Centrifugal Pump Operation & Performance

Centrifugal pumps are widely used in drinking water, wastewater, distribution, collection, and treatment systems. Their actual performance depends on both the pump and the hydraulic system connected to it.

An operator should understand how flow, head, system resistance, pump speed, valve position, and impeller condition interact. This makes it possible to recognize abnormal performance before a pump fails or treatment is affected.

How a Centrifugal Pump Produces Flow

A centrifugal pump uses a rotating impeller to add energy to liquid.

Liquid enters near the center of the impeller and is accelerated outward as the impeller rotates.

The casing then directs the liquid toward the discharge and converts part of the velocity energy into pressure.

A Pump Does Not Produce One Fixed Flow

A common mistake is to assume that a centrifugal pump always produces the flow shown on its nameplate or design sheet.

Actual flow depends on:

• pump speed;
• impeller diameter;
• system resistance;
• static head;
• valve position;
• pipe condition;
• pump condition.

The pump and the system determine the operating point together.

Pump Curve

A pump curve shows the relationship between flow and head for a specific pump operating at specified conditions.

For a typical centrifugal pump:

• head is highest at low flow;
• head decreases as flow increases.

The curve may also show:

• efficiency;
• horsepower;
• NPSH required;
• different impeller diameters;
• different operating speeds.

System Curve

A system curve shows the head required by the piping system at different flow rates.

System head may include:

• static elevation;
• required discharge pressure;
• pipe friction;
• valve and fitting losses;
• equipment losses.

Friction loss increases as flow increases, so the system curve usually rises with flow.

Operating Point

The centrifugal-pump operating point occurs near the intersection of the pump curve and system curve.

At that point:

• pump head equals required system head;
• the corresponding flow is the actual operating flow.

Changing System Resistance Changes Pump Flow

If system resistance increases, the operating point moves.

For a typical centrifugal pump, increased resistance generally causes:

• lower flow;
• higher pump discharge head.

Possible causes include:

• partially closing a discharge valve;
• filter fouling;
• pipe blockage;
• increased pipe roughness.

Decreasing System Resistance

If system resistance decreases, a typical centrifugal pump generally moves toward:

• higher flow;
• lower head.

This may occur when:

• a valve opens;
• a blockage is removed;
• a parallel flow path is opened;
• a dirty filter is cleaned.

Best Efficiency Point

The Best Efficiency Point, or BEP, is the region where a centrifugal pump operates near its highest hydraulic efficiency.

Operating close to BEP generally supports:

• better energy efficiency;
• lower vibration;
• lower hydraulic stress;
• better bearing and seal life.

Operating Far from BEP

Operating too far from BEP can contribute to:

• internal recirculation;
• vibration;
• seal damage;
• bearing loading;
• reduced efficiency;
• unstable operation.

The acceptable operating range depends on the pump and manufacturer guidance.

Shutoff Head

Shutoff head is the head developed by a centrifugal pump at approximately zero flow.

This occurs when the discharge is closed while the pump is operating.

Shutoff head is generally the highest head shown on the pump curve.

Do Not Operate at Shutoff Indefinitely

Operating a centrifugal pump against a closed discharge for too long can cause:

• heating of the liquid;
• internal recirculation;
• seal damage;
• pump damage.

Any permitted closed-valve operation should follow manufacturer and facility procedures.

Runout

Runout describes operation at very high flow and low head near the far-right portion of the pump curve.

Operating excessively far into runout can cause:

• motor overload;
• poor efficiency;
• cavitation risk;
• mechanical stress.

Throttling a Centrifugal Pump

Partially closing a discharge valve increases system resistance.

The system curve becomes steeper and the operating point shifts toward lower flow.

Throttling can control flow, but hydraulic energy is lost across the valve.

Why Throttling Reduces Flow

Throttling does not directly slow the impeller.

Instead, it increases the head the system requires.

The pump then operates at a different point on its curve.

Do Not Throttle the Suction Side as a Normal Flow-Control Method

Restricting pump suction can reduce available pressure at the pump inlet.

This can increase the risk of:

• cavitation;
• loss of prime;
• poor pump performance.

Normal centrifugal-pump flow control is generally performed on the discharge side or through speed control, subject to system design.

Variable-Frequency Drives

A Variable-Frequency Drive, or VFD, changes motor speed by controlling electrical frequency.

Changing pump speed changes:

• flow;
• head;
• power requirement.

VFDs are commonly used for:

• pressure control;
• flow control;
• tank-level control;
• energy management;
• soft acceleration and deceleration.

Affinity Laws

The centrifugal-pump affinity laws describe approximate relationships between pump speed, flow, head, and power for geometrically similar operating conditions.

For changes in pump speed:

Q₂ ÷ Q₁ = N₂ ÷ N₁

H₂ ÷ H₁ = (N₂ ÷ N₁)²

P₂ ÷ P₁ = (N₂ ÷ N₁)³

where:

• Q = flow;
• H = head;
• P = power;
• N = pump speed.

Flow Changes Directly with Speed

If speed decreases to 80% of the original speed:

Q₂ = Q₁ × 0.80

If original flow is 1,000 gpm:

Q₂ = 1,000 × 0.80 = 800 gpm

Head Changes with the Square of Speed

Using the same 80% speed:

H₂ = H₁ × 0.80²

If original head is 100 feet:

H₂ = 100 × 0.64 = 64 ft

Power Changes with the Cube of Speed

Again using 80% speed:

P₂ = P₁ × 0.80³

If original power is 50 horsepower:

P₂ = 50 × 0.512

P₂ = 25.6 hp

This shows why reducing centrifugal-pump speed can provide substantial energy savings in appropriate systems.

Affinity Laws Are Approximate

The affinity laws are useful but should not be treated as exact for every field condition.

Actual performance can differ because of:

• pump efficiency changes;
• motor efficiency;
• system curve shape;
• minimum speed limitations;
• hydraulic instability.

Impeller Diameter Changes

Changing effective impeller diameter also changes pump performance.

A smaller trimmed impeller generally produces:

• less flow;
• less head;
• lower power requirement.

The correct manufacturer curve should be used for the actual installed impeller diameter.

Pump Efficiency

Pump efficiency compares useful hydraulic output with mechanical energy supplied to the pump.

A simplified expression is:

Pump Efficiency = Hydraulic Power Output ÷ Mechanical Power Input × 100

Efficiency varies with operating point.

Water Horsepower

For water, hydraulic power can be estimated using:

Water Horsepower = Flow, gpm × Head, ft ÷ 3960

Example:

A pump delivers 1,000 gpm against 80 feet of head.

Water Horsepower = 1,000 × 80 ÷ 3960

Water Horsepower = approximately 20.2 hp

Brake Horsepower

If pump efficiency is known:

Brake Horsepower = Water Horsepower ÷ Pump Efficiency

Efficiency must be expressed as a decimal.

If water horsepower is 20.2 hp and pump efficiency is 75%:

BHP = 20.2 ÷ 0.75

BHP = approximately 26.9 hp

Motor Horsepower

The motor must be capable of supplying the required pump input power under expected operating conditions.

Motor sizing should include appropriate design margin and follow manufacturer and engineering requirements.

Operators should not assume the nameplate horsepower represents actual continuous power use at every operating point.

Pump Performance Monitoring

Useful centrifugal-pump operating data include:

• flow;
• suction pressure;
• discharge pressure;
• pump speed;
• motor current;
• power;
• vibration;
• bearing temperature;
• seal leakage.

These values are most useful when compared with historical normal conditions.

Calculating Pump Head from Pressure Gauges

If suction and discharge gauges are near the same elevation and velocity differences are small, a simplified estimate of pump head can be based on pressure difference.

For water:

Pump Head, ft ≈ Pressure Increase, psi × 2.31

Example:

Suction pressure is 10 psi and discharge pressure is 55 psi.

Pressure increase:

55 - 10 = 45 psi

Head added:

45 × 2.31 = approximately 104 ft

More complete calculations may require elevation and velocity corrections.

High Discharge Pressure and Low Flow

If a centrifugal pump shows:

• higher than normal discharge pressure;
• lower than normal flow;

possible causes include:

• partially closed discharge valve;
• blocked pipeline;
• dirty filter;
• higher downstream pressure;
• increased system resistance.

Low Discharge Pressure and Low Flow

Possible causes include:

• wrong rotation;
• worn impeller;
• low pump speed;
• air in the pump;
• loss of prime;
• suction restriction;
• cavitation;
• internal damage.

Normal Pressure but Low Flow

This condition may result from:

• flow-meter problem;
• partial blockage;
• changed system configuration;
• pump wear;
• incorrect valve position.

Operators should compare multiple measurements rather than relying on one instrument.

High Flow and Low Head

High flow with low head may occur when system resistance is lower than expected.

Possible causes include:

• open bypass;
• unexpected valve position;
• pipe break;
• very low downstream level;
• operation near runout.

Parallel Pump Operation

Two or more centrifugal pumps can operate in parallel when their discharges connect to a common system.

Parallel operation primarily increases available flow.

Total flow is not necessarily equal to twice the single-pump flow because the system curve changes as flow increases.

Example of Parallel Pump Behavior

If one pump delivers 800 gpm alone, two identical pumps in parallel may deliver more than 800 gpm but less than 1,600 gpm total, depending on system resistance.

Each pump operates at the common system head.

Unequal Parallel Pumps

Parallel pumps should have compatible performance characteristics.

Pumps with very different curves can share flow unevenly and may operate poorly.

Operators should use approved system operating procedures.

Series Pump Operation

Pumps operating in series pass approximately the same flow through each pump.

Their heads are added.

Series operation is used when greater total head is required.

A simplified relationship is:

Total Head ≈ Head of Pump 1 + Head of Pump 2

Parallel Versus Series

Remember the general distinction:

• parallel pumps primarily increase flow capacity;
• series pumps primarily increase head capability.

Lead-Lag Operation

Multiple-pump stations often use lead-lag sequencing.

The lead pump starts first.

A lag pump starts when additional capacity is needed.

Triggers may include:

• high wet-well level;
• low system pressure;
• high demand;
• insufficient flow from one pump.

Pump Rotation Scheduling

Control systems may alternate lead duty among pumps to balance:

• run time;
• starts;
• wear.

Balanced run hours can help prevent one pump from aging much faster than the others.

Frequent Cycling

Excessive starts and stops can increase wear on:

• motors;
• starters;
• contactors;
• couplings;
• check valves.

Possible causes include:

• poor control setpoints;
• small wet-well working volume;
• incorrect pressure-band settings;
• oversized pumps;
• level-sensor problems.

Minimum Flow

Many centrifugal pumps require a minimum continuous flow to avoid excessive internal recirculation or heating.

The minimum acceptable flow depends on the pump design.

Operators should follow manufacturer limits rather than assume that any low flow is acceptable.

Maximum Flow

Operating beyond the recommended maximum flow may increase:

• motor load;
• cavitation risk;
• vibration;
• hydraulic instability.

Efficiency and Energy Cost

A pump operating far from BEP can consume more energy for each gallon moved.

Energy performance may improve through:

• correct pump selection;
• appropriate VFD control;
• reduced unnecessary throttling;
• clean pipelines and filters;
• restored pump condition;
• proper operating sequence.

Compare Pump Performance Over Time

Operators can establish a baseline using:

• flow;
• head;
• motor current;
• speed;
• power;
• vibration.

If the same pump at the same speed gradually produces less flow at similar system conditions, possible causes include:

• impeller wear;
• increased internal clearances;
• system restriction;
• instrument error.

Performance Testing

A basic pump performance check may compare measured:

• flow;
• suction pressure;
• discharge pressure;
• speed;
• electrical load;

with the expected pump curve.

The comparison should use the correct:

• pump model;
• impeller diameter;
• speed.

Do Not Compare to the Wrong Pump Curve

A pump curve for another impeller diameter or speed may produce misleading conclusions.

Verify:

• equipment identification;
• installed impeller;
• motor speed;
• VFD operating frequency.

Wear Affects Pump Performance

Centrifugal pump performance can decline because of:

• impeller wear;
• erosion;
• corrosion;
• increased wear-ring clearance;
• internal recirculation.

Performance loss may appear as lower flow or lower head under similar conditions.

Wear Rings

Some centrifugal pumps use wear rings to limit leakage between high-pressure and low-pressure areas inside the pump.

As clearance increases:

• internal recirculation increases;
• efficiency can decrease;
• pump capacity can decrease.

Impeller Fouling

Debris, scale, or solids can reduce effective impeller performance.

Possible symptoms include:

• reduced flow;
• vibration;
• reduced head;
• higher or unusual motor load.

VFD Minimum Speed

Reducing pump speed too far may create problems such as:

• insufficient cooling;
• poor motor performance;
• inadequate flow;
• unstable hydraulic operation;
• failure to meet minimum process requirements.

Minimum allowable speed should follow system and equipment requirements.

Control Point Matters

A VFD may be controlled by:

• pressure;
• flow;
• tank level;
• wet-well level;
• process demand.

The location and quality of the control signal affect system response.

Common Centrifugal Pump Performance Mistakes

• Assuming a pump always produces one fixed flow.
• Ignoring the system curve.
• Operating far from BEP without recognizing the effect.
• Running against a closed discharge for too long.
• Operating excessively near runout.
• Throttling the suction side to control normal flow.
• Applying affinity laws without considering their assumptions.
• Assuming two pumps in parallel produce exactly twice the flow.
• Confusing parallel operation with series operation.
• Comparing measured performance with the wrong pump curve.
• Ignoring pump wear when performance gradually declines.
• Failing to compare flow, head, current, and speed together.
• Ignoring excessive pump cycling.

A Practical Centrifugal Pump Performance Review

1. Identify the correct pump and pump curve.
2. Confirm pump speed and impeller diameter.
3. Measure or review actual flow.
4. Review suction pressure.
5. Review discharge pressure.
6. Estimate total pump head.
7. Compare the operating point with the pump curve.
8. Check how close operation is to the preferred range or BEP.
9. Review motor current or power.
10. Review vibration, noise, and bearing condition.
11. Check valve positions and system resistance.
12. Compare current performance with historical data.

What to Remember for the Exam

• A centrifugal pump does not produce one fixed flow.
• Actual operating flow depends on both the pump curve and system curve.
• The operating point is near the intersection of the pump and system curves.
• Increasing system resistance generally reduces centrifugal-pump flow.
• Decreasing system resistance generally increases flow.
• BEP means Best Efficiency Point.
• Operating far from BEP can increase vibration, wear, and energy use.
• Shutoff head occurs at approximately zero flow.
• Runout occurs at very high flow and low head.
• Throttling the discharge increases system resistance and reduces flow.
• Normal flow control should not be accomplished by unnecessarily restricting pump suction.
• A VFD changes pump speed and can control flow and head.
• Under the affinity laws, flow changes approximately in direct proportion to speed.
• Head changes approximately with the square of speed.
• Power changes approximately with the cube of speed.
• Water horsepower for water is approximately Q × H ÷ 3960.
• Parallel pumps primarily increase available flow.
• Series pumps primarily increase available head.
• Two identical pumps in parallel do not necessarily produce exactly twice the single-pump flow.
• Always compare performance with the correct curve for the actual impeller diameter and speed.