Question: Problems encountered on-site with electromagnetic flowmeters
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Site conditions: A spiral pump with a flow rate of 200 m³/h is installed in the horizontal pipe (DN250); there is a valve behind it, and another elbow follows. One meter further down, the pipe turns vertically upward, and an electromagnetic flowmeter is installed at a point about 2 meters up in the vertical pipe. The situation is as follows: after starting the pump, the flow rate measured by the electromagnetic flowmeter increases gradually from zero, reaching 180–190 m³/h quite quickly. After that, it remains around 180. At this point, the valve is closed to two-thirds of its position, and the instantaneous flow rate measured by the flowmeter drops by only about 10 m³. Given this, is there something wrong with the flowmeter? Another situation is this: when the valve is fully open, the water level downstream in the pipeline (several hundred meters away) is not full of the pipe – it’s only about one quarter full. Is this normal?
The flow meter should be normal. There are no issues with the installation. The flow rate is primarily determined by the output of the pump; even when the valve is closed, the pump’s head remains high, so the flow velocity of the fluid at the valve increases, and the decrease in flow rate is not significant. The pipes further downstream are not lined because, as water flows downstream, gravity causes it to flow along the lower wall of the pipes; as a result, a small amount of air enters outside the water outlet at that location, leading to a situation where the outlet is not fully filled with water.
Vertical installation does not cause the half-tube phenomenon, and the display is normal
According to Yokogawa’s electromagnetic flowmeter, the measurement range for 250mm is 53–1767 m3/h; the minimum value that ensures a 1% accuracy is 24 m3/h. Therefore, it is an issue with the accuracy of your meter. Based on your flow rate, an electromagnetic flowmeter with a DN150mm size should suffice, as this will ensure the required accuracy for minimum values.
The whole process seems to be fine. 1. The pump’s power is around 80%, and the display is normal. Close the valve to 2/3, but do not turn off the pump; the flow rate decreases only slightly. 2. The outlet is far away, which relates to the head issue and results in the outlet not being filled to capacity.