Pump Impellers: 3 Different Types and How to Choose the Right One

An impeller is the heart of a pump. As the rotating component that transfers the motor’s energy onto a fluid, it plays a vital role in maintaining the flow and pressure in a pump.

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Impellers do a marvelous job of transferring liquids by creating centrifugal force while minimizing agitation and maximizing the pump’s efficiency.

Centrifugal pump impellers vary in diameter, material, and number of vanes. The size of the protective wall around them, also known as a shroud, also varies. The presence or lack of a shroud classifies impellers as either open, semi-open, or closed.

The right impeller should fit well, withstand the liquid you’re pumping, and be able to resist wear and tear well, all without costing a fortune.

Consider these factors when making a selection:

• Size or diameter of the impeller
• Nature of the liquid to be pumped
• Material of the impeller
• Cost and maintenance of the impeller

We’ll show you how to select the right pump for the job, but first, here are the three pump impeller types to choose from.

1 – Open Impellers

All centrifugal pump impellers have rotating blades, called vanes, but in open impellers, the vanes don’t have any covering, resulting in an open design.

An open design makes an impeller suitable for handling liquids with suspended solids. Sewage is a good example and passes through an open design easily.

Another advantage of an open design is that it enables easy cleaning and repair of the vanes since they’re not covered by a metal plate.

However, open impellers are weak due to the lack of support around their vanes. They struggle to generate pressure, rendering them less efficient than semi-open and closed impellers.

As a result, they tend to be used in small pumps that don’t have to handle many operations.

Open impellers operate at a higher NPSH (net positive suction head) to prevent cavitation and increase pump efficiency.

What is Cavitation?

Cavitation is the formation of cavities around the impeller where pressure is usually low. They form when the liquid’s pressure drops below vapor pressure and turns into bubbles.

As the bubbles move to high-pressure areas of the pump, they gain tremendous energy that causes them to implode on the impeller, forming cavities.

The cavities cause mechanical damage that leads to noise, vibration, and deterioration of the entire pump, making it less efficient.

Oversized pumps may run at a higher pressure and flow rate than their application requires. That pressure is far from the ideal operating point, known as the best efficiency point (BEP).

It’s best to modify the impeller in such pumps to avoid energy waste. You can do so by trimming the vanes to reduce the impeller’s diameter. This process results in an overall circumferential speed reduction at the impeller’s outlet.

When the outside diameter is reduced, so are the flow rate and head, but this doesn’t alter the rotational speed. This process improves impeller capacity, which saves energy and reduces costs.

Trimming an impeller is considered when:

• Open system bypass valves create an excess flow rate
• An oversized throttle pump provides head that exceeds process requirements
• The flow rate and operating head exceed process requirements

Note: Limit trimming to 75% of an impeller’s diameter. Trimming 5-10% more than the recommended 75% increases the net positive suction head required (NPSHr) due to recirculation between the impeller inlet and discharge.

A bit of clearance space must be left between the impeller vanes and casing to minimize the liquid’s recirculation.

2 – Semi-Open Impellers

Semi-open impellers have a metallic back wall that strengthens their vanes more than open impellers. However, the front side remains open.

Their efficiency is between that of open and closed impellers, making them suitable for medium-sized pumps.

Semi-open impellers can handle liquids with higher levels of solids, such as mud, better than closed impellers. They’re able to do so because they aren’t restricted by their back wall, as closed impellers are.

A shroud’s mass determines the impeller speed, so semi-open impellers rotate faster than closed impellers but slower than open impellers.
Semi-open impellers can handle mud and fibrous material, such as paper pulp, well. However, constant pumping of these solids wears on the impellers as the solids rub against the vanes repeatedly.

As they wear out, the space between the casing and impeller vanes, known as clearance, increases. This causes slipping and recirculating of the liquid, leading to pump inefficiency.

You can correct the wearing out of semi-open impellers by making axial adjustments. A small variation in axial clearance simultaneously changes the axial size of the front and back clearances, greatly improving the pump’s performance.

However, a large axial clearance at the front will decrease the pump’s head and inefficiency, but it won’t affect the shaft power.

Axial clearance mostly affects volumetric efficiency. As clearance increases, the volumetric efficiency decreases, making it the main factor in how much axial clearance is needed.

3 – Closed Impellers

Closed impellers have a back and front covering, making them stronger than both open and semi-open impellers. They’re used in large pumps and provide adequate flow at low NPSH.

Their design is more complicated than open and semi-open impellers because more materials, like cast iron and bronze, are required to fabricate them. This also causes them to be costly.

Closed impeller vanes usually have a single backward curvature, but there are wider types with double-curved vanes featuring twisted suction ends. The latter are referred to as Francis or mixed flow vanes.

Closed impellers transfer clean liquids with low viscosity, like fresh or salt water. As such, they’re often used in water treatment plants. Any contact with solids clogs them, and cleaning becomes difficult due to their enclosed design.

Closed impellers are very efficient because the liquid flows through the impeller’s eye and is directed between the two shrouds in a circular movement.

These impellers generally lose efficiency as the wear ring clearance increases. Wear rings control the discharge fluid that may flow back to low pressure.

The table below summarizes the differences between open, semi-open, and closed impellers:

• Vanes are open on both sides
• Handle suspended solids well
• Operate at a higher NPSH

• Have a back wall covering
• Handle few amounts of solids
• Operate at medium NPSH

• Vanes are covered on all sides
• Handle clean liquids only
• Operate at low NPHS

Let’s now take a look at what factors you should consider when purchasing an impeller.

How to Choose the Right Impeller

Material

Consider whether the impeller’s material can resist abrasion and corrosion. Stainless steel is the most common impeller material because of its anti-corrosive, anti-heat, and anti-contamination properties.

Bronze is a good choice when pumping liquids that contain salt. In such a case, gunmetal, a type of bronze, is used for the impeller.

Cast iron is cheaper than brass or bronze, and it’s resistant to alkali corrosion. But, if you’re dealing with acids, cast iron impellers aren’t a good choice because they’ll rust.

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A good choice when pumping acids is an impeller coated with polymers. The extra layer is applied on areas prone to wear, and the impeller remains protected as long as there’s no damage to the coating.

Type of Liquid

If you’re pumping wastewater, the water may contain solids, foamy matter or stringy substances. In this case, the best option is an impeller that allows the passage of solids without clogging.

When dealing with a viscous liquid, you’ll need an impeller optimized to perform well with viscous liquids. This is because high viscosity slows down an impeller’s functionality and affects pump performance.

Compatibility

Sometimes, pump manufacturers make oversized pumps to leave room for viscosity changes. This wastes energy because the pump is operating at high pressure.

When getting a new impeller, have its diameter trimmed to ensure compatibility with the pump. Pumps are designed with shaft casings that can accommodate different impeller sizes.

Cost

The cost-effectiveness of an impeller is of utmost importance. That means considering all the above suggestions, including maintenance costs, and getting a good quality impeller at a price that won’t drain your pocket.

Choose the Right Pump for the Application

Pump impellers are classified as one of three types: Open, semi-open, or closed. Their classification is based on whether they have a protective shroud or not.

Open impellers handle solids well and operate at a higher net positive suction head (NPSH), while closed impellers handle liquids well and operate at a lower NPSH. Semi-open impellers share common features of the two.

To choose the right impeller, first determine what kind of liquid will be pumped. Is it corrosive or highly viscous?

The material used also determines how well an impeller can withstand wear and tear when subjected to different temperatures, pressures, and liquid types.

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Battery-powered backup pumps are the most common backup system, designed to take over when your main pump loses power or stops functioning. These systems typically use a 12-volt deep-cycle battery—the same type used in some trolling boats or golf carts—with costs ranging from $100 to $300 or more. The battery’s capacity, measured in amp-hours (Ah), indicates how long it can power the pump. A battery with 75Ah to 120Ah will allow several hours of intermittent pumping, which can often stretch over several days during an extended blackout. Most models can move 800 to 2,000 gallons per hour (gph) from your sump pit, with higher-gph models better suited for homes prone to significant water influx during storms.

How they work: Battery-powered systems use a float switch to activate the pump when the water level in the sump pit rises. Drawing power from the battery, the pump discharges the water through the same discharge pipe used by your primary sump pump. 

Battery life: A fully charged deep-cycle battery typically provides 7 to 8 hours of active pumping time, which roughly translates to several days’ worth of intermittent pumping during a sustained blackout. 

Pros
● They generally cost less than combo or water-powered units. 

● They can operate for several days on a charge, providing effective coverage for most outages.

● They’re relatively easy to install for DIYers with basic plumbing knowledge. A contractor can install one in 2 to 4 hours. 

● Higher-amp-hour batteries can provide longer run time if necessary.

Cons
● Batteries need to be maintained, checked regularly, and typically replaced every five to seven years.

● Run time is limited, so if a blackout lasts longer than a few days, your home may be vulnerable. 

● They might not fit. These systems share a pit with your primary pump, so you’ll need to make sure that there’s enough room for both units.

It might sound counterintuitive, but when your basement is threatened by rising water levels, one solution is to throw more water at the problem. Water-powered backup pumps use the pressure from your home’s municipal water supply to create suction that removes water from the sump pit. Because they aren’t powered by electricity, these systems have unlimited run time: As long as you have enough water pressure, they can bail you out. But they’re not an option for homeowners using well water, and they come with a few quirks and installation requirements.

How they work: Similar to a battery-powered unit, a float switch activates the pump when water levels in your sump pit rise. The system uses the force of pressurized municipal water flowing through your pipes to create suction, pulling water out of the sump pit and discharging it away from your home. Generally, a water-powered pump will remove 1 gallon of sump water for every 2 gallons of municipal water used. It’s not the most efficient trade-off, but when your basement is at risk of flooding, you’ll be glad for the extra help. Your water bill may be affected, too. To prevent backflow and protect your potable water supply, most water-powered backup pumps come with an integrated backflow prevention valve. Local codes may require an additional device to meet safety standards. 

Pros 
● They have unlimited run time. As long as your home has municipal water pressure, these systems can run intermittently, making them a reliable option for extended emergencies.

● Water-powered pumps are generally lower-maintenance than battery-operated units because they have fewer components to maintain (no batteries to charge or replace).

● Water-powered pumps are typically mounted above the sump pit on a rafter or on a wall, so space isn’t a concern if you have a small sump pit.

Cons
● You need to have municipal water service to create the suction needed for these units to work. If you’re on well water, these are a no-go.

● Installation can be more complex and pricier than for battery-powered units. For most, connecting a water-powered pump to your home’s plumbing will require professional installation, often involving modifications to your water supply lines, adding to the cost.

● They’re generally not as powerful as electric systems. Water-powered pumps generally have lower pumping rates than their battery-powered counterparts, and they may struggle to keep up during very heavy rainfall or in high-volume flooding.

● Their reliance on water usage can be a concern in drought-prone areas or regions with water conservation measures in place. Check to make sure these are legal in your municipality. 

● These systems require close proximity to your sump pit and your home’s water supply. If your water supply lines aren’t nearby, installation could be difficult or even unfeasible without significant replumbing.

Combo units offer an all-in-one solution by combining a primary electric sump pump with a battery-powered backup in a single streamlined system. These pumps automatically switch between the main and backup systems, ensuring that your basement stays dry during power outages or when the primary pump fails. By using the same discharge line for both pumps, combo units simplify installation. They’re especially relevant if you’re already looking to replace an aging or malfunctioning primary pump since you’ll be shopping for—and installing—only one unit.

How they work: Combo units consist of a standard electric sump pump for regular operation and a backup pump that kicks in only when needed. If the main pump malfunctions or power goes out, the backup system is activated, driven by a float switch that monitors rising water levels. Like the two other types, most combo systems have an alarm to tell you when the backup pump is running (or, for battery units, when maintenance is required). 

Pros 
● Installation is simpler. With both pumps using the same discharge line, installation is more straightforward, reducing the need for extensive plumbing work.

● You’ll shop (and install) only once. If you need to replace your primary pump, a combo unit allows you to deal with a backup at the same time. 

Cons 
● Combo units have a higher up-front cost than battery-powered backup units. If you recently purchased a primary pump, you might not want to replace it. 

● Like other battery-powered systems, combo units require regular battery checks and replacements every five to seven years to make sure the backup is ready when it’s needed.

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