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

Common examples of interfaces where material levels can exist include liquid/gas, solid/gas, and liquid/liquid. Liquid/gas interfaces are commonly found in level measurement systems for tanks and other containment vessels. Solid/gas interfaces generally found in solids handling systems are indicative of the inventory of solids in the vessel. Liquid/liquid interfaces are often found in processes where a separation of the liquids occurs due to the different specific gravities of the liquids above and below the liquid/liquid interface. Other interfaces and multiple interfaces, such as slurry/liquid, slurry/gas, slurry/liquid/gas, and liquid/liquid/gas can exist in some processes.

The material level is important to determine the inventory of the material in the vessel. These levels may be well defined or not. Liquid level in a tank is an example of a well-defined interface because (assuming no agitation) the liquid/gas interface arranges itself to be horizontal. The gas/solid interface in a vessel would usually not be well defined because solids generally do not arrange themselves to be horizontal, so a solids level measurement may not be indicative of the inventory in the vessel. Therefore, accurate measurement of level is dependent upon the process and the level measurement system.

Level measurement systems generally consist of a level sensor and its associated electronics.

As defined herein, level sensors are the devices that are used to sense the interface between two or more materials. Level sensors generally exploit one of a number of properties of the material or its interface. Level sensors may be in contact with the material or not. Some level sensors are designed to be in contact with the material and may or may not have moving parts. Others are designed to be in contact with the environment of the material. Still other level measurement systems allow the level sensors to be located externally and have no contact with the material or its environment.

The electronic circuitry associated with the level sensor is the level transmitter. The level transmitter processes the signal(s) produced by the level sensor. The level sensor and level transmitter may be separate and distinct pieces of equipment, or both may be integrated into one assembly. Nonetheless, level measurement systems that produce an electronic output will have both level sensor and level transmitter functionality.

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

In addition, calibration checks of the fourth flowmeter were performed under operating conditions using relatively inaccurate calibration techniques to check the operation of this flowmeter. It was anecdotally reported that the flowmeter calibration was originally checked by having a person in one manhole manually measure the liquid level in the flowmeter primary element and yell that level to a person above ground. He would then turn and yell the value down to another person in an adjacent manhole, where the transmitter was located, who would compare that value with the output of the transmitter.

This may sound reasonable because there was sewage flowing at all times. However, the flow rate was not steady and tended to increase and decrease during relatively short periods of time, which meant that the transmitter measurement could change by the time the level was measured by a person near the flowmeter primary element and communicated to the person near the transmitter. Aside from only checking the calibration at one operating flow rate, this calibration check was performed manually while the flow rate was changing, which introduced significant additional sources of calibration error. 

Some years prior to my visit, a new transmitter for the fourth flowmeter was installed in the same manhole as the flowmeter primary element, so the level and transmitter output could be checked by one person without yelling. This technique was better than the previous technique by eliminating the verbal communications (yelling up and down manholes) because the flowmeter primary element and transmitter were located in the same manhole. This installation also included a ruler on a sight glass that could be isolated from the flowmeter primary element and be used to calibrate the transmitter zero and span without having to measure the liquid level in the flowmeter primary element.

Read more about the fourth flowmeter next month.

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

A. U-tube down

B. Inverted U-tube

C. U-tube horizontal (parallel to grade)

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.

Few Coriolis mass flowmeters are mounted in the horizontal plane (Answer C), so mounting the flowmeter with an inverted U-tube orientation (Answer A) that hydraulically removes liquid from the flowmeter would be 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 single-path self-filling and self-draining Coriolis mass flowmeter that forms a loop, jumps up 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.