2000. If you are getting this type of dam- age and require that flow, then maybe, just maybe you’ve chosen the wrong pump. So in- stead of putting that money into a new impeller, you should think about getting the correct pump for the job. Looking at the NPSHR curve would help. Understanding NPSHR before you pick a pump could save someone a lot of money! Next month, let’s take a look at boiling water. Bill Bill Corey may be contacted via e-mail to michele@worldwidedrillingresource.com 47 WorldWide Drilling Resource ® MAY 2016