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Level Measurement (Part 1 of 2)by David W Spitzer and Walt Boyes
Some level measurement systems measure the level of the material. For example, float location and the amount that a capacitance probe is covered with material are indicative of the material level.
However, many level measurement systems inherently do not measure level --- they infer level. Most notable is hydrostatic level measurement that infers level using fluid density. Whereas level measurement is (theoretically) supposed to determine the location of an interface, hydrostatic measurement measures the effective mass of the fluid and infers the location of the interface. Therefore, the (inferred) level measurement will vary when the density of the material in the vessel changes.
Other examples of inferred level measurement include ultrasonic and radar level measurement systems where the measured distance from the sensor to the material is used to infer the level of the material.
The most common application of level measurement systems is to determine the inventory of a contained material. Although this may be desirable for the efficient operation and safety of the process, level measurements are also used for accounting purposes.
Excerpted from The Consumer Guide to Non-Contact Level Gauges.
Safety First: Placing the Priority on the Worker Over the Instrument by David W Spitzer
Last month we examined the installation of
impulse tubing in high-pressure steam flowmeter applications. In a seminar some years ago, one of my
students posed an adamant objection to the manner in which transmitters are
taken out of service in steam service.
Fortunately, the student not only knew the procedure as to how to remove
the transmitter from service, but also why it was done according to this
procedure. The proposed procedure may
not be the best in a technical sense, but it makes perfect sense if you value
your and your co-workers' wellbeing.
Removing a differential pressure
transmitter from service in a "normal" installation is relatively
straightforward. You should start by
opening the bypass valve to create a hydraulic jumper across the transmitter that
protects the transmitter from pressure transients that might create a large
differential pressure across the transmitter.
The high-side and low-side shutoff valves can then be closed to isolate
the transmitter. Putting the transmitter
back in service entails ensuring that the bypass valve is open (creating the
hydraulic jumper) before opening the high-side and low-side shutoff
My student's objection was that
using this procedure in high-pressure steam service could cause injury to the
instrument technician and any people who happened to be nearby. In steam service, the impulse tubing is designed
to create condensate seal legs that isolate the transmitter from live
steam. Opening the bypass valve first
will cause flow to occur in the impulse tubing because the upstream pressure
tap is at a higher pressure than the downstream pressure tap. This flow will cause the condensate seal to
be removed from the impulse tubing and expose the transmitter and its
associated impulse tubing to live steam.
Aside from potentially damaging the transmitter, this sudden thermal
shock could cause the normally cold impulse tubing to fail and potentially
To remove a transmitter from
steam service, the student recommended first simultaneously closing the
high-side and low-side shutoff valves before opening the bypass valve. This procedure effectively maintains the condensate
seals so live steam does not reach the transmitter or most of the impulse
tubing. Similarly, returning the
transmitter to service should be performed by first closing the bypass valve
and then simultaneously opening the high-side and low-side shutoff valves.
This procedure can allow the transmitter to be exposed
to an excessive differential pressure and potentially damage the
transmitter. However, it also reduces
the potential for injury --- which makes perfect sense.
This article originally appeared in Flow Control magazine.
How Rising Density Affects DP Steam Flow Measurementby David W Spitzer
For a fixed differential pressure measurement
(say 100 inches of water column) in a differential pressure steam flow
measurement system, increasing density causes the compensated steam mass flow
Decrease proportional to the square root of
Decrease proportional to the steam density
Increase proportional to the square root of
Increase proportional to the steam density
Differential pressure flow
measurement devices are commonly applied to measure steam flow. In addition, pressure and temperature
measurements are typically implemented using individual or multivariable
transmitters. The compensated steam mass
flow rate can be calculated using flow computer functionality that can be
implemented in a flow computer or other computational device.
For differential pressure
flowmeters, the volumetric flow (Q) is proportional to the square root of the
ratio of the differential pressure (?P) divided by the fluid density (?). However, the mass flow (W) is the
product of the volumetric flow and density. Therefore, for a given raw differential pressure
measurement, the compensated mass flow of steam is proportional to the square
root of the steam density. Answer C is
Additional Complicating Factors
Flow computers typically use steam tables to
determine the steam density. Using other
devices with more rudimentary density algorithms (such as the gas laws) may
result in significant mass flow measurement error.
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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