Centrifugal Pumps - Stable and Unstable Flow

The characteristics of stable and unstable flow in centrifugal pumps. Understand the behavior differences between stable head-flow characteristics where differential head progressively falls with increasing flow rate, and unstable head-flow characteristics where differential head rises to a maximum before falling.

Flow Condition	Differential Head (h)	Flow Rate (q)	Characteristics	Stability Status
Stable Head-Flow	Progressively falls	Increasing	Linear negative relationship	Stable
Unstable Head-Flow	Rises to max then falls	Increasing	Non-linear with peak	Unstable - Oscillates
Stable Operation	Predictable	Controlled	Recommended for design	Safe
Unstable Operation	Unpredictable	Modulating	Flow oscillates, pipeline vibrates	Not Recommended

Flow Condition

Differential Head (h)

Flow Rate (q)

Characteristics

Stability Status

Stable Head-Flow

Progressively falls

Increasing

Linear negative relationship

Stable

Unstable Head-Flow

Rises to max then falls

Increasing

Non-linear with peak

Unstable - Oscillates

Stable Operation

Predictable

Controlled

Recommended for design

Safe

Unstable Operation

Unpredictable

Modulating

Flow oscillates, pipeline vibrates

Not Recommended

Stable Head-Flow Characteristic

The differential head (h) progressively falls with increasing flow rate (q). This creates a stable operating condition where the pump behavior is predictable and consistent.

Unstable Head-Flow Characteristic

The differential head (h) rises to a maximum and then progressively falls with increasing flow rate (q). An unstable pump may start to oscillate between two possible combinations of head-flow, causing the flow rate to modulate and the pipeline to vibrate.

Recommendations

It is recommended that centrifugal pumps are designed and operated under stable conditions to ensure reliable and safe operation.