In Pumps 101, we reviewed the basics of centrifugal pump impellers. We explored how impellers move fluid, discussed the basics of impeller geometry and introduced a number of impeller styles. These concepts are where we will pick up for the 201-level discussion on selection and application of centrifugal pump impellers.
Proper pump and impeller selection involves a number of considerations. First, we need to know to where and at what rate the fluid needs to be moved. This combination of required head and flow rate is called an operating point. The operating point is directly related to the geometry of the required impeller. Applications with long vertical runs (high head) require an impeller with a larger outer diameter than those with shorter vertical runs. Additionally, applications moving 1,000 gallons of water will need an impeller with taller vanes than an application moving 100 gallons in the same amount of time. As an application’s head or flow requirements increase, the impeller and its corresponding pump typically increase in size as well.
Another consideration with a direct relation to impeller size is the application’s expected solids content. Many applications will expect to see a variety of solids. These solids may be small abrasive debris like sand or metal shavings, stringy fibrous materials or large solids similar in size to a baseball, if not larger. The selected pump and impeller must pass these solids while avoiding damage from clogs and abrasive wear. Additional considerations must also be made for equipment downstream of the pump. While you may be able to select a pump with the ability to pass a certain type of solid, you cannot assume the downstream piping, valves and other process equipment will have the same solids handling capabilities. Knowing the expected solids in the stream is not only critical to selecting the proper size of the pump and impeller, but also to choosing the best impeller style for the application.
One of the most common solids handling impellers is the open channel style. Typically found in sewage and wastewater, this impeller’s geometry includes channels between its vanes with an open face toward the inlet. The space between the vanes provides a smooth path that allows the impeller to push incoming solids from the impeller eye out to the volute and eventually through the pump’s discharge. The open face of the impeller allows for improved solids passage with less likelihood of a clog. While the open channel impeller style has long been an effective method of pumping solids, there is a growing trend of clogging due to fibrous materials. Stringy materials such as flushable wipes, nylon and denim have become more prevalent, causing issues for historically reliable pump styles.
To combat this issue, chopper and cutter pump styles can be applied to cut down troublesome solids. The sharp stationary blades sit in close proximity to the impeller. If the stringy material starts to wind around the impeller, the cutting mechanism cuts the solids before they permanently affix themselves to the rotating assembly. Without a chopper or cutter assembly, enough of the stringy material can wrap around and constrict the rotating components.
Another option for solids handling is the vortex or recessed impeller style. This style sits recessed high in the casing and induces fluid movement via the vortex created by the impeller’s rapid rotation. While this method is not as efficient, it does provide a number of solids passage benefits. A primary benefit is the large free space providing minimal obstacles for solids passage. The only limiting factor for solids passage for a vortex style is how large of a solid can fit through the pump’s entry. An additional benefit of this style is minimal contact between impeller and solids. Abrasive solids can be pumped with minimal contact, thereby reducing wear damage to the impeller. This is especially beneficial in coastal regions prone to sand intrusion.
In mountainous regions, applications typically have higher elevation compared to flatter terrain. Constructing long stretches of piping around this is not only a difficult task, but also costly. A common method to simplifying piping in this region is to use smaller high pressure polyvinyl chloride (PVC) piping as an alternative to the large metal piping. These smaller pipes are able to navigate the terrain until they reach flatter ground where a larger pipe header can be placed. Keep in mind that just because these applications are high pressure does not mean there is a higher demand to move more fluid. This is where a high pressure impeller comes in to meet these high pressure applications. The tight clearances, enclosed vanes and large outer diameters are geometric features that help achieve high pressure pumping in a smaller pump size than standard pressure impeller pump options.
High pressure applications have their own set of solids handling considerations. Due to these applications often using smaller piping, solids passage size must be considered for the whole system and not just the pump. Often, pump manufacturers who offer high pressure pumps will include a strainer over the inlet, preventing large solids from entering the pump. This is ideal for high pressure applications with minimal expected solids but can cause clogging if enough solids build up around the strainer face. In those high pressure applications with large quantities of expected solids, a grinder system may be required. Often paired with a high head impeller, the grinder system cuts up solids into small pieces that pass through a strainer plate at the pump’s inlet. This is similar to a chopper or cutter pump style; however, the grinder cuts the solids down to pass through small high pressure piping where the former styles would likely cause clogs.
Proper pump and impeller selection requires numerous considerations outside the scope of what was covered in this series. However, understanding the various styles is often one of the most critical steps. The ability to select the proper style will allow you to narrow in on manufacturers who offer that product or specialize in a specific application.