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18. The performance of a centrifugal pump was determined using water; the flow rate was Q = 11 m3/h, the vacuum gauge reading at the pump inlet was Pb = 165 mmHg, the pressure gauge reading at the pump outlet was Pc = 0.18 MPa. The vertical distance between the two pressure measurement points, namely those of the pressure gauge and the vacuum gauge, was h0 = 0.5 m. The shaft power was N = 1.10 kW. What are the values of the head H (in meters) and efficiency η (%) for this centrifugal pump? A 16.30 44.40 B 20.53 55.90 C 20.59 56.10 D 21.10 57.50 Should a positive or negative value be used for this vacuum gauge in the calculations? What’s wrong with this approach to applying Bernoulli’s equation? The answer given is A, with a head of 16.3 m. I seek help from experts; thank you so much! ! !
It’s P/pg; I mistyped it in haste
The pump inlet vacuum gauge reading Pb=165 mmHg indicates that the inlet is not under vacuum but instead under backflow.
Both the inlet pressure and outlet pressure must be maintained at the same pressure reference (gauge pressure or absolute pressure) before the pressure difference can be calculated； Unless the inlet is at high pressure, the inlet of a pump is generally specified in terms of absolute pressure or vacuum level (vacuum level refers to the pressure that is lower than atmospheric pressure, measured with atmospheric pressure as a reference; it is a pressure difference, where vacuum level = atmospheric pressure – absolute pressure) ； If all calculations are done based on relative pressure (gauge pressure), the inlet gauge pressure is -165 mmHg (which is a negative pressure), while the outlet gauge pressure is 0.18 MPa ； If calculations are all done based on absolute pressure, then the absolute pressure at the inlet will be atmospheric pressure minus 165 mmHg, while the absolute pressure at the outlet should be atmospheric pressure plus 0.18 MPa. Only after that can the pump head be calculated; only with an accurate head value can the efficiency be determined correctly. I haven’t carried out the actual calculations, but I hope this helps you
Thank you, your explanation made it much clearer
What exactly does backflow mean? I couldn’t find any relevant explanations in the textbook, so I’m not quite sure. Is positive pressure generated? How can we determine whether there is backflow or a vacuum based on the mercury column? Could you please explain it again?
The range of the vacuum gauge is from 0 to -0.1 MPa; therefore, in the case of a vacuum, the gauge reading should be negative. The title states that the reading is 165 mmHg, with no negative sign; therefore, my understanding is that the inlet is not at vacuum. But there seems to be a problem with the question; if it’s positive pressure, a vacuum gauge cannot be used for measurement – instead, a vacuum pressure gauge or a regular pressure gauge is required. Backflow is the opposite of suction; it can be understood as pouring from a higher place into the pump.
This post was last edited by 3983596_FPPZ on 2018-12-9 at 17:07. There are two ways in which the inlet liquid level can relate to the pump’s inlet height: suction and backflow. Suction occurs when the liquid level at the pump inlet is lower than the pump’s inlet, requiring the pump to draw liquid from a lower level; in this case, the inlet pressure is negative (lower than atmospheric pressure). Ordinary centrifugal pumps need to have vacuum created either on the pump itself or in the inlet pipeline before starting, so that the pump can be filled with liquid before it can operate. Self-priming pumps are used precisely for dealing with situations involving suction； Backflow occurs when the liquid level at the inlet is higher than that at the pump’s inlet; once the inlet valve is opened, the liquid flows automatically into the pump. At this time, the inlet pressure is generally positive (higher than atmospheric pressure), and it is necessary to evacuate the air from the inlet pipeline and inside the pump before the pump can be started