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Level Measurement (Part 2 of 2)by David W Spitzer and Walt Boyes
Level measurements can be used to keep tanks in the plant within reasonable operating limits. Tanks that become empty can potentially upset production due to the inability to feed the downstream process. They can also cause a lack of flowing material that can damage equipment, such as pumps. On the other hand, tanks that overflow can potentially create environmental hazards.
Level measurements may be used to manipulate the flow of material through the plant, such as in a sewage treatment plant where a high level in the primary collection tank would increase the flow through the entire treatment plant.
Level measurements may be used to determine the interface between two materials. In some applications, the location of a liquid/liquid interface can be important to ensure that the separation occurs properly.
Most level measurement applications may seem routine but they can become relatively difficult to implement when additional factors are considered. For example, there are many level measurement systems that can be used to measure the level of a liquid in a vessel. However, the number of viable measurement systems decreases significantly when the vessel is agitated and operates at 200 degrees Celsius and 40 bar. Foaming and the effects of filling and emptying the vessel can further limit the viability of many level measurement technologies. Level measurement applications are rarely "cookbook" and typically require careful engineering.
Excerpted from The Consumer Guide to Non-Contact Level Gauges.
Considering Capillary Tubing: How the Application Affects Tubing Length Requirementsby David W Spitzer
In the last two months we examined the
installation of impulse tubing in high-pressure steam flowmeter applications
and removal of high-pressure steam flow transmitters from service. We found that measurement error can occur due
to the different densities of the liquids in the impulse tubes in
non-horizontal impulse tubing runs. A
similar effect can occur when diaphragm seals are used with differential
pressure transmitters. Differential
pressure transmitters with diaphragm seals are not commonly applied to flow
measurements, but they are quite commonly applied to level measurements.
A common "rule of thumb" is
to purchase the capillary tubes of the same length for both the high-pressure
and low-pressure sides of the transmitter.
This would seem to make (mechanical) sense for flowmeters because the
flowmeter element taps are generally close to one another, so the distances
from the transmitter to each tap is similar.
In level applications, the
nozzles can be in quite different locations.
For example, a transmitter may be located 1 meter from the lower nozzle
(near grade) and 8 meters from the upper nozzle (near the top of the vessel). The "rule of thumb" would stipulate that the
transmitter have (say) 10 meters of capillary tubing on both sides of the
transmitter. This would seem reasonable
for the upper nozzle and excessively long (and costly) for the nearby lower
nozzle (where the capillary tubing would typically be coiled for
convenience). However, this analysis
only takes physical dimensions into consideration.
The capillary tubing
contains liquid that transmits the pressure from the diaphragm seal to the
transmitter. The density of this liquid
changes with its temperature so the installation should be designed to maintain
the liquid in both capillaries at the same temperature. This is similar to the concept that was
discussed for steam flow measurement transmitters.
However, the liquid in the capillary tubing is
different from the steam flowmeter seal in the sense that the liquid in the
capillary tubing is captive. Therefore,
expansion of the liquid in one capillary will cause the liquid volume to change
and affect the differential pressure measurement. By its nature, the differential pressure
transmitter will approximately cancel the expansion and density affects for
capillary tubes of equal length (and liquid volume) when the capillaries are at
the same temperature. However, unequal
lengths of capillary tubing (and unequal liquid volume) on each side of the
transmitter can affect the measurement because the different capillary lengths
(and volumes) result in different amounts of expansion. Therefore, it is desirable for the capillary
tube lengths to be not only equal, but also at the same temperature.
This article originally appeared in Flow Control magazine.
The Difference Between Component and System Accuracyby David W Spitzer
A flow measurement system consists of a
flowmeter element, transmitter and indicator.
If each of the three components has an accuracy of 1 percent, the
performance of the flow measurement system is approximately:
None of the above
The performance of the flow
measurement system should be inferior to the performance of the flow element
itself. Answer C could be viable (given "approximate" in the wording of the question).
However, the magnitude of the performance of the other two components (1
percent) causes Answer C to be incorrect.
It is not often that all of the
components in a measurement system are in error by the maximum amount in the
same direction at the same time.
Therefore, the errors for the three components in this system would not
be mathematically added to obtain 3 percent.
Answer A is not correct.
The overall accuracy of measurement systems is
typically calculated by taking the square root of the sum of the squares of the
effect of each component. For this
system, this would be calculated as the square root of (12 + 12
+ 12), or 1.73 percent. It
might appear that Answer B is correct, however further investigation would show
that the flowmeter element has a percentage of rate error whereas the
transmitter and indicator have percentage of full scale errors. This means that the transmitter and indicator
exhibit flow errors in excess of 1 percent at flows below 100 percent of full
scale flow. Therefore, the calculation
that results in 1.73 percent performance is only correct at full scale flow. Answer D is correct.
Additional Complicating Factors
Definitive calculations to determine the overall
accuracy of a measurement system entail analysis of many factors that affect
the measurement including dimensional, physical, process, operational,
calibration, ambient and measurement considerations.
This article originally appeared in Flow Control magazine.
ABOUT SPITZER AND BOYES, LLC
In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 500 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.
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