PUMPS / REPLACEMENT
Replacing a centrifugal pump: match the duty, not just the maximum flow
An old duty sheet says “10 m³/h at 20 m head”. A replacement catalogue lists a maximum flow of 20 m³/h and a maximum head of 30 m. Both numbers are higher. Does that make it a suitable replacement?
Not from those two limits alone. They do not establish what the pump can deliver at the required combination of flow and head. Ask for the performance curve for the proposed configuration and compare it with the system’s requirements.
The numbers above are fictional, not a recommendation. This discussion concerns a centrifugal pump for non-hazardous water transfer. A different liquid, pump type or critical service needs its own assessment.
Keep the flow and head together
Write the required flow and head as a pair, with units and the operating conditions they refer to. Head describes the energy added per unit weight of liquid; it is not simply the vertical height of the pipe. The system requirement also reflects endpoint conditions and losses through the flow path.[2]
Before using the old sheet, establish what it records: a design requirement, a measured condition or a catalogue entry. If the pipe route, tank levels, valves or process have changed, the earlier requirement may no longer describe the work the replacement must do.
Send the document with that distinction visible. If the flow or head is unknown, leave it unknown and ask what information is needed to establish the duty. A maximum from a product listing is not a substitute for the missing operating requirement.
The pump works with the system it is connected to
For the illustrated centrifugal-pump arrangement, the operating point lies where the pump head–flow curve meets the system curve.[1] A pump curve alone describes one side of that relationship.
The Hydraulic Institute explains how valve positions and system changes can alter operating conditions, while speed or impeller changes alter the pump curve.[3] Ask which speed, impeller and other stated conditions the proposed curve represents. A family-level plot is not yet evidence for a particular configuration.
- 1Pump head–flow curve
- 2System curve
- 3Their intersection: operating point
Conceptual centrifugal-pump curves without numerical scales. Horizontal axis Q: flow; vertical axis H: head. This is not a curve for the fictional quote, a selected model, or proof that the required duty is met.
A matching curve is only part of the comparison
Now put the liquid and its temperature beside the curve conditions. The DOE selection guidance identifies properties such as composition, temperature, solids and viscosity as relevant to selection, and discusses suction conditions.[2] A water curve should not be silently treated as evidence for another liquid.
For a replacement enquiry, record the water service and temperature range, where the liquid enters from, and any known contamination or changes. Include the actual operating range if the process varies. Do not describe the liquid as “water” if the mixture or solids are uncertain.
Have the pump supplier or responsible engineer assess the inlet conditions, available versus required net positive suction head (NPSH) and the proposed operating range. Those checks are separate from seeing a flow–head pair on a chart. Do not invent a suction margin from the sketch.[4]
Bring the motor and the pipe connections into the same comparison
Hydraulic suitability does not answer whether the replacement fits or can be driven as installed. Put the proposed motor and pump documents beside the existing installation information. Ask for an explicit response to differences.
Useful records include supply voltage and frequency, speed and drive arrangement, motor details, connection type and dimensions, orientation, footprint and mounting. Add the wetted-material and seal specifications for review against the service. These are enquiry inputs; a matching connection or nominal rating is not an approval.
| Area | Information to send | Response to request |
|---|---|---|
| Duty | Information to sendRequired flow/head pair or operating range; basis and units | Response to requestProposed curve/configuration and assessment against the duty range |
| Liquid and inlet | Information to sendWater service, temperature range, supply arrangement and known changes | Response to requestAssessment of materials, seal, suction conditions and remaining unknowns |
| Power and drive | Information to sendNameplate, supply frequency/voltage, speed and drive details | Response to requestMotor/drive configuration and any installation changes |
| Mechanical fit | Information to sendConnection and mounting drawings, dimensions and orientation | Response to requestA drawing showing differences, not just a model-name match |
Ask for a candidate and a list of unresolved conditions
A useful reply identifies the exact pump and motor configuration, its curve, relevant drawings, and the conditions under which the supplier assessed it. It also separates confirmed information from points that still need the site’s input.
Compare those records with the required duty and installation before ordering. If a supplier proposes changing the speed, connections or pipework, treat that as a changed installation for the responsible engineer to review—not as a drop-in replacement.
The aim is to reach a specific comparison: what the candidate can do in the intended system, what has to change, and what remains unverified. Two reassuring maximum numbers cannot provide that answer.
Prepare a replacement enquiry
Open supplier questions
We are reviewing a centrifugal-pump replacement for water transfer. Existing pump and motor: [model / nameplates / available drawings] Service and temperature range: [known information / unknowns] Required flow and head, or operating range: [values, units and source; state if unknown] System changes: [pipe route, levels, valves, process or none known] Inlet/supply arrangement: [description / available records] Power and drive: [voltage, frequency, speed, drive arrangement] Connections and mounting: [drawings / dimensions / orientation] Please identify the proposed pump and motor configuration, its performance curve and stated curve conditions, mechanical drawings, installation changes, and any information still needed to assess hydraulic and suction suitability. Please do not treat the catalogue maximum flow and maximum head as a confirmed duty-point match.
Sources
- KSB — Operating point
Operating point and changes to pump/system curves. HTML text, not the underlying engineering standard.
- U.S. DOE — Pump Selection Considerations
October 2005, pages 1–2: liquid properties, flow/head, suction and selection considerations. Historical engineering guidance, not current statutory requirements.
- Hydraulic Institute — Combined Pump & System Curves
Tutorial text on curve interaction, changing conditions, speed and impeller changes. ANSI/HI 14.3 itself was not accessed.
- Hydraulic Institute — Pump Principles / NPSH
NPSH section: available and required NPSH and the need for a suitable margin. No numerical example or margin adopted.