Stage By Stage Design Of Impellers In A Multi-stage Gas-liquid Multiphase Pump Considering The Compressibility Of Gas

In the development of subsea oilfield,multiphase pumps directly transport the mixture of oil,gas and water from subsea oilwells through a single pipeline.Multiphase transport technology can simplify equipment,decrease back-pressure of the wellhead and save capital costs.Due to the compressibility of gas,the flow rate of the medium decreases when stage increases,along with the flow parameters in impellers.In order to improve the hydraulic performance of the rotodynamic pump,the study on the hydraulic design stage by stage of a multi-stage rotodynamic multiphase pump is conducted.Commercial software CFX is used to do the numerical simulation.The change law of flow parameters in the inlet of impellers is investigated and advice of stage by stage design is given.Numerical simulations of a five-stage multiphase pump in different operating conditions are done to study the change characteristics of inlet flow parameters of impellers.Results show that the inlet volume flow and inlet liquid flow angle decrease substantially after the compression of the first stage while at a lower rate after the compression of other stages.When other two operating conditions are kept unchanged,the extent of inlet volume flow and inlet liquid flow angle decreasing is bigger with the increasing rotational speed or increasing inlet gas volume fraction.According to the research,the stage by stage design method of multiphase pump is proposed.If the inlet gas volume fraction is smaller than 10% or bigger than 90%,all impellers can be designed uniformly.If it is between 10%~30%,the first two impellers should be designed individually,the structure of latter stages keeps constant with the second impeller.If the inlet gas volume fraction is between 30%~50%,the first three impellers should be designed individually,while the structure of other two stage keeps constant with the third impeller.When the inlet gas volume fraction is between 50%~90%,with a high gas compressibility but a low differential pressure of the multiphase pump,only the first three stage should be designed individually.According to the design method,stage by stage design of the five-stage multiphase pump is conducted.The numerical simulation results show that the hydraulic efficiency increases by 3.29% and 7.25% when the inlet gas volume fraction is 30% and 50%.The hydraulic loss in impellers improves obviously,which verifies the validation of the method of stage by stage design.