Pump Selection – Key Considerations
Jay Kelly, Co-founder Floco Process and Comprex North America
Selecting the right pump can pose a significant challenge due to the extensive array of options available for different applications. Each type of pump comes with distinct advantages and disadvantages that necessitate careful evaluation. In this article, we provide a comprehensive overview of prevalent pumping technologies and delve into the limitations associated with pump selection per applications.
Selecting the right pump depends on several important factors, and it’s crucial to consider your project goals and design objectives. One key factor to keep in mind is the cost of downtime, which can have a major impact on your decision-making process. Sometimes, just one hour of downtime can be as expensive as purchasing a new pump. In such cases, the initial price of the pump becomes less important compared to its reliability. However, for less critical applications, factors like the pump’s price, ongoing operational costs, and its overall reliability play a more significant role in determining which pump offers the best value.
Two other important factors that significantly impact pump selection are the properties of the fluid being handled and the specific conditions of the process. Various variables, including viscosity, pressure, flow rate, density, chemical compatibility, temperature range, vapor pressure, the presence of suspended or dissolved solids, and the consistency of the fluid, all need to be carefully taken into account when choosing the right pump. Furthermore, unique considerations specific to each application must be addressed. For instance, is the product sensitive to shear forces or prone to foaming? Does it have a potential for ignition or explosion? Is there a requirement for high-purity materials and finishes in handling the process fluid? Is clean-in-place capability necessary for the application, or are there environmental and safety concerns that must be considered? These questions are essential in making an informed decision.
PD vs. Centrifugal Pump Selection
Once all project details are understood, the next step is to decide which type of pump is the best fit for the application. For a novice the sheer number of pump technologies can be overwhelming. It is important to first understand the difference between positive displacement (PD) and centrifugal pumps and when to use each technology.
The conventional definition states that a PD pump captures and transfers a fixed amount of fluid per cycle. A centrifugal pump imparts velocity onto fluid through rotational force. This is not incorrect, but it may not be immediately useful to someone tasked with make a pump selection on a new application. A more practical difference is that centrifugal pumps have a high level of volumetric slip, causing them to limit pressure output. As centrifugal pumps experience more pressure downstream, they produce less flow. While this can also be true of PD pumps, they tend to experience significantly less volumetric slip, allowing them to push past line resistances with little to no change in volumetric displacement. Opposed to a most centrifugal pumps, pressure will continue to increase until a secondary limiting component relieves pressure or the pump motor overloads.
Consequently, centrifugal pumps are typically better suited for applications requiring higher flow rates at low to medium pressures. Additionally, they work optimally with thinner fluids, typically below 500 cP. If your process fluid exceeds 200 cP, it’s advisable to consult the pump manufacturer. While some Positive Displacement (PD) pumps can achieve similar flow rates to centrifugal pumps, the cost can become prohibitive as flow increases. It’s also worth noting that the pressure generation characteristics of PD pumps necessitate a heightened level of safety considerations to prevent the risk of catastrophic over pressurization..
Positive Displacement (PD) pumps excel when it comes to handling viscous fluids. In fact, it’s quite common for a rotary PD pump to maintain 100% volumetric displacement with viscosities exceeding 500 cP. PD pumps are also frequently employed in low-viscosity scenarios, typically in applications with low flow rates and higher pressures. Additionally, PD pumps are the top choice for applications that demand precision metering or dosing, making them an ideal solution for these specific needs.
Selecting the Right PD Pump
Once you’ve determined that a Positive Displacement (PD) pump is the optimal choice for a particular application, the next decision is to specify the type of PD pump that suits your needs. PD pumps can be categorized into two primary groups: reciprocating and rotary pumps.
In essence, reciprocating pumps operate by displacing fluid through the linear motion of components like diaphragms, pistons, or plungers. The direction of fluid flow is regulated by check valves. Conversely, rotary pumps utilize a motor-driven rotor or multiple rotors to displace fluid.
In practice, piston and plunger pumps are typically best suited for high-pressure, lower-viscosity applications. However, there are exceptions, notably in applications such as concrete pumping, fracking, grease lubrication systems. Diaphragm pumps, on the other hand, offer versatility and can efficiently handle a wide range of products, especially at lower pressures.
Because reciprocating pumps operate with linear pumping mechanisms, it’s more common to utilize compressed air to drive them. However, motor-driven reciprocating pumps equipped with cam shafts are also utilized. Additionally, plunger and piston pumps excel in applications that demand high-pressure flow through a nozzle, such as paint sprayers, pressure washers, inline cleaners, and glue application systems.
Rotary Positive Displacement pumps encompass a diverse range of pump types, as depicted in Image 1. These pumps are primarily distinguished by their rotor and stator/housing designs. A common characteristic among them is the minimal to no clearance between the rotor and housing or stator.
Vane, eccentric disc, progressive cavity, and peristaltic pumps are examples of designs that feature contacting rotors or vanes. This attribute empowers these pumps to excel in displacing liquids with lower viscosities. Progressive cavity pumps, in particular, exhibit remarkable versatility, capable of handling high to low viscosity products, even those with substantial suspended solids, while delivering a smooth, pulsation-free flow. Certain peristaltic pumps offer the same advantages, with the exception that they introduce a moderate degree of pulsation and necessitate periodic hose replacement. An added benefit of some peristaltic pump designs is their ability to operate in a dry state. It’s important to note that a limitation of pumps with contacting components is the wear and heat generated from friction. For this reason, it’s particularly vital that progressive cavity and eccentric disc pumps process fluids possess lubricating characteristics.
Internal gear, external gear, lobe, and circumferential piston pumps all feature multiple rotors designed with internal clearances. Gear pumps, specifically, do not require timing gears, as the rotors perform both fluid displacement and idler gear rotation functions. This design characteristic typically results in lower costs and simplified maintenance procedures.
However, internal and external gear pumps tend to have a shorter lifespan when commissioned to pump fluids containing various types of suspended solids, particularly abrasive slurries. This vulnerability arises from their internally lubricated bearing design.
Internal gear pumps have are great for pumping of highly viscous materials. Additionally, their straightforward design and fewer components simplify maintenance, reducing downtime and operational disruptions. On the other hand, external gear pumps have a edge in scenarios where higher pressure and lower flow rates are required, and they are often are selected in precision metering applications
Lobe pumps can handle a wide range of fluids. They are a go-to for handling fluids with suspended solids and are well-suited for hygienic applications due to their smooth design, while circumferential piston pumps, an innovation of traditional lobe pumps, offer tighter clearances that make them superior for moving low to high viscosity liquids. However, circumferential piston pumps have limitations in high-temperature applications as thermal expansion can cause rotor contact with the housing, potentially seizing the pump. Manufacturers have addressed this issue by providing hot clearance rotor options that increase tolerances for elevated-temperature tasks.
Centrifugal Pump Selection
Centrifugal pumps are among the most widely used pumps globally, available in numerous designs. They are known for their well-established reliability, ease of maintenance, and remarkable adaptability.
Common types of centrifugal pumps include end suction, split case, and multi-stage pumps. End suction pumps are the most widely used among centrifugal pumps. They are highly versatile, with many manufacturers offering multiple impeller and volute options to accommodate a broad spectrum of applications. Open-face impellers are often employed in end suction pumps, as they are capable of effectively pumping fluids with moderate suspended solid concentrations while maintaining favorable Net Positive Suction Head Required (NPSHr) performance. The simplicity of their design makes end suction pumps cost-effective and easy to maintain. Examples of each of the technologies are depicted below in image 2.
Split case pumps are better suited for higher flow rates. For clean fluids these pumps tend to offer better efficiency and reliability. Because split case pumps have fully enclosed impellers fitted with wear rings, they are less ideal for fluids containing suspended solids. They can be more challenging to maintain and often come at a higher up-front cost.
Multistage centrifugal pumps contain multiple enclosed impellers configured in series to allow for higher pressure capabilities. Boiler feed, irrigation, and water booster systems are a few examples of where these pumps are utilized. As these pumps increase in size, they become expensive and difficult to maintain. Multistage pumps are available in both vertical and horizonal configuration.
Pump Selection Conclusion
In conclusion, the ability to make informed choices when selecting pumping technologies plays a pivotal role in the efficient operation of systems across various industries. While this article provides a foundational understanding of some key principles, it should serve as a starting point for those seeking a more in-depth exploration of each pump technology.