Water hammer

Any change in flow velocity faster than the pipework can absorb it. Four triggers cover most of what happens on site: a valve closed quickly by hand or by an actuator, a solenoid valve snapping shut on a dosing skid or an appliance, a pump starting or stopping direct-on-line, and a power failure that drops the pump out while the column is still moving. On a rising main the power failure is the worst of the four. Flow reverses through the pump before the non-return valve has seated. A valve left shut and forgotten is one of the everyday causes of pump damage we see, and it bites both ways. Shut it quickly on a live line and you get a surge. Run the pump into it and you get dry running instead.

The pressure rise from a sudden stop depends on three things: the density of the medium, the speed the pressure wave travels in that pipe, and the change in velocity. It does not depend on the length of the pipe, and it does not depend on the pump's duty pressure. So a low-pressure system can still see a surge several times its normal working pressure. Wave speed follows the pipe material and wall thickness. A rigid steel or cast iron main carries a faster wave and a harder surge than plastic, which flexes and takes some of it out. Add the surge to the working pressure, then compare that against the rating of the weakest component in the line. Usually that is a gauge, a flanged joint, or the check valve itself.

The pump is rarely the cause and usually the casualty. On a power failure the column reverses through the pump before the non-return valve seats. The impeller spins backwards, then stops hard when the valve finally closes. Repeat that and you loosen the impeller on its shaft, wreck the mechanical seal and put the shock through the bearings. The low-pressure half of the cycle can also drop the medium below its vapour pressure, so it flashes into vapour and then collapses much as it does in cavitation. In the pipework the same event fractures brackets and hangers, cracks fittings, and works flanged joints until they weep. The pump then gets replaced like for like and fails the same way, because the fault was never in the pump. Most of what goes wrong on an installation is a system fault presenting as a pump fault, and water hammer is the clearest case of it there is.

Two arrangements produce most of the water hammer on a commercial or industrial site. The first is a pressure booster set switching pumps against a closed system. Every start and stop is a velocity change, and a waterlogged or undersized accumulator vessel removes the only cushion the system has. Sizing the set against the real demand profile, rather than a peak somebody guessed at, is half the cure, which is what booster set sizing works through. The second is a long rising main fed by a submersible pump or a borehole pump. The column carries real momentum, and a power cut stops the pump instantly. Ask about the pipework, the valve positions and the control philosophy before you look at the pump.

Slow the velocity change down. Start the pump gently: a soft starter, or a pump with integrated speed control such as the Grundfos CRE, ramps up and down instead of switching. On a duty and standby set that also stops the changeover throwing a transient into the line every time. The trade-off is real, because a drive costs more than a direct-on-line starter and adds a component that can fail. Next, choose the non-return valve on how it closes rather than on price. A spring-assisted or nozzle check valve seats before flow reverses; a swing check closes late and slams. Then design the pipe velocity down, anchor and bracket the pipework properly, and set actuator closing times to suit the line rather than the actuator's default. Where a transient analysis calls for a surge vessel or an air valve, that is a specialist item and it sits outside the pump set. What we can do is select and size the pump against the real duty, specify the control gear that starts and stops it gently, and get the non-return arrangement right. Send us the flow, head, medium and the pipework layout and we will tell you what the duty needs.