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

Float sensors are typically suspended in the vessel whereby its location (height) reflects the location of the surface of the material. Various transmitter designs are available to sense the height of the float and generate a level measurement.

Magnetostrictive level sensors generally consist of a probe on which a float moves so as to locate itself at the surface of the material. Electrical pulses within the probe are reflected back to the transmitter by a magnetic field formed around the probe by a magnet embedded in the float. The level measurement is derived from the amount of time measured for the pulses to travel to and return from the float.

Retracting level sensors are typically floats or weights that are removed from the material during normal operation. Periodically, the float/weight is lowered into the vessel until contact with the material is made. The level measurement is derived from the distance measured prior to the sensor reaching the material.

Level measurement systems that have non-mechanical sensors that contact the material include capacitance, hydrostatic, contact radar, and submerged ultrasonic level measurement technologies. In addition, ultrasonic level sensors that measure liquid level while submerged in the liquid are also available.

Capacitance sensors are typically probes that are inserted into the material, however other shapes and arrangements are available. As the material level increases, the probe becomes more covered, and the capacitance between the probe and the material increases. The increase in capacitance is proportional to the covering of the probe and related to the electrical conductivity (of conductive materials) or the dielectric constant (of insulating materials). The transmitter electronics processes this information and produces a signal proportional to the material level.

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

In particular, the previous one-point calibration technique under operating conditions could not check the flowmeter zero. As a result, the flowmeter zero was not checked for years. The more recent installation allowed the transmitter to be zeroed and spanned using the ruler on a sight glass connected to the purge tubing for the flowmeter primary element.

However, my findings indicated that the transmitter zero was significantly out of adjustment. How was it possible to make this determination despite flowmeter calibration being performed and witnessed by both parties every few months? The answer was quite simple: read the instructions.

The differential pressure transmitter was connected to a tap on the flowmeter primary element. The instruction manual for the flowmeter primary element indicated that the zero should be performed by closing the valve at the tap and removing a plug to allow liquid to drain down to the level that would occur under zero flow conditions. Performing this procedure revealed that the ruler used to calibrate the transmitter was positioned improperly and caused the flow measurement system to measure significantly higher than the actual flow. This resulted in significantly higher billing for the first district, especially on days when surcharges applied.

Next month, read about what happened to cause this flowmeter to be fixed.

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 vapor 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) would entail modification of the upstream and downstream piping. The flag position (Answer D) would typically be most practical.

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 upstream (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.