Regarding closing the outlet valve before starting the centrifugal pump
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The outlet valve must not be opened before starting a centrifugal pump, in order to protect the motor from overload. However, there is a problem: if the outlet valve is open when the pump is started, why doesn’t the centrifugal pump start pumping fluid? I read a document that said, “The low output is caused by the pressure head dropping below the conveying height.” I’ve never been able to understand what this means; I hope everyone can offer some guidance. Also, what does the pump head mean? Is it the head pressure? The definition of head is the energy per unit weight of a fluid. Could you explain it in simple terms?
Since a centrifugal pump creates suction by the rotation of its impeller to generate vacuum, filling it with liquid, releasing air, and closing the outlet valve before starting can prevent air from entering the pump; otherwise, it will not be able to draw water up. I believe that having the outlet valve open when starting the pump affects the vacuum level, increasing the resistance to fluid flow and reducing the height to which the liquid can be pumped, which in turn lowers the actual head pressure of the pump. The head is the head pressure of pump machinery, referring to the distance that a unit weight of liquid can be lifted, also known as head.
When a centrifugal pump is started with the outlet valve open, flow rate may not be optimal in some cases, but it sometimes doesn’t have much of an impact; it mainly depends on whether the inside of the pump is filled with liquid at the time of startup and whether there is any gas present. In simple terms, the plunger determines whether the liquid phase can fill the pump body by its own gravity ； Raising the height of the tower itself is intended to provide the pump at the bottom of the tower with sufficient head pressure for filling.
The pump outlet valve is open, and due to the relatively high pressure in the outlet section, it is easy for the fluid at the outlet to flow back into the pump; if there is gas present, the pump can easily become evacuated.
If there is gas inside the pump, it will cause gas entrapment; a very low negative pressure is generated at the center of the impeller, resulting in the pump being unable to deliver fluid or create pressure!
When starting a general centrifugal pump, it is necessary to close the pump outlet valve in order to prevent the excessive starting current from damaging the motor. Operating the outlet start pump can easily cause cavitation in the pump, which hinders its proper operation. The head of a pump, or the pressure developed by a pump, refers to the energy that the pump imparts to each unit weight of liquid. The unit of a pump’s head is commonly expressed as the height of a liquid column of the fluid being pumped (in meters of liquid column).
However, centrifugal pumps equipped with frequency converters can do this, as such pumps allow it to monitor various parameters on-site or in a controlled environment, and to increase the frequency control signal. In this way, the pump can reach its operating capacity as quickly as possible without running dry
The outlet valve must be closed when starting a centrifugal pump; the main purpose of this is to protect the motor. As can be seen from the performance curve of the centrifugal pump, when the flow rate is 0, the power output is also 0, and as the flow rate increases, the motor’s power consumption increases as well. Therefore, closing the outlet valve at startup helps to protect the motor, and it is not intended to prevent gas from entering. Additionally, the head is the height of fluid that a centrifugal pump can deliver at its optimal operating point; it generally increases as the flow rate increases. However, when the flow rate reaches a certain level, increasing it further results in a decrease in head. If the flow rate is not as expected, it is necessary to examine the flow and head performance curves of the centrifugal pump to determine its operating point. If the flow rate exceeds the optimal operating point, then the head will definitely decrease.
It is mainly to prevent the motor from tripping due to overload. Because the larger the outlet valve is opened, the greater the load becomes. The starting current is already high; with the outlet valve still open, the load will definitely be very large, causing a trip!
The head of a pump refers to the pressure difference between the inlet and outlet of the pump (in Pa), which is converted into a height difference (in meters) using the formula Δp=ρgh； Similar to the concept of head, that is, h)
During power fluctuations, the heavy oil pump, the intermediate-stage pump, and others come to a stop. So when restarting them, should the outlet valve be closed first and opened again after startup? ? ?