Inline Pump Motor: Motor Selection for Vertically Mounted Pumps

An inline pump motor is a flanged three-phase induction motor that sits vertically on top of a centrifugal pump whose suction and discharge ports share one axis. The motor shaft points down, turns the pump shaft through a rigid coupling and, in most designs, also carries the axial force created by the impeller in its own bearing. Its power is chosen from the pump's flow and head: an inline pump delivering 140 m³/h of cooling water at 20 m needs about 10 kW of shaft power if pump efficiency is taken as 0.76, and runs on an 11 kW, 4-pole motor.

In an inline pump the motor is not just the drive but part of the pump structure: its bearing, flange and mounting direction determine pump life. Details that could be minor on a horizontal pump (suitability for vertical mounting, axial load, position of drain holes) are therefore key selection items here. Other pump pages are on Pump Motors; suitable motors and current prices on Buy Electric Motors Online.

What is an inline pump and where is it used?

Inline is the name of a casing and installation form; the principle is centrifugal. Suction and discharge flanges have the same diameter and lie on the same axis, so the pump fits into the pipeline like a valve. Motor and pump head sit on top of the casing; one manufacturer calls this a "top-pull-out" design: for service, motor, pump head and impeller are lifted out together while the casing stays in the pipework [1].

There is another product people think of as an inline pump: the small wet-rotor circulator in heating systems. In those the motor is integral with the pump, the rotor turns in the water and it cannot be replaced by a standard motor; these products are covered separately on the circulation pump page. This page covers dry-rotor inline pumps driven by a separate standard IEC motor.

• Central heating and district heating circuits [1].
• Chilled water and water-glycol in air-conditioning, chiller and cooling-tower circuits [2].
• Building water supply and industrial cooling [1].
• Any line in pipe shafts and tight plant rooms where space must be saved.

How it differs from end-suction and split case pumps

In an end-suction pump water enters at the shaft end and leaves upward; the pipeline turns 90 degrees at the pump, and pump and motor stand on a baseplate on the floor. In a split case pump a double-suction impeller handles large flow, the casing splits horizontally and a wide foundation is needed. In an inline pump the pipe continues straight; space needed is small, but the motor is up high, the centre of gravity is high and the whole motor weight rests on the pump casing.

According to one manufacturer's catalogue, inline pumps with motors below 11 kW can be installed in horizontal or vertical pipes, while pumps with motors of 11 kW and above are installed only in horizontal pipes with the motor vertical. The same source states that the motor must never point downwards [1].

How the motor drives the pump: flange, coupling and axial load

The motor bolts by its flange to the pump head or motor stool. A rigid two-part coupling joins motor shaft and pump shaft [1]. No belts or gearboxes are used; motor speed is pump speed.

Axial load in the motor bearing

One manufacturer states that in its inline pumps radial and axial forces are absorbed by the fixed bearing at the motor drive end, so the pump needs no bearing of its own [1]. Even with a hydraulically balanced impeller some axial force remains, and the weight of rotor, coupling and impeller is added. The front bearing of a motor fitted to an inline pump must therefore be chosen to carry this load in the vertical position. For vertical operation the motor must be ordered as suitable for vertical mounting and the axial load must be confirmed with the pump manufacturer.

V1 and V18: vertical flange types

In IEC mounting codes, V1 is a vertical motor with the large through-hole flange (B5) at the bottom and the shaft pointing down; V18 is the same arrangement with the small tapped flange (B14). Inline pump motors mostly run as V1; V18 is also seen at small ratings. The flange dimensions (spigot diameter, pitch circle) must fit the seat on the pump head exactly. The exact type and flange size come from the pump manufacturer's drawing; at DRG, mounting type and vertical use are specified with the order note.

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How many kW is an inline pump motor? Market models

Inline pump families range from models slightly bigger than a boiler circulator to high-pressure series of hundreds of kilowatts. Each pump has a single motor; twin-head inline pumps have two separate motors with one on standby, so installed power is the sum of both but only one runs at a time [1].

In this pump group, motor options of roughly 0.12–630 kW are seen depending on the manufacturer's model and capacity [1][2]. As port size (DN) and impeller diameter grow, flow and head rise and so does the motor step; at the same DN the 2-pole version delivers far more head than the 4-pole one and needs far more power. Weight grows with power too: in one manufacturer's catalogue the smallest 0.25 kW model weighs about 50 kg and the largest 200 kW model about 2,300 kg [2]. Much of this weight is the motor, and on large models the pump feet and foundation carry it.

Overlap with the DRG range

Inline pumps needing 0.25–355 kW motors at 1000/1500/3000 rpm have a counterpart in the DRG range. The 0.12 and 0.18 kW steps fall below the range and high-pressure series above 355 kW fall above it; both are outside the DRG range.

Worked example: 140 m³/h, 20 m

An inline pump is being selected for a shopping centre's chiller circuit to overcome the closed-loop friction losses: 140 m³/h and 20 m, with pump efficiency read from the curve as 0.76.

• Q = 140 / 3600 = 0.0389 m³/s
• P = ρ·g·Q·H / η_pump = 1000 × 9.81 × 0.0389 × 20 / 0.76 ≈ 10.0 kW
• About 10% margin: 11.0 kW → standard rating 11 kW
• 4 poles are chosen: low head and noise inside the building

DRG-160M-4 runs at 1465 rpm and draws 20.4 A at 400 V; its front bearing is 6309-C3 and shaft diameter 42 mm. Being 11 kW, under the rule above the pump is fitted in a horizontal pipe with the motor vertical [1]. If speed is reduced to 80% by inverter at night, power falls to 0.8³ ≈ 0.51 times, about 5.1 kW. For the speed–power relation see our affinity laws article, and for your own values the Pump & Fan Motor Power Calculator.

Motor selection checklist

The hydraulic selection and flange connection of the pump are the job of the pump maker or designer; the motor supplier provides a motor matching the given shaft power, speed, mounting and ambient conditions. Items specific to inline pumps:

• Pump nameplate: DN, impeller diameter, P2 and poles (many makers' model codes contain all four [2]).
• Old motor's mounting: V1 (B5 flange) or V18 (B14 flange); flange outer diameter, pitch circle and spigot diameter.
• Shaft diameter and length, how the coupling fixes to the shaft end (key, clamping).
• Is the pump head made for the pump maker's special motor flange? If so, a standard motor may not fit.
• Pipe direction: horizontal or vertical pipe; motor vertical or horizontal.
• If chilled water is pumped, condensation in the motor: the drain hole must be open and point down [1].
• Lifting clearance above the motor for service; a crane point on large models.

Speed and mounting notes

Heads in building heating and cooling circuits are low, so 4 poles are common; small-DN models needing more head use 2 poles [2]. The motor speed must match the poles on the pump nameplate. One manufacturer recommends considering vibration damping for pumps with motors of 11 kW and above and treating it as mandatory from 90 kW [1]. In chilled-water circuits, ambient air can condense inside the motor when it stops; on a vertical motor the lowest drain hole is kept open. In the DRG range motors of 4 kW and above are wound 400/690 V and suit star-delta starting; they also run on the inverters common on inline pumps.

Matching DRG motors

DRG Motor is a manufacturer, supplier and seller of electric motors. Our range covers 0.25–11 kW aluminium and 0.55–355 kW cast iron frames; 1000, 1500 and 3000 rpm, 3-phase, 230/400 V or 400/690 V windings, B3, B5, B14 and B35 mounting (with order note). For an inline pump a B5 flanged motor is ordered with a V1 vertical-use note, and at small ratings a B14 flanged motor with a V18 note; the axial load is confirmed with the pump maker. Ratings around the worked example:

DRG MOTOR motors are guaranteed against material and manufacturing defects for 2 years from the delivery date (warranty certificate). For general motor topics in heating and cooling plants see our HVAC fan and pump motor article.

DRG motors for inline pumps

• DRG-112M-4 – 4 kW – 1450 rpm – 112M cast iron
• DRG-132M-4 – 7.5 kW – 1460 rpm – 132M cast iron
• DRG-160M1-2 – 11 kW – 2945 rpm – 160M cast iron
• DRG-160M-4 – 11 kW – 1465 rpm – 160M cast iron
• DRG-180L-4 – 22 kW – 1470 rpm – 180L cast iron

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Inline pump motor: frequently asked questions

Can a foot-mounted (B3) motor be fitted to an inline pump?

No. The motor seats on the pump head by its flange and works as part of the pump; a flanged motor (B5 or B14) ordered as suitable for vertical use is needed.

Can an inline pump motor be installed horizontally?

According to one manufacturer's catalogue, pumps with motors below 11 kW may also be installed with a horizontal shaft; at 11 kW and above the motor must be vertical. In no case is the motor installed pointing downwards.

Can the motor of a wet-rotor circulator be replaced?

No. In wet-rotor pumps the motor is integral with the pump; on failure the pump head or the pump is replaced. A standard motor fits only dry-rotor inline pumps.

How many amps does an 11 kW inline pump motor draw?

The 4-pole DRG-160M-4 draws 20.4 A at 400 V and 21.5 A at 380 V; the 2-pole DRG-160M1-2 draws 19.6 A at 400 V.

Send us the pump nameplate, the old motor's flange size and the mounting direction, and we will decide the right DRG model and order note together. Phone and WhatsApp: +90 (542) 666 11 11; e-mail: [email protected]; or our contact page. Store: Buy Electric Motors Online.

Sources

1. Grundfos – TP, TPD, TPE, TPED in-line pumps data booklet – Series 100: 0.12–0.25 kW; Series 200: 0.12–2.2 kW; Series 300: 0.25–132 kW; Series 400 (25 bar): 5.5–630 kW; radial and axial forces taken by the fixed drive-end motor bearing; ≥11 kW only in horizontal pipes with vertical motor; motor never pointing down; drain hole.
2. Wilo – Atmos GIGA-I, full range and technical data – inline pump with IEC standard motor; model code gives DN, impeller diameter, P2 and poles; P2 steps 0.25–200 kW; model weights.