What Causes Water Hammer in Pump Systems?
If you have ever heard a loud bang from a pipe just after a pump stops, you may have experienced water hammer.
It can sound like something has gone seriously wrong. Sometimes, it is simply an irritating knock. But that noise is only the symptom of what is happening inside the system.
Water hammer is a hydraulic transient caused by a rapid change in flow velocity. That change creates a pressure wave which travels through the pipework, potentially putting significant stress on pumps, valves, pipework and other components.
So, what causes it, and more importantly, how can you reduce the risk?
What causes water hammer?
At its simplest, water hammer occurs when the velocity of liquid flowing through a closed pipeline changes rapidly.
Unlike air, water does not compress under normal conditions. This is an important part of understanding water hammer. When moving water is forced to slow down or stop suddenly, there is very little room for that change to be absorbed by compression. The moving water has both momentum and kinetic energy, so this sudden change can produce a pressure wave through the system.
The water’s kinetic energy is transferred into pressure energy and elastic energy within the liquid and pipework, while some energy is dissipated through friction, turbulence, vibration and sound.
This can happen when:
- A pump starts or stops suddenly
- A pump trips following a power failure
- A valve closes rapidly
- A check valve closes following pump shutdown
- A pipeline is filled too quickly
- Flow conditions change significantly elsewhere in the system
So, how much pressure can this actually create?
This is where the Joukowsky equation comes in:
Δp = ρaΔv
Where Δp is the change in pressure, ρ is the liquid density, a is the pressure-wave velocity, and Δv is the change in flow velocity.
The important point is that even a relatively small change in velocity can produce a substantial pressure change because pressure waves travel very quickly through water and pipework.
The Joukowsky equation provides a useful first indication of the potential surge, but real systems are more complicated. Pipe material, diameter, length, valve characteristics and the timing of the flow change can all influence the actual transient.
Why do pumps cause water hammer?
Pump starts and stops are some of the most common causes of pressure transients.
With a fixed-speed pump, starting the motor can move the pump rapidly from zero speed towards its operating condition, accelerating the water through the system. If that change in flow velocity happens quickly enough, it can create a pressure transient.
Shutdown presents the opposite problem.
The pump stops producing head, but the water in the discharge pipe does not simply stop at the same instant. It is still moving. If the flow decelerates too quickly, the resulting pressure wave can travel back through the system.
A sudden pump trip or power failure can make this particularly severe because the pump has no opportunity to control how quickly the flow decelerates.
This is why pump selection and control strategy need to be considered together. Getting the pump duty point right is important, but it is only part of the picture. The way the system starts, stops and responds to changing demand matters too.
What role does the non-return valve play?
The non-return valve, or check valve, has a simple job: stop water flowing backwards when the pump stops.
But how it does that matters.
If the pump stops and the flow reverses before the check valve closes, the valve may close against an established reverse velocity. That sudden change in flow can create another pressure surge.
This means choosing a non-return valve is not simply a case of finding one that meets the required pressure and flow rating. Its closing characteristics also need to suit the pump and the wider system.
Does pipework affect water hammer?
Absolutely. Water hammer is not just a pump issue.
The length, diameter and material of the pipe all influence how a pressure transient behaves.
A long pipeline contains a much larger moving column of water than a short one. When the flow changes, the pressure wave travels along the pipe and can reflect from valves, tanks and other changes in the system.
Pipe diameter matters too. For a given flow rate, a smaller pipe produces a higher flow velocity. This means that a given change in flow rate can result in a greater change in velocity, increasing the potential for a significant pressure transient.
The pipe material and wall characteristics also influence the speed at which the pressure wave travels.
Put simply, the pump, pipework, valves and controls all form part of the same hydraulic system. Changing one part can affect how the whole system behaves.
What happens if the pressure drops too far?
When people talk about water hammer, the focus is usually on excessive pressure. But a severe transient can also cause pressure to fall significantly.
If the pressure drops sufficiently low, the continuous column of water can separate. This is known as column separation.
A vapour cavity can then form in the pipeline. As the pressure recovers, that cavity can collapse, and the separated water columns can come back together. The resulting impact can create another pressure surge.
So, a single transient event can involve both significant pressure drops and subsequent pressure increases.
Can variable-speed pumps reduce water hammer?
They can, and this is one of the advantages of controlling pump speed.
A variable-speed drive can control how quickly the pump accelerates and decelerates rather than switching immediately between zero and full speed. This allows the flow velocity to change more gradually, reducing the potential severity of the resulting pressure transient.
The same principle applies during shutdown. Instead of stopping the pump abruptly, the controller can reduce speed in a controlled manner before stopping it.
That does not mean a variable-speed pump will automatically eliminate water hammer. Poorly selected valves, excessive flow velocities, unsuitable pipework or other hydraulic conditions can still create transients.
However, controlling the rate at which flow changes can make a significant difference to how the system behaves.
What does a pressure vessel do?
Pressure vessels provide hydraulic buffering within many booster set systems.
The vessel contains pressurised water and can accommodate smaller changes in demand without requiring the pumps to respond immediately to every fluctuation. This can help maintain more stable pressure and reduce unnecessary pump cycling.
A pressure vessel can also help damp pressure fluctuations, but bigger is not automatically better. Increasing the size of the vessel is not, by itself, an answer to every water hammer problem.
Other forms of surge protection can also be used depending on the application. Surge arrestors, for example, are designed to absorb or control pressure transients and can be part of a wider surge protection strategy.
As with the rest of the system, the right approach depends on the application, pump arrangement, controls, pipework and operating conditions.
Reducing water hammer starts with system design
A pump can be correctly selected for its required duty and still produce undesirable transients if the wider system has not been considered.
At Trebles, we design and manufacture complete systems with the whole application in mind. Pump duty, controls, pressure vessels, valves and associated equipment all need to work together to provide stable system operation.
For straightforward boosting applications, look at our Booster Sets: