Water hammer is a pressure wave caused by a rapid change in water velocity. In irrigation systems, it can occur when a pump stops suddenly or a valve closes quickly. Knocking in the pipes is a clue. The timing of the knock and a pressure recording can help identify what happened.
What is water hammer
When water changes speed rapidly, the compressibility of the liquid and the elasticity of the pipe allow the disturbance to propagate like a wave. The wave can be reflected where the path, diameter or flow conditions change. Depending on the maneuver and the section of the network, the pressure may rise above the normal value or fall below atmospheric pressure. Queiroz et al., water hammer test
For a sudden change in velocity in a full pipe, the Joukowsky equation estimates the magnitude of the initial pressure change as |Δp| = ρ × c × |Δv|. Here, ρ is the water density, c is the wave speed and Δv is the change in water velocity. With density in kilograms per cubic metre and speeds in metres per second, the result is in pascals. For a simple pipe section of length L, 2L/c is the approximate round-trip time of the wave. It helps compare the duration of the operation with the response time of that section. UFCG, Joukowsky relation · UTFPR, propagation time
At a given wave speed, a larger change in water velocity tends to produce a larger initial pressure change. Wave speed also depends on the pipe material, diameter, wall thickness and support conditions. In field and laboratory studies, Soares and colleagues estimated about 1,130 m/s in a cast-iron water main, 440 m/s in a PVC test setup and 350 m/s in an HDPE test setup. These values apply to the systems studied. The design review must check maximum and minimum pressures, accounting for wave reflections, the valves and pump, and possible water column separation. Soares et al., field and laboratory studies
Main causes in irrigation
Sudden pump stop
A power outage can stop pumping and trigger a pressure drop that travels along the supply main. Flow can reverse depending on the elevation difference and the system response. A sudden slowdown of reverse flow at the check valve can cause a pressure surge. The response depends on the pipeline layout, pump inertia and the closing characteristics of the check valve. Bermad, transients during power outage · Ferreira et al., simulation of pumping failure
Fast closing of valves
When an irrigation-zone valve rapidly cuts off flow, a pressure surge can travel upstream. Assess the zone-switching sequence and valve closure time together with the pump and the other valves in the system. Valve position alone does not indicate how much flow is available or how much pressure the pump can generate. Grundfos, valve closure and water hammer
Trapped air and water column separation
During filling, air already in the pipe must escape in a controlled manner. In some shutdowns, water columns can separate. If air enters the pipe, some of it can escape rapidly when the columns rejoin. Sudden closure of an air valve under these conditions can also cause a pressure spike. The analysis needs to consider air inlet and outlet points, including on relatively flat terrain. Bermad, air and pressure waves in irrigation
Poorly positioned check valve in wells
For installations with a submersible pump, Grundfos describes cases in which the placement of check valves or a leak in the lower check valve can leave a section of the discharge pipe under a partial vacuum. On the next start, water can move through that section and strike the stationary water column. To inspect the installation, check the location of the check valves relative to the water level and consult the pump manufacturer’s instructions. Grundfos, water hammer in submersible pumps
Signs that the network is experiencing water hammer
- Knocks or clicks when starting up, shutting down or changing sectors.
- Vibration in pipes, fittings and above-ground pipe assemblies during these operations.
- Leaks recurring at the same points, joints that come apart or recurring valve damage.
- Tubes that deform inwards, a sign compatible with low internal pressure in susceptible tubes.
These signals guide the investigation. Leaks and ruptures can also have other causes. To distinguish the scenarios, write down the section, the maneuver, the sector in operation and whether there was a power interruption. The technical team can compare these occurrences with a pressure record capable of capturing rapid variations. A steady-state pressure reading may miss the peak or dip. Soares et al., measurement of transients with transducers
How to protect piping against water hammer
The first step is to identify which maneuvers lead to pressures outside the allowable range. The assessment brings together the terrain profile, the length and dimensions of the pipes, the flow rates, the pump curves and the maneuvering times. Transient simulation allows you to test, for example, shutdown due to power outages and changing sectors before choosing protection. UFU, simulation and comparison of devices in the Capim Branco water pipeline
Operation measures
- Gradual filling. Starting with a controlled flow can reduce the impact at the end of filling. The procedure must allow for air to escape.
- Controlled closing. If the valve allows the closing speed to be regulated, the time must be defined for the network and checked during commissioning.
- Scheduled start and stop. The controller can smooth out normal maneuvers. A power failure must be evaluated separately because it can interrupt pump control.
Protection devices
- Check valve. Limits reverse flow. The type and speed of closure influence the peak that may occur when the water column returns.
- Relief valve. Discharges water when pressure exceeds the set point. Its capacity must match the expected pressure surge. Protection against low pressure is assessed separately.
- Surge-anticipating valve. Certain models use the initial pressure drop to open before the return wave arrives. The device discharges part of the water and closes in a controlled manner. Bermad, 735-M surge-anticipating valve data sheet
- Air valve. Depending on the model, it admits air when pressure falls below atmospheric pressure or releases air during filling and operation. Models with controlled air discharge can reduce the pressure spike associated with air valve closure. See how the functions differ in the guide to air valves in irrigation.
- Hydropneumatic surge vessel. It can supply or receive water during the transient, mitigating pressure drops and peaks. In a UFU case study, it was the device with the best result among the alternatives simulated in that pipeline. UFU, study of the Capim Branco pipeline
Sizing needs to consider operating pressures and extreme transient pressures when selecting pipes, fittings and valves. To review the operating point, see how to choose the pump for irrigation.
FAQ
Are water hammer and cavitation the same thing?
Water hammer is the propagation of a pressure wave following a rapid change in flow. Cavitation involves the formation of vapor when the local pressure drops low enough and the subsequent collapse of the cavities when the pressure rises. The phenomena can occur together. If the transient produces separation of the liquid column, the reunion of the columns may cause another pressure spike.
Can drip irrigation systems experience water hammer?
Yes. Assess the risk in the supply pipeline, the mainline and the irrigation zones subject to filling, drainage or rapid closure. In subsurface drip irrigation, soil particles can also enter through the emitter when pressure falls below atmospheric pressure during drainage. Address this risk when designing the air intake and selecting emitters. Embrapa, clogging of emitters
Does irrigation automation help avoid the problem?
It can help to execute a defined sequence of maneuvers for the network. The design needs to verify that the pump and valves respond as expected in normal operation and during a failure. Learn how components integrate into automated irrigation on farm.
Conclusion
If the pipes knock or leaks keep returning to the same spot, record what happened and which operation was underway. For an existing system, collect the pipeline layout, pipe diameters, pump model and valve locations. For a new system, include valve and pump operations and power outages in the design analysis.
The Perin Irrigação can evaluate the scenario based on the municipality, crop, area and water source. Speak to the team via WhatsApp (14) 99824-5219 or see options irrigation systems. If you prefer, send an email.
contato@perinirrigacao.com.br
Sources consulted
Research conducted on November 7, 2026.
- Queiroz et al. Development of water hammer simulator equipment. Annals of the XXI ENMC and IX ECTM, 2018.
- Soares et al. Water hammer assessment considering the dynamic effects associated with the piping material. XVIII Brazilian Symposium on Water Resources, 2009.
- Ferreira, Barbosa and Castro. Computational modeling of water hammer in plastic conduits. DAE Magazine, 2016.
- Tonin. Analysis of water hammer in the raw water pipeline of the Capim Branco system. UFU, 2024.
- Moura. Joukowsky relation, equation 4.10. UFCG, 2025.
- UTFPR. Analysis of transients and pressure wave round trip time.
- Bermad. How to reduce water hammer in irrigation systems.
- Bermad. Protection of the irrigation system against waves associated with power outages.
- Bermad. 735-M surge-anticipating valve. Data sheet.
- Grundfos. Water hammer.
- Grundfos. Water hammer in submersible pumps.
- Almeida. Clogging of emitters in localized irrigation. Embrapa, Documents 182, 2009.
