Submersible Volute vs. Centrifugal Pumps – Differences and Applications
Submersible screw pump vs. centrifugal – differences and applications
When we deal with inquiries about a pump for a well or borehole on atria.sk, the first question we must answer is not the brand or price, but the pumping principle. In the market we basically encounter two fundamentally different constructions of submersible pumps – centrifugal (often multi-stage, with impellers) and screw (with a flexible stator insert and rotor). Both categories have their fixed place in practice, but confusing them or choosing "by eye" almost always leads to premature wear, unnecessary energy consumption, or the fact that the pump simply cannot deliver the water where it needs to go. In this article, we will look at how both constructions work, how they differ in terms of hydraulics, what water they can handle, what their lifespan and maintenance are, and finally we will project this into specific types of wells and boreholes that we commonly encounter with customers.
Principle of a submersible centrifugal pump
A centrifugal pump works on the principle of converting the kinetic energy of a rotating impeller into the pressure energy of a liquid. Inside the pump body is one or (typically in submersible well pumps) several impellers connected in series – we speak of a multi-stage pump. Water enters the first stage, the impeller gives it speed, the diffuser converts this speed into pressure, and the same process continues through the subsequent stages until the desired pressure (delivery head) is achieved at the outlet. The more stages, the higher the delivery head the pump is capable of creating.
This modular construction is the reason why centrifugal submersible pumps of the type IBO 3" STM 20 are so widespread – the manufacturer simply produces a range of pumps with the same motor and hydraulic unit, just with a different number of stages, thus covering the entire range of required delivery heads from low (a few tens of meters) to high (150 or more meters).
The impellers of a centrifugal pump are usually made of technical polymer (e.g. noryl) or stainless steel, depending on the pump class. The motor rotor is mounted in an oil- or water-filled chamber, separated from the hydraulic part by a mechanical seal. This arrangement is sensitive to abrasive particles – sand acts like sandpaper on bearings and seals, so manufacturers agree that the content of solid particles should not exceed approximately 50 mg/l, in the case of higher quality models with ceramic seals up to 150 mg/l. At a higher sand content, the lifespan is dramatically reduced – in practice, we have seen cases where an incorrectly installed pump in a sandy well "died" within three months, although the normal lifespan with clean water is 8–15 years.
Principle of a submersible screw (rotary) pump
A screw pump works on a completely different principle – it is a positive displacement, not a centrifugal pump. Inside it is a metal (usually chrome-plated or stainless steel) eccentric-shaped rotor, which rotates inside a flexible rubber (elastomeric) stator insert. The rotor does not rotate coaxially, but performs a planetary motion – thus creating sealed chambers between itself and the stator, which gradually move from suction to discharge and "transfer" the enclosed water like in closed pockets. That is why this type is also called a Moyno pump (after the inventor of the principle).
Thanks to this principle, the screw pump is exceptionally tolerant to mechanical impurities and sand – the rubber stator partially adapts to the sand grain that gets into the chamber and "passes" it on, without the risk of jamming as with an impeller. Therefore, screw pumps are the first choice precisely in narrow boreholes with a higher sand or silt content, where a centrifugal pump would quickly fail. A typical representative of this construction in our range is Submersible screw pump for narrow borehole STING 2", designed precisely for narrow boreholes with a diameter of 2 inches, where a classic 3" or 4" pump physically does not fit.
Hydraulic properties – flow, pressure and characteristics
The difference between the two types is clearly reflected in the shape of the hydraulic characteristic, i.e. how the flow rate changes in relation to the delivery head (pressure).
A centrifugal pump has a decreasing characteristic – at zero delivery head it delivers the maximum flow rate, and with increasing head (pipe resistance, height difference) the flow rate gradually decreases to zero at the point of a closed valve. This means that the flow rate of a centrifugal pump is always a compromise between how much water we need and to what height/pressure it needs to be delivered. When designing, we always look for an operating point where the pump curve intersects with the pipe network curve.
A screw pump, on the other hand, has a flow rate that is practically independent of pressure (within a reasonable range) – it is a positive displacement pump, so for a given number of revolutions it delivers approximately the same amount of water regardless of whether it is pumping to 20 or 60 meters in height (limited naturally by the motor power and maximum allowable pressure). This is an advantage in variable delivery head, but a disadvantage in dry operation or at closed discharge – without a flow switch, the pressure in the system can rise to the point of damaging the stator or the piping.
Practical consequence: with a centrifugal pump, it is worthwhile to accurately dimension the head, otherwise we will either get an unnecessarily high flow without pressure or, on the contrary, insufficient flow. With a progressing cavity pump, flow regulation is simpler (often via a frequency converter or throttling), but dry running and discharge blockage protection must always be ensured.
Sensitivity to sand, silt, and mechanical impurities
This is one of the most common questions we address when selecting a pump for a specific well. The reality in Slovakia is that a large portion of dug wells and many boreholes (especially older, poorly constructed ones, or those drilled into less stable rock) produce water with a certain amount of sand, especially after heavy rain or when the water level drops.
- Centrifugal pump – suitable for clean to slightly contaminated water. Sand causes erosion of impellers, bearing wear and clogging, and in extreme cases, rotor jamming. Recommended solid particle content up to 50–150 mg/l depending on the model.
- Progressing cavity pump – tolerant to higher sand and fine impurity content, typically handling 150–200 mg/l and more in catalog data, and in some models even significantly more for short periods. The flexible stator can "pass" sand particles without jamming.
Despite the higher sand tolerance of progressing cavity pumps, it is still true that no pump appreciates large amounts of coarse sand or gravel. If a well or borehole continuously draws in a lot of sand, it is a sign that the cause should be addressed (undersized or damaged well casing, improperly placed filter, or pump positioned too low, drawing sediment from the bottom). In such cases, we recommend also installing an Sand filter 1" for suction, which will capture mechanical impurities before they reach the water distribution system in the house, or the pressure tank and fixtures, where they would cause premature wear.
Borehole diameter and physical dimensions of pumps
Progressing cavity pumps generally have a more cylindrical, "slender" shape with a longer body, making them an ideal choice for narrow 2" (approximately 50 mm) boreholes, where standard 3" or 4" centrifugal pumps simply cannot fit. The STING 2" is a typical solution for such cases – old, narrow boreholes where only a hand pump or small pump was considered during construction, and today we need to deliver higher performance without enlarging the borehole (which would mean new drilling work and significant costs).
Centrifugal pumps are manufactured in 3" and 4" diameters, with the IBO 3" STM 20 being a versatile solution for standard boreholes with a diameter of at least 3 inches (approximately 76 mm) and above. Precise differences between the various diameters and how to measure the diameter of your own borehole are discussed in detail in the article "What borehole diameter do I need: the difference between 2", 3", and 4" pumps."
Overview comparison of both types
| Property | Centrifugal pump | Progressing cavity pump |
|---|---|---|
| Principle | Rotational, impellers | Volumetric, rotor in flexible stator |
| Typical diameter | 3", 4" | from 2" upwards |
| Sand tolerance | low to medium (~150 mg/l) | high (200 mg/l and more) |
| Flow characteristic | decreases with head | almost constant |
| Dry running | short-term tolerant | very risky for the stator |
| Seal/stator lifespan | seal – long lifespan | elastomer – wear, replacement |
| Typical use | standard wells and boreholes, clean water | narrow boreholes, sandy/muddy water |
Sensitivity to dry running and starting
Dry running (without water in the pump) is dangerous for both constructions, but for different reasons. In a centrifugal pump, the lack of water means loss of cooling and lubrication for the mechanical seal and bearings – the motor overheats, the seal burns out, and in the worst case, the entire rotor jams. In a progressing cavity pump, the situation is even more critical – the rubber stator is lubricated and cooled precisely by the water being pumped, and without it, the stator irreversibly burns and deforms after just a few seconds to tens of seconds of dry running. That is why it is absolutely recommended to install a float switch or electronic dry-running protection for progressing cavity pumps, ideally in combination with a pressure or flow switch.
Starting a progressing cavity pump also requires a slightly higher starting torque, as the rotor must overcome initial friction in the stator (especially if the stator has dried out after a long period of inactivity) – this is why some models recommend a frequency converter with an increased starting torque or a soft start.
Water quality, chemical composition, and temperature
The rubber stator in a progressing cavity pump is also sensitive to the chemical composition of the water – aggressive water with high iron, manganese, hydrogen sulfide content, or extreme pH can gradually degrade the stator rubber faster than it would with standard drinking water. In centrifugal pumps with stainless steel or technopolymer impellers, this sensitivity is lower, although even there, iron and manganese can cause scaling and clogging of diffusers over time.
Water temperature also plays a role – most standard pumps (of both types) are designed for water up to 30–35 °C, occasionally up to 40 °C in continuous operation with sufficient flow around the motor for cooling. For warmer water (e.g., geothermal boreholes), specialized pumps are required, which can be discussed separately – the standard range for wells is not suitable for this.
Practical scenarios from everyday practice
To move beyond the theoretical level, let's look at typical situations that customers most often contact us about:
Scenario 1: Old narrow borehole after a hand pump
A cottage, a borehole drilled 30–40 years ago, with a pipe diameter of only 2", originally equipped with a hand or diaphragm pump. The customer wants an automatic water supply to the house. Solution: progressing cavity pump STING 2", because a 3"/4" centrifugal pump physically cannot fit into such a borehole, and older boreholes often also show slight sanding after long periods of inactivity.
Scenario 2: New 4" well with clean water
A newly drilled well with certified construction, stable clean water, water level depth of 25 m, required head to the house and pressure tank of about 35–40 m. An ideal solution here is a centrifugal pump, for example the IBO 3" STM 20 series with the appropriate number of stages according to the required head (a detailed calculation process is described in the article "What power and head to choose for a pump based on well depth").
Scenario 3: Dug well with occasional sanding after rain
A classic dug well with a diameter of 1–1.2 m, after heavy rainfall, increased turbidity and sand appear. If the well is wide enough for a 4" pump, a centrifugal pump with higher sand tolerance can be considered in combination with a sand filter at the discharge, or directly switch to a progressing cavity pump if the sanding is more pronounced and recurring. In such cases, we always recommend adding an Sand filter 1" for suction, which will protect both the pump and the subsequent piping and fixtures in the house.
Installation and pump mounting – what to pay attention to
Regardless of the pump type, several basic principles apply, which are discussed in detail in the separate article "Installation of a submersible pump in a well or narrow borehole":
- The pump is always suspended on a stainless steel rope or suitable discharge pipe, never just on the electrical cable.
- The minimum distance from the pump to the bottom of the well/borehole should be at least 0.5–1 m to avoid suction of settled sediment.
- The pump must be permanently submerged by at least the recommended distance below the suction basket (usually at least 0.3–1 m depending on the model) to prevent exposure when the water level drops.
- The electrical connection must be via a properly dimensioned circuit breaker, and in the case of progressing cavity pumps, ideally also via dry-running protection.
- When starting a progressing cavity pump for the first time, check the direction of rotation (in the wrong direction, the stator can be damaged even after a short run).
Service, wear and maintenance costs
From the perspective of long-term ownership costs, both types of pumps have their "weak" components. In the case of centrifugal pumps, it is most often the mechanical seal and bearings, or the wear of impellers in the presence of sand. The lifespan in clean water and proper installation typically ranges between 8-15 years, significantly less in worse conditions.
In the case of helical pumps, the typical consumable item is the elastomeric stator itself, which in some models can be replaced separately without the need to buy a new pump – this can significantly reduce long-term operating costs, as stator replacement is much cheaper than a new pump. The frequency of replacement depends on water quality and sand content, but roughly speaking, in moderately demanding conditions, we are talking about an interval of several years.
Common faults, their symptoms and solutions (jammed rotor, burned seal, leaking wiring, worn stator, system airlock) are discussed in detail in the article "Most common faults of submersible pumps and how to solve them" – we recommend reading it before you decide to return or replace the pump, as some problems can be resolved without the intervention of a service technician.
Energy consumption and efficiency
Centrifugal pumps achieve high energy efficiency when operating at the optimal point of the characteristic curve (where the pump curve intersects the pipe network curve near the point of maximum efficiency). Problems arise when the pump is significantly oversized or undersized in relation to the actual head – operation outside the optimal point means unnecessary energy consumption and faster wear.
Helical pumps often have an advantage over small centrifugal pumps in terms of efficiency at low flows and high pressures, as they do not require multiple stages to achieve high pressure at low flow. On the other hand, centrifugal pumps are often more advantageous at higher flows. A precise comparison of specific STING and IBO models in terms of performance, flow and consumption can be found in the separate article "Comparison of STING and IBO pumps – what to choose."
How to decide – summary of selection criteria
When making the final decision, we recommend following these steps, which are discussed in more detail in the article "How to choose a pump for a well – key selection criteria:
- Measure the diameter of the well/borehole – if the diameter is less than 3" (76 mm), you practically automatically move towards a helical pump of type STING 2".
- Determine the water quality – if the well/borehole constantly or frequently produces sand or silt, prefer a helical pump or a centrifugal pump with an additional sand filter.
- Calculate the required head – the depth of the water level + the height difference to the consumers + pressure losses in the pipe + the required pressure in the pressure tank.
- Consider the required flow rate – number of people, consumers, possibly garden irrigation.
- Plan dry run protection – especially important for helical pumps.
A detailed procedure for correctly choosing a specific model according to the diameter of the well and the depth of the borehole can also be found in the articles "How to choose a submersible pump for a well or borehole" and "What diameter pump do you need – 2" or 3" borehole," which continue on this topic.
Most frequently asked questions (FAQ)
Can I replace a centrifugal pump with a helical pump in the same borehole without modifications?
In most cases, yes, provided the diameter of the borehole is suitable for both types and the electrical connection is compatible. However, you need to recalculate the head and flow, as the characteristics of both types differ significantly, and in the case of a helical pump, add dry run protection if it is not already installed.
Is a helical pump always a better choice in sandy water?
In most cases, yes, as it is more tolerant to mechanical impurities. However, you should not forget that even a helical pump has its limits – in the case of extreme amounts of sand or gravel, the primary issue (e.g. damaged well casing or incorrect installation depth) should be addressed, not just changing the pump type.
Why does a helical pump not pump at all when first started, even though it is connected?
The most common cause is the incorrect direction of motor rotation (in three-phase motors, it is enough to swap two phases) or a dried-out stator after a long period of inactivity, which requires a slightly higher starting torque. If the problem does not disappear after checking the direction of rotation, the pump should be checked by a service technician.
What is the price difference between a centrifugal and a helical pump?
Helical pumps with a small diameter (2") are often in a similar or slightly higher price range than comparable centrifugal pumps of a larger diameter, due to the more complex manufacturing technology of the stator and rotor. However, the decisive factor should be the diameter of the borehole and the water quality, not just the purchase price itself.
Can a helical pump be used in a wide 4" borehole with clean water?
Technically yes, but it is not typically the most economically advantageous solution – in clean water and with a sufficient borehole diameter, a centrifugal pump is usually more advantageous, as it often has a lower purchase price and simpler maintenance at comparable performance.
How often should the stator be replaced in a helical pump?
The exact interval depends on water quality and sand content, but roughly speaking, in moderately demanding conditions, it is a matter of several years of operation. A signal for inspection is a drop in flow or pressure at an unchanged pump setting.
Conclusion
Choosing between a helical and a centrifugal submersible pump is not a question of "which is better" in an absolute sense, but a question of which one meets the specific conditions of a given well or borehole. A narrow borehole, sandy water, the need for a stable flow at varying head – these are situations where a helical pump of type STING 2" will show its advantages. A standard, sufficiently wide borehole with clean water and a clearly defined head, on the other hand, is the domain of reliable and cost-effective centrifugal pumps of the IBO range. In any case of uncertainty regarding the diameter of the borehole, water quality or required head, it is worth consulting – an incorrectly chosen pump will either wear out quickly or provide unsatisfactory performance for a long time, and replacement after a few months of operation will always be more expensive than a correct choice from the start.
Do you have a question about this topic?
Can't decide or are you dealing with a specific situation in your home? Write to us – we are happy to help.