Glossary

ATEX is the name for two European directives on explosive atmospheres: one setting the duties on employers to assess and classify a hazardous area, the other setting the requirements for equipment sold for use in one. An ATEX pump is a pump certified under the second.

Pump cavitation is vapour forming and collapsing inside a pump because the local pressure has fallen below the medium's vapour pressure. It is boiling brought on by pressure, not by heat. The bubbles form at the impeller inlet and implode as the pressure recovers, taking metal off the vane each time and cutting the pump's head, flow and efficiency.

Clean-in-place, or CIP, cleans the inside of process equipment by circulating cleaning fluids through it while it stays assembled. Tanks, pipework, valves and pumps are all cleaned in position, at a controlled flow velocity, temperature, chemical concentration and time, so the result can be repeated and evidenced rather than judged by eye.

Corrosion resistance is a material's ability to withstand chemical attack in a specific medium, at a specific concentration and temperature. It is not a fixed property of a material and not a league table. The same grade of stainless steel can be right in one liquid and perforate in the next.

Dry running is a pump turning with no liquid in its wet end, either because the suction supply has run out or because a valve is shut. The mechanical seal is usually the part that fails, since the seal faces rely on the pumped liquid to lubricate and cool them. What it costs you depends on the pump type.

Duplex stainless steel is a stainless with two phases in its microstructure, roughly half austenite and half ferrite. The mix gives it more than double the room-temperature yield strength of a standard austenitic such as 316L, and far better resistance to chloride stress corrosion cracking. It is a family, not a grade.

Effluent is liquid waste discharged from a process, a works or a building, treated or partly treated. It sits between raw sewage and clean water. The gross solids have been screened or settled out, but suspended solids, grit and grease stay in it. That difference decides the pump.

An EPDM seal is a sealing element in ethylene propylene diene monomer rubber, the default elastomer for water, steam, dilute acids and caustic cleaning chemistry. It is the standard fit on hygienic and potable pumps, and it swells and softens in mineral oils, fuels and hydrocarbons.

A float switch is a level-sensing device that starts and stops a pump as the liquid level in a wet well, sump or tank rises and falls. The float rides on the surface, and the level at which it tips or lifts is what makes and breaks the pump's control circuit.

Hastelloy is a registered trademark of Haynes International, not a material specification. It covers a family of nickel-based corrosion-resistant alloys, each with its own chemistry and its own chemical window. Writing Hastelloy on a pump enquiry names a trademark. It does not tell anyone which grade the duty needs.

An impeller is the rotating vaned part of a centrifugal pump that adds energy to the liquid. It turns the shaft's rotation into velocity at the vane tips, and the casing around it turns that velocity into pressure. Its shape sets both the pump's efficiency and what solids it will pass.

A macerator pump reduces rag, fibre and soft solids to a size the pumping system can pass without blocking. In industry the cutting is done either by an inline macerator fitted in the pipeline ahead of a separate pump, or by a grinder pump that cuts at its own inlet and pumps in one unit.

A mechanical seal is the shaft seal on a pump: two flat faces, one turning with the shaft and one fixed in the housing, pressed together with a very thin film of the pumped liquid between them. That film lubricates the faces and does the sealing. Lose it and the seal fails.

A non-Newtonian fluid is one whose viscosity changes with the shear applied to it, so it has no single viscosity figure. Most media that give pumps trouble are non-Newtonian. Sewage sludge, slurries and many food products thin out as they are worked, so the viscosity you measure at rest is not the viscosity the pump sees.

Passivation is a chemical treatment that removes free iron and other contamination from a stainless steel surface so the chromium-rich oxide film can re-form over it. The film forms naturally in air. Passivation is the process step that restores it after welding, machining or grinding.

A peristaltic pump moves medium by squeezing a flexible hose or tube against a curved track with rollers or shoes. The medium touches nothing but the bore of the hose, so the pump has no seals, valves or moving parts in the wetted path.

A positive displacement pump moves a fixed volume of medium with every stroke or shaft revolution, then forces it into the discharge whatever the pressure downstream. Flow follows pump speed rather than system head, so it holds its rate to within a few per cent as the pressure changes.

A pressure switch is a pressure-sensing device that starts and stops a pump to hold a system between two set points. It closes and starts the pump when the pressure falls to the cut-in setting, and it opens and stops the pump when the pressure rises to the cut-out setting.

A PTFE seal is a sealing element made from polytetrafluoroethylene, specified where the medium would attack a rubber one. In a pump it is usually a bellows, a wedge or a PTFE-jacketed O-ring with an elastomer core, because PTFE is a thermoplastic with no elastic memory of its own.

A rotary lobe pump curve plots flow against pump speed at a stated differential pressure, not head against flow. Because the pump displaces a fixed volume per revolution, speed sets the flow. The chart adds absorbed power and NPSHr at the same speed, and shows how much flow slip costs you as pressure rises.

A self-priming pump can evacuate air from its suction line and establish flow without being flooded by hand first. It keeps a reservoir of liquid in its casing, mixes the entrained air with that liquid, and separates the air out on recirculation until the suction line is full and the pump is passing medium.

Shear sensitivity is the tendency of a medium's structure to break down under the mechanical stress a pump applies to it. Emulsions destabilise and separate, flocculated sludge loses its floc, and polymer solutions thin permanently. A shear sensitive pump is one chosen to move that medium gently, usually a positive displacement type running slowly.

A solids-handling pump is a pump built to move liquid carrying rag, grit or soft debris without clogging. It does that by keeping a large clear passage through the impeller and casing, measured as free passage in millimetres, rather than by being stronger than an ordinary pump.

Super duplex is a higher-alloyed grade within the duplex stainless steel family, not a separate material. It carries roughly 25% chromium and 3 to 5% molybdenum with more nitrogen than a standard grade, which lifts its pitting resistance equivalent number to 40 or above. That number is the definition.

Viscosity is a medium's resistance to flow: how hard it is to shear one layer of it past the next. It is the third question we ask when picking a duty, after flow rate and the pipework, because it decides the pump type. Thin media suit a centrifugal pump. Thick media want positive displacement.

A Viton seal is a sealing element in FKM fluoroelastomer, the rubber specified for mineral oils, fuels and hydrocarbons. Viton is Chemours' trade name for one family of FKM, not a material specification. It is the inverse of EPDM: strong where EPDM is weak, and weaker on hot water and steam.

Water hammer is the pressure surge that travels through pipework when a moving column of liquid is stopped or turned abruptly. A valve slams shut, a pump trips on power failure, or a check valve closes late. The column's momentum has nowhere to go, so it becomes a pressure wave that hammers along the line.

WRAS approved means a water fitting or material has been certified by the Water Regulations Approval Scheme as meeting the UK water fittings regulations. The approval applies to fittings that carry or receive water from the public mains supply, so it covers potable duties and says nothing about a wastewater pump.