Types of Centrifugal Pumps: Technical Classification and Selection Criteria

When to use a single-stage or multi-stage pump?

The single-stage pump has one impeller and generates the full head in a single step; the multi-stage pump has two or more impellers in series, whose heads add up. FBCN is single-stage, with 53 models and catalog head up to 135 m. The decision must start from the flow–head point and the NPSH, not from an isolated universal limit.

The single-stage pump is constructively simpler, cheaper and easier to maintain — the full head comes from a single conversion of kinetic energy into pressure. When the required head exceeds what one impeller can generate at the available rotation without compromising mechanical stress and NPSH, the multi-stage arrangement becomes the natural path.

Multi-stage pumps have two or more impellers in series within the same casing. The head from each stage adds up, allowing very high pressures (up to hundreds of meters) with smaller and mechanically less stressed impellers. Typical multi-stage applications: boiler feed, oilfield water injection and high-pressure systems. The FB Bombas FBCN line is single-stage (53 models covering up to 135 m head) — suitable for the vast majority of industrial B2B applications.

What is the difference between horizontal and vertical centrifugal pumps?

The horizontal pump keeps the shaft parallel to the floor and eases access to the motor, bearings and rotating assembly. The vertical pump reduces footprint and can work with naturally flooded suction in wells or tanks, but makes access to the lower components harder. FBCN and FBOT are horizontal.

Horizontal centrifugal pumps have the shaft parallel to the ground. They are the most common configuration in industrial process because they facilitate motor alignment, bearing maintenance and casing drainage. The FB Bombas FBCN and FBOT Series are horizontal. FBCN has back pull-out construction — a feature that allows removal of the entire rotating assembly (impeller, shaft, bearings and bearing housing) from the rear without disconnecting process piping, significantly reducing maintenance time.

Vertical pumps have the shaft perpendicular to the ground. Their advantages are: (1) smaller footprint; (2) naturally flooded suction when installed in wells or tanks below the pump level; (3) natural casing drainage by gravity. Their disadvantages are more critical alignment, difficult maintenance of lower bearings and higher cost. They are common in raw water systems, industrial drainage and turbine pumps for deep wells.

What changes between end-suction, split-case and in-line pumps?

In the end-suction pump, suction enters axially and discharge leaves radially; in the split-case, the split casing allows access to the impeller without removing the nozzles; in the in-line, suction and discharge are aligned in the piping. FBCN is described in the catalog as end-suction and back pull-out. The official scope of ASME B73.1 covers exactly this family: horizontal, end-suction, single-stage pumps with centerline discharge.

In end-suction pumps (axial suction), the suction nozzle is aligned with the impeller axis (axial inlet) and the discharge nozzle exits radially from the volute. It is the most common configuration in industrial process — the entire FB Bombas FBCN Series is end-suction per ASME B73.1. FBOT is also end-suction, in close-coupled construction where the motor is flanged directly to the pump without intermediate coupling, eliminating shaft alignment and reducing footprint.

In split-case pumps, the casing is split into two halves by a plane containing the shaft. The impeller sits between the two halves. The advantage is access to the impeller without removing the nozzles — faster maintenance in large pumps. Common in high-flow pumps (cooling water, intake) and horizontal fire pumps. In in-line pumps, suction and discharge are aligned in series in the same pipe run — which simplifies installation and eliminates elbows. Typically used in booster and circulation pumps.

Which impeller to choose: closed, semi-open or open?

The closed impeller, with two shrouds, prioritizes efficiency for clean liquids. The semi-open accepts moderate solids with higher clogging tolerance; the open favors passage of fibers or larger solids, with lower efficiency. The standard FBCN uses a closed impeller.

The closed impeller has blades enclosed by two shrouds (front and back) — a geometry that minimizes internal recirculation, hence the most efficient. Suitable for clean liquids or those with fine particles. It is the standard geometry of the FB Bombas FBCN Series.

The semi-open impeller has only the back shroud, with blades exposed on the suction side. It is indicated for liquids with moderate suspended solids or pulps — the absence of the front shroud reduces clogging risk between blades and casing. Hydraulic efficiency is lower than closed impeller. The open impeller has only blades attached to the hub, without shrouds.

Used for fluids with large solids, fibers or applications where clogging is the main concern — at the cost of significantly lower hydraulic efficiency.

How are the FBCN and FBOT series classified?

FBCN is radial, single-stage, horizontal, end-suction, back pull-out and closed-impeller; the catalog registers 53 models, DN 25 to DN 300, up to 2,200 m³/h and 135 m. FBOT is radial, single-stage, horizontal and end-suction close-coupled, intended for thermal oil up to 350 °C. These are catalog limits, not a model selection.

Applying the five criteria above to the FB Bombas centrifugal lines: the FBCN Series is radial (radial flow), single-stage, horizontal, end-suction with back pull-out construction and closed impeller — fully compliant with ASME B73.1. There are 53 models divided into 43 standard (DN25 to DN150) and 10 large-capacity (DN200 to DN300), covering flow up to 2,200 m³/h and head up to 135 m.

The FBOT Series is radial, single-stage, horizontal, end-suction close-coupled with cooled sealing chamber — designed specifically for thermal oil up to 350°C. Both series undergo hydraulic acceptance testing per ANSI/HI 14.6 and ISO 21940 G2.5 dynamic balancing.