Negative pressure at Pump Suction side during surge

Hello Bushra,

I assume your transient event is an emergency pump shutdown. In such a case, the downstream side of the pump would experience a "downsurge" (low pressure wave) immediately following the shutdown as the water column tries to stop, and the upstream side of the pump would experience an "upsurge" (high pressure wave). However, these wave propagate and reflect off end points and boundaries, then may travel back to the pump.

For a typical pump station where you have a Reservoir > short pipe > pump, the upsurge wave on the upstream side of the pump will quickly travel to the reservoir and reflect back, eventually causing a downsurge (low pressure) wave that can cause negative pressure. All of this can happen regardless of the flow. So, even if a check valve is closed in the pump station and the flow on the upstream side is zero, the energy of the pressure wave will still travel and reflect. Often in a case like this where the valve closes and the flow is zero, the oscillation and reflection of the pressure wave may continue even to the end of the simulation without noticeable decay. This is because in the absence of flow, there is no headloss to help dampen the energy from the pressure waves. See more in the following article from our Wiki: Transient pressure wave not dampening or unexpected lack of headloss

If you have a very short pipe between the pump and the upstream reservoir, the results may be skewed due to length or wave speed approximations, so you may need to use a smaller timestep to capture the most accurate results in the short pipeline. You can also try one of the other transient friction methods as mentioned in the first article above, to check if the piping in question is sensitive to it.

Next, consider if the pump NPSH is adequate. If the predicted negative pressures are not influenced by any HAMMER assumptions, are not acceptable, and you cannot change the upstream boundary HGL or pump elevation, consider the inertia of the pump. A greater inertia (such as a flywheel) will slow down the pump shutdown and can have a positive impact on the resulting transient pressure waves.