Hydraulic Imbalance in Multistage Pumps — Causes and Correction

Multistage centrifugal pumps achieve high pressure by passing the fluid through multiple impeller stages in series. Each stage adds a pressure increment — and each stage also generates an axial hydraulic thrust force. In a correctly designed and maintained multistage pump, these forces are balanced by the pump's internal balance arrangement. When they are not — due to uneven wear, non-genuine replacement stages, or incorrect assembly — the resulting axial imbalance can destroy bearings and shaft seals in a fraction of their normal service life.

How Axial Thrust Is Generated in Multistage Pumps

Each impeller in a multistage pump generates axial thrust directed toward the suction side of the pump — a consequence of the pressure differential between the front and rear faces of the impeller disc. In a single-stage pump, this thrust is managed by a balance disc, balance drum, or thrust bearing. In a multistage pump like the Lowara e-SV series, the stages are typically arranged in opposing pairs or groups, with the hydraulic forces of one group partially cancelling the forces of the other.

The axial force balance is designed for the pump as manufactured — with each stage producing the same pressure rise at the same flow rate. This balance depends on:

• All stages having the correct impeller trim diameter
• All stages having the same wear ring clearances
• The diffuser and stage casing geometry being correct for each stage position
• The hydraulic performance of each stage being within specification

When any of these conditions is violated, the thrust balance is disturbed and the residual axial load is absorbed by the thrust bearing — which was not designed to carry sustained axial load beyond a defined maximum.

How Imbalance Develops in Service

Uneven Stage Wear

In a pump handling water with suspended particles or mildly abrasive media, wear ring clearances increase over time. If stages wear at different rates — due to flow maldistribution or localised particle concentration — the pressure rise contribution of each stage diverges from its design value. Stages with increased clearance contribute less head; stages with tighter clearance contribute relatively more. The net effect is a shift in the axial load distribution that the balance arrangement must compensate — potentially exceeding the thrust bearing capacity.

Mixed Genuine and Non-Genuine Stages

A particularly problematic scenario occurs when a repair replaces some stages with non-genuine components while retaining others. Non-genuine diffuser stages manufactured to commercial tolerances may have slightly different hydraulic characteristics from the genuine Lowara stages. When mixed in the same pump, the stages produce slightly different pressure increments — creating an inter-stage hydraulic imbalance that the balance arrangement cannot fully correct.

Incorrect Assembly Sequence

Multistage pumps must be assembled with stages in the correct sequence. Each stage position is designed for the specific pressure level and flow conditions at that point in the pump. Some Lowara multistage designs use different impeller trims at different stage positions — assembling them in the wrong sequence changes the hydraulic loading distribution and affects the balance.

Failure Modes and Diagnosis

Hydraulic imbalance in a multistage pump typically manifests through:

• Premature thrust bearing failure — characterised by axial play development and elevated axial vibration
• Shaft seal leakage — axial shaft displacement from bearing wear allows the mechanical seal to operate outside its designed clearance envelope
• Progressive performance degradation — as worn stages contribute less head, the pump H-Q curve drops below specification
• Elevated axial vibration component — measurable at the motor bearing positions