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

Displacer hydrostatic level sensors are based upon the principle that submerging a displacer (float) in a fluid causes its weight to change due to buoyancy. Therefore, increasing level increases the submergence of the displacer and causes its weight to decrease. This information is used to infer the height of the fluid that is dependent upon knowledge of the specific gravity of the fluid. Displacer sensors may be located directly in the vessel or connected to the vessel with external piping.

A contact radar level measurement sensor is typically a probe that becomes covered by the material as the level changes. The transmitter sends a radar frequency pulse down the probe that is reflected back to the transmitter at the surface of the material. The reflected signal takes more time to return to the transmitter when the probe is less covered. The covering of the probe is determined by measuring the time it takes the reflected signal to return to the transmitter. The strength of the reflection (and the ability to utilize this technology) is determined by the dielectric constant of the material.

Submerged ultrasonic technology emits ultrasonic energy upwards from a sensor located the bottom of a vessel. The ultrasonic energy is reflected back to the transmitter by the liquid/gas interface. The level measurement can be derived from the amount of time measured for the ultrasonic energy to travel to and return from the interface. The ability to use this technology is dependent upon the ability of the interface to reflect ultrasonic energy back to the sensor.

Excerpted from The Consumer Guide to Non-Contact Level Gauges.

These high flow measurements increased the proportion of sewage allocated to the first district, increasing the allocation of treatment costs to the first sewage district, which increased its sewage treatment rate per thousand gallons of sewage. As a result, the first sewage district paid more than it should — every day. More insidious and financially injurious were the surcharges that were applied during days with high flows that typically occurred due to significant wet weather events.

Some years later, the ruler used to calibrate this flowmeter fell off and was reportedly discovered on the floor of the manhole. No one knew how this happened, but a new ruler was affixed, and the transmitter was calibrated. Observation of the calibration using the zero procedure in the instruction manual ensured that the ruler was properly located. After the new ruler was affixed, the flowmeter proceeded to consistently measure lower by an amount almost equal to the previously described zero offset.

Not to be outdone, the other three sewage flowmeters were improperly installed. As in the previously discussed flowmeter, the liquid level at a particular location in the flowmeter primary element should be used to determine the flow rate. However, the transmitters that were used would not measure in that location due to their size, so they were instead installed a few diameters upstream of the flowmeter. In other words, the level should have been measured at a location indicative of the flow rate but was instead measured at a more convenient location where the level has an unknown relationship with flow rate. Further analysis revealed that the levels at these locations were typically higher than those at the proper location in the flowmeter primary element, so the flow measurement system tended to measure higher than the actual sewage flow rate, which exacerbated the overbilling.

Read about the total plant flowmeter next month.

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 vapor 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 vapor 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.