Understanding Recirculation in Pumps

Devices

Portable balancer & Vibration analyzer Balanset-1A

$2,353.83

Parts

Vibration sensor

$107.26

Parts

Optical Sensor (Laser Tachometer)

$147.78

Devices

Balanset-4

$8,107.90

Parts

Magnetic Stand Insize-60-kgf

$54.82

Parts

Reflective tape

$11.92

Devices

Dynamic balancer “Balanset-1A” OEM

$2,067.80

Definition: What is Recirculation?

Recirculation is a flow instability that occurs in centrifugal pumps and fans when operating at flow rates significantly below the design point (best efficiency point or BEP). At low flows, fluid partially reverses direction, flowing backward from the discharge region back toward the suction, creating unstable recirculating patterns at the impeller inlet or discharge. This phenomenon generates low-frequency vibration pulsations (typically 0.2-0.8× running speed), noise, efficiency loss, and can cause severe mechanical damage through cyclic loading, cavitation, and heating.

Recirculation is one of the most destructive operating conditions for pumps because the unsteady hydraulic forces can be enormous, triggering bearing failures, seal damage, shaft fatigue, and even impeller structural failure in severe cases. Understanding and preventing recirculation is critical for pump reliability.

Types of Recirculation

1. Suction Recirculation

Occurs at impeller inlet (suction side):

• Mechanism: At low flow, fluid entering impeller eye has wrong flow angle
• Separation: Flow separates from vane suction surfaces
• Reverse Flow: Separated fluid flows backward out of impeller eye
• Onset: Typically at 60-70% of BEP flow
• Location: Concentrated near impeller shrouds

2. Discharge Recirculation

Occurs at impeller discharge (outlet):

• Mechanism: High-pressure discharge fluid flows backward into impeller periphery
• Path: Through clearance gaps (wear rings, side gaps)
• Mixing: Recirculated flow mixes with main flow, creating turbulence
• Onset: Typically at 40-60% of BEP flow
• More Severe: Generally more damaging than suction recirculation

3. Combined Recirculation

• Both suction and discharge recirculation present simultaneously
• Occurs at very low flows (< 40% BEP)
• Most severe vibration and damage potential
• Should be avoided through minimum flow protection

Vibration Signature

Characteristic Pattern

• Frequency: Sub-synchronous, typically 0.2-0.8× running speed
• Example: 1750 RPM pump showing 10-20 Hz pulsations
• Amplitude: Can be 2-5× normal operating vibration
• Unstable: Frequency and amplitude vary, not constant
• Random Component: Broadband increase from turbulence

Flow Dependence

• High Flow: No recirculation, low vibration
• Moderate Flow (80-100% BEP): Minimal recirculation, acceptable vibration
• Low Flow (50-70% BEP): Suction recirculation begins, vibration increases
• Very Low Flow (< 50% BEP): Severe recirculation, very high vibration
• Shutoff: Maximum recirculation, maximum vibration and damage rate

Additional Indicators

• High axial vibration component
• Noise increase (roaring or rumbling)
• Performance loss (head and flow below curve)
• Temperature increase from hydraulic losses

Consequences and Damage

Immediate Effects

• Severe Vibration: Can exceed alarm limits in minutes
• Noise: Loud turbulent noise
• Efficiency Loss: Power consumption high for delivered flow
• Heating: Hydraulic losses converted to heat

Mechanical Damage

• Bearing Failure: High cyclic loads accelerate bearing wear
• Seal Damage: Vibration and pressure pulsations damage seals
• Shaft Fatigue: Alternating bending stress from hydraulic forces
• Impeller Damage: Vane fatigue cracking from cyclic loading

Hydraulic Damage

• Cavitation: Recirculation zones prone to cavitation
• Erosion: High-velocity recirculating flow erodes surfaces
• Vortex Cavitation: Vortices in recirculation zones cavitate

Detection and Diagnosis

Vibration Analysis

• Look for sub-synchronous components (0.2-0.8×)
• Test at multiple flow rates
• Identify flow rate where pulsations begin (recirculation onset)
• Compare to pump performance curve predictions

Performance Testing

• Measure actual head-flow curve
• Compare to design curve
• Deviation at low flow indicates recirculation
• Power consumption higher than curve prediction

Acoustic Monitoring

• Distinctive turbulent roaring sound
• Broadband noise increase
• Can be heard and felt at pump casing

Prevention and Mitigation

Operating Strategies

Minimum Flow Protection

• Install automatic minimum flow recirculation line
• Valve opens below safe minimum flow (typically 60-70% BEP)
• Recirculates discharge back to suction or tank
• Prevents operation in recirculation zone

Operating Point Control

• Avoid operation below minimum continuous flow
• Use variable speed drive to match pump to demand
• Multiple smaller pumps rather than single large pump (better turndown)
• Staged operation of parallel pumps

Design Solutions

• Inducer: Axial inlet stage to stabilize suction flow
• Low-Flow Impellers: Special designs for low-flow operation
• Proper Sizing: Don’t oversize pump (avoid chronic low-flow operation)
• Wider Operating Range: Select pumps with flat curves tolerating flow variation

System Design

• Design system for pump operation near BEP
• Provide adequate NPSH margin to reduce cavitation in recirculation zones
• Control valve placement to minimize suction throttling
• Bypass or recirculation systems for minimum flow assurance

Industry Standards and Guidelines

Minimum Continuous Flow

• API 610: Specifies minimum continuous stable flow for centrifugal pumps
• Typical Values: 60-70% of BEP flow for radial pumps, 70-80% for mixed flow
• Thermal Consideration: Also limited by temperature rise at low flow

Performance Testing

• Factory tests verify recirculation onset point
• Field performance tests to confirm
• Acceptance criteria for vibration at minimum flow

Recirculation represents one of the most severe operating conditions for centrifugal pumps. Its characteristic sub-synchronous vibration signature, severe pulsation amplitudes, and potential for rapid mechanical damage make understanding recirculation onset conditions, implementing minimum flow protection, and avoiding chronic low-flow operation essential for pump reliability and longevity in industrial service.

---

← Back to Main Index