Pump Balancing: How to Reduce Vibration and Extend the Life of Mechanical Seals

The effect of imbalance on pump operation

Imbalance in a pump impeller arises from the same causes as in other equipment:

Causes of imbalance:

• Blade erosion: uneven wear from the pumped liquid, especially where it contains abrasive particles
• Cavitation damage: destruction of the blade surface by imploding cavitation bubbles
• Corrosion: chemical corrosion in an aggressive medium
• Deposit build-up: accumulation of salts, scale and pumped products
• Repair defects: replacing or weld-repairing individual blades without re-balancing afterwards

The consequences of imbalance for pumps:

• Shaft vibration: creates radial runout that is transmitted to the seal
• Increased bearing load: service life can drop from 5 years to 6–12 months
• Unstable delivery: pulsations in pressure and flow
• Increased energy consumption: 10–15% of wasted electricity costs
• Noise and vibration: transmitted through the pipework to the whole system

The mechanical seal problem

The mechanical seal is one of the most vulnerable components of a centrifugal pump. It works in demanding conditions: high pressure, an aggressive medium and constant friction.

How imbalance destroys the seal:

1. Radial shaft runout: vibration from imbalance causes shaft runout (deviation of the axis from its nominal position)
2. Loss of contact integrity: the seal faces periodically separate and slip
3. Accelerated wear: uneven wear of the faces, overheating
4. Leaks: the appearance of a leak and the need for frequent seal replacement

Balancing a pump impeller in good time is a direct saving on expensive mechanical seals.

The link between imbalance and cavitation

Cavitation is the formation and collapse of vapour bubbles in a liquid when the pressure falls below the saturated vapour pressure. It is a dangerous phenomenon that destroys impeller blades.

How imbalance and cavitation interact:

Cavitation causes imbalance:

• Cavitation erosion destroys the blades unevenly
• This creates a mass asymmetry → imbalance

Imbalance worsens cavitation:

• Vibration from imbalance creates flow pulsations
• Unstable flow intensifies cavitation
• A vicious circle develops

The characteristic sound of cavitation is "grinding gravel". If you hear this sound and the vibration is unstable, deal with the hydraulic problem first, then balance.

Methods of balancing pumps

Method 1: Balancing the impeller on a machine

When it is used: during a pump overhaul, when the impeller has been removed.

Advantages:

• High balancing accuracy for the individual component
• You can simultaneously re-machine and weld-repair worn blades
• Controlled workshop conditions

Procedure:

1. The impeller is mounted on a balancing mandrel (a dummy shaft)
2. Balancing is carried out in 1 or 2 planes
3. After the pump is assembled, a check balance in the assembled state is recommended

Method 2: Balancing the pump as an assembly (on site)

When it is used: for installed pumps, without dismantling.

Advantages:

• Minimal downtime (2–4 hours instead of several days)
• Balancing of the whole system, accounting for the coupling, alignment and the actual bearings
• Savings on disassembly/reassembly

Procedure:

1. Vibration sensors are fitted to the pump bearing housings
2. A laser tachometer is aimed at the coupling or shaft
3. Balancing is carried out by the 3-run method (see the methodology article)
4. Correction weights are fitted to the rear disc of the impeller (where accessible)

📖 Detailed methodology: Balancing with the Balanset-1A step by step

A combined approach

In practice, a combination of methods is often used:

1. During repair: balancing the impeller on a machine (rough balancing)
2. After assembly: finishing the balance of the assembled pump in situ

This ensures minimal vibration of the whole unit, accounting for all factors: coupling, alignment and pipework stiffness.

Practical recommendations

When pump balancing is needed

• After an overhaul: always balance after replacing the impeller or shaft, or repairing blades
• When vibration appears: if the pump has started to vibrate or run noisier than usual
• After cavitation damage: cavitation destroys blades unevenly → imbalance
• Periodically for critical pumps: boiler feed pumps and circulating pumps — once every 1–2 years
• When seals are replaced frequently: if a seal lasts less than a year, check the balance

The specifics of balancing different pump types

Centrifugal pumps:

• Usually balanced in 1 plane (if the impeller is narrow)
• For double-suction designs — in 2 planes
• Balance quality grade: G6.3

Multistage pumps:

• Each impeller is balanced separately
• Then the entire assembled rotor is balanced
• A more complex procedure that requires experience

Submersible pumps:

• Balanced on a special rig
• Hydrodynamic forces are taken into account

Checks before balancing

You must check:

1. Pump-to-motor alignment: shaft misalignment produces vibration at 2× — balancing will not help
2. Bearing condition: play is not permissible
3. Absence of cavitation: the characteristic "gravel" noise, unstable vibration
4. Cleanliness of the impeller: deposits themselves create imbalance
5. Fixings: all bolts must be tightened

After balancing

What to check:

• Vibration reduced into zone A or B (per ISO 10816)
• Stability of the pressure and flow readings
• No leaks through the seal
• Bearing temperature (must be within limits)

Periodic checks:

For critical pumps, vibration monitoring once a quarter is recommended. A gradual rise in vibration at the rotational frequency (1×) is a sign of developing imbalance.