What is the most likely cause of severe vibration when a new horizontal centrifugal pump is initially started?

Severe vibration during the initial start-up of a new horizontal centrifugal pump is most commonly attributed to misalignment between the motor and the pump. When the motor and pump shafts are not properly aligned, it can create an imbalance during operation. This misalignment causes uneven forces on the components, leading to excessive vibration as the system tries to operate under these conditions.

Proper alignment is crucial for the smooth operation of centrifugal pumps. It ensures that both the motor and pump shafts are in line with each other, which allows for even distribution of forces and minimizes wear on bearings and other components. In newly installed systems, alignment issues can frequently arise due to installation errors or settling of the equipment.

The other options, while they can lead to operational issues over time, are less likely to cause severe vibration immediately upon start-up. Worn bearings typically develop issues after prolonged use. Low water supply can affect performance but usually does not cause severe vibration upon initial start. Incorrect pump size can lead to inefficiencies or potential operational problems, but it does not generally result in immediate severe vibration upon startup. Thus, the primary cause of severe vibration right at the start is often due to misalignment of the motor and pump.