12.6 Pump testing
It is accepted practice to construct and test a scale model of any large new circulating water pump design. This is an economic method of evaluating the likely performance of the full size pump and the cost of any corrections or adjustments is negligible compared with making modifications to the full size pump itself. Model tests enable the hydraulic characteristics of the full size pump to be accurately predicted. This is particularly relevant to concrete volute pumps which cannot be works tested.
Scale models are also used to verify design proposals for the suction intake to the pumps. The model normally comprises the forebay, intake screen chamber, draft tube, volute and discharge pipework. Comprehensive tests are carried out to demonstrate the acceptability of the intake system over the complete range of operating conditions, taking account of the combinations of pumps operating and the variation in suction water levels.
In determining the full size pump performance from the results of the model tests, the normal affinity laws are applied to demonstrate that the guarantees on head and efficiency at the rated flowrate, and the shape of the head/flow and power/flow characteristics are likely to be met when the full size pump is eventually tested on site.
Site tests on circulating water pumps are carried out in accordance with CEGB Site Test Code No. 6. To overcome any contractual problems in determining head losses at suction and discharge, the pump is deemed to be that part of the system from the open water level at draft tube inlet to the discharge measurement point.
Flow measurement is considered the most difficult aspect of circulating water pump site testing, and various methods are currently employed. On stations with long straight open culverts, the current meter-traversing technique has been found to give satisfactory results and is relatively easy to install and use. For other stations where there are no suitable open channels, the isotope dilution method is widely used. This involves the injection of a tracer of known concentration at a constant rate upstream of the pump and measurement of the concentration at a point downstream of the injection point where adequate mixing will have taken place. The dilution technique is completely independent of the velocity of the pumped fluid, or the dimensions of the pipe-work, and yields highly accurate results. The principal disadvantage arises from the strict safety precautions necessary when using radioactive tracers and the resulting inflexibility of the test programme.
An alternative flow measurement technique now developed is the use of ultrasonic flowmeters, with multiple transmission paths being included to take
account of the velocity profile across the flow path. Although the measurement transducers have to be installed with a high degree of accuracy to ensure correct flow readings, the facility is then available for use throughout the life of the station.