Merging old and new in level measurement

Nov. 8, 2017
Differential pressure makes great strides with simple innovations

Wireless is often a good application for level measurement because in many cases, the response time is somewhat slower, the distances tend to be quite long between sensors, and there is little interference from things like pipe racks and closely packed equipment.

Despite advances in level sensing and communications technology, a significant percentage of level measurements still rely on “plain old” differential pressure measurement and the equation  directly correlating the differential pressure to the associated height of the column of liquid above the lower pressure tap. Many intelligent transmitters include calculation capability and therefore, will be able to transmit an output as level rather than differential pressure by simply dividing the differential pressure by the density. (Mind the units and if necessary associated conversion factors.)

We also know that every pressure transmitter measures differential pressure—in many cases, the one side is open to atmosphere and hence the psig output. When measuring level, this means that for an open vessel (i.e. tank or non-pressurized vessel), there is no need for the low-pressure leg to be connected to a second nozzle. The accompanying figure shows how the transmitters may be mounted to the vessel wall—the top nozzle and capillary are dashed lines. To ensure that it remains in liquid all the time, the lower nozzle is located just below the lowest liquid level. Using a capillary and diaphragm seal system has the advantage of being isolated, and therefore less susceptible to contamination by the process. For this reason, most of the time a capillary is used to connect the second (top) nozzle with its flush, flanged diaphragm seal while the lower nozzle has the transmitter’s high-pressure side directly mounted to the vessel and associated nozzle by a second flush, flanged diaphragm seal.

Care must be taken with capillary seals, not only in selecting the fill fluid but also during installation. Mechanical protection provides support and protection from kinks or restrictions in the capillary that will cause errors in the measurement and premature capillary failure. To provide mechanical integrity, capillaries are normally run inside channel, which can be mounted to Unistrut® for support.

Intelligent devices, however, can replace the capillary system to the top transmitter with a transmitter itself, or with an “electronic remote sensor” option replacing the capillary with a cable. Because it is not possible to have two transmitters calibrated for zero measurement error, the two-transmitter option introduces some error in the measurement, however, if the level (i.e. differential pressure) is large enough to mask this error and digital communications are used, the two-transmitter option may be less expensive in the long term than a long capillary. Using two wireless transmitters provides the option of using two transmitters rather than a long capillary and also offers the option of not needing to install a power conduit.

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For applications where there is a possibility of material build-up in the nozzle and/or solids, extended flanged diaphragm seals should be used as they bring the pressure diaphragm closer to the process and ‘fill’ the nozzle with the diaphragm itself, thus preventing plugging or bridging of the measurement ports. There are, however, a few cautionary items to consider. The diaphragms are designed to have a close tolerance for the pipe internal diameter—always be sure to have the welder pencil grind the inside of the nozzle to remove any possible slag so you do not damage the diaphragm during installation. Another consideration is to keep the diaphragm approximately half an inch behind the surface of the inner vessel wall, as this will reduce the erosive wear on the diaphragm while also ensuring that any solids buildup on the resulting ledge cannot build up to a level where it will adversely affect the measurement.

One challenge associated with measuring level via differential pressure is the assumption that the liquid density is constant or alternately what is the actual density value. All of us know that assume can come back to bite us—so why assume the density when, by using a second set of nozzles a fixed distance apart, the above equation can be solved for density? Placing these nozzles below the lowest liquid level will provide a calculated density reflecting the actual liquid in the vessel.  I normally place the nozzles one meter or three feet apart with the lower tap on the same centerline as the level measuring transmitter(s), as this makes density conversion calculation easier (kg/m3 or lb/ft3) while also providing enough distance to get a reasonable measurement. While manufacturers may allow for closer spacing between the density measurement nozzles, the necessary separation can consume a significant part of the vessel volume, especially on smaller vessels. If space is an issue, in some cases it is necessary to install the top density measurement nozzle above the low liquid level, while the lower nozzle is always below the low-low liquid level.

I have also used this lower liquid density measurement along with an alternate level sensing technology such as ultrasonic, float, magnetorestrictive or another alternative providing the top of surface measurement and then using the difference between the ‘true’ level as compared to the calculated density compensated level a rough approximation of the liquid/liquid interface can be estimated. Of course, you could always use a different level sensing technology such as guided wave radar able to detect the interface – but that would mean another device and nozzle.

Advances in technology such as intelligent devices, and digital communications make it possible to use traditional measurement technologies in new ways. Regardless, though applied in innovative ways, they continue to rely on basic principles.

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