Solids-handling pump

Free passage, or free ball passage, is the diameter of the largest sphere that will travel through the wet end without touching anything. It is quoted in millimetres and it is the first figure to ask for. "Solids handling" on its own is a category, not a specification.

The figure is capped by the discharge bore. A DN80 pump cannot pass a 100 mm solid, whatever the impeller does, so free passage and connection size get chosen together. It also varies widely inside a single range: Grundfos publishes a free passage of 35–125 mm across the SE/SL Range 52 pumps, depending on the size and on which hydraulic variant is fitted. Two pumps with the same badge on the same range are not interchangeable on this.

BS EN 12050-1:2015, the standard for wastewater lifting plants handling faecal matter, treats the two families separately. Clause 4.3.5 sets a minimum free ball passage for the plant. Discharge sizing then splits in two, with clause 4.3.6 for non-macerating plants and clause 4.3.7 for macerating ones, so the standard sizes the pipe differently depending on which family you fit.

Free passage has a blind spot, and it is a wide one. Rag does not arrive as a sphere. A ball-passage test says nothing about a rope of wipes twice the length of the impeller. That is why the impeller geometry matters as much as the number beside it.

Every gain in free passage is paid for somewhere. Usually in efficiency, and always in how comfortable the pump is away from its duty point.

Grundfos puts the vortex trade plainly: the benefit over a channel impeller is the reduced risk of clogging, it is the better choice where the liquid carries a lot of sand, and its efficiency is typically lower.

So commit on the running hours. On a small station that blocks every other week, take the vortex and pay the power. The call-outs cost more than the kilowatts. On a duty running most of the day for the next 20 years, that efficiency gap is real money and a channel or screw centrifugal hydraulic earns its keep. Work out which one you have before you choose, because the answer flips.

Hidrostal's screw centrifugal impeller was invented by the company's founder, Martin Staehle, and developed in Peru. It was built for the fisheries industry, to move fish off the boat for processing without damaging them. Moving large soft objects gently was the original brief, and wastewater turned out to want the same thing.

The single screw-shaped vane gives a large free passage with low shear and low pulsation. Hidrostal fits its compact pumps with a 50 mm free passage on a 50–80 mm discharge, and its submersible range runs to 2,500 l/s and 100 m of head on motors of 0.55–400 kW. The impeller is offered in lines aimed at different media: the open Q and K lines for coarse suspended solids and for fibrous material that tends to become entangled, including wet wipes and synthetic fibre; a closed V line for highly abrasive media such as sand and chippings; and a closed F line for gentle conveying, which is where the fish, fruit and vegetables still sit.

Low shear is not only a food-industry concern. A pump that shreds what it cannot pass hands the pieces on to whatever is downstream, and on a works with fine screens or a dewatering plant that is somebody else's blockage rather than no blockage. The Hidrostal submersible is the range we reach for when a station has a rag history and the duty is big enough to care about efficiency.

A cutter belongs in the line when something after the pump needs the solids reduced. It does not belong there simply to stop a pump blocking, because that treats the symptom and puts a wearing part in the flow path to do it.

Four ways it gets done, and they are not interchangeable:

• Chopper pump. Ebara's ChopX does the primary cut with the impeller's leading edges turning in tight clearance across a cutter bar at the suction, then a second cut behind the impeller where the sharpened vane edges pass over spiral grooves in the backplate. Ebara points it at municipal sludges, scum and fat, lift-station waste, food waste, manure and abattoir waste.
• Grinder pump. The Grundfos SEG carries a grinder at the inlet, runs 0.9–4 kW, and reaches 10 l/s (36 m³/h) and 49 m of head. The grinder is there so the discharge can be a small-bore pressurised main where a gravity sewer is not an option. It buys you the pipe, not the pump.
• Inline macerator. Vogelsang's RotaCut sits in the pipe rather than in the pump, with cutting screens giving 4–50 mm free passage and an integrated heavy material separator that drops out stones and metal before they reach the cutter head. It protects the pumps, valves and pipework behind it.
• Twin-shaft grinder. Vogelsang's XRipper runs two shafts at different speeds so that material settling between the rotors is raked back out, and is aimed at wet wipes, wood, fabric and general trash in channels and inflows.

Decide it by naming the thing downstream that cannot take the solids whole. A small-bore pressurised main, a digester, a dewatering plant, a heat exchanger. If you can name it, specify a cutter and size the pump behind it. If you cannot, you want free passage, and the macerator and grinder range is the wrong aisle. Our grinder pumps and the macerator entry cover the cutting side in more depth.

Water UK ran the largest in-depth investigation of UK sewer blockages and published it in December 2017. Wipes made up around 93% of the material causing the blockages it examined, and most of those wipes were never sold as flushable. That is what a municipal solids-handling pump is up against, and it is why a specification written around grit or gravel misses the point.

The enquiry that lands here most often is a station clogging constantly on rag, with a request for a better solids-handling pump. We have declined to quote that. The pump already in the wet well was a good solids-handling pump, and putting the same class of machine back in would have blocked the same way. What the site needed was rag removal upstream, a macerator or a chopper pump, which was a system change nobody was asking us to sell. If a pump you already own is blocking, the first thing to establish is what came out of it when it was lifted.

Grit is the second cause and it works differently: it does not stop the pump, it wears it. Grit gets through because the grit removal on the works is not doing its job, clearances open up, and the pump loses head and efficiency long before anybody notices the free passage was never the problem. Where a works has a known grit problem, specify hard materials for the wet end rather than a bigger passage. Hidrostal offers hi-chrome, ductile iron, stainless steel and duplex across its ranges for exactly this.

Third is dry running, which usually traces back to suction conditions that were never right, or to a valve somebody shut and forgot. On a centrifugal pump the part that goes first is the mechanical seal, because it needs liquid to lubricate it and there is nothing else keeping it alive. When one of these pumps comes back, look at the seal before you look at the impeller.

Above roughly 3–4% solids, and certainly on thickened sludge, the centrifugal family runs out of road and the answer moves to positive displacement: a rotary lobe, a progressing cavity, or a double disc pump. Viscosity pushes the same way, but the solids figure is the one a site can usually quote. A rotodynamic pump asked to move something too thick will not fail dramatically, it will simply not make the duty, and everyone will spend a fortnight blaming the impeller.

The other wrong answer is the oversized one, and it arrives through margin stacking. The site states a flow. The consultant adds margin for future duty, reasonably. The contractor adds margin. The installer adds a little more head. Nobody in that chain is careless and the pump that comes out of it is far too big. Two things follow. Power rises with flow and head, so a 3 kW application finishes up on an 11 kW motor and stays uneconomical for the life of the installation. Worse, a centrifugal pump run off the end of its curve wants back pressure it is not getting, cavitates, and puts vibration into the pipework while it fails its own seals.

Free passage is where this bites hardest, because a big passage is the easiest margin in the world to add and it looks free. It is not. Every millimetre of passage you take beyond what the medium needs comes out of the efficiency and out of how well the pump behaves at part load.

Send the medium before the pump. The pump falls out of it.

• The largest solid the pump must pass, and what it actually is. Rag, wipes, grit, stones and lumps of product are five different problems.
• Flow in l/s or m³/h, and the total dynamic head worked out properly: the static difference, then the pipe length, bore, bends and valves.
• What the medium is. Screened or unscreened sewage, storm water, industrial effluent, or sludge with a dry solids content. Give the temperature too.
• Whether the site has grit, and whether the grit removal works.
• The installation. Wet well on an auto-coupling, dry well alongside, or inline.
• Anything downstream that cannot take the solids whole.
• Duty and standby, or a single pump.

If this is a replacement rather than a new station, add what is already installed and what the blockage looked like when the pump came out. That one detail decides whether the answer is a different impeller or a screen, and it is the difference between fixing the station and buying the same problem again. The pump selection guide walks the whole sequence.