Ask for the types of centrifugal pump and you will usually get a list: single stage, multi stage, end suction, axial flow, submersible, self-priming. The list is not wrong, but it is hard to use, because the items on it answer different questions. A pump can be single stage and end suction and radial flow at the same time.
This article sorts that list into three classifications that do not overlap, then uses them to answer the question the list never answers: which of these types can be built in polypropylene, and which cannot.
Key takeaways
- The types of centrifugal pump are usually listed as if they were alternatives. They are not: three independent classifications describe the same machine.
- The three are flow direction through the impeller (radial, mixed, axial), number of stages (single, multi), and mechanical arrangement (overhung or between bearings, horizontal or vertical, end suction or inline).
- Single stage and end suction are not synonyms. API 610 lists a horizontal end-suction single-stage pump and a vertical inline single-stage pump as different type codes.
- Multiple stages become the economical choice when one impeller would have to develop more than about 150 psid, according to the US DOE handbook.
- Self-priming is not a hydraulic class. It describes how the casing is filled before start-up, and most centrifugal pumps are not self-priming.
- Polypropylene is a realistic wet-end material for horizontal single-stage end-suction pumps, and much less common in multistage and vertically suspended designs.
- Choose in a fixed order: head first, then piping, then floor space, then chemistry, then maintenance access. A later answer can overturn an earlier one.
The Three Ways to Classify a Centrifugal Pump
One pump, three answers
Take a horizontal single-stage end-suction pump in chemical service. Ask three questions and you get three labels that are all true at once. What does the fluid do as it leaves the impeller? It leaves at right angles to the shaft, so the pump is radial flow. How many impellers are on the shaft? One, so it is single stage. How is it built and mounted? The impeller overhangs the bearings, the shaft is horizontal, and the suction nozzle is on the axis at one end, so it is an overhung, horizontal, end-suction pump.
None of those labels excludes the others. They are orthogonal: changing one does not force a change in the other two.
Why most lists mix the dimensions
The list of types of centrifugal pumps that a buyer usually sees is a flat sequence of seven or eight names, and it mixes all three classifications into one line. A typical version runs single stage, multi stage, axial flow, radial flow, mixed flow, self-priming and submersible. That sequence invites the reader to pick one, which is impossible: axial and radial flow are flow types, multi stage is a stage count, and submersible describes where the pump sits rather than how it works.
API 610, the standard for centrifugal pumps in petroleum and petrochemical service, does the opposite. It sorts pumps into three families by mechanical arrangement — overhung, between bearings and vertically suspended — and then splits overhung pumps into six type codes, because the same overhung impeller can be horizontal and end suction, horizontal and centreline mounted, or vertical and inline.
The useful version of the list therefore has three columns, not one.
| Classification | The question it answers | Types under it |
|---|---|---|
| Flow direction | How does the fluid leave the impeller? | Radial flow, mixed flow, axial flow |
| Number of stages | How many impellers are in series? | Single stage, multi stage |
| Mechanical arrangement | How is the shaft supported, how is the pump oriented, and how does liquid enter? | Overhung or between bearings; horizontal or vertical; end suction or inline; submersible |
One related distinction is worth keeping separate, because the two get confused. A centrifugal fan is classified mainly by wheel design rather than by stage count or suction geometry; that taxonomy is set out in our guide to types of centrifugal fans.
Single Stage vs Multi Stage: Where the Head Comes From
One impeller or several in series
A single stage centrifugal pump has one impeller on one shaft. The US Department of Energy handbook defines a stage as “that portion of a centrifugal pump consisting of one impeller and its associated components”, and notes that most centrifugal pumps are single-stage, containing only one impeller. A multistage pump puts several impellers on a common shaft inside one casing and routes the discharge of each impeller to the suction of the next, so the pressure rises in steps.
The same handbook gives the threshold that decides the question. A single impeller asked to develop more than 150 psid between suction and discharge is “difficult and costly to design and construct”; adding stages is the more economical route above that point. That is roughly 10 bar of differential pressure. In most plant rooms the single stage centrifugal pump is the default, and the multistage machine appears only where that threshold is crossed.
What the extra stages cost
Stages buy head and charge for it in length, parts and downtime. A multistage casing is longer, so the rotor has more overhang or more bearings to carry it; each stage adds an interstage seal and a set of clearances that wear; and taking the pump apart for a seal change means working through the stack rather than removing one impeller. For a duty that a single impeller can meet, those costs buy nothing.
| Dimension | Single stage | Multi stage |
|---|---|---|
| Impellers | One | Two or more in series |
| Where the head comes from | A single impeller | Added stage by stage |
| When it becomes the economical choice | Below about 150 psid differential | Above about 150 psid differential |
| Axial length | Short | Long |
| Parts between the impeller and the seal | Few | Interstage seals and clearances at every stage |
| Disassembly for a seal change | Remove the impeller | Work through the stack |
End Suction vs Inline: It Is the Suction Geometry
The 90-degree turn
An end suction centrifugal pump takes liquid in along the axis of the shaft at one end of the casing, and discharges it at right angles to that axis. The geometry is old enough to be standardised: ISO 2858 covers end-suction centrifugal pumps rated to 16 bar, specifies their nominal duty point and principal dimensions, and names the pump with three numbers — inlet diameter, outlet diameter and nominal impeller diameter, so that 80-50-250 means an 80 mm inlet, a 50 mm outlet and a 250 mm impeller. Flange sizes in the standard run from 50 mm to 200 mm.
An inline pump takes liquid in and discharges it along the same axis, so it can be dropped into a straight run of pipe without elbows. That is the whole difference between an end suction centrifugal pump and an inline one: the first turns the flow 90 degrees, the second does not.
Why inline is not automatically vertical
Because the two are separate properties, and treating them as one is the most common error in this subject. API 610 makes the separation explicit. Its overhung family contains a horizontal pump with a foot support (OH1) and a vertical inline pump with a bearing bracket (OH3); both are single stage, both have an overhung impeller, and they differ in orientation and suction geometry, not in hydraulic class.
So the correct statements are narrow ones. An end-suction pump is normally horizontal. An inline pump is normally vertical. Neither is a definition, and a horizontal inline pump or a vertical end-suction pump is not a contradiction.
| Dimension | End suction | Inline |
|---|---|---|
| Inlet direction | Along the shaft axis, at one end | Along the same axis as the outlet |
| Outlet direction | 90 degrees to the inlet | Same axis as the inlet |
| Piping needed | An elbow and a suction line | Fits into a straight run |
| Floor area | Larger | Smaller |
| Typical orientation | Horizontal | Vertical |
| What API 610 calls it | OH1 or OH2 (horizontal overhung) | OH3, OH4 or OH5 (vertical overhung) |
Horizontal vs Vertical: It Is the Shaft Orientation
Footprint and headroom
A horizontal centrifugal pump lays the shaft parallel to the floor, so the machine spreads out and needs less height above it. A vertical pump stands the shaft up, so it needs less floor area and more headroom. Which one is available is usually decided by the building rather than by the hydraulics, and it is worth settling before the pump is chosen rather than after.
Vertically suspended pumps take the idea further: the impeller hangs below the support and is immersed in the liquid it is pumping, which is how a sump or a wet pit is served without a suction line. API 610 gives this family its own type codes because the arrangement, not the impeller, defines how it is installed and maintained.
What orientation changes at maintenance time
A horizontal centrifugal pump is serviced at floor level with normal lifting gear. A vertical pump is often serviced from above, and the motor, coupling and sometimes the whole rotating assembly have to be lifted out to reach the wet end. That changes the maintenance plan, the spares held, and how long an outage takes — which is the subject of the next two sections.
| Dimension | Horizontal | Vertical |
|---|---|---|
| Shaft orientation | Parallel to the floor | Vertical |
| Floor area / headroom | Larger footprint, lower headroom | Smaller footprint, higher headroom |
| Access for service | At floor level | From above, usually with lifting gear |
| Suction | Normally a suction line from a tank or sump | Often immersed in the liquid |
| Typical setting | Pump room, skid, plant floor | Sump, pit, booster set, confined space |
Which of These Types Can Be Built in Polypropylene
What a solid thermoplastic wet end changes
A polypropylene pump changes the economics of the casing. A solid PP volute and impeller can be moulded rather than cast and machined, and the material does not corrode in the fluids listed on a compatibility chart. What it cannot do is carry the same stress as metal, hold a fine tolerance at temperature, or take a mechanical load the way a steel casing does. Those limits are not evenly distributed across the classifications above, which is why some types are common in PP and others are not.
The pattern is easiest to see in the mechanical arrangement. A horizontal single-stage end-suction pump has one moulded volute, one moulded impeller, no interstage seals, and a shaft that passes through a single seal. Every one of those parts has a straightforward thermoplastic version. A multistage pump adds interstage seals and a stack of close clearances that a polymer cannot hold as well as metal; a vertically suspended pump adds a long shaft, immersed bearings and a column, none of which plays to a polymer’s strengths.
That same parts list, split by whether the fluid touches each item and which ones are consumables, is set out in centrifugal pump parts.
The types we build, and the types we do not
Two separate statements belong here, and they should not be merged. The first is what is common in the market. Polypropylene is a mainstream material for horizontal single-stage end-suction pumps, and it is rare in multistage and vertically suspended designs. That is an observation about what is available, not a claim that the others are impossible — sealless magnetic-drive and self-priming pumps in PP do exist.
The second statement is our own catalogue. We build the horizontal single-stage end-suction pump, in solid PP, from 0.75 to 7.5 kW, rated for liquid up to 60 °C. We do not build multistage, vertically suspended, magnetic-drive or self-priming pumps. If the duty needs one of those, the honest answer is that we are the wrong supplier, not that the pump does not exist.
For the type we do build, the PP centrifugal pump range is the product side, and the selection method that goes with it is in the polypropylene pump selection guide. Where the alternative being weighed is metal rather than another polymer, the trade-off is set out in the polypropylene pump vs stainless steel comparison, and the scrubber-loop case is worked through in the PP pump for wet scrubber article.
| Type | How common in polypropylene | In our catalogue |
|---|---|---|
| Horizontal single-stage end suction | Mainstream | Yes, 0.75–7.5 kW |
| Vertical inline, single stage | Uncommon | No |
| Multistage | Rare | No |
| Vertically suspended / sump | Rare, usually metal or lined | No |
| Magnetic drive | Available from specialist makers | No |
| Self-priming | Available | No |
What the Type Costs You After Installation
Frame-mounted or close-coupled
Within one type code there is still a structural choice. A frame-mounted pump has its own bearing bracket and is joined to the motor by a flexible coupling on a common baseplate, which is the arrangement API 610 calls OH1. A close-coupled pump mounts the impeller directly on the motor shaft and has no coupling at all, which is the arrangement API 610 calls OH5.
The trade is predictable. Frame-mounted costs floor space and an alignment step, and repays it by letting the motor be serviced or replaced without opening the pump, and by carrying higher power. Close-coupled removes the alignment and the footprint, and hands back the problem that a motor failure usually means dismantling the whole unit.
The maintenance ladder
Cost after installation tracks structural complexity more reliably than purchase price does. One impeller, one seal and one volute is the shortest maintenance path; every added stage, every immersed bearing and every long shaft adds an item that has to be checked, cooled, flushed or lifted out. That is why the type decision and the maintenance plan should be made together, and why a cheaper pump of a more complex type is rarely cheaper over its life. The same test applies to a polypropylene pump: the type has to suit the duty before the material can.
For chemical transfer within our range, the polypropylene chemical pump page lists what is solid PP and what is not, which is the list to check against the maintenance plan.
A Ten-Minute Selection Sequence
Ask these in order
Five questions, asked in this order, resolve most type decisions before any datasheet is opened. The order matters because each answer narrows the field that the next question is asked in.
First, how much head does the duty need? That sets the stage count, and the 150 psid figure is the point where a single impeller stops being the economical answer. Second, how does the piping arrive and leave? That sets the suction geometry. Third, how much floor area and how much headroom does the site give? That sets the orientation. Fourth, what is the fluid, at what concentration, at what temperature? That sets the material. Fifth, how often can the plant afford to stop the pump? That sets the mounting arrangement and the spares.
Reading the answers
The sequence is designed so that a late answer can overturn an early one, and it should be allowed to. A duty that needs a multistage pump for head may be ruled out by a fluid that only polypropylene handles, and the honest response is to change the process or the pump type rather than to force a material into a shape it does not suit. The same applies at the other end: a horizontal end-suction pump chosen for its maintenance access is the wrong answer if the only space available is a pit. A polypropylene pump that satisfies all five questions is a good answer; one that satisfies four is a compromise waiting to fail.
For water-side duties at the low end of the range, the PP water pump range covers the same construction without the chemical-service specification, and the industrial water pump range shows where it sits in the wider package.
| Order | Question | What it decides |
|---|---|---|
| 1 | How much head does the duty need? | Stage count |
| 2 | How does the piping arrive and leave? | Suction geometry: end suction or inline |
| 3 | How much floor area and headroom is available? | Orientation: horizontal or vertical |
| 4 | What is the fluid, at what concentration and temperature? | Wet-end material |
| 5 | How often can the plant afford to stop the pump? | Mounting arrangement and spares holding |
Frequently Asked Questions
How many types of centrifugal pump are there?
The number depends on the classification. By flow direction there are three: radial, mixed and axial. By stage count there are two: single and multistage. By mechanical arrangement the count is open-ended, because API 610 alone defines six overhung, five between-bearing and seven vertically suspended type codes. Any single number answers only one of those questions.
What is the most common type of centrifugal pump?
The single-stage pump. The US DOE handbook states that most centrifugal pumps are single-stage, containing only one impeller, and the horizontal end-suction version of that design is the one most often installed in general industrial service.
Is a single stage pump the same as an end suction pump?
No. Stage count and suction geometry are separate classifications. API 610 lists a horizontal end-suction single-stage pump as OH1 and a vertical inline single-stage pump as OH3: both are single stage, and they are different machines. The two descriptions often travel together because the horizontal end-suction single-stage pump is the most common combination, not because they mean the same thing.
What is the difference between horizontal and vertical centrifugal pumps?
The shaft orientation. A horizontal pump spreads out across the floor and is serviced at floor level. A vertical pump stands up, needs less floor area and more headroom, and is usually serviced from above. Orientation is independent of stage count and of suction geometry, so a vertical pump can still be single stage and inline.
Can centrifugal pumps be made of polypropylene?
Yes, and the type matters. Polypropylene is a mainstream wet-end material for horizontal single-stage end-suction pumps, where a single moulded volute, a moulded impeller and a single shaft seal all have workable thermoplastic versions. It is rare in multistage and vertically suspended designs, where interstage clearances and immersed bearings favour metal. Magnetic-drive and self-priming pumps in PP do exist, from specialist makers.
Which type of centrifugal pump is best for chemical service?
There is no single answer, but the order of decision is fixed. Chemistry decides the material first, because a fluid outside the material’s limits removes it regardless of how well the hydraulics fit. Then head decides the stage count, the piping decides the suction geometry, and the site decides the orientation. In polypropylene the combination that survives all four is usually the horizontal single-stage end-suction pump.
Sources
- U.S. Department of Energy. DOE Fundamentals Handbook: Mechanical Science, DOE-HDBK-1018/1-93, “Pumps”. The three flow classifications (radial, mixed, axial), the definition of a stage, the statement that most centrifugal pumps are single-stage and not self-priming, and the 150 psid threshold above which multiple stages become the economical choice. engineeringlibrary.org
- American Petroleum Institute. API 610 / ISO 13709, Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries. Type codes OH1–OH6, BB1–BB5 and VS1–VS7; the separation of overhung pumps by orientation and suction geometry, and the close-coupled arrangement. Summarised at wermac.org
- International Organization for Standardization. ISO 2858:1975 / EN ISO 2858:2010, End-suction centrifugal pumps (rating 16 bar) — Designation, nominal duty point and dimensions. The three-number designation, the 16 bar rating, and the 50 mm to 200 mm flange range. standards.iteh.ai
