Why is the cavitation head value lower on a closed-type test bench compared to an open-type one?
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The net positive suction head at the test bench was 3.2 m, while when the same pump was tested on a closed-loop setup, the net positive suction head was 2.7 m. What is the reason for this difference? Please, experts, provide an analysis
The net positive suction head measured on a closed-test bench is accurate.
The open test bench does not account for net positive suction head
Why are open types not allowed? What’s the difference?
As the valve opening decreases, the flow velocity through the valve increases, thereby reducing the pressure at the valve and causing local cavitation. At the same time, the valve opening decreases, resulting in a poor condition of the liquid flow; bubbles are likely to form during this flow. These bubbles enter the pump inlet along with the liquid, causing cavitation to occur in the pump prematurely. If the pump cavitation is greater than 5m, this phenomenon will disappear, and the results for open and closed systems will then be essentially the same.
I just asked – for closed-type tanks, vacuum extraction is used to conduct cavitation head tests, and this method is much more accurate compared to using on/off valves in open-type tanks. However, vacuum pumps sometimes aren’t powerful enough, resulting in insufficient vacuum levels; when the cavitation head is too low, such tests cannot be carried out