Given a set of required hydraulic conditions (head and flow), a defined set of fluid properties and a specified duty cycle, many pump users would think it would be relatively easy to select the correct pump for an application. Sometimes it is simple, but most often it is not.
In reality, every centrifugal pump is designed for only one operating point of flow and head; all other points on the manufacturer's curve are, to some degree, simply a hydraulic compromise and departure from the best efficiency point (BEP). Good pump designers scrutinize common industrial processes and design their pumps to specifically match the corresponding head and flow rate as required. In rare cases, the user will have the manufacturer design a pump specifically for the application.
What selection criteria do you evaluate when buying a pump? Some people look first at the pump's initial cost, while others look at the operating conditions and the proximity to the pump's BEP. Some may also consider whether the impeller is at maximum or minimum trim. Giving major deliberation to the pump efficiency and expected energy consumption is always a good decision.
For simplicity, this article will discuss centrifugal pumps and fluid that exhibits Newtonian properties comparable to water. Assume the fluid is 68 F with a specific gravity of 1.0, a viscosity of less than 5 centipoise, and little to no suspended solids (SS), entrained air or non-condensable gases.
There is no set of universal rules for all pump selection cases except this: Be pump smart and informed. Analyze several pump selection aspects, not just one. Note that the right pump may not be a centrifugal pump. What may be the correct pump for one fluid may not be for a different fluid.
Selecting a pump with an operating point at or to the near left of the BEP is a commonly accepted and conservative decision, but do not be afraid of operating points to the right of the BEP. If the operating points are not too far to the right, there will still be plenty of margin before the end of the curve. Selecting a pump near the BEP will yield the most efficient pump with the least amount of vibration and radial forces acting on the shaft. Ensure that your calculations for the system design curve, also known as system resistance curve (SRC), are correct because the pump will operate where its curve intersects the system curve.
Do not expect to run near or at the end of the curve. This area is fraught with recirculation, cavitation and net positive suction head (NPSH) margin issues, regardless of the manufacturer. As I candidly state in my pump and fluid dynamics course, "There are demons and dragons in the area at the end of the curve."
Selecting a pump that will operate near the minimum flow limits is just as risky as choosing one that will operate at the end of the curve (see my November 2015 Pumps & Systems column, "Follow These Steps for a More Reliable Pump," for a detailed discussion on minimum flow). Be aware of when manufacturers claim a lower acceptable minimum flow than others for a similarly designed pump. The pump will operate in that minimum flow region, but the added costs for mechanical seals, bearings and unplanned downtime will exceed initial savings. Most manufacturers' warranties will not cover wear and/or failure of the mechanical seals and bearings.
As a general rule do not select a pump with the maximum size impeller. Selecting a maximum "wheel" (impeller) leaves no margin for mistakes in your calculations. Unacknowledged factors could increase system friction because of age, corrosion or other factors such as marine growth or fouled heat exchangers. Knowledgeable sources suggest selecting impellers that are no more than 90 to 95 percent of the maximum diameter. This decision allows for future growth in the system.
Many users may specify impellers that are less than the maximum diameter while sizing the driver to be non-overloading for the maximum impeller at or near the end of the curve. Problems with vane passing frequencies may arise; at maximum diameter, the impeller vanes are close to the volute cutwater (or diffusor vanes), creating pressure fluctuations that cause unacceptable vibrations. Pump manufacturers can incorporate good design practices to control the distance ratio from the impeller tip to the cutwater/diffusor edge.
Minimum impeller diameters are less efficient than larger impellers for a given pump. Be careful using the affinity laws for any approximations as you approach minimum diameter, because the calculation accuracy is significantly less as you go from maximum to minimum diameter. I recommend using the original equipment manufacturer's (OEM) published curves, if available. Do not trim the impeller less than the OEM's stated minimum, especially in a lift or self-priming application.