A question regarding the centrifugal oil pumps in refining units
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Our company’s refinery unit, specifically the vacuum distillation unit, originally used single-stage centrifugal pumps for pumping the heavy oil at the bottom of the towers; due to failures, these were replaced with multi-stage centrifugal pumps of the same type, but then the problem arose that the pumps were unable to deliver sufficient flow. Normal operation was restored after switching to a single-stage centrifugal pump. After the replacement, the same phenomenon occurs. Why is this?
The reason is as follows: the inlet pressure of the bottom pump in the atmospheric and vacuum distillation tower is relatively low, slightly higher than the saturated vapor pressure of the oil, which is sufficient to ensure the stable operation of a single-stage pump； However, as the pumping capacity of a multi-stage pump increases, the inlet pressure of the oil between the stages becomes lower, causing the oil to vaporize easily and leading to cavitation. This is why, as the original poster mentioned, no flow is achieved; but when a single-stage pump is used, flow is obtained again.
The operating characteristic curves of single-stage pumps and multi-stage pumps are different. When a single-stage pump is designed, the operating pressure and flow rate fall on the curve of optimal performance; thus, the desired flow rate can be achieved at that operating pressure. In contrast, with a multi-stage pump, its performance curve shows a certain flow rate corresponding to higher pressures. When the operating pressure is low, the pump’s performance curve deviates significantly, resulting in a sharp drop in efficiency. At the same time, the self-sealing between pump stages does not meet the sealing requirements; when the outlet pressure is too high, regulation is necessary through backflow or the pump outlet valve. As a result, the pump’s flow rate decreases significantly. This problem occurs not only with oil pumps but also with water pumps.
All I know is that some of our high-pressure centrifugal pumps require a certain pressure at the inlet in order to start.
The advantage of multi-stage pumps is high head and low flow rate. This is the difference from a single-stage pump; in other words, a multi-stage pump with the same power cannot achieve the high flow rate of a single-stage pump, which is what the original poster meant by saying that switching to a multi-stage pump does not increase the flow rate.