mag

Malaysia Water Research Journal
Test
Table 2. The relationship between prototype and model values.
Water Level
MSL
Major
Pump 1
Major
Pump 2
Major
Pump 3
Major
Pump 4
Run 1 +1.80 √ √ √ √
Run 2 +1.50 √ √ √
Run 3 +0.90 √ √
Run 4 +0.40 (Min level) √
3 RESULT AND DISCUSSION
3.1 Vortices
Ideally, the flow of water into a pump should be uniform, steady, without swirl
and without air, either entrained from a free surface or released from local low
pressure regions. Lack of uniformity can lead to reduction of efficiency. Unsteady
flow will result in fluctuating loading of the propeller, leading to noise and
vibration. Swirl in the intake can cause a change in flow, reduction in the pump
efficiency and power. It may result in vortices leading from the free surface or
from a bounding solid surface into the pump. These vortices can become strong
enough for the cores to be air filled or cavitation. Vortices from the water surface
can draw air continuously into the pump; solid surface vortices, often called
submerged vortices, provide discontinuities in the flow around the propeller
blades.
The free vortices are generated by rotation and separation combined with
the effects of accelerated flow at the pump intake. Coherent subsurface swirl
is often apparent, sometimes penetrating into the intake but again, since only
gravitational and inertial forces are involved Froude Number similarity accurately
predicts the magnitude of such effects. In the extreme, air core vortices will be
generated which may have serious consequences. Such vortices, with air core
extending near to a model intake, will be subjected to scale effects. In a full-size
installation, due to the absence of scale effects, the transition from a surface
dimple and coherent subsurface swirl to an air core will occur more readily than
in the model.
In order to achieve the objectives, it is necessary to investigate the
performance of the pump sump as designed and to make modifications
to overcome any encountered problems. The assessment of the pump sump
involved observing the approach flow pattern towards each operating pump
intake with the aid of dye tracer. The tests were undertaken under steady
conditions with each duty pump operating at the specified design flow rate and
water levels. When a single pump in operation it was conducted at lower water
level of 260 mm (from the bottom sump) and at 400 mm water level when all
major and minor pumps are in operation. Observations were made from the
dosing of the blue dye at the entrance and immediately downstream of the
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Institut Penyelidikan Hidraulik Kebangsaan Malaysia (NAHRIM)
National Hydraulic Institute of Malaysia (NAHRIM)