Discussion on centrifugal pump operation

The return line should not be opened; it is meant to maintain the minimum flow rate of the pump. In my opinion, before starting the pump, close all outlet valves; once the pressure has risen to an appropriate level, fully open the return line, and finally open the outlet valves to adjust the pressure.

The outlet can be closed when the pump starts. I’ve heard of a return line for the first time; generally, it’s not used

When I was operating centrifugal pumps before, the manual valve on the return line was kept open, and a pneumatic control valve was installed between the two manual valves. This pneumatic control valve was closed when the pump started up (it could be controlled either manually or automatically). In other words, the pipeline formed by these three valves was closed when the pump started. Once the pump was running properly and the outlet valve was fully open, the return valve would be opened only when it was necessary to adjust the flow rate – this is the method of flow regulation using the return line in pump operation

Before starting a centrifugal pump, open the inlet valve first to allow oil to flow into the pump; the outlet valve and the return valve remain closed. At this point, the pump’s outlet vent valve can be opened to release air, or the pump can be started directly. If the pressure is unstable, use the outlet vent valve to release air; once the pressure stabilizes, slowly open the outlet valve. It is important to monitor whether the pump’s pressure remains stable until the outlet valve is fully open, and then the process is complete. If a downstream station requires the adjustment of flow rate or pressure, a return valve can be used for this purpose; the larger the return valve is opened, the lower the flow rate or pressure will be. The second issue is that draining the liquid hydrocarbon inside the pump prevents leaks in the pump’s mechanical seal, which could otherwise lead to safety problems ； Liquid hydrocarbons expand when heated, which can damage mechanical seals.

The outlet valve should be closed to start the pump, allowing it to operate at low power, which is beneficial for energy savings and for protecting the pump.

Could you explain in detail the specific reason for opening the return valve first? PS: I completely agree with the fifth floor’s opinion!

Our factory’s operating procedures also require the use of a return line, but I don’t think it’s necessary.

Your procedures need to be changed.

The evacuation of liquefied hydrocarbon pumps is intended to prevent excessive pressure buildup in enclosed spaces due to rising temperatures, which could threaten the safety of the equipment.

When starting a centrifugal pump, it is required to do so without any load, that is, with both the outlet and the return valve closed. This is done because the starting current is relatively high when the pump starts, and this measure helps prevent the motor from being damaged. Therefore, under normal circumstances, the practice of fully closing the outlet and return flow is adopted. In practical operation, especially when starting liquid hydrocarbon pumps in summer, cavitation often occurs. This is due to a small amount of vaporization within the pump body; if the pump is not properly filled before starting, the motor’s speed of 2950 revolutions per minute, combined with the pressure buildup, leads to increased vaporization of the heated material, making it difficult to start the pump. In such cases, allowing some reflux or flow at the outlet can help ensure successful startup. Additionally, liquid hydrocarbons have a low density which results in a low starting load, and modern motors come equipped with excellent overload protection; therefore, it is possible to have some backflow. However, if the pump can start normally, it is not recommended to turn it on. Liquefied hydrocarbon pumps are emptied after each use; nowadays, mechanical seals are generally used, and pumps that do not require cooling water do not need it. But there is some truth to your unit’s requirement to empty it; I think the reason isn’t expansion and pressure relief (which can be achieved by relieving pressure in the pipelines connected to the pump). The real reason is concern over minor leaks in the mechanical seal. If the material is something like propylene, which is highly volatile, this can result in very low temperatures at the seal location, even below zero degrees in summer. If that area is cooled by water, it can freeze and crack, leading to disasters. I encountered this accident myself; it was extremely dangerous, with nothing but vaporized mist everywhere. Therefore, it is necessary to empty the liquid hydrocarbon pump that uses cooling water in machine pumps.