Transient pressures (commonly called water hammer) occur in any pipeline when the velocity of flow changes. Valve closure and pump trip are common causes of water hammer but other circumstances such as start-up tests under non-standard conditions may generate unexpected transient conditions. Water hammer pressures can sometimes cause serious damage to pipeline equipment such as non-return valves and pumps, or even cause pipeline rupture.
The main difficulty in appreciating the nature of water hammer is its invisibility. It is easy to appreciate the effect of a reflected wave from a sea wall because the wave pattern can be directly observed; this is not possible in a closed pipeline. So any computer simulation of water hammer should include a graphical representation of the conditions.
Hytran software does just this, providing a real-time visualisation of the pressure waves traveling the pipeline, and also time-dependent pressure transients at fixed points such as pump stations. This graphical presentation of results provides not just a summary of the calculation; it assists in a greater understanding of the phenomenon.
Hytran also makes extensive use of graphics in the development of the hydraulic model. A simple drag-and-drop technique is used to generate a flowsheet representation of the pipe system and a wide range of equipment items can be simulated. These include multiple pumps; check valves, discharge tanks, control valves and air chambers.
One aspect of water hammer is the phenomenon of column separation or vapour cavity formation. This occurs when the pressure in the pipeline temporarily falls to absolute zero as a negative pressure wave travels by; high points in the pipe system are particularly vulnerable. As the returning positive wave arrives, the vapour cavity collapses, causing a new set of transients superimposed on the original pressure wave. This often results in significantly higher pressures than those caused by the initial conditions.
The two screen images are taken from the Hytran computer program illustrate this phenomenon.
The simulation is a 3km water pipeline with a single pump discharging 174 l/s to a reservoir with a static rise of 41m. There are two high points in the system prior to the reservoir. The simulation is of pump trip. Local pressure transients are displayed at the pump (the blue line on the inset graph) and at the first high point (the red line on the inset graph).
In screen-image No.1, the blue line shows steady-state conditions for 5 seconds prior to pump trip, after which the pump head falls rapidly to zero. Shortly afterwards, the pressure at the first high point falls to absolute zero as a result of the elevation of the pipe in respect of the hydraulic grade line. A vapour cavity occurs at this point. Approximately 25 seconds later the vapour cavity collapses as the columns rejoin causing a peak transient pressure or head at the pump of more than twice the pump shut-off head. The cycle repeats itself every 20 seconds thereafter, with the second peak significantly attenuated due to friction in the system. But by this time the damage may be done.
What options are available to reduce the peak pressures? There are many commercially available equipment items such as air chambers, bladder or Charlatte for example; pressure relief valves, surge tanks etc.
A solution to the problem shown in screen-image No.1 could be as simple as a discharge tank located at the high point, its purpose being the prevention of the formation of the vapour cavity and therefore its subsequent collapse. Screen-image No.2 shows just this. The discharge tank measures approximately 1.5m diameter with the head in the tank 1.2m above the pipe center-line. The tank is connected to the pipeline via a 100mm diameter pipe fitted with a non-return valve. As the negative pressure wave caused by the pump trip passes the high point, water is drawn into the pipeline from the discharge tank filling the cavity that would otherwise be formed. Consequently the high pressure caused by the collapsing cavity cannot occur. In fact, as the blue line on the inset graph shows, the peak head at the pump after trip is not just lower that the shut-off head of the pump but lower than the duty head.
A water hammer analysis should be performed on all pipelines as part of an overall risk assessment. The visual presentation of the transients from a software program such as Hytran, graphically presenting the various case studies, will assist the engineer is understanding the system and, if water hammer is a potential problem, designing a suitable protection system.