A Method for an Alternative Flow Measurement in a Centrifugal Compressor
Measuring the flow entering a centrifugal compressor is critical for process control, condition monitoring as well as for the protection of the compressor from surge. Usual methods (such as orifices, ultrasonic flowmeters and others) may cause permanent pressure losses, may be costly, or deliver weak, noisy signals. By using the natural acceleration found in the inlet of a centrifugal compressor, a built-in flow measurement can be incorporated into the machine with non-intrusive static pressure taps. This measurement can be used by the package PLC to determine compressor flow rates and can be used for process and surge control, thus reducing the need for orifice plates. By removing orifice plates from the compressor station, improved operating efficiency can be realized through reducing station piping head loss. While other non-intrusive flow measurement devices exist, all are external to the compressor and must be purchased separately at significant cost. By measuring the natural pressure drop from the inlet flange to the inlet of the impeller, correlations to standard flow nozzle measurements can be made. Correlating this pressure data to standard flow measurement devices installed during the factory test allows a flow calibration to be established. This paper describes the theory in using the natural pressure drop that occurs in the inlet system of a centrifugal gas compressor as a flow measurement reading. It further explains the cost savings to end-users in both first cost and operating costs compared to independent flow measurement devices. It also describes their use as both a primary flow measurement as well as surge control. Comparisons to independent flow measurements and application history are also made.