Causes of water hammer
Ziruba Josef. Water Hammer in Pipe-Line Systems. Elsevier, 1993, English, 363 pages.
It has been stated that water hammer develops due to any change in pressure or discharge in a conduit. The causes of such changes may be most varied. Let us mention at least some of those, which frequently induce large changes in pressure.
A liquid flows from a reservoir through a pipe-line provided with a valve at its downstream end. The discharge in the pipe-line varies as the valve is being closed. The variation in discharge induces water hammer. If the closing process is not suitably controlled, the largest changes in pressure usually occur in the final stages of closing.
A reservoir may feed an entire pipe-line network, for example, a water-supply network with a number of offtakes, which may be adjusted independently. Every adjustment induces water hammer phenomena in the network and their effects are mutually superimposed.
Another relevant example is the pumping of a liquid into a reservoir. A pump is installed at the upstream end of a pipe-line followed by a check valve which prevents the outflow of the liquid from the reservoir when the pump is switched off. After the pump has been stopped, the liquid continues moving due to its own inertia and the pressure in the pipe-line drops. Sometimes, this drop may be so large as to cause cavitation. In the next stage, the liquid starts flowing back towards the pump. Its flow is, however, checked by the check valve, which closes.
This produces an increase in pressure which may imperil the entire system. The increase and drop in pressure are repeated at regular periods until the entire phenomenon fades out.
A still more dangerous situation may develop, if the check valve does not prevent the backflow of the liquid through the pump in time. The liquid may then flow from the reservoir at a high velocity. If the check valve suddenly stops the backflow at this stage, the resulting effects of water hammer may be still more pronounced.
When the generator of a water turbine is disconnected from a power network, the turbine speed starts to increase. Consequently, the turbine controller closes the inflow to the turbine, thus creating water hammer in the penstock. Water hammer effects in the penstock are created by any changes in discharge through the turbine, caused by changes in the connected power network, by the operators, or by breakdowns. Sometimes, the entire system becomes unstable due to the mutual influence of a turbine equipped with a controller and to an unsteady flow in the penstock. In such case, even small variations in pressure in the penstock may increase steadily and perilously.
Air entrapped in a pipe-line is a frequent cause of water hammer. Sometimes, it may reduce the effects of water hammer. If the air is in the form of minute bubbles dispersed throughout the stream of the fluid, it increase the compressibility of the fluid and reduces its average density, thus also reduces the effects of water hammer. Similarly, if air accumulates in the form of larger bubbles at suitable points of the conduit, it also has a damping effect on the development of water hammer. If, however, larger bubbles move through the pipe-line, they may reach, for example, the outflow which is restricted by the valve. While a bubble passes through the valve, the velocity of the liquid flowing in the pipe-line may increase considerably, because the pressure losses in the valve are much lower for the air than for the liquid. Once the bubble has escaped from the pipe-line, the liquid starts flowing through the valve again and the pressure losses in the valve are increased. This effect induces water hammer of a character similar to that caused by a rapid opening and a subsequent partial closing of a valve. A similar effect may take place not only at the outflow, but also at the point of any large source of a local resistance in a pipe-line system.
Air can enter the conduit in various ways. The entire pipe-line is filled with air before any liquid is let in. Air may also get into the pipe-line during operation, for example, by being additionally sucked in by the pumps, through air-inlet pipes or valves, or dispersed in the liquid, etc. Solid substances may also be present in a pipe-line, either intentionally, for example in the case of hydralic transport, or accidentally. These solid substances may induce water hammer directly, for example, by the sudden clogging of some part of the pipe-line. Apart from this, they may also unfavourably affect water hammer produced by other causes. They may increase the density of the flowing mixture and reduce its compressibility thus increasing the effects of water hammer. Solid particles may settle in a pipe-line and reduce its cross-sectional area so that the velocity of the flowing liquid is higher for the same discharge and the pressure variations at water hammer are increased.
There are many other causes of water hammer. In more complex systems especially, the cumulative effect of several types of devices which influence water hammer may have an adverse effect. However, even in simple cases, for example
in pumping water into a reservoir, manipulations very unfavourable with regard to water hammer may take place. For example, after the failure of the pump, the operator may start it again. Much depends on the instant of this starting. If it is done at a time when the entire water hammer effect has died down, it is an operation for which the system must have been designed. If, however, the pump is started sooner than that, unfavourable effects may appear, due to the superimposition of water hammers produced by both the failure and the restarting of the pump, so that the pressure in the pipe-line may increase much more than when the pump is restarted during the steady state.