There is gas in the inlet pipeline of the centrifugal pump
Thread Content
There are two heat exchangers in series at the inlet of our centrifugal pump; previously they were plate heat exchangers, but now they have been replaced with shell-and-tube heat exchangers. The section from the exit of the heat exchangers to the pump inlet is at a higher level than the exit of the heat exchangers itself, and there is no vent valve in that area. Now I’m trying to start the pump, and I noticed that there was gas at the pump’s inlet. The gas was removed by using the exhaust valve on the pump’s outlet. When the pump was started, it worked fine at first; but once the outlet valve was opened to one-third, the pressure dropped, and the pump began to vibrate significantly, with noise levels increasing greatly as well. The preliminary judgment is that there is gas in the inlet pipeline, resulting in insufficient inlet flow. I hope the experts here can offer some guidance.
According to what you say, even after importing exhaust gas, gas still remains, causing abnormalities in the pump!
This post was last edited by zdl1966 on 2018-10-20 at 20:08. 1. The pump flow rate may be set too high, resulting in the evacuation of all the liquid in the heat exchanger, and even of the liquid inside the tower. 2. A liquid storage tank is installed in front of the pump, with a vent on the top of the tank set at the height of the tower; the liquid flows from the bottom of the tank to the pump inlet.
Cavitation may occur in the pump; shell-and-tube heat exchangers have a higher pressure drop than plate heat exchangers. Moreover, there is a U-shaped section from the outlet of the heat exchanger to the inlet of the pump, which increases the pressure drop at the pump inlet, resulting in an effective NPSHa that is lower than the pump’s NPSHr. Solution: Reduce the installation height from the heat exchanger outlet to the pump inlet, arranging the piping in a gradually descending manner to avoid an U-shaped configuration. The best solution is to move the series heat exchanger to the pump outlet.
An automatic control valve is added at the pump outlet; the opening of this valve is adjusted according to the liquid level in the tower, so that an appropriate amount of liquid remains in the tower bottom to prevent gas from entering the pump.
What was explained on the 4th floor was very clear: if a centrifugal pump suffers from cavitation, volumetric pumps will not.
Thank you. Although positive displacement pumps do not take cavitation into account, when selecting such pumps, it is necessary to ensure that the pump inlet is below the lowest liquid level in the suction tank. If the suction inlet height of the positive displacement pump is higher than the liquid level in the suction tank, it takes longer for the gas drawn in to be completely removed from within the pump, making it difficult for the pump to draw liquid properly. If the liquid inhaled vaporizes partially inside the pump, and since the volumetric flow rate of gas is much higher than that of liquid, the pump will discharge a mixture of gas and liquid. If a large amount of gas vaporizes, the amount of liquid discharged will be very small, making it difficult to achieve the designed flow rate.
Try increasing the diameter of that inverted U-shaped pipe! This bend is considered from a manufacturing perspective; it cannot be made lower!
Yes, for now we can only rely on pumps to slowly remove it in small amounts.
It might be cavitation; the heat exchanger has been replaced, which increased the resistance, and the margin against cavitation is not sufficient