Part 1 of a two-part series
This guide explains the forces and load paths. Part 2 explains how the same forces can appear as bearing, seal, coupling, and shaft symptoms.
Radial thrust: where it comes from
Radial thrust acts generally perpendicular to the shaft centerline. In a single-volute centrifugal pump, pressure around the impeller can become uneven, particularly away from the design operating condition. The resulting hydraulic force is transferred through the impeller and shaft into the bearings and supporting structure.
Double-volute or other balancing features may reduce some radial force, but they do not make the pump independent of operating point. Wear, erosion, recirculation, impeller damage, solids, viscosity, and a changed system curve can alter the pressure distribution. Avoid relying on a single formula or a generic force value without the pump geometry, speed, fluid, and operating condition that define it.
Axial thrust: pressure and momentum along the shaft
Axial thrust acts along the shaft centerline. It can result from pressure differences across impeller surfaces, momentum changes, wear-ring clearances, balance holes, back vanes, opposed impellers, or other pump-specific features. The bearing arrangement and casing configuration determine how the load is carried.
Axial load is not identical across all ANSI process-pump designs. A replacement review should identify the impeller configuration, balance features, shaft arrangement, seal chamber, bearing arrangement, and direction of rotation. A casing or flange match by itself does not prove that the replacement has the same axial-load behavior.
Why the operating point changes load
The hydraulic force pattern changes as flow moves along the pump curve. Operation close to the manufacturer’s preferred region may provide a more balanced hydraulic condition, while operation far from that region can increase radial force, recirculation, vibration, or shaft movement. The applicable preferred operating region (POR), allowable operating region (AOR), minimum continuous stable flow, and maximum flow should come from the selected pump’s documentation or project requirements.
Do not apply one universal “safe percentage of BEP” to every pump. Check the actual curve, impeller, speed, liquid, system curve, control state, and any manufacturer limits. The pump system fundamentals guide explains how the pump and system curves establish the installed operating point.
Hydraulic load versus installation load
Similar symptoms can come from different sources. A load review should distinguish hydraulic forces from pipe, alignment, foundation, and rotating-assembly problems.
| Load source | Direction | Typical cause | Evidence to collect |
|---|---|---|---|
| Radial hydraulic thrust | Across shaft centerline | Pressure imbalance, off-design flow, internal wear | Flow, head, speed, curve, vibration direction and trend |
| Axial hydraulic thrust | Along shaft centerline | Impeller pressure distribution or balance configuration | Impeller design, bearing arrangement, axial vibration or position |
| Pipe strain | Force/moment at nozzles | Misfit, thermal movement, unsupported piping | Cold/hot condition, alignment and nozzle checks |
| Coupling/alignment load | Radial and axial | Soft foot, offset/angular misalignment, coupling issue | Alignment readings, soft-foot check, coupling condition |
| Rotating-assembly load | Radial/axial | Unbalance, bent shaft, rubbing, damaged bearing | Runout, phase, spectrum, clearance and inspection evidence |
The Hydraulic Institute overview of basic hydraulic loads and the related Pumps & Systems technical article provide broader industry context. Use them to frame the question, then apply the specific pump and project data.
What the bearing system must absorb
The bearing system transfers radial and axial forces while maintaining shaft position and rotation. The actual response depends on bearing type, preload or internal clearance, lubrication, temperature, housing fit, shaft stiffness, coupling, and the load history. A bearing can be damaged by a hydraulic load, but it can also be damaged by lubrication, contamination, installation, electrical current, misalignment, or a rotating defect.
Keep the concept separate from a life multiplier. Without a defined bearing, load spectrum, speed, lubrication, reliability basis, and operating history, it is not responsible to promise a specific life increase or failure interval.
What changes after speed, impeller, or process changes?
- VFD speed: changes pump head, flow, power, and the operating point; verify the approved speed envelope and load response.
- Impeller trim or replacement: changes hydraulic performance and can alter pressure distribution, power, clearances, and NPSH.
- Specific gravity or viscosity: changes force, power, and hydraulic behavior; verify the liquid basis.
- Piping or control changes: change the system curve, valve state, bypass flow, suction condition, or parallel-pump interaction.
- Process change: can move the pump outside the intended region even when the pump hardware is unchanged.
When a replacement is considered after a process change, send the original model and size, measured duty, speed, liquid properties, operating region, seal arrangement, bearing configuration, and any failure evidence. The ANSI Pump Selection & Replacement worksheet helps organize that information.
What to record before doing load analysis
- Flow, suction pressure, discharge pressure, head, speed, and control state.
- Specific gravity, viscosity, temperature, solids, gas, and vapor-pressure information where relevant.
- Vibration amplitude, direction, phase or spectrum, and the time relation to the operating condition.
- Bearing temperature, lubrication condition, leakage, seal history, and coupling condition.
- Shaft runout, alignment, soft foot, pipe strain, foundation, and baseplate evidence.
- Impeller trim, clearances, wear-ring condition, balance features, and previous repair history.
Take readings before disassembly whenever it is safe. Compare readings at more than one stable operating condition if the process permits. A single post-failure photograph rarely distinguishes hydraulic load from installation or rotating-assembly load.
Need to compare an installed pump’s duty with a replacement?
Send the model and size, measured flow and head, speed, liquid data, vibration direction, bearing and seal history, drawings, and any alignment or pipe-strain records. ANSI Pumps Pro can help identify which hydraulic, dimensional, material, and mechanical checks remain open.
Continue to Part 2: bearing and seal life
Part 1 explains the forces. Part 2 explains how radial and axial thrust can appear as bearing heat, vibration, seal leakage, shaft movement, and coupling wear, and gives a symptom-to-evidence inspection sequence.
FAQ
What is radial thrust in a centrifugal pump?
Radial thrust is a force generally perpendicular to the shaft centerline, often associated with uneven pressure distribution around the impeller and casing as operating conditions change.
What is axial thrust in a centrifugal pump?
Axial thrust acts along the shaft and can result from pressure distribution, momentum, impeller geometry, balance features, clearances, and the pump’s bearing arrangement.
Does operating near BEP eliminate hydraulic load?
No. It may provide a preferred hydraulic condition for the selected pump, but the actual load depends on design, operating point, fluid, speed, condition, and project or manufacturer limits.
Can pipe strain look like hydraulic thrust?
Yes. Pipe strain, soft foot, misalignment, foundation movement, and rotating defects can produce radial or axial symptoms similar to a hydraulic problem, so field evidence should be compared.
What should be checked before replacing a pump after a bearing failure?
Check the duty and operating point, speed, liquid, impeller and balance configuration, bearings, shaft, alignment, pipe strain, lubrication, seal arrangement, and the complete failure history.
