Calpeda GM 50 Cast Iron Submersible Sewage and Drainage Pump

The GM 50 is the cast iron member of Calpeda's submersible family, and the designation key says exactly that: GM is the cast iron pump, GX the stainless one, GQ the cast iron and stainless combination. Calpeda puts it on civil and industrial wastewater that is not aggressive to the pump materials, and on dirty water carrying solids to 45 mm on the GMC or 50 mm on the GMV. It is a 2-pole machine on a G 2 threaded or DN65 flanged delivery, and at 26.5 kg to 31 kg with its cable it is heavy enough that Calpeda's own handling rule, two people to a package over 25 kg, applies to every build in the range.

Size it against the 50 Hz tables rather than a range summary. Three impeller diameters cover each wet end, and the letter at the end of the designation is the impeller, not the motor frame.

Note where the 48 m³/h headline lands. It is the last tabulated point on the 1.5 kW GMC 50/AE, at 1 m of head, and nobody specifies a pump there.

Channel or vortex, and why the GM 50 is the odd one out

Calpeda's other submersible ranges pass the same sphere on either wet end. The stainless GXC and GXV both take 35 mm; the GQN and the GQV both take 50 mm. Free passage is not what separates those pairs, so the vortex is a straight trade of duty for rag tolerance. The GM 50 does not work like that. Calpeda rates the single-channel GMC at 45 mm and the free-flow GMV at 50 mm, so on this range the vortex build wins twice, on solids as well as on fouling, and the channel build has to earn its place on head alone.

It does earn it, and the size of the gap is in Calpeda's own tables rather than up for argument. The same three motor ratings carry both wet ends.

Take a duty of 30 m³/h at 5 m. The 1.1 kW GMC 50/BE lands on it exactly. Nothing in the vortex range reaches it: the largest GMV 50/AE gives 4.3 m at that flow, on twice the rated output the channel build needed. Going vortex on the GM 50 does not cost you a frame size. Past about 30 m³/h it costs you the duty.

So the question is what the wastewater is carrying, and Calpeda answers it directly. Where the liquid has a high solid content or carries filamentous particles, the instructions send you to the free-flow construction, and the GMV is named on that list. The GMC is not. Read that as the wipes test. If the chamber takes foul drainage off occupied buildings, specify the GMV, because a single passage that binds costs you a lifted pump and a call-out every time it happens. If it takes screened effluent, surface water or plant room drainage, the GMC is the better pump on the curve and the cheaper one to run.

Neither build is a macerator. If a good solids-handling pump is already blocking on rag, another impeller will not fix it and we will say so before quoting.

Threaded or flanged is an installation decision

The port is where the GM 50 stops being a portable pump. The GMC 50 and GMV 50 carry a G 2 ISO 228 threaded delivery, which suits a hose or a screwed riser and a pump you lift out on a chain. The GMC 50-65 and GMV 50-65 carry a DN65 flange to EN 1092-2 PN 10, and those two are the builds Calpeda lists for fixed installation on automatic coupling feet and guide rails.

That matters more than the extra 15 mm of bore. On a guide rail set the coupling foot bolts to the floor of the sump with the delivery pipe, two guide tubes run up to a bracket at the cover, and the pump seals on its own weight as it lands. Inspection becomes a lift on a chain instead of an entry into the chamber, which on a foul sump is the difference between a routine job and a confined-space procedure. Order the threaded build for that duty and you have bought the wrong pump, whatever the curve says.

Whichever you take, keep the internal diameter of the delivery pipe at or above the pump connection, fit a check valve against back flow, and arrange a gate valve and union so the pump can come out without draining the system. Where slime collects in the bottom of the pit, stand the pump clear on a support.

Where the GM 50 stops

Liquid temperature stops at 35°C, pH at 6 to 11, and liquid density at 1100 kg/m³. Maximum immersion is 10 m with a suitable cable length, and the continuous duty rating holds only while the motor is submerged. Maximum permissible working pressure in the pump body is 25 m, which is 2.5 bar, and the tallest curve in the range never comes near it.

Calpeda allows 30 starts an hour at regular intervals. That limit is a chamber calculation rather than a pump one, and on a tight sump it decides the frame before the duty point gets a say. Exceed it and the motor is what pays for the undersized wet well.

Calpeda's tables hold for a medium at 1.0 kg/dm³ and 20 cSt or below, which is to say a medium behaving as water. Sludge that thickens beyond that is a different hydraulic problem, and the figures above stop describing it.

The GM 50 draws through the base of the pump, works the medium with a cast iron single-channel or free-flow impeller, and discharges through a G 2 threaded port or a DN65 flange. The motor is a 2-pole submersible induction motor running at about 2900 rpm at 50 Hz, IP X8 for continuous immersion, insulation class F, with a triple impregnation humidity-proof dry winding. Calpeda builds it to EN 60034-1, EN 60335-1 and EN 60335-2-41. Sound pressure is under 70 dB(A) at minimum immersion depth and falls away as the pump submerges.

The two impellers

A single-channel impeller carries the whole flow down one passage cast through the vane. The medium is worked by that vane, which is why the pump holds its head as flow rises, and the passage itself is the limit on what will fit: 45 mm on the GMC.

A free-flow impeller sits recessed up in the casing and drives the medium by the vortex it sets up in the open chamber ahead of it. Most of what you pump never reaches a vane, so there is nothing in the path for a length of fibre to hang on and gather more behind it. That open chamber is also why the GMV takes the larger 50 mm solid. You pay for both in efficiency, for every hour the pump runs.

The seal stack and the oil chamber

The shaft runs through two mechanical seals with an oil chamber between them, which Calpeda describes as protection against dry running. The two faces are not the same specification. The upper seal is ceramic against carbon with NBR elastomers. The lower one, the seal that meets the sewage, runs silicon carbide against silicon carbide, which is the harder pairing and the one that survives grit in the medium.

Dry running is still what finishes the arrangement fastest. Take the liquid away and the faces run against each other with nothing between them. On a submersible that usually traces back to a float that has stopped swinging or a chamber pulled down further than the design allowed, not to the pump.

The chamber holds 0.5 litres. Calpeda specifies white oil suitable for food machinery and pharmaceutical use, and adds that on the GMC and GMV a normal SAE 10W-30 engine oil will also do. Do not fill the chamber completely; enough air has to stay inside to take up the expansion when the oil warms. There can be slight pressure behind the plug, so open it with that in mind.

The electrical side, which differs from the rest of the family

Three-phase GMC and GMV run on 230 V or 400 V ±10%, drawing 1.9 A, 2.7 A and 3.8 A at 400 V across the 0.75, 1.1 and 1.5 kW builds. The cable is 10 m of H07RN-F 4G1.5 mm² plus 2x0.5 mm², with no plug. Those two extra cores are the reason the cable is not a plain four-core: the motor carries two thermal protectors in series, inserted between two different phases, and they are brought out for connection to your control box.

Read Calpeda's qualification on them carefully, because it is the part people miss. The protectors guard against overloading, not against operation with a blocked rotor. The control box therefore needs an overload-protective device with a curve D characteristic set to the nameplate current, and a hot-wire ammeter relay coupled with the control contactor on top of that. A rag-bound impeller is a stalled rotor, which is precisely the case the thermal protectors do not cover.

Single-phase GMCM and GMVM run on 230 V ±10% with a float switch, a thermal protector and an incorporated capacitor already fitted, on 10 m of H07RN-F 3G1.5 mm² with a CEI-UNEL 47166 plug. Full load current is 4.5 A and 6.5 A, with 16 µF and 25 µF capacitors rated at 450 V. Watch the input against the output when sizing the supply: the 1.1 kW GMCM 50/BE draws 1.5 kW at the input. Single-phase stops at 1.1 kW, so the 1.5 kW A builds are three-phase only.

Other voltages, 60 Hz and a motor suitable for operation with a frequency converter are all listed by Calpeda as available on request.

Reading the designation

Calpeda's key runs GM for the series, then the impeller letter, then M for single-phase, then the port, then the impeller diameter. C is the single-channel impeller and V the free-flow vortex. No M means three-phase. The 50 is the delivery port in millimetres, and on the flanged pumps the 65 that follows it is the flanged port diameter. So a GMCM 50-65C is a single-phase single-channel pump on the DN65 flange.

The last letter is the one worth knowing. It is the impeller diameter, and it runs the opposite way to the numbers: the C builds are the 0.75 kW pumps and the A builds the 1.5 kW ones, on a common pump body. If you are matching an existing unit, photograph the nameplate and send us the full designation and the serial number rather than the size.

Checking it over

With three-phase, check the direction of rotation before it goes in. Running backwards costs delivery and can damage the mechanical seal, and the arrow on the pump is what the impeller should follow. To look at the impeller, take out the screws (12.20) and lift off the casing cover (12.00), then turn the impeller by hand to check it is free. The impeller comes off with the nut (28.04), and the GMV impeller has threaded dismantling holes for the purpose.

For the seal, drain the oil chamber through the plug (14.46) and its washer (14.47) with the hole turned downward, then reach the mechanical seal (36.00) by removing the impeller key (28.20), the screws (14.24) and the pump casing (14.00). After pumping muddy liquid, run the pump briefly on clean water before it stands idle.

Where it must not go

The exclusions here are Calpeda's, and they are absolute. Explosive or flammable environments are out, and putting the pump anywhere the atmosphere is potentially explosive is forbidden outright. If your chamber is a classified hazardous area, this is not the pump. Tell us the zone and we will quote a certified unit against it instead.

The pump must also not be used in garden ponds, tanks or swimming pools while people are in the water. For pond and pool duties Calpeda requires a residual current device with I∆N no greater than 30 mA in the supply circuit. The unit is earthed in every case, including on a non-metallic delivery pipe, and the supply needs a disconnecting switch with at least 3 mm of contact separation on all poles.