Partially Closed Valve Effects on Electromagnetic Flow Meter Accuracy

As the need to manage water resources increases, the ability to accurately measure use becomes crucial. In industry, measurement inaccuracies can cost either the supplier or consumer large amounts of money or even result in a depleted resource. Generally, flow meters need a flow profile to be fully developed to optimize measurement accuracy. In order to produce accurate measurements, most flow meters require a straight section of pipe immediately upstream of the meter. The straight pipe helps to develop the flow profile and produces flow conditions that are as ideal as possible. In many field installations, space restrictions lead to meters being installed with reduced straight pipe between a flow disturbance and the meter. Electromagnetic flow meters are referred to by several different names including: magnetic flow meter, mag meter or mag. The electromagnetic flow meter has grown in popularity in recent years as technology has improved and costs have decreased. Manufacturers commonly claim that their meters can produce highly accurate flow measurements, some as low as ±0.10%, with reduced upstream straight pipe requirements as low as 0 pipe diameters between a disturbance and the meter itself. The results of this study provide quantified error associated with a partially closed butterfly valve on electromagnetic flow meter accuracy. The results show the effect of four valve openings (25%, 50%, 75%, and 100% open), at five locations downstream of the valve (1, 3, 5, 10, and 20 pipe diameters), and at two different meter orientations to the valve shaft. It was concluded in the study that twenty pipe diameters of length are required to produce accurate measurements (within the manufacturer’s specified accuracy) when a magnetic flow meter is installed downstream of a throttling butterfly valve. Surprisingly, the largest effect to the meter’s accuracy unexpectedly occurred when the upstream butterfly valve was fully open and not when it was in a throttling position