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77 Field Instrumentation GEN Rev)

3.8 Flow Measurement Errors

We have already discussed the pros and cons of each type of flow detector commonly found in a generating station. Some, such as the orifice, are more prone to damage by particulate or saturated steam then others. However, there are common areas where the flow readings can be inaccurate or invalid
Erosion: Particulate, suspended solids or debris in the piping will not only plug up the sensing lines, it will erode the sensing device. The orifice, by its design with a thin, sharp edge is most affected, but the flow nozzle and even venturi can also be damaged. As the material wears away, the differential pressure between the high and low sides of the sensor will drop and the flow reading will decrease.
Over ranging Damage to the DP Cell
Again, as previously described, the system pressures are usually much greater than the differential pressure and three valve manifolds must be correctly used.
Vapour Formation in the Throat
D/P flow sensors operate on the relation between velocity and pressure. As gas requires less pressure to compress, there is a greater pressure differential across the DP cell when the gas expands on the LP side of the sensor. The flow sensor will indicate a higher flow rate than there actually is. The turbulence created at the LP side of the sensor will also make the reading somewhat unstable. A small amount of gas or vapour will make a large difference in the indicated flow rate. The opposite can occur if the vapour forms in the HP side of the sensor due to cavitations or gas pockets when the fluid approaches the boiling point. In such an instance there will be a fluctuating pressure drop across the DP cell that will give an erroneously low (or even negative) DP reading.

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