Weir Type Flowmeters

• Weir type flow meters are used to measure open channel flow where the level of the channel is measured against a weir opening.
• The flow meter is based on gravitational impact of an open weir. If the rate of flow exiting the weir is same as the flow entering the system, the level will remain constant
• Ultrasonic and bubbler type flow meters are typically used for measuring the height of the weir level
• If ultrasonic level measurement is used, typically, a PVC pipe will be provided to minimize false echos
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• The wet leg measured DP is 0 at full and negative at empty vessel
• The wet leg manifold does not have any equalizing valve to eliminate the effect of valve parsing. A parsing equalizing valve would cause the low side filled product to drop down, reducing the low side pressure. If this occurs, we would expect a higher DP (high minus low) and would of course result in a higher level measurement
• To verify 100% level i.e. 0 dp, vent both high side and low side manifolds
• To verify 0% level i.e. lowest dp (negative), vent high side manifold first. Then, the lowside must me vented at the top tapping point. By this way you measure atmospheric pressure at high side and wet leg pressure at low side. This low dp should be equal to the LRV
• Some transmitters allow the setting of 0% level at 0 dp and 100% level at made-positive lowest dp. This is done be adding an offset or zero