Materials, connections and finish. Cast iron or standard stainless instead of polished, flanged or threaded connections instead of hygienic clamps, and selection driven by abrasion and chemical resistance rather than cleanability. The pumping principle is identical; the price is substantially lower.

Sludge and slurry in water treatment, chemical transfer, paper coatings, adhesives, waste and biogas feedstock, oils and lubricants. The common thread is fluid that is viscous, carries solids, or must not be sheared - conditions where a centrifugal pump either blocks or damages the product.

Because sludge has variable solids content including rag and grit, and the process needs a defined flow rate regardless of discharge head. A positive-displacement pump delivers that; a centrifugal pump's output varies with head and it blocks on rag. Lobe pumps also pass solids that would jam tighter-clearance types.

Usually yes. A wider running clearance tolerates grit and thermal expansion and greatly extends life in abrasive duty, at the cost of more slip and lower volumetric efficiency. In dirty service that is normally the right trade, and it is a specification decision to make at purchase.

Stainless, hardened steel, and elastomer-coated or fully elastomeric rotors. Hardened rotors resist grit abrasion; elastomer rotors tolerate contact better and run more quietly, but have temperature and chemical limits. Selection should follow the fluid's abrasiveness and chemistry rather than defaulting to stainless.

Products that solidify or become too viscous to pump when cold - bitumen, chocolate, heavy waxes and some adhesives. A jacket around the casing circulates steam or hot fluid to keep the product mobile, and it also allows the pump to be cleared after shutdown rather than setting solid inside.

Better than tight-clearance pumps, but abrasion is still the main wear mechanism and life is finite. Hardened or elastomer-coated rotors, opened clearances and moderate speeds all extend it. For heavily abrasive slurry a dedicated slurry or peristaltic pump may outlast a lobe pump considerably.