Another type of protection device is the pump bypass. The following figure shows a booster pumping system. When the booster pumps shut down, the resulting reduction in flow generates pressure waves on both sides of the pump. The wave traveling upstream is a positive transient and the wave traveling downstream is a negative transient.

Figure 14-18: Booster Pumping System with Bypass

Depending on the relative lengths of the upstream pipeline (LS) and the downstream pipeline (LR) and the magnitude of the velocity changes, a pump bypass connection can act as a transient protection element. Water continues past the booster station if the downstream pressure falls below the upstream pressure, thus limiting the pressure rise upstream of the booster station and the pressure drop downstream.

The next figure shows the transient analysis results for such a system. These results show that the bypass opened to transfer water from the upstream pipeline to the downstream pipeline, which helped to attenuate or control the maximum and minimum pressure transients on the upstream and downstream sides of the station.

Figure 14-19: Booster Pump Shutdown

The effectiveness of a booster-station bypass depends on the specific booster pumping system and the relative lengths of the upstream and downstream pipelines. If the low-pressure surge generated on the discharge side of the pump is still greater than the high-pressure surge generated on the suction side of the pump (which tends to occur if LR < LS), the bypass will not open. For systems in which the bypass may not open, other transient protection devices are necessary. Each system should be individually analyzed to assess the occurrence of excessive high- and/or low-pressure transients and determine strategies to control potentially excessive pressures.