Flow rate measurement of wet steam flow by clamp-on ultrasonic flow meter

Steam is widely used as a medium for thermal energy transport in industry, such as in heat supply and drying processes. It is therefore important to evaluate precisely the steam flow rate at the demand end from the viewpoint of energy management. The state of steam in the steam piping of industrial facilities without the superheating process is usually wet steam resulting from causes such as heat loss, heat supply, and work. It is well known that steam wetness often causes measurement error of the steam flow meter. However, there has been no established standard for estimating the measurement error since it is difficult to evaluate the effects of steam wetness quantitatively in plants and factories. A clamp-on ultrasonic steam flow meter is suitable for use in factories since pipe processing is unnecessary. We conducted experiments to clarify characteristics of the measurement error of a clamp-on ultrasonic flow meter owing to steam wetness considering the influence of pipe diameter. The total mass flow rate of the steam through the test section was measured using a calibrated Coriolis flow meter downstream of the condenser to obtain a reference value. The measurement error of the clamp-on ultrasonic flow meter was estimated by comparison with the reference value. The flow rate difference between these two flow meters was found to increase with increasing steam wetness. Moreover, the clamp-on ultrasonic flow meter measurement was almost the same as the flow rate of only the gas phase in the wet steam flow. From the experimental results, we proposed a correction method for measurement using the clamp-on ultrasonic flow meter in a wet steam flow by correcting the density in the flow rate calculation. Moreover, we clarified the applicability of the clamp-on ultrasonic flow meter in piping of various diameters.