What are the negative effects of operating a centrifugal pump at low flow rates on equipment safety?
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What are the negative effects on equipment safety when the reflux pump operates at low flow rate as a centrifugal pump during the startup of the distillation tower? Is there no simpler way other than opening the pre-heater at the outlet of the standby pump (the bypass valve across the outlet) and opening the bypass lines at the inlet and outlet of the pump body?
Adding variable frequency is not good for the motor when used over a long period of time, as it can easily lead to cavitation
It can cause damage to the impeller and motor. In addition to opening the pre-heating line at the outlet of the standby pump (the bypass valve at the outlet) and opening the bypass lines at the pump’s inlet and outlet, it is also possible to adjust the motor’s frequency.
A low-flow bypass can be installed. Pumps operating at low flow rates for extended periods are prone to cavitation.
Long-term operation at low flow rates can damage the pump, and the temperature of the material at the outlet will increase
Too low a flow rate can cause the temperature of the fluid inside the pump to rise, or even lead to its vaporization. We encountered a pump with too low a flow rate earlier; it was pumping oil at a temperature of around 40°C. Due to the low flow rate, after operating for over an hour, the temperature of the pump body rose above 160°C, and the pump began to smoke. Fortunately, it was detected in time, resulting only in some damage to the seal, without causing any major accidents. It is generally better to use a bypass control method, allowing the bypass to handle a portion of the flow. It’s simply a matter of automatic versus manual control; to simplify things, one can install a minimum flow protection valve. When the flow rate in the main pipeline falls below the pump’s minimum flow requirement, the secondary pipeline will automatically open to a certain degree, ensuring that the sum of the flows in the main and secondary pipelines remains at a constant level, namely the minimum flow value. Once the flow rate in the main pipeline exceeds this minimum value, the secondary pipeline will automatically close again, thus protecting the pump. Minimum flow protection valves are widely used in boiler feedwater systems. However, using frequency conversion is not an unacceptable approach; if speed reduction through frequency conversion is employed to lower the flow rate, the head pressure will also decrease significantly, to the point where it may no longer meet the process requirements. The pump speed is directly proportional to the flow rate, while it is proportional to the head in a quadratic manner; a slight decrease in speed results in a significant drop in head.
Set up a bypass return flow; it’s best to have a minimum return valve – it’s explained clearly above!