Two centrifugal pumps with the same casing become two completely different machines when equipped with different impellers. One delivers high efficiency in clean water but clogs within weeks when used with a fluid containing particles; the other does not clog but performs the same task using more energy.
This option, listed in the catalog as "open-fan," "semi-enclosed," and "closed-fan," is the line item most frequently overlooked in bid comparisons. This guide covers how to install these three impeller types, the trade-off between solids handling capacity and efficiency, how wear affects efficiency, and which type to select for which fluid.
What does the impeller do?
In a centrifugal pump, the fluid enters the impeller at a point near the axis of rotation. The impeller accelerates the fluid and throws it outward from the center; as the fluid slows down in the volute casing, the kinetic energy it has gained is converted into pressure.
How efficient this conversion is depends on how smoothly the fluid flows between the blades. Whether the impeller is closed or not comes into play right here: closing it regulates the flow but narrows the passage.
Three types of gears
Open impeller
It consists of a central chamber and wings attached to it; the wings have no side walls. The fluid flows freely between the wings.
- Solid materials: the best choice. In liquids carrying suspended solids, sand jamming does not occur as easily as it does in a closed impeller.
- Strength: Since the side plate is not fixed, blade stresses are significantly lower.
- Maintenance: Internal components are visible; inspection and maintenance are easy.
- Limitations: It operates within a narrow specific speed range, and its efficiency is slightly lower than that of a closed impeller.
In our stainless steel series, the open-fan model is described in the data sheet as “recommended for liquids containing suspended solids that pose a risk of clogging”; the external mechanical seal also makes maintenance and replacement easier.
Semi-enclosed impeller
A wall has been added behind the blades. This provides greater strength compared to the open-type impeller.
It can handle mixed solid–liquid flow, but does so at the expense of efficiency. The definition on our data sheet clearly expresses this balance: "It strikes a balance between the solid-passing capacity of an open fan and the efficiency of a closed fan; it is used in liquids containing a moderate level of particles."
Closed-loop impeller
Walls are added to both the front and back sides of the wings. The liquid flows through a closed channel.
- Efficiency: highest — hydraulic efficiency is superior in clean or low-particle-content fluids.
- Strength and Load: It has high strength and reduces axial thrust loads.
- Operating range: It can operate over a wider specific speed range.
- Solids: Poor. It cannot operate effectively in liquids containing solids, and it is difficult to clean when clogged.
- Cost: Manufacturing is more expensive due to its complex design.
It is typically used in large pumps and clean water applications. It is also preferred for high-temperature service—the air-cooled series with a closed impeller, which we covered in our guide to hot oil pumps.
Comparison
| Open | Semi-enclosed | Closed | |
|---|---|---|---|
| Hydraulic efficiency | Low–medium | Medium | Highest |
| Solids | Best | Medium | Weak |
| Wing span | Lowest | Medium | High strength |
| Specific speed range | Narrow | Medium | Wide |
| Cleaning / maintenance | Easy (parts are visible) | Medium | Difficult |
| Wear compensation | With clearance adjustment | With clearance adjustment | Wear ring replacement |
| Cost | Low | Medium | High |
Setting Up the Trade-Off Correctly
The choice comes down to a single question: What's in your liquid?
- Clean liquid, no particles: closed impeller. The difference in efficiency adds up over the years on your energy bill.
- Moderate level of particles, no fibers: semi-closed impeller. Reduces the risk of clogging to an acceptable level without sacrificing too much efficiency.
- Suspended solids, risk of clogging: open impeller. Here, efficiency is a secondary concern—the efficiency of a stopped pump is zero.
The most costly mistake is installing a closed impeller in a liquid containing particles with the goal of "maximizing efficiency." A clogged pump not only halts production but also incurs costs for packing and gaskets while it is being disassembled and cleaned.
Clearance and Wear: Where Does Efficiency Go?
The clearance between the impeller and the casing directly determines the pump’s efficiency. As the clearance increases, a portion of the pumped fluid leaks back from the discharge side to the suction side; this internal leakage does no work but consumes energy.
The method of compensating for wear varies depending on the type of impeller:
- In open and semi-enclosed impellers: wear can be compensated for by readjusting the clearance between the blades and the casing. This is a maintenance procedure that can be performed in the field and restores efficiency.
- In closed impellers: efficiency decreases as the wear ring clearance increases. In contrast, adjusting the impeller–casing clearance does not create a blade wear problem as critical as that in open impellers.
Practical conclusion: If the flow rate in a closed-impeller pump decreases over time while the pressure remains constant, the first thing to check is the wear ring. In an open-impeller pump, the same symptom is often resolved by adjusting the clearance. This distinction corresponds to the “flow rate has dropped, pressure remains the same” entry in our troubleshooting and maintenance guide, specifically regarding the impeller side.
Specific speed: What determines the shape of the impeller?
Specific speed is a dimensionless parameter that combines a pump’s flow rate and head into a single value and determines the impeller’s geometry. At low specific speeds, the flow is entirely radial (narrow, large-diameter impeller, high head, low flow rate); as the specific speed increases, the geometry shifts toward mixed flow, and as it increases further, it shifts toward axial flow (propeller-like).
What is important for our discussion is this: while a closed impeller can operate over a wider range of specific speeds, an open impeller is limited to a narrow band. Therefore, at the extremes where very high head or very high flow rate is required, the choice naturally narrows down to the closed impeller.
Number of steps: odd or even?
Another term that is often confused with “impeller type” is “number of stages.” In a single-stage pump, the head is generated by a single impeller. In a two-stage pump, the fluid passes through two impellers in sequence, resulting in a higher head than a single-stage pump at the same flow rate.
In other words, the stage is selected based on pressure, while the impeller type is selected based on the fluid’s properties. These are two independent decisions: the impellers in a two-stage pump can be open, semi-closed, or closed.
In our guide on TDH and system curves, we’ve provided a step-by-step explanation of how to calculate the required head.
Standard Pump: Why Is the Size Standard Important?
Once the decision to use a pump has been made, there’s one detail that many people overlook: whether the pump conforms to the size standard. TS EN ISO 2858 defines the classification, nominal duty point, and main dimensions of end-suction centrifugal pumps (16 bar). ISO 5199, on the other hand, specifies the technical requirements for this class of pumps.
In practical terms, this means that a pump that complies with the standard can be replaced years later with a model from another manufacturer that also complies with the same standard, without requiring any modifications to the system. A non-standard casing, however, requires work on the piping, base, and couplings when replacement is necessary. The main dimensions in our standard pump series comply with this standard; in non-standard complementary models, dimensions may vary from manufacturer to manufacturer.
How performance is verified is also governed by a separate standard: ISO 9906 defines hydraulic performance tests and acceptance classes for customer acceptance of rotodynamic pumps. To avoid debate over the expected value in a pump with a reduced impeller diameter, the acceptance class is specified in the specifications.
Reducing the impeller diameter
If the selected pump remains above the calculated point, the solution is not to throttle the valve but to reduce the impeller diameter. The energy throttled at the valve is converted into heat and is billed hourly; reducing the impeller diameter, however, permanently brings the pump to the correct point.
Reducing the diameter is done within the manufacturer’s specified limits—if the ratio falls below a certain level, efficiency and flow pattern are compromised. We discuss the effect of changes in speed on flow rate, head, and power using affinity equations in our pool pump guide.
Ingredients and Temperature
The material of the impeller is just as important as the impeller type in determining its service life. In fluids containing abrasive particles, the impeller wears out before the casing and is typically the first part to be replaced.
In our stainless steel series, the casing material is available in AISI 304, 316, or 316L; the fluid discharge temperature ranges from −15 °C to +110 °C, and up to 120 °C with a special mechanical seal. Please refer to our material selection guide for chemical compatibility and material limitations.
Conditions on the suction side also directly affect the impeller: cavitation begins at the impeller inlet and erodes the metal there. The calculation of the suction line is provided in our NPSH guide.
Symptom → cause → initial examination
| Symptom | Possible cause | Initial Check |
|---|---|---|
| Flow rate has dropped, pressure remains the same (closed impeller) | Wear ring clearance has increased | Wear ring measurement |
| Flow rate has decreased (open/semi-closed impeller) | Blade–casing clearance has increased | Clearance adjustment; impeller wear |
| The pump clogs periodically | Impeller type is not suitable for the solid content of the fluid | Particle size and fiber content; switch to an open impeller |
| High energy consumption for the same workload | Impeller type is too open or worn | Efficiency curve; internal leakage |
| Pitting and pits at the impeller inlet | Cavitation | NPSHa calculation; suction line |
| The impeller is wearing out rapidly | Abrasive particles; material is inadequate | Particle hardness; material selection |
| Vibration has increased; imbalance | Impeller is worn on one side or clogged | Vibration measurement; impeller cleaning and balancing |
Election Checklist
- Liquid composition: Are there any particles? What is their size? Does it contain fibers?
- Consequences of clogging: If the pump stops, will production stop? If so, efficiency is a secondary concern.
- Abrasiveness: Are the particles hard? Was the impeller material selected accordingly?
- Maintenance access: Can the clearance adjustment on an open impeller be performed on-site?
- Operating point: Has the TDH been calculated, and is the selected impeller efficient at that point?
- Stage: Can the required head be achieved with a single impeller?
- Is there a diameter reduction factor (to avoid the need for throttling with a throttling valve)?
- Temperature and material: Is the fluid temperature within the series limit?
- Suction conditions: Is the NPSHa sufficient? The impeller is the first point where cavitation occurs.
Frequently Asked Questions
Is an open fan or a closed fan better?
"Better" depends on the fluid. A closed impeller provides higher hydraulic efficiency in clean fluid; an open impeller does not clog in fluid containing solids. Installing a closed impeller in a fluid containing particles for the sake of efficiency is the most common and most costly mistake—because the efficiency of a pump that has stopped is zero.
What is the center of a semi-enclosed impeller?
Thanks to the wall added behind the blades, it is more durable than an open impeller and more permeable than a closed impeller. It can handle mixed solid–liquid flow, but does so at the expense of efficiency. This represents a reasonable balance for liquids containing moderate levels of particles.
My impeller is worn out—can its efficiency be restored?
It depends on the impeller type. In open and semi-closed impellers, some of the efficiency can be recovered by readjusting the blade-to-casing clearance; this is a procedure that can be performed in the field. In closed impellers, efficiency loss is mostly due to the wear ring clearance, and the ring must be replaced.
Should I get a single-stage or a two-stage one?
This decision is independent of the impeller type and depends on the required head. In a single-stage pump, the head is generated by a single impeller; in a two-stage pump, the fluid passes through two impellers in sequence, resulting in a higher head at the same flow rate. First, calculate the total head (TDH), then decide on the stage configuration.
My pump is too powerful—what should I do?
Throttling the throttling valve works, but the throttled energy is converted into heat, and you pay for it every hour. A permanent solution is to reduce the impeller diameter within the manufacturer’s specifications or switch to speed control. Furthermore, excessive throttling pushes the pump into the low-flow rate range, creating a risk of recirculation.
Why is the impeller the first part to wear out?
Because that is the part that comes into contact with the fluid at the highest relative velocity. In fluids containing abrasive particles, the impeller wears out before the casing. If cavitation occurs, the damage also begins at the impeller inlet. For this reason, the impeller material is selected specifically for abrasive applications.
Summary
The choice of impeller is a trade-off: solids-handling capacity or hydraulic efficiency? An open impeller handles solids and is easy to maintain; a closed impeller provides the highest efficiency but is prone to clogging; a semi-closed impeller falls somewhere in between. If your fluid contains particles and clogging is halting production, the efficiency debate takes a back seat.
The compensation for wear also varies by type: in open and semi-closed gears, adjusting the backlash restores efficiency, while in closed gears, the load falls on the wear ring. The number of stages, however, is a separate decision and depends on the head.
If you provide us with the particle content of your fluid, as well as your flow rate and head requirements, we can work together to determine the appropriate impeller type and model. You can review our stainless steel centrifugal pump and standard pump lines, or get in touch with us for technical support.