Why does a twin-screw pump stop pumping when the outlet pressure rises?
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This post was last edited by HLBO on 2023-12-5 at 16:57. There is a twin-screw pump on site, and due to the high viscosity of the medium, there have been continuous signs of vacuum formation. At first, I thought that since the viscosity of the medium was too high, the pipelines couldn’t keep up with the flow rate (that’s just my assumption), which was causing the vacuum; so I reduced the outlet valve and increased the flow rate through the bypass valve in order to improve the situation. The adjustments did work initially, but as the viscosity of the oil increased further, vacuum formation started again. A couple of days ago, I accidentally turned off all the bypass valves of the screw pump; as a result, the outlet pressure rose from 0.7 MPa to 1.0 MPa. The signs of vacuum immediately disappeared. I tried it several more times, and with the flow rate delivered by the pump remaining unchanged (the flow meter is located after the outlet valve, while the bypass valves are before it), whenever the outlet pressure increased, the signs of vacuum improved. I would like to ask the teachers: was it an error in the initial judgment – namely, that the high viscosity at the inlet caused a vacuum condition due to insufficient flow in the pipelines – or was there some other reason?
A twin-screw pump is a common positive-displacement pump that uses two intermeshing screws rotating within the pump chamber to push the liquid forward in a spiral motion along the pump casing, thereby achieving continuous fluid transport. The flow rate of the pump is primarily determined by the speed of the screw, while the outlet pressure is influenced by factors such as the pump’s mechanical structure, the properties of the fluid (such as viscosity), and the system resistance. In your case, there may be several reasons for the disappearance of the cavitation phenomenon in the twin-screw pump: 1. Reducing the pump’s flow rate and increasing the outlet pressure can improve the sealing between the pump chamber and the fluid; this is especially true in cases of high viscosity, as the pump’s sealing performance becomes more sensitive under such conditions. In this case, a better seal is formed inside the pump chamber, which can reduce pumping out of the pump. 2. As the fluid viscosity increases, a higher pressure is required to allow the fluid to flow smoothly through the pump. Turning off the cross-flow increases the system resistance and raises the outlet pressure; this shifts the pump’s operating point to a higher head (pressure) point on its performance curve, which helps to prevent cavitation and vaporization in the chamber. 3. The high-viscosity medium forms a good lubricating film within the pump chamber, which aids in sealing and reducing leaks inside the pump, thereby improving the pump’s suction performance. 4. Increasing the pressure may also raise the temperature of the fluid, thereby reducing its viscosity and making it easier for the pump to draw it up and transport it. Although this thermal effect may not be significant, it can also have certain positive effects in some cases. In short, by increasing the outlet pressure, it may help improve the sealing conditions within the pump, reduce internal leakage, and prevent vacuum formation. However, this operation must be carried out with caution, as excessive pressure may cause the pump to overload or lead to premature wear of its internal components. The correct approach is to take into account the properties of the fluid, the performance curve of the pump, as well as the actual operating conditions of the system, and to adjust the operating parameters of the pump appropriately in order to ensure its proper operation and the stability of the system. For future operations, it is recommended to make adjustments based on the pump’s manual and performance curve, as well as the specific conditions of use. If the problem persists, it is recommended to consult the pump manufacturer or relevant fluid dynamics professionals. .
Generally, there are certain requirements for the suction pressure of a pump, which is also related to the temperature and viscosity of the fluid. Within the range of process conditions, it is acceptable to use a control valve to keep the flow rate and pressure within that range; it is not acceptable to go outside this range. For example, if the temperature is too low, the viscosity of the fluid becomes extremely high, which increases the load on the pump as well as the outlet pressure. It is recommended to compare the process requirements of the pump with the actual production conditions on site to determine whether the pump is being used within those process requirements. For phenomena related to the manufacturing process, the causes should be sought from this perspective; moreover, risky adjustments beyond the specified conditions should not be made, for reference only.