Friction Loss & System Resistance

Whenever water moves through a pipe or treatment component, some hydraulic energy is lost because of resistance. This loss is commonly called friction loss or head loss. Operators need to understand friction because it affects pressure, pump performance, flow capacity, energy use, and the hydraulic grade line.

Friction loss is not caused by one factor alone. It depends on flow, pipe diameter, pipe length, internal roughness, valves, fittings, and other restrictions in the system.

What Is Friction Loss?

Friction loss is hydraulic energy lost as water moves through a system.

It occurs because moving water must overcome resistance from:

• pipe walls;
• valves;
• elbows;
• tees;
• meters;
• filters;
• changes in direction;
• changes in diameter;
• other flow restrictions.

Friction loss is usually expressed in feet of head.

Friction Loss Reduces Available Head

As water flows through a pipeline, hydraulic grade decreases in the direction of flow.

The lost head is no longer available to provide:

• pressure;
• elevation gain;
• flow through downstream equipment.

This is why pressure measured downstream is often lower than pressure measured upstream.

Friction Loss Increases with Flow

One of the most important hydraulic relationships is:

higher flow generally produces greater friction loss

When operators increase flow through the same pipe, velocity increases and energy loss rises.

This effect is usually nonlinear, meaning that doubling the flow can increase friction loss by much more than two times.

Pipe Diameter Has a Major Effect

Smaller pipe diameter produces higher velocity for the same flow.

Higher velocity increases friction loss.

Therefore:

• small pipes generally produce greater head loss;
• large pipes generally produce lower head loss for the same flow.

This is one reason pipe diameter has such a strong effect on system capacity.

Example: Same Flow Through Different Pipes

Suppose the same 600 gpm is sent through two pipes.

A smaller pipe has:

• less cross-sectional area;
• higher velocity;
• greater friction loss.

A larger pipe has:

• more cross-sectional area;
• lower velocity;
• lower friction loss.

The exact head loss depends on pipe material, length, diameter, and the hydraulic equation used.

Pipe Length

Friction loss increases with pipe length.

For the same pipe and flow:

• twice the pipe length produces approximately twice the friction loss;
• half the pipe length produces approximately half the friction loss.

This linear relationship with length is useful when friction loss is given per unit length.

Friction Loss per 100 Feet

Pipe-friction tables often express loss as:

feet of head loss per 100 feet of pipe

Example:

A pipe has a friction loss of 4.0 feet per 100 feet at the operating flow.

The pipeline is 750 feet long.

Total Loss = 4.0 ft/100 ft × 750 ft

Total Loss = 30 ft

The pipe produces approximately 30 feet of friction head loss.

Pipe Roughness

Rougher pipe surfaces generally produce greater resistance.

Pipe roughness can increase because of:

• corrosion;
• tuberculation;
• scale;
• biological growth;
• sediment;
• age and deterioration.

As internal roughness increases, the same flow may require more head.

Older Pipe Can Lose Hydraulic Capacity

A pipeline may still be structurally intact but carry less flow efficiently because internal roughness has increased.

Operators may observe:

• lower downstream pressure;
• greater pump head requirement;
• reduced flow;
• increased pumping energy.

Minor Losses

Losses through fittings, valves, entrances, exits, and other localized components are often called minor losses.

The word minor does not mean they are always small.

In systems with many valves and fittings, these losses can be significant.

Examples of Local Resistance

Components that create additional resistance include:

• elbows;
• tees;
• check valves;
• partially closed valves;
• meters;
• strainers;
• reducers;
• expansions;
• filter media;
• control valves.

Partially Closed Valves

A partially closed valve creates additional resistance.

This causes:

• greater head loss;
• lower downstream pressure;
• reduced flow in many systems.

An unexpectedly low flow may therefore result from a valve position problem rather than a pump failure.

Restrictions and Blockages

Accumulated material can increase system resistance.

Examples include:

• debris in strainers;
• scale in piping;
• partially blocked suction lines;
• clogged filters;
• solids accumulation in wastewater piping.

Operators may detect these conditions through increasing differential pressure or decreasing flow.

Differential Pressure Shows Head Loss

Pressure measurements before and after a component can be used to determine pressure loss.

Differential Pressure = Upstream Pressure - Downstream Pressure

For water:

Head Loss, ft = Differential Pressure, psi × 2.31

Example:

Pressure before a strainer is 48 psi and after the strainer is 43 psi.

Differential Pressure = 48 - 43 = 5 psi

Head Loss = 5 psi × 2.31 = 11.55 ft

The strainer is producing about 11.6 feet of head loss.

Increasing Differential Pressure Can Indicate Fouling

If differential pressure across a component increases over time at approximately the same flow, resistance is increasing.

This may indicate:

• filter loading;
• strainer blockage;
• scale buildup;
• debris accumulation.

Trend data can help identify when cleaning or maintenance is needed.

Hazen-Williams Equation

Water-distribution calculations often use the Hazen-Williams equation to estimate friction loss in pressurized water pipes.

The equation relates friction loss to:

• flow;
• pipe diameter;
• pipe length;
• pipe roughness coefficient.

The C-factor represents the hydraulic smoothness of the pipe.

A larger C-factor generally represents a smoother pipe and lower friction loss.

Hazen-Williams C-Factor

The C-factor depends on:

• pipe material;
• age;
• internal condition;
• deposits.

Operators should use the value specified by the applicable design reference or problem rather than assume one universal value.

Darcy-Weisbach Equation

The Darcy-Weisbach equation is another method for calculating friction loss.

It uses:

• friction factor;
• pipe length;
• pipe diameter;
• velocity;
• gravity.

It is broadly applicable and is based on fundamental hydraulic relationships.

Most operator exam problems will provide the formula or sufficient information if a detailed Darcy-Weisbach calculation is required.

Equivalent Length Method

Valves and fittings may sometimes be represented by an equivalent length of straight pipe.

For example, several fittings might create the same resistance as an additional 80 feet of pipe.

If the actual pipe is 500 feet long:

Effective Length = 500 ft + 80 ft = 580 ft

Friction loss can then be estimated using the total effective length.

System Resistance

System resistance is the total hydraulic resistance the pump must overcome at a particular flow.

It can include:

• static elevation difference;
• pipe friction;
• valves and fittings;
• meters;
• treatment equipment;
• required discharge pressure.

As flow increases, the friction component of system resistance increases.

Static Head and Friction Head

It is useful to separate:

• static head, which comes mainly from elevation and pressure requirements;
• friction head, which depends on flow through the system.

A simplified relationship is:

Total System Head = Static Head + Friction Head

Static Head Does Not Depend on Flow

If a pump lifts water from one open tank to another, the elevation difference between the two water surfaces is essentially the static head.

That elevation difference exists whether flow is high or low.

Friction loss, however, changes with flow.

System Curve

A system curve shows how much head the piping system requires at different flow rates.

At zero flow:

• friction loss is approximately zero;
• system head may equal the static head.

As flow increases:

• friction increases;
• required system head increases.

The system curve therefore generally rises as flow increases.

Pump Curve and System Curve

A pump curve shows the head a pump can produce at different flows.

The system curve shows the head the system requires at different flows.

The point where the two curves intersect is the approximate operating point.

At that point:

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

Increasing Resistance Changes the Operating Point

If system resistance increases, the system curve becomes steeper.

For a typical centrifugal pump, the operating point usually shifts toward:

• lower flow;
• higher pump head.

Possible causes include:

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

Decreasing Resistance Changes the Operating Point

If system resistance decreases, the operating point may shift toward:

• higher flow;
• lower pump head.

This can occur when:

• a valve opens further;
• a blockage is removed;
• a cleaner pipe path becomes available;
• parallel piping is placed in service.

Parallel Pipes Reduce Resistance

When parallel flow paths are available, total hydraulic resistance can decrease.

The flow divides between branches according to their resistance.

A lower-resistance branch generally carries more flow.

Opening a second parallel line may therefore increase total system flow.

Series Components Add Head Loss

When water flows through components in series, their head losses add.

For example:

Total Head Loss = Pipe Loss + Valve Loss + Meter Loss + Filter Loss

If:

• pipe loss = 18 ft;
• valve loss = 4 ft;
• meter loss = 3 ft;
• filter loss = 12 ft;

then:

Total Head Loss = 18 + 4 + 3 + 12 = 37 ft

Example: Total Pump Head Requirement

A pump must overcome:

• 70 ft static head;
• 25 ft pipe friction;
• 8 ft valve and fitting losses.

Total required head:

70 + 25 + 8 = 103 ft

The pump must provide approximately 103 feet of head at that operating flow.

Friction Loss and Energy Cost

Additional friction requires additional pump energy.

Examples of conditions that can increase energy use include:

• undersized piping;
• partially closed valves;
• dirty filters;
• rough or scaled pipe;
• blocked strainers.

Reducing unnecessary resistance can reduce pumping energy.

Throttling

Throttling a valve intentionally increases system resistance to reduce flow.

This can be an effective control method, but energy is lost across the valve.

Operators should understand that a throttled valve does not make the pump produce less energy efficiently in every case. It increases resistance and changes the operating point.

Variable-Speed Pumping

Variable-speed drives can change pump speed and shift pump performance.

Reducing speed can often reduce flow and head without creating as much artificial resistance as throttling.

Actual energy savings depend on the system and control strategy.

Suction-Side Resistance

High resistance on a pump suction line is especially important.

Possible causes include:

• clogged suction strainer;
• partially closed suction valve;
• undersized suction piping;
• long suction line;
• excessive fittings;
• low source level.

Excessive suction loss can reduce pump performance and contribute to cavitation.

Discharge-Side Resistance

High discharge resistance may cause:

• lower pump flow;
• higher discharge pressure;
• operation away from the preferred pump operating region.

Operators should compare flow and pressure readings with normal historical values.

Pressure Patterns Can Reveal Resistance

Suppose a pump normally operates at:

• 70 psi discharge pressure;
• 900 gpm.

Later it operates at:

• 82 psi discharge pressure;
• 650 gpm.

If pump speed is unchanged, the combination of higher pressure and lower flow may suggest increased downstream resistance.

Possible causes include:

• closed valve;
• blocked pipe;
• dirty filter;
• another restriction.

Pressure Loss Can Also Indicate Leakage

Not every pressure problem is caused by resistance.

Unexpected low pressure may also result from:

• large demand;
• main break;
• leak;
• pump problem;
• low storage level;
• open hydrant;
• incorrect valve configuration.

Operators should evaluate pressure together with flow and system conditions.

Filter Head Loss

As particles accumulate in filter media, resistance increases.

Operators may observe increasing:

• head loss;
• differential pressure.

Head-loss trend is one indicator used in filter operation.

Collection-System Resistance

Wastewater force mains also experience friction loss.

Resistance can increase because of:

• solids accumulation;
• air pockets;
• partially closed valves;
• pipe roughness;
• smaller effective diameter.

Increased force-main resistance can change pump flow and run time.

Air in Pipelines

Air pockets can affect hydraulic performance.

Depending on the system, trapped air may:

• reduce effective flow area;
• increase head loss;
• create unstable flow;
• affect pump performance.

Proper air-release and vacuum-control devices may be important in pipelines with significant elevation changes.

Use Trends to Detect Increasing Resistance

Operators can track:

• flow;
• suction pressure;
• discharge pressure;
• differential pressure;
• pump speed;
• motor current;
• filter head loss.

Changes in these values can reveal developing resistance problems.

Common Friction-Loss Mistakes

• Assuming friction loss is independent of flow.
• Ignoring pipe diameter when comparing head loss.
• Assuming all pressure loss is caused by elevation.
• Ignoring valves and fittings.
• Using friction loss per 100 feet without multiplying by actual pipe length.
• Forgetting to include equipment head losses.
• Assuming minor losses are always insignificant.
• Ignoring increased pipe roughness with age.
• Confusing static head with friction head.
• Assuming a pump always produces the same flow regardless of system resistance.
• Ignoring suction-side restrictions.
• Failing to investigate increasing differential pressure.

A Practical System Resistance Problem Method

1. Identify the required flow.
2. Determine static elevation or pressure requirements.
3. Determine pipe length and diameter.
4. Determine friction loss for the pipe at the operating flow.
5. Add valve, fitting, meter, filter, and equipment losses.
6. Add all losses in series.
7. Combine static head and friction head.
8. Compare required system head with pump capability.
9. Check whether the calculated flow and pressure are reasonable.
10. Use actual operating data to verify assumptions where possible.

What to Remember for the Exam

• Friction loss is hydraulic energy lost as water moves through a system.
• Friction loss is commonly expressed in feet of head.
• Higher flow generally increases friction loss.
• Smaller pipe diameter generally increases friction loss for the same flow.
• Longer pipe produces more friction loss.
• Rougher pipe generally produces greater resistance.
• Valves, fittings, meters, filters, and restrictions create additional head loss.
• Differential pressure can be converted to head loss using approximately 2.31 ft/psi for water.
• Losses through components in series are added together.
• Static head and friction head are different.
• Static head is largely independent of flow, while friction head increases with flow.
• A system curve shows required system head at different flows.
• The pump operating point occurs near the intersection of the pump curve and system curve.
• Increasing system resistance usually reduces flow for a typical centrifugal pump.
• Partially closed valves, fouled filters, and pipe blockages increase resistance.
• Excessive suction resistance can contribute to poor pump performance and cavitation.
• Increasing differential pressure at the same flow can indicate fouling or blockage.
• Operators should compare pressure, flow, and historical trends when troubleshooting resistance problems.