Under negative pressure, once the water temperature reaches a certain value, the flow rate of the centrifugal pump begins to fluctuate; find a solution
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Recently, a centrifugal pump was tested using water; it draws liquid from a flash separator that is under negative pressure (around -80 kPaG). The pump is installed at a distance of about 9 meters from the liquid surface. The inlet pipe has a diameter of 3 inches, while the outlet pipe has a diameter of 2 inches. The designed flow rate is 12 m3/h. When the water temperature is below 57°C, the pump operates smoothly; once the temperature rises above 57°C, the flow rate begins to decrease and fluctuates frequently, while the pressure and temperature inside the flash separator remain relatively stable. For some reason, it’s taken several days to fix it. The cavitation issue of the pump has also been checked; theoretically, cavitation should not occur. The flow velocity in the inlet pipeline is also below 1 m/s. The pressure drop in the inlet pipeline during calculation is also not significant. Exhaust was also tried. But none of them solved the problem. A pressure balance line has not yet been installed at the pump inlet. Based on the test data, it seems that gas begins to appear at 57°C, causing the flow rate to decrease. Dear classmates, what caused this problem, and how can it be solved?
This post was last edited by lovebird on 2015-7-16 at 15:14. Under such negative pressure, the boiling point of water is 57°C; for -80 KPAG, which corresponds to an absolute pressure of 180,066 KPAA, the boiling point is 57.798°C. At this point, the water begins to boil and vaporize. The boiling point of a liquid increases as pressure rises.
Reaching the water’s bubble point is caused by cavitation in the centrifugal pump, which is what is commonly referred to as the vacuum phase.
The boiling point of water at -80 kPag (i.e., 21.32 kPaA) is approximately 61.4°C; the water in the flash separator has not reached its boiling point, and no boiling was observed. Based on the current data, the pressure of water in front of the impeller inside the pump may reach below -80 kPa, but it is not possible to measure the pressure at the impeller directly. The pressure at the pump inlet has not reached -80 kPag. The problem is that the flow rate starts to decrease at a water temperature of 57°C. During the testing, the water temperature never exceeded 60°C.
From the perspective of cavitation margin verification, cavitation should not occur theoretically. Moreover, no vibration or noise from the pump was heard at the site. How can this current phenomenon be resolved?
You said the pump is 9 meters below the liquid level; therefore, the pressure at the inlet of the pump impeller needs to be increased by 9 meters of hydraulic head, so it should be much higher than the vaporization pressure. In such operating conditions, the water is already close to a boiling state (the atmospheric pressure at the same location varies, and it differs from place to place; therefore, it’s quite possible that the atmospheric pressure is 97–98 kPa). If air is present before the fluid reaches the pump inlet, this can cause air locking in the pump. It’s not clear whether there is an exhaust line installed in front of the pump, leading to the gas space of the separator
The atmospheric pressure varies in different regions, and there is also resistance in the pipes. So I think the pressure here is close to the boiling point of water at 57 degrees Celsius.
It is caused by the vaporization of water; water vapor appears inside the pump, resulting in unstable outlet flow. In severe cases, the pump may not be able to deliver water due to vapor lock. Vapor lock can occur alongside cavitation, or it may not happen at all.