“Energy efficiency” has become a byword for consumer-friendly environmental efforts. Fiona Burlig and Erica Myers of Energy Policy Institute at the University of Chicago explained to Forbes, “Energy efficiency programs promise to be a ‘win-win’: by reducing energy consumption, they lower households’ electricity bills and greenhouse gas emissions at the same time. That’s why when lawmakers are allocating spending with an eye to both the pocketbook and the climate, energy efficiency is always a go-to—filled with what appears to be low-hanging fruit.” But does the same hold true for businesses?
We would say yes — and one of the best ways in which you can do so in a pump-focused business is by trimming your pump’s impeller. In this post, we will discuss impeller trim, how it works, and its impact on your pump’s performance.
What is an Impeller’s Trim?
Before we begin to discuss impeller trimming and the importance of impeller trimming in pumps, we need to discuss the fundamentals of centrifugal pumps. At their most basic, centrifugal pumps use rotation to cause fluid to flow through the pump’s casing and be discharged. That rotational element is key to making the pump into a pump, and the part that causes it is called an impeller.
Connected to the pump’s motor either with a shaft or (in our case) magnets, the impeller spins within the casing (the most common design is called a volute), moving the liquid as it enters the chamber and forcing it into the attached pipe system. Impellers can take on a lot of different shapes, and we will discuss some of their different configurations below, but the best way to conceptualize them is to a think of a propeller. Similarly, when you think about the volute, bring to mind the rough shape of a nautilus shell.
The circle-shaped impeller sits within the volute, and has certain clearances based on the impeller’s diameter. That diameter is called an impeller’s trim. While it’s ultimately constrained by the volute’s size, changing its trim can have a significant impact on a pump’s performance.
How Impeller Trimming Works
Trimming an impeller simply means decreasing the impeller’s diameter, and the impeller trimming method you use will likely depend on several factors. One pump may have several different impeller trim sizes available, but it is standard to have the largest size be the defunct impeller.
The impeller trimming method when you need less trim gets a little more complicated. Using a lathe or some such similar equipment, you will measure the desired amount of reduction. One common methods will have you chamfer the impeller while the lathe spins, shaving away part of the material using an implement angled at 45 degrees. The resulting angle on the impeller will be 90 degrees due to the way the implement is positioned during the turning process. However, understand that there are other ways to trim your impeller. These include:
- Oblique Trimming: Removing material at an angle so that the edge of the impeller sits at less than 90 degrees.
- Triangle Trimming: Removing a triangle-shaped section from the innermost edge of the impeller.
- Semicircular Trimming: Removing a semicircle-shaped section from the innermost edge of the impeller
Depending on your trimming style and material, you may need to chamfer the impeller several times before you reach your desired trim and grind it to remove any leftover burrs.
Types of Impellers
A pump impeller can come in numerous different sizes and designs. However, impellers have a handful of standard archetypes that inform nearly every end product. The nonprofit website Nuclear-Power.net does an excellent job breaking down those examples by their constituent parts.
The first part involves the material surrounding the spokes, arms, or vanes of the impeller. All impellers contain vanes, but specific configurations depend on end-use scenarios and required efficiencies. Impeller configurations include:
- Open Impellers: The simplest and most cost-effective design only features the impeller shaft and vanes. Typically used only in small pumps and when pumping slurries as it allows solids to pass thru the pump more easily.
- Semi-Open Impellers: A single shroud covers one side of the impeller blades, increasing efficiency. Common in both small and medium pumps.
- Closed Impellers: A pair of shrouds encapsulate the vanes, creating a highly efficient and expensive impeller suitable for large pumps.
Another factor to consider in impeller design is the way in which the vanes radiate from the impeller’s central vertex. They can be backward curved, forward curved, or radial (i.e., radiating straight out like sunbeams). Additionally, impellers may only allow liquid to enter from one point (single suction) or from both sides (double suction).
Effect of Impeller Trimming on Performance
The effect of impeller trimming on flow and performance depends on a number of factors, all of which you should take into account before undertaking any action. To oversimplify, the smaller the impeller diameter becomes, the less flow and head the impeller will produce. Because most pump manufacturers design their casings to accommodate multiple impellers, the first thing you should do prior to trimming is to consult the included impeller size chart. These charts include performance curves, which show the resultant efficiency. Trimming an impeller often increases efficiency — but not always.
For instance, take care with how much material you remove while trimming. According to the U.S. Department of Energy, “Trimming should be limited to about 75% of a pump’s maximum impeller diameter, because excessive trimming can result in a mismatched impeller and casing. As the impeller diameter decreases, added clearance between the impeller and the fixed pump casing increases internal flow recirculation, causes head loss, and lowers pumping efficiency.”
It also makes sense to employ the impeller trimming calculation for yourself so that you can determine the effect of trimming on flow, pressure, and power. The U.S. Department of energy provides the formulae for impeller trimming affinity laws. In short, the laws state:
- Flow is proportional to shaft speed or impeller diameter
- Pressure is proportional to the square of shaft speed or impeller diameter
- Power is proportional to the cube of shaft speed or impeller diameter
Considerations for Impeller Trimming
Like many things in life, the rules and guidelines we’ve mentioned in the previous sections don’t always work perfectly smoothly in real life. Even if you’ve made all your measurements and calculated every formula, you’ll need to take some considerations into account before you begin to trim your impeller. These include:
- Impeller trimming that decreases the diameter by more than 10 percent can decrease the net positive suction head required (NPSHr)
- Trimming too much material from the impeller can lead to turbulent flow.
- Cutting material from the impeller while failing to remove material from the volute can lead to increased vibrations that affect pump efficiency
If you’re considering trimming a centrifugal pump, contact us at March Pumps. We’ve been manufacturing industry-leading centrifugal pumps since 1962, and our pumps service many kinds of industries, including chemical transfers, electrostatic painting, water treatment, microchip manufacturing, beverage manufacturing and more.