DAVID W SPITZER'S E-ZINE (August 2022)

The amount billed each day was calculated by multiplying the daily flow by the billing rate per thousand gallons and then adding a surcharge if the flow during the day exceeded a set amount. The surcharge arrangement in the contract could be thought of as a penalty structure for not maintaining a tight sewage system and thereby allowing excess water to enter the system and require treatment. 

Surcharges that were applied on days with high sewage flow accumulated over time and prompted the first district to hire a flowmeter expert (me) to examine the existing flowmeters and investigate their appropriateness and performance. Investigation indicated that all four flowmeters were appropriate for the application, but three of the four flowmeters were installed incorrectly, which caused their measurements to be higher than the actual flow by amounts that could not be quantified.

In addition, the zero on the fourth (and largest) flowmeter was found to be elevated. My recollection is that the zero offset was quantified and caused the measurement to be high by approximately 20% of the typical daily flow. Stated differently, the first sewage district received invoices that were higher than they should have been.

Next month, we will discuss the fourth flowmeter.

This article originally appeared in P. I. Process Instrumentation magazine.

A. U-tube down

B. Inverted U-tube

C. Horizontal (parallel to grade)

D. Flag position

Coriolis mass flowmeters in gas service must be completely full of gas to measure accurately. The U-tube down orientation (Answer A) could accumulate liquid and should not be used for gas applications.

The inverted U-tube orientation (Answer B) and horizontal orientation (Answer C) could be acceptable but would entail modification of the upstream and downstream piping. The flag position (Answer D) would be the most practical answer.  

Additional Complicating Factors

Not all Coriolis mass flowmeters have U-tube geometry, and some of these geometries can allow liquid to accumulate in the flowmeter.

For example, a Coriolis mass flowmeter with delta-shaped tubes can accumulate liquid in the downstream (lower) portion of the tubes when mounted in a flag position (Answer D), so the upstream and downstream piping should be modified to mount the flowmeter with its delta-shaped tubes inverted (Answer B) or horizontal (Answer C).

Similarly, a single-path self-filling and self-draining Coriolis mass flowmeter that forms a loop, jumps up (sideways if mounted in flag position) and then forms another loop must be installed in the horizontal plane (Answer C) to remove all liquid from the system because any other orientation can allow liquid to accumulate in the flowmeter.

This article originally appeared in P. I. Process Instrumentation magazine.