Pressure, Head & Hydraulic Grade

Pressure and head are fundamental hydraulic concepts in water and wastewater systems. Operators use them to understand pumps, tanks, pipelines, distribution systems, force mains, filters, and many other processes where water moves under pressure.

Pressure describes force applied over an area. Head expresses hydraulic energy as the height of a column of liquid. Because head can represent pressure, elevation, velocity, and energy loss in the same units of length, it is especially useful for analyzing water systems.

Pressure

Pressure is force applied over a unit area.

In water and wastewater operations, pressure is commonly expressed in:

• pounds per square inch, or psi;
• feet of water, when pressure is expressed as head.

Pressure gauges on pumps, pipelines, filters, and distribution systems usually display psi.

Head

Head represents the energy of water as an equivalent vertical height of water.

Head is commonly expressed in feet.

Using feet of head allows several forms of hydraulic energy to be compared directly.

Important types of head include:

• pressure head;
• elevation head;
• velocity head;
• friction head loss.

Pressure Head

Pressure head is the amount of hydraulic energy caused by water pressure.

For water under normal operating conditions, the commonly used relationship is:

1 psi = approximately 2.31 feet of water

and:

1 foot of water = approximately 0.433 psi

These relationships are extremely useful in operator calculations.

Convert Pressure to Head

To convert pressure in psi to feet of head:

Head, ft = Pressure, psi × 2.31 ft/psi

Example:

A pressure gauge reads 60 psi.

Head = 60 psi × 2.31 ft/psi

Head = 138.6 ft

The pressure corresponds to approximately 139 feet of water head.

Convert Head to Pressure

To convert feet of water head to psi:

Pressure, psi = Head, ft × 0.433 psi/ft

or:

Pressure, psi = Head, ft ÷ 2.31

Example:

A tank water surface is 100 feet above a pressure gauge.

Pressure = 100 ft × 0.433 psi/ft

Pressure = 43.3 psi

Ignoring friction and other losses, the gauge would experience approximately 43 psi.

Why Elevation Creates Pressure

Water stored at a higher elevation has gravitational potential energy.

When that water is connected to a lower point through a full pipeline, the elevation difference creates pressure.

This is why elevated storage tanks can maintain distribution-system pressure without a pump running continuously.

Elevation Head

Elevation head is the hydraulic energy associated with the elevation of water relative to a selected reference point.

If one point is 50 feet higher than another, there is a 50-foot difference in elevation head.

The reference elevation can be:

• mean sea level;
• a facility datum;
• another defined reference.

The exact reference is less important than using the same reference consistently throughout the calculation.

Pressure Head Plus Elevation Head

For many basic operator problems, hydraulic energy can be understood by combining:

Hydraulic Head = Elevation Head + Pressure Head

Velocity effects may also be important in a complete energy analysis, but for many distribution-system and static-pressure problems, elevation and pressure are the main components.

Hydraulic Grade

The hydraulic grade at a point represents the elevation to which water would rise in an open vertical tube connected to that point.

It combines:

• elevation head;
• pressure head.

The basic relationship is:

Hydraulic Grade = Elevation + Pressure Head

Hydraulic Grade Line

The Hydraulic Grade Line, commonly abbreviated HGL, is an imaginary line connecting the hydraulic grade at different locations in a system.

It represents the level to which water would rise in piezometer tubes connected along the pipeline.

The HGL is useful for understanding:

• system pressure;
• elevation effects;
• friction loss;
• pump effects;
• storage-tank influence.

Reading Pressure from the Hydraulic Grade Line

Pressure head at any point can be found from:

Pressure Head = Hydraulic Grade Elevation - Pipe Elevation

Example:

The hydraulic grade at a location is 520 feet. The water main is at elevation 430 feet.

Pressure Head = 520 ft - 430 ft = 90 ft

Convert to pressure:

Pressure = 90 ft ÷ 2.31

Pressure = approximately 39 psi

Elevation Changes Pressure

If hydraulic grade remains approximately constant, pressure decreases as elevation increases.

Likewise, pressure increases as elevation decreases.

This is an important concept in hilly distribution systems.

For approximately every 2.31 feet of elevation increase, pressure decreases by about 1 psi, ignoring friction and other changes.

Example: Pressure Change with Elevation

A pipeline rises 46.2 feet between two locations.

Ignoring friction:

Pressure Change = 46.2 ft ÷ 2.31 ft/psi

Pressure Change = 20 psi

The higher location will have approximately 20 psi less pressure than the lower location.

Static Pressure

Static pressure is the pressure in a system when water is not flowing at the point being evaluated.

Under static conditions:

• friction loss caused by flow is essentially absent;
• pressure is primarily determined by hydraulic grade and elevation.

Static pressure is often higher than pressure measured while significant flow is occurring.

Residual or Dynamic Pressure

When water flows, energy is lost through friction and other hydraulic resistance.

The pressure available during flow is therefore usually lower than static pressure.

This operating pressure may be called:

• residual pressure;
• dynamic pressure;
• flowing pressure.

The exact term used depends on the application.

Why Pressure Drops During Flow

As water moves through a pipe, energy is lost because of:

• friction against the pipe wall;
• valves;
• fittings;
• meters;
• changes in direction;
• changes in pipe diameter;
• other restrictions.

These losses reduce the hydraulic grade in the direction of flow.

Hydraulic Grade Falls in the Direction of Flow

In a pipeline without a pump, the hydraulic grade normally decreases in the direction of flow because energy is lost to friction.

The greater the friction loss, the steeper the hydraulic grade line declines.

Factors increasing friction loss include:

• higher flow;
• smaller pipe diameter;
• rougher pipe;
• longer pipe;
• additional valves and fittings.

Pumps Raise Hydraulic Grade

A pump adds energy to the water.

Across an operating pump, the hydraulic grade increases by approximately the head added by the pump, subject to the details of the system.

This added head allows water to:

• move to a higher elevation;
• overcome friction;
• maintain pressure;
• enter storage;
• flow through treatment equipment.

Storage Tanks Establish Hydraulic Grade

An open storage tank connected to a distribution system strongly influences the system hydraulic grade.

At the tank water surface:

• pressure relative to atmosphere is zero;
• hydraulic grade is approximately equal to the water-surface elevation.

As the tank level rises, available hydraulic grade increases. As the tank level falls, hydraulic grade decreases.

Example: Elevated Tank Pressure

An elevated tank water surface is at elevation 680 feet.

A customer service connection is at elevation 570 feet.

Ignoring friction:

Pressure Head = 680 ft - 570 ft = 110 ft

Pressure = 110 ft ÷ 2.31

Pressure = approximately 47.6 psi

Pressure Zones

Distribution systems with large elevation differences may be divided into pressure zones.

Pressure zones help prevent:

• excessively high pressure at low elevations;
• inadequate pressure at high elevations.

Pressure zones may use:

• separate storage tanks;
• booster pumps;
• pressure-reducing valves;
• control valves.

Pressure-Reducing Valves

A pressure-reducing valve, or PRV, reduces downstream pressure.

PRVs are often used where:

• water flows from a high-pressure zone to a lower-pressure zone;
• low-elevation areas would otherwise experience excessive pressure.

The valve creates a controlled head loss.

Booster Pumps

Booster pumps add head to increase pressure or move water into higher-elevation areas.

A booster station may be used when existing hydraulic grade cannot provide adequate pressure to a higher zone.

Pressure Gauges

Pressure gauges provide useful information for:

• pump operation;
• filter condition;
• distribution-system pressure;
• pipeline troubleshooting;
• detecting restrictions or equipment problems.

Gauge readings should be evaluated together with equipment status, flow, and elevation.

Gauge Elevation Matters

A pressure reading applies at the elevation of the gauge connection.

If two gauges are at significantly different elevations, they can show different pressures even when connected to the same hydraulic grade.

This is not necessarily an instrument problem.

Pressure Difference Across Equipment

Operators often compare pressure before and after equipment.

The difference may indicate:

• head loss;
• filter loading;
• restriction;
• valve condition;
• pump performance.

This difference is commonly called differential pressure.

Differential Pressure

Differential Pressure = Upstream Pressure - Downstream Pressure

Example:

Pressure before a filter is 52 psi and pressure after the filter is 44 psi.

Differential Pressure = 52 psi - 44 psi = 8 psi

Convert to head loss:

8 psi × 2.31 ft/psi = 18.5 ft of head

The equipment is producing approximately 18.5 feet of head loss.

Head Loss

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

Losses can occur through:

• pipe friction;
• valves;
• fittings;
• meters;
• filters;
• treatment equipment.

Head loss is covered in greater detail in the Friction Loss & System Resistance article.

Velocity Head

Velocity head represents kinetic energy caused by the velocity of moving water.

It is expressed as:

Velocity Head = V² ÷ 2g

where:

• V = velocity;
• g = acceleration due to gravity.

Velocity head is important in complete hydraulic-energy calculations but is often relatively small compared with pressure and elevation head in typical distribution-system problems.

Total Energy Head

A complete hydraulic-energy analysis considers:

Total Head = Elevation Head + Pressure Head + Velocity Head

As water moves through a real system, energy is also lost through friction and other resistance.

Pumps add energy, while friction and control devices remove hydraulic energy.

Hydraulic Grade Line Versus Energy Grade Line

The hydraulic grade line represents:

Elevation Head + Pressure Head

The energy grade line represents:

Elevation Head + Pressure Head + Velocity Head

The energy grade line therefore lies above the hydraulic grade line by the amount of velocity head while water is flowing.

Negative Gauge Pressure

If the hydraulic grade falls below the pipe elevation, gauge pressure may become negative.

Negative pressure conditions can be important because they may:

• increase the risk of contaminant intrusion;
• contribute to cavitation at pump suction;
• indicate inadequate system pressure.

Operators should investigate unexpected low or negative pressures promptly.

Suction Pressure

Pump suction conditions strongly affect pump performance.

Low suction pressure may result from:

• low source level;
• blocked suction piping;
• closed valve;
• excessive suction lift;
• high friction loss;
• high flow.

Very low absolute pressure at pump suction can lead to cavitation.

Total Dynamic Head

Total Dynamic Head, or TDH, is the total head a pump must develop under operating conditions.

It accounts for the hydraulic energy difference between the suction and discharge sides of the pumping system, including elevation, pressure, velocity, and losses as applicable.

DEP pump training identifies TDH as the amount of energy a pump must develop to move liquid through the system.

Static Head

Static head is primarily the elevation difference between the source water surface and the discharge water surface when flow-related losses are not included.

Static head can be:

• positive when water must be lifted;
• reduced when the source elevation assists the flow.

Dynamic Head Includes Flow Losses

When flow occurs, friction and other losses must be added to the static requirement.

A simplified concept is:

Total Dynamic Head = Static Head + Friction and Other Losses

Actual pump-system calculations may also include pressure and velocity differences.

Example: Simplified Pump Head

A pump must move water from a reservoir water level at elevation 420 feet to a tank water level at elevation 500 feet.

Static head:

500 ft - 420 ft = 80 ft

If friction and minor losses at the design flow total 25 feet:

TDH = 80 ft + 25 ft = 105 ft

The pump must provide approximately 105 feet of head at that operating flow.

Specific Gravity Affects Pressure Conversion

The familiar relationship of 2.31 feet per psi applies approximately to water with a specific gravity near 1.0.

For liquids with a different specific gravity:

Pressure, psi = Head, ft × Specific Gravity ÷ 2.31

For most basic water-operator exam problems, water is assumed unless another liquid or specific gravity is provided.

Common Pressure and Head Mistakes

• Confusing feet of head with feet of elevation.
• Using 2.31 in the wrong direction during conversion.
• Forgetting that higher elevation usually means lower pressure when hydraulic grade is unchanged.
• Comparing pressures at different elevations without considering elevation.
• Assuming static pressure and flowing pressure are the same.
• Ignoring friction loss when water is flowing.
• Assuming a pump creates flow without also considering the system head it must overcome.
• Confusing hydraulic grade with pipe elevation.
• Using nominal tank height instead of actual water-surface elevation.
• Ignoring gauge location when interpreting pressure readings.

A Practical Pressure and Head Problem Method

1. Identify the elevations of the important points.
2. Identify known pressure values.
3. Convert psi to feet of head when necessary.
4. Place all heads on the same elevation reference.
5. Determine hydraulic grade.
6. Account for elevation differences.
7. Account for pumps adding head.
8. Account for friction and equipment losses when flow occurs.
9. Convert the final pressure head back to psi if required.
10. Check whether the result is physically reasonable.

What to Remember for the Exam

• Pressure is force per unit area and is commonly measured in psi.
• Head expresses hydraulic energy as a height of liquid.
• For water, 1 psi is approximately 2.31 feet of head.
• For water, 1 foot of head is approximately 0.433 psi.
• Pressure head is energy caused by pressure.
• Elevation head is energy caused by elevation.
• Hydraulic grade equals elevation head plus pressure head.
• The hydraulic grade line connects hydraulic-grade elevations through a system.
• Pressure head equals hydraulic grade minus pipe elevation.
• Increasing elevation generally decreases pressure if hydraulic grade remains the same.
• Static pressure is measured without significant flow-related friction loss.
• Flowing pressure is usually lower because friction consumes hydraulic energy.
• Pumps add head and raise the hydraulic grade.
• Open storage-tank water-surface elevation establishes hydraulic grade at the tank.
• Pressure-reducing valves intentionally reduce downstream hydraulic grade.
• Differential pressure can be converted to head loss using 2.31 ft/psi for water.
• Total head includes elevation, pressure, and velocity head.
• Total dynamic head represents the head a pump must develop under operating conditions.
• Always consider elevation when comparing pressure readings at different locations.