[Truncated abstract] Centrifugal pumps are designed to operate at a capacity that maximizes the efficiency of the pump. Operation below this design capacity results in reduction of pump efficiency as the geometric configuration of the impeller and casing no longer provide an ideal flow pattern. Consequently there are changes in the flow field within the pump, including flow separation and regions of localized, non-uniform, unsteady flow. This flow interacts with rotating and stationary components inside the pump creating additional disturbance and hydraulic excitation. It is anticipated that the local hydrodynamic and global hydroacoustic excitation due to partial-flow operation will affect the structural vibration measured on the pump. In this study, the effects of partial-flow operation on the vibration signal at the pump bearing housing are measured on a number of industrial double-suction pump units. These are a particular class of centrifugal pump commonly used in high volume applications such as water distribution. The aims are to understand how the vibration signals change at the different operating conditions and to determine which parameters are best suited to monitoring the observed changes. Comparison is made between the response, under similar operating conditions, of pumps both within sets of identical size and design, across sets of similar design and different sizes, and between different designs. The pumps are all in-service industrial units. In double-suction pumps the impeller motion in the axial direction (along the line of the shaft) is not constrained. Due to symmetry in impeller design, axial stability is maintained by equal and opposite hydraulic forces on the two opposing sides of the impeller. The potential for loss of axial hydraulic balance during partial-flow operation is examined from a theoretical perspective, based on a literature review, and by measurement of the axial displacement of the shaft. Structural vibration is measured using accelerometers mounted at the non-drive end bearing housing in the horizontal and axial orientations. Changes in signal contribution and characteristics are examined using a variety of qualitative and quantitative techniques. Test signals are used to assess the limitations of the techniques and the effect of parameter selection on the interpretation of the signals.