Pumps move water, wastewater, sludge and chemicals around every treatment plant. Transfer pumps are chosen for flow and head; dosing (metering) pumps are chosen for accuracy at small flows against a given pressure.
Most pump problems in the field come from buying on flow alone, or from wetted materials that were never checked against the fluid.
What falls under this category
- Centrifugal pumps
- End-suction, vertical multistage and split-case pumps for clean-water transfer, booster and high-pressure duties.
- Submersible pumps
- For raw sewage, effluent and drainage, with impellers designed to pass solids.
- Positive-displacement pumps
- Progressive cavity and lobe pumps for sludge and viscous fluids.
- Dosing pumps
- Solenoid and motor-driven diaphragm pumps, peristaltic pumps and piston pumps for adding chemicals in controlled amounts.
- Dosing skids
- Pre-assembled packages of chemical tank, pumps, valves, instruments and containment.
Specifications for transfer pumps
- Duty point
- Required flow and total dynamic head. Ask for the pump curve with your duty point marked. Pumps last longest and use least energy near their best efficiency point.
- NPSH
- Net positive suction head available at your installation must exceed what the pump requires, with a margin, or the pump will cavitate.
- Efficiency and motor
- Pump efficiency at the duty point, motor power and motor efficiency class (for example IE3 or IE4).
- Materials and seals
- Casing, impeller and shaft materials, and mechanical seal type and faces, chosen for the fluid, its chloride level and any abrasive solids.
- Solids handling
- Free passage size and impeller type for wastewater and sludge.
Specifications for dosing pumps
- Flow range and back pressure
- Maximum flow in L/h at the pressure of the injection point. Select a pump whose normal dose sits comfortably inside its range, not at the bottom of it.
- Accuracy and turndown
- Stated steady-state accuracy and the range over which it holds.
- Wetted materials
- Pump head, diaphragm, valve balls and seals (for example PVDF, PP, PTFE, stainless steel) checked against a compatibility table for the specific chemical and concentration.
- Control
- Manual setting, 4–20 mA, pulse input from a flowmeter or fieldbus communication, depending on whether the dose follows flow or a measured value.
- Accessories
- Foot valve, injection valve, pressure relief valve, back-pressure valve, pulsation dampener and calibration column.
Information to send suppliers with your enquiry
- Fluid name, concentration, temperature and viscosity. For chemicals, include the safety data sheet.
- Required flow range and the pressure at the injection or discharge point.
- For transfer pumps: suction conditions and, ideally, the system curve.
- Control signal available and power supply.
- Installation location and any hazardous-area requirements.
Common sourcing mistakes
- Oversizing dosing pumps so they run at a small fraction of capacity, where accuracy is poorest.
- Ignoring off-gassing chemicals such as sodium hypochlorite and hydrogen peroxide, which can vapour-lock ordinary pump heads. Degassing heads exist for this.
- Leaving out a back-pressure valve when dosing into a low-pressure line, which allows siphoning and overdosing.
- Buying transfer pumps on flow alone without checking head and NPSH.