The use of variable frequency drives as a final control element in the petroleum industry

The concept of using a variable frequency drive to control electric driven centrifugal pumps has been well known for some time, but seldom used in the petroleum industry. The reasons given for this range from: misinformed economic data, unease of using unfamiliar electrical technology, to an ignorance of how these devices react as a control device. One still sees the majority of flow, pressure and temperature control being performed by using constant speed centrifugal machines being throttled by a control valve to provide the desired control of fluids. This paper sets out to provide application guidelines on the correct selection of a variable frequency drive for use with a centrifugal pump, to provide flow, pressure or temperature control. The topics covered include the effect on the pump-system curve when using a variable frequency drive, the economic considerations of a variable frequency drive vs. a control valve, the correct selection of a drive for a given pump and fluid characteristics, the correct planning and installation considerations for VF drives, tuning a control loop when a VF drive is used as the final control element. Also discussed are the additional benefits which a VF drive can bring including reduced fugitive emissions, the enhanced diagnostic information about the running motor which can be brought into the control system, and the reduced wear on bearings and seals in a pump by using a VF drive. The paper concludes by giving some application examples of where VF drives have been used in both the upstream and down stream sectors of the petroleum industry, as a final control element