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Non-Contact Level Measurementby David W Spitzer and Walt Boyes
Non-contact level measurement systems include laser, non-contact radar, and ultrasonic level measurement technologies. In general, these level measurement systems consist of a level sensor located above the material surface that emits a signal and processes the returns (reflections) of that signal.
However, in solids applications, material entering and leaving the vessel affect the solid/gas interface. As such, this interface is typically not horizontal, so a single level measurement may not be representative of the amount of material in the vessel. In these applications, the level measurement reflects the level at one point in the vessel. Level measurements can change rapidly when the material level that is sensed changes rapidly. In some applications, multiple level measurements may be needed to provide a more accurate indication of the inventory of material in the vessel.
For example, a rat-hole may form as solid material leaves the vessel. If the level measurement reflects a point in the rat-hole, the measured level will decrease (as expected). However, if the material remaining on the sides of the vessel falls and fills in the rat-hole, the level will abruptly (and unexpectedly) increase. Sensors should be located such that the measured level represents the actual level while avoiding rat-hole affects. Multiple sensors may be needed if an appropriate location cannot be found.
In addition, most non-contact level sensors cannot accurately measure distances that are close to the sensor itself. Sensors are typically installed to allow the transmitter to disregard measurements at these distances. This is often called the blanking distance.
Excerpted from The Consumer Guide to Non-Contact Level Gauges.
Considering the Cooling of Capillary Tubing: How a 'Rule of Thumb' Can Result in a Measurement Errorby David W Spitzer
Last month we determined that the capillary
tubing for differential pressure measurements should generally be the same
length --- even though the added physical length may seem wasteful, expensive
and cumbersome. Often, the excess
capillary tubing for the closer nozzle or tap is coiled for convenience.
In one such flowmeter application, the capillary tubes
were of equal length --- following the "rule of thumb" to use capillary tubes
of equal length. However, the excess
capillary tubing to the closer tap (approximately 4 meters) was coiled inside
the base of the transmitter enclosure.
This would not be a problem if the temperature of the capillary tubing
inside the base and exposed to the closer tap were the same as the temperature
as the exposed capillary tubing to the farther tap.
In this installation, the instrument enclosure was cooled
so the temperature of the capillary tubing was different. In particular, the temperature of the
capillary tubing to the closer tap was colder than that to the farther
tap. Compounding the problem, the
exposed capillary tubing to the farther tap was exposed to sunlight during part
of the day. These conditions affect the
measurement in the same direction so the measurement error would be greater
than either individual error.
One of the capillary tubes was coiled while the other was
not. Although it might seem that this
would affect the measurement, the measurement should not be affected because
the pressure increases/decreases in the coiled impulse tube effectively cancel
so the static pressure at the transmitter is dependent upon the elevations of
the tap and the transmitter.
In many applications the transmitter enclosure is heated
and can create similar problems. This
often occurs in colder climates where the capillary tubing to the closer tap
will be warmer than the capillary tubing to the farther tap that is exposed to
the cold climate. These conditions
affect the measurement in the same direction so the measurement error would be
greater than either individual error --- albeit in the opposite direction as
compared to an installation where the instrument enclosure is cooled.
Remember that following a "rule of thumb" may be a
good thing, but doing so without a complete understanding can create problems.
This article originally appeared in Flow Control magazine.
Expressing Pump Discharge Pressureby David W Spitzer
Reducing the motor speed of a centrifugal pump
reduces both the discharge pressure of the pump and energy consumption of the
motor. When the motor is operated at 80
percent speed, what is the approximate pump discharge pressure and motor energy
consumption expressed as a percent of full speed discharge pressure and full
speed motor energy consumption respectively?
65 precent pressure and 50 percent energy
65 precent pressure and 80 percent energy
80 precent pressure and 50 percent energy
80 precent pressure and 80 percent energy
Variable speed drives are
increasingly applied in many applications to reduce energy consumption.Reducing the speed of a centrifugal pump
reduces the pump discharge pressure by the square of its speed.Therefore, the discharge pressure will be
approximately 64 percent of the full speed discharge pressure when the pump is
operated at 80 percent of speed.Answer
C and Answer D are not correct.
Reducing the speed of a
centrifugal pump reduces the energy consumption by the cube of its speed. Even a small speed reduction such as 5
percent can save almost 15 percent of the energy consumed at full speed. In this case, operating at 80 percent speed
reduces the energy consumption to approximately 51 percent of the energy
required to operate the pump at full speed.
Therefore, Answer A is correct.
Additional Complicating Factors
Many variable speed drive applications and
installations require attention to detail to ensure that the motor and its
mechanical equipment can function properly in operation. In addition, failure to attend to certain
details of lower speed operation can damage the equipment.
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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