Improving power efficiency of underloaded pipelines with variable frequency drives

The paper considers the option to reduce power consumption in the operation of the underloaded trunk pipeline by engaging additional pump units with lower rotor r.p.m. control using variable frequency drive. Under these conditions, the operating point of the pumps (delivery rate, head) approaches to the rated output, thus increasing the efficiency of the pumps and decreasing the electricity power consumption for pumping operations. The paper presents comparative calculations of operating conditions for a section of the trunk pipeline with a specified pumping capacity of 35 million t/year (60 % of the design load) when one, two and three main line pumps with variable frequency drives are engaged. The power consumption for pumping is calculated using the dependence of the variable frequency drive and motor efficiencies on the electric motor load and rotational speed. Based on the results of calculations, the electric energy saving is determined when additional pumping units are engaged. The experience has shown that when the second main line pump was engaged, the electric energy saving for pumping was 3.7% compared to the condition with one pump running. However, if the third pump is engaged, the saving is only 1.9 % – the decrease is due to lower motor and variable frequency drive efficiencies in the underloaded condition.