Centrifugal or positive displacement
Almost every industrial pump falls into one of two families, and they behave in opposite ways. A centrifugal pump imparts velocity to the fluid and converts it to pressure; its flow varies with the system pressure it works against. A positive displacement pump moves a fixed volume per revolution; its flow is essentially independent of pressure, which is why it can build damaging pressure against a closed discharge.
| Characteristic | Centrifugal | Positive displacement |
|---|---|---|
| Flow vs pressure | Falls as head rises | Nearly constant regardless of head |
| Viscous fluids | Efficiency drops sharply | Handles high viscosity well |
| Self-priming | Generally not, unless specifically designed | Usually yes |
| Closed discharge | Tolerable briefly; overheats | Dangerous — needs relief protection |
| Typical duty | High flow, moderate head, thin fluids | Metered dosing, viscous or shear-sensitive fluids |
| Common types | End suction, multistage, submersible, slurry | Diaphragm, gear, peristaltic, screw |
Defining the duty point
The duty point is the flow rate and total head the pump must deliver. Total head is not the vertical lift alone — it is static lift plus friction losses through pipe, fittings and valves, plus any pressure the discharge has to overcome. Friction losses are often the larger part on a long run, and they rise with the square of flow.
Oversizing is the most expensive habit in pump procurement. A pump selected well to the right of its best efficiency point runs hot, wastes energy, and loads the bearings and seal, which is why oversized pumps fail more often than correctly sized ones.
NPSH and cavitation
Cavitation occurs when pressure at the pump inlet falls below the vapour pressure of the liquid. Vapour bubbles form and then collapse violently as pressure recovers in the impeller, eroding metal and producing the characteristic sound of pumping gravel. It destroys impellers and it is entirely avoidable.
Every pump has an NPSH required (NPSHr) figure. Your installation has an NPSH available (NPSHa), set by the suction arrangement, liquid temperature and vapour pressure. NPSHa must exceed NPSHr with a real margin — a suction lift, a hot liquid or a restricted suction line all reduce NPSHa, and all three together are the classic cavitation case.
Material and seal selection
Wetted materials are chosen against the fluid, and the seal is chosen against the fluid, the pressure and the temperature. For chemical duty this decision matters more than the hydraulic selection, because the wrong elastomer can fail within days.
- Cast iron and bronze suit clean water and neutral fluids
- Stainless steel suits corrosive process fluids and hygienic duty
- Non-metallic and lined pumps handle acids, hypochlorite and other aggressive chemistry
- Hardened and high-chrome materials are used for abrasive slurry service
- Mechanical seals suit most duties; double seals with a flush are used for hazardous or crystallising fluids
- Sealless magnetic drive pumps eliminate the seal entirely where leakage is unacceptable
Dosing pumps for chemical treatment
Chemical dosing is a positive displacement application. The requirement is an accurate, repeatable, adjustable small flow against system pressure — a duty a centrifugal pump cannot meet. Diaphragm metering pumps dominate water treatment dosing for coagulants, hypochlorite and metabisulfite.
- Size the pump so the normal dose falls in the middle of its turndown range, not at the extreme
- Confirm elastomer and head material compatibility with the specific chemical
- Include a calibration column, back pressure valve and pulsation damper where accuracy matters
- Provide relief protection — a positive displacement pump against a blocked line will damage something
What to include in a pump enquiry
- Flow rate and total head, or the full system detail needed to calculate head
- Fluid, concentration, temperature, specific gravity, viscosity and solids content
- Suction arrangement — flooded suction or suction lift, and the pipe run
- Required materials of construction and seal arrangement
- Motor supply voltage, frequency and any hazardous area classification
- Duty and standby requirement, quantity, delivery city and required date