Calpeda BNM(S) Bronze Close Coupled Centrifugal Pump

The Calpeda BNM is an end suction water pump with a bronze wet end. The hydraulics, the dimensions and the curves are the NM, NMS range, which Calpeda publishes at 1–660 m³/h and 3–95 m on motors from 0.25 to 90 kW. What the B in front of the designation changes is the metal: bronze CC480K EN 1982 in the pump casing, in the lantern bracket on the close coupled build and in the casing cover on the stub-shaft build, plus a bronze impeller and an AISI 316 shaft.

Bronze is what you specify when the water would eat a cast iron casing. That is the whole reason this version exists, and the limits that come with it are worth stating plainly. Calpeda rates the BNM for clean liquids, non-explosive and non-flammable, without abrasives and non-aggressive to the pump materials, with a solids content up to 0.2%. The material change moves where "non-aggressive to the pump materials" falls. It does not turn the pump into a chemical pump, and it does nothing about grit. Send us the water analysis and the temperature, not just the word for the water, and we will tell you whether bronze is the right answer or whether the duty wants stainless.

The trade-off is pressure. Maximum permissible working pressure on the bronze version is 10 bar. Calpeda's 16 bar rating belongs to the ductile iron casing, so if the system needs 16 bar, bronze is not the build for it. That catches people out on tall buildings and on boosted mains, where the closed-valve head plus the inlet pressure is what the casing has to hold, not the duty point.

The other thing worth checking before you order is whether you need bronze at all. Specified as insurance, it is money spent on a problem the water does not have, and the cast iron NM on the same curve costs less and holds more pressure. Specified because the water genuinely attacks iron, it pays for itself the first time you do not have to lift a corroded casing out. We would rather ask the question than sell you the dearer casing.

Size it against the real duty. On an EN 733 pump the common failure is not the pump at all, it is a duty point that was arrived at by margin stacking: the customer states a flow, the consultant adds margin, the contractor adds margin, and the installer adds a little more. Margin stacking is how a 3 kW duty ends up with an 11 kW motor on it, and how a centrifugal pump ends up running off the end of its performance curve with no back pressure to work against. That is where cavitation, failed seals and vibration come from.

The BNM is a single-stage end suction centrifugal pump. Liquid enters axially at the suction flange, the impeller throws it outwards, and it leaves radially through the delivery flange on top of the casing. A single mechanical seal in carbon, ceramic and NBR keeps the liquid off the shaft. Two-pole units run at about 2,900 rpm at 50 Hz and four-pole units at about 1,450 rpm.

Reading the designation

Calpeda's own worked example is BNM(S)4 EI 32/16A/B. B is the bronze version, and no B means the pump is cast iron. NM is the series, S is the stub-shaft version, and 4 marks the 4-pole build, so no 4 means 2-pole. EI means the pump carries the I-MAT frequency converter. Then 32 is the delivery port diameter in millimetres, 16 is the nominal impeller diameter, A is the impeller size and /B refers to a revision. Calpeda's operating instructions write the same family as B-NM. If you are matching an existing pump, send us the whole string off the nameplate together with the serial number.

Close coupled or stub shaft

BNM and BNM4 are close coupled: the motor has an extended shaft that drives the impeller directly, up to 22 kW on the 2-pole build and 15 kW on the 4-pole. BNMS and BNMS4 use the stub-shaft bracket instead, which takes a standard IEC motor and carries an integrated thrust bearing. The stub-shaft build is also the one that goes to IP 55 as standard, against IP 54 on the close coupled version. On the bronze stub-shaft pumps the casing cover is bronze and the lantern bracket stays cast iron GJL 200 EN 1561, which is what you can see in the photographs.

Where the bronze version is offered

Calpeda marks the bronze build model by model in the data sheet rather than offering it across every size. BNM runs to a DN80 delivery port, BNM4 and BNMS to DN100, and BNMS4 to DN150. If a cast iron model you have been quoted has no bronze equivalent, there is usually a bronze pump on a neighbouring curve that will cover the same duty point, and that is a question worth asking before you settle the specification.

Standards and limits

Main dimensions and performance follow EN 733, with additional sizes for completion, and flanges are PN 10-16 to EN 1092-2 with PN 10 on DN 200. Motors are constructed in accordance with EN 60034-1 and EN 60034-30-1, and the electropumps comply with European Regulation 547/2012. Published performance carries tolerances to UNI EN ISO 9906:2012.

Two published limits are easy to miss and both bite on a boosting duty. Sound pressure and permitted starts per hour both step with motor rating. On the 2-pole pumps that runs 70 dB(A) and 60 starts per hour to 2.2 kW, 80 dB(A) and 40 starts to 7.5 kW, 85 dB(A) and 20 starts to 30 kW, and 90 dB(A) and 10 starts to 75 kW. The 4-pole pumps hit the same steps at 7.5, 30, 75 and 90 kW. A pump sized well above the draw-off will run into the starts limit long before it runs into anything else.

The pump is not self-priming. Fill it completely with liquid before starting and never run it dry, and where the pump sits above the water level, fill the suction pipe and the pump through the priming hole. On a drinking water duty Calpeda requires a thorough flush with clean water before start-up to clear preservatives, test liquid and grease. A small leak from the mechanical seal during the first run is normal, because the seal faces are lubricated by the liquid being pumped.