Calpeda SD Submersible Borehole Pump

The SD is Calpeda's multistage submersible borehole pump, and it is a very different machine from the sewage and drainage submersibles it sits beside in the catalogue. It hangs in the water on the end of the rising main, screwed on an ISO 228 connection, with the motor underneath the pump end and the water it delivers passing the motor to cool it. Calpeda lists it for water supply systems, for civil and industrial applications, for fire fighting and for irrigation.

One naming note before you order anything. Calpeda's 50 Hz data sheet prints the range as SDP, and the pumps are designated 4SDP for a 4-inch well and 6SDP for a 6-inch, with M added for single phase. Quote the whole designation, including the stage identification and the number of impellers.

What the range actually covers

The 4-inch frames, all three-phase figures:

The 6-inch frames:

The 52 family carries a designation break worth catching before you order. Frames to 18.5 kW print as 6SDP 52; from 22 kW upwards Calpeda prints them as 6SDP B52, so the two 37 kW frames at the top of the range are the B52/25A and the B52/28A. The B is part of the model, not a footnote.

Read down the family column first and across second. The family number sets the flow band, and within a family the stage count sets the head. A 4SDP 4/5C has five stages and gives 33 m at zero flow; a 4SDP 4/48C has 48 and gives 309 m. That is about 6.5 m a stage, and it holds right across the family.

Size against the borehole, not against the catalogue

The number that decides a borehole pump is not the one at the top of a curve. It is the pumping water level once the hole has drawn down, and the driller has it from the test pumping. Take the level under drawdown, add the lift from there to the discharge point, add the friction in the rising main, and select against that total dynamic head at the flow the hole will actually give.

Work an example. Say the duty is 6 m³/h at 130 m of total dynamic head. Off Calpeda's 50 Hz tables, the 4 kW 4SDP 6/34C gives 132 m at 6 m³/h, so that is the pump. Going a frame down to the 3 kW 4SDP 6/27C gives 107 m and leaves you short. Worth noting on the same duty: the 6-inch 6SDP 20/30 also runs 4 kW and gives 134 m at 6 m³/h, which is 2 m more for a hole half as wide again. If the bore is 4-inch, the 4SDP is not a compromise on that duty. It is the same pump in a thinner casing.

That is the general shape of it. Choose the bore size on what has been drilled, not on the assumption that bigger is stronger.

The failure that puts most borehole pumps back on the lorry

Almost every borehole pump that dies young takes more water than the hole gives. The mechanism is worth spelling out, because it is not obvious from the curve.

The SD is cooled by the water going past its motor on the way into the strainer. Calpeda gives that as a minimum flow velocity: 0.1 m/s over a 4-inch motor and 0.5 m/s over a 6-inch. Draw the level down towards the inlet and the flow falls, the velocity falls with it, and the motor is running in still water long before it is running in air. Calpeda's own instruction is short on this. Never run the pump dry, not even for a short trial run.

Two things follow, and both belong in the specification rather than in the commissioning notes. Calpeda asks that the delivery connection sits at least 1 m below the deepest dynamic level while the pump is running, with an automatic control system to stop the pump if the water falls below that. It also asks for electrodes fitted to protect against dry running. Treat both as part of the pump, not as extras.

There is a third case people miss. If the well or the tank is a good deal wider than the pump, the water can rise past the motor without ever flowing over it fast enough. Calpeda's answer is a cooling flow shroud, a flow inducer sleeve fitted around the unit to force the water past the motor at velocity. Any hole noticeably wider than the pump wants one.

Water quality decides how long it lasts

Sand is the slow killer, and Calpeda puts a number on it: 150 g/m³ for the 4SDP and 300 g/m³ for the 6SDP. Above that you are grinding the stages, and the pump loses head for a long time before it stops.

That number is a question for the driller, not for us. A borehole that was never properly developed will keep producing fines, and no pump in this range is built to survive it. Calpeda asks you to check at first start that the sand in the water has disappeared or is minimal. If it has not, stop and sort the hole out. Setting the pump clear of the bottom helps as well, far enough up that sand and mud do not build around the motor.

Where Calpeda's headline and its tables disagree

Calpeda's product page gives the head range to 650 m. The 50 Hz tables go further: the 11 kW 6SDP 20/76 is published at 691 m at zero flow, and four other families pass 640 m. Select off the table for the family you are buying, not off the range.

Where the SD stops

Liquid temperature stops at 35°C on a 4-inch motor. On a 6-inch it moves with the frame: Calpeda's motor table allows 40°C up to 15 kW, 35°C from 18.5 to 30 kW and 30°C at 37 kW, so the 30°C printed in the general operating conditions is the worst case rather than the rule. Starts are limited to 30 an hour on the 4-inch and 20 on the 6-inch, which on a borehole feeding a small pressure vessel is a real constraint. The pump is rated for continuous duty, and Calpeda asks you to avoid long periods running against a closed discharge.

Above 7.5 kW you cannot start it direct on line. Calpeda requires star/delta, a soft starter, stator impedance or an autotransformer, to hold down both the starting current and the torque. That is switchgear and panel space, and it belongs in the budget from the start.

Calpeda states that installing the pump in an environment with a potentially explosive atmosphere is forbidden.

Water enters the SD through a stainless filter above the motor, passes up through a stack of impellers and diffusers, and leaves through a delivery casing with a built-in non-return valve on a screwed ISO 228 connection. Each stage takes the water from the one below and adds its share of head. The motor is a 2-pole induction motor running at about 2900 rpm at 50 Hz, sized to NEMA standards for connection to the pump, protected to IP 68 and rated for continuous duty.

Two hydraulic builds, one principle

The 4SDP uses radial floating impellers and the 6SDP uses radial impellers. The materials follow the same split. On the 4SDP the diffusers are polycarbonate, the impellers are NORYL, the shaft is AISI 304 and the bearing bushes are polyacetal. On the 6SDP the diffusers are NORYL, the impellers are LEXAN, the shaft is the harder AISI 431 and the bearing bushes are NBR or HNBR. Everything structural on both, the external jacket, stage casings, delivery casing, suction lantern, filter and screws, is AISI 304.

Calpeda notes that the 4SDP 1, 2, 3 and 6 electropumps comply with European Regulation 547/2012.

Setting depth and the rising main

The rising main is not just pipe. Calpeda asks for a pressure gauge, a check valve no more than 7 m from the pump outlet, and further lift-type check valves at least every 50 m of straight vertical pipe above the pump to hold off water hammer. Where the total length of delivery piping passes 100 m, add one or more non-return valves. Fit a gate valve at the surface to regulate delivery, head and absorbed power.

Depth of immersion should always exceed 1 m, and the gap between the motor and the well filter should exceed 1 m. The well diameter has to be wide enough along its whole length for the pump to pass with clearance all round, which is the sort of thing worth confirming against the driller's log before the pump is on site.

Threaded delivery pipes must be tightened properly. Calpeda's warning is blunt: a joint that unscrews drops the pump down the well. It advises spot welding metal pipes to the threaded joints, and proper connections on plastic pipe. Clamp the power cable to the delivery pipe about every 3 m, lower the pump on a safety rope or chain of non-perishable material, and never hang it on the cable.

Where the pump has to run horizontally, Calpeda asks for the axis at least 0.5 m above the bottom of the sump or tank, a supplementary check valve because the built-in valve does not seal properly on its side, and an arrangement that lets the air out at start-up.

The electrical side

Three-phase 4-inch motors run on 230 V or 400 V and 6-inch motors on 400 V or 400/690 V, both ±10%. Single-phase 4SDPM builds run on 230 V up to 2.2 kW, with a capacitor of 20 to 80 µF at 450 V depending on the frame and a full load current from 3.6 A to 14.6 A. Insulation class is F on the 4-inch motors.

The control panel is yours to build, and Calpeda is specific about what goes in it: a disconnecting switch with at least 3 mm of contact separation on all poles, a motor protector with a curve D characteristic set to the nameplate current, the capacitor on a single-phase build, and electrodes for dry-run protection. At 7.5 kW and above, add the reduced-voltage starting.

Pumps normally arrive with the motor and the pump end separate, the exception being the 4-inch units, which come assembled. Torque the suction lantern nuts crosswise to 10 Nm on a 4-inch motor, starting from the one opposite the cable.

Start the pump with the gate valve nearly shut and wait until the delivery pipe is clear of air, then open up. On three-phase, check the direction of rotation by comparing pressure or flow before and after swapping two phases. The right way round is obvious once you look at the gauge. Check that the absorbed current stays inside the rating, and throttle the gate valve if it does not.

On a generator supply, switch the generator on before the motor and off after it, both times without load. Getting that sequence wrong can damage the generator as well as the motor.

Reading the designation

Calpeda's key runs the well diameter in inches, then SDP for the series, then M for single phase, then the stage identification, then the number of impellers, and finally the motor flange coupling. So a 4SDPM 6/14 is a single-phase 4-inch pump on the 6 hydraulic with 14 impellers, and a 6SDP 52/3A 4SM is a three-phase 6-inch pump on the 52 hydraulic with three impellers on a 4-inch motor flange. When you are replacing an existing set, read the whole string and the serial number off the nameplate.