If you've ever heard a loud bang or thud from your piping system when a pump starts or stops, you've experienced water hammer. It's one of the most common — and most underestimated — causes of damage in industrial pumping systems. Left unaddressed, it bursts pipes, destroys valves, cracks fittings, and shortens pump life significantly.
The good news is it's largely preventable. And for most fixed-speed pumping applications, a properly specified soft starter is the most cost-effective solution.
What causes water hammer?
Water hammer is the term for pressure surges within a piping system caused by rapid changes in flow. There are two primary mechanisms.
The first is sudden valve closure. When a valve closes quickly, the leading edge of the fluid column stops moving — but the fluid behind it doesn't. That kinetic energy converts to pressure and sends a shockwave back upstream through the pipe. The math is unforgiving: water flowing at just 10 ft/s through a pipe with an instantaneously closed valve generates an additional 657 PSI of pressure. Most systems aren't designed to absorb that repeatedly.
The second mechanism is water column separation and closure — where the liquid column within a pipe separates under low pressure conditions and then suddenly slams back together, generating an equally damaging shockwave. This is particularly common during pump stop events.
Both mechanisms can be triggered by the same thing: pump starts and stops that change flow and pressure too rapidly.
Preventing water hammer: control the rate of change
The core principle of water hammer prevention is straightforward — the slower flow changes, the lower the pressure surge. This applies to both valve closure and pump speed transitions.
Valve control is one approach. The relationship between valve closure time and pressure surge magnitude is well established: slower closure means lower surge. Motorized valves with controlled closure timing can reduce water hammer risk significantly in the right applications.
Electronic motor control — soft starters and variable frequency drives — addresses the problem at the pump itself, controlling how quickly the motor accelerates and decelerates. This prevents the sudden flow changes that trigger both valve slam and column separation, while simultaneously reducing mechanical stress on the motor, shaft, and connected equipment.
Soft starter or VFD: which is right for your pump?
Both soft starters and variable frequency drives can control pump acceleration and deceleration to mitigate water hammer. The choice depends on whether you need speed control during operation, or only during starting and stopping.
Soft starters control speed during the start and stop cycle only. Once the pump reaches full speed, the soft starter is bypassed and the system runs at full efficiency — energy losses of less than 0.5%, zero harmonics, and no harmonic filters required. Capital cost is significantly lower than a VFD.
VFDs control speed continuously, during both start/stop and run. This is the right choice when varying flow during operation produces measurable system efficiency gains — for example, in variable-demand water distribution systems. But that capability comes at a cost: higher capital expenditure, energy losses of 4–6% during run, and harmonic distortion that typically requires additional filtering.
The decision rule is simple: if varying run-time speed produces proven system efficiency gains, use a VFD. In all other cases, a soft starter is the better choice — lower cost, higher run efficiency, and fully capable of preventing water hammer through controlled starting and stopping.
Not all soft starters handle pumps equally
This is where specification decisions matter. Not all soft starters provide the same level of control over pump acceleration and deceleration — and for water hammer prevention, the quality of that control is everything.
Basic soft starters manage voltage or current during starting. More advanced units offer direct acceleration and deceleration control, with selectable profiles that can be matched to the specific characteristics of your pump and system curves. This matters because the optimal stopping profile for an open system lifting water to height is different from the optimal profile for a long horizontal pipe with high flow velocity.
The right profiles for common pumping scenarios:
- Early acceleration — gets the pump to speed quickly to ensure water lubrication of bearings, then provides precise flow control through the rest of the start
- Early deceleration — ideal for open systems without non-return valves; stops flow while maintaining forward rotation as water drains back
- Constant deceleration — best for low-head, high-flow, long-pipe systems where gradual momentum dissipation prevents pressure surges
- Late deceleration — ideal for flat system or pump curves, providing extended gradual control through the critical speed range before completing the stop
When specifying a soft starter for pumping applications, confirm it offers selectable acceleration and deceleration profiles — not just a single adjustable ramp time.
Water hammer is predictable, calculable, and preventable. For the majority of fixed-speed industrial pumping applications — water and wastewater, mining dewatering, building services, irrigation, oil and gas — a soft starter with proper acceleration and deceleration control is the most cost-effective solution available.
The key is matching the technology to the application: understanding your system curve, your pump curve, and selecting a soft starter with the control sophistication to handle both correctly.
Benshaw engineers work with customers across water and wastewater, mining, and heavy industry to specify the right motor control solution for pumping applications. To discuss your application, visit benshaw.com.