December 2009
10
TechnicalExclusive
Considerations for
Point Level Monitoring of
Powder and Bulk Solids
T
By Joe Lewis
The use of point level monitors for
powder and bulk solids in modern
Estimated Usage of Common Point Level Technologies
industry dates back to at least 1934.
These devices, also known as bin level
1.2%
1.5%
indicators, are quite resilient. This ex2.8%
emplifies the importance of point level
monitors. So what is point level monitoring?
Point level monitoring is monitoring
33.1%
29.3%
for the presence or absence of material
at a predetermined point within a bin,
silo, or other vessel. This can include
powders, granular, or other forms of
32.1%
bulk solids in storage or a processing
application. The use of sensors to detect
the presence or absence of material
is commonplace and there are many
choices of technologies (Figure 1). HighVibrating Element
Tilt Switch
level detection prevents overfilling and
Rotary Paddle
Diaphram Switch
controls fill shutoff. Low-level detection
RF
Admittance
Other
prevents material outages.
The selection of a point level sensor
technology is impacted by many facFigure 1: Estimated usage of point level technologies
tors. The most common are:
* Temperature
* Mounting Location
inside the bin and the ambient conditions surround* Bulk Density
ing the sensor outside the bin can impact the choice
* Particle Size
of technology, brand, and version within the technol* Dielectric Constant (εr)
ogy category.
* Corrosion/Abrasion
Temperature affects electrical/electronic compo* Material Buildup
nents and the use of nonmetallic wetted parts (materials in contact with the material). Internal bin and
Temperature
ambient temperature extremes are what you need
The most commonly used sensors are in contact
to know. Each sensor considered should clearly state
with the material being monitored. The temperature
these temperature limits.
The technology with greatest range of temperature
capability will typically be the mechanical and elecMin
Internal
Max
Internal
Min
Ambient
Max
Ambient
Technology
Bin Temp
Bin Temp
Temp
Temp
tromechanical technologies. Refer to Table 1.1
This data is synthesized from the specification
Diaphragm
Same
Same
-40°F (-40°C) 250°F (121°C)
for standard versions of devices from a variety of
Tilt
Same
Same
-40°F (-40°C) 250°F (121°C)
brands. Generally, all of these technologies will be
Rotary Paddle
300°F (149°C) -57°F (-49°C)
200°F (93°C)
acceptable for 80% of the point level applications in
powder and bulk solids. Other selection criteria will
RF Admittance
176°F (80°C)
-40°F (-40°C)
150°F (65°C)
typically be used to make a final choice. The appliVibrating Element -22°F (-30°C) 230°F (110°C) -22°F (-30°C)
140°F (60°C)
cation must fit within the temperature limits of the
Cap Prox Switch
Same
Same
-13°F (-25°C) 176°F (80°C)
chosen technology.
Table 1: Typical internal bin and ambient temperatures for common point level technologies.
11
www.PowderBulkSolids.com
Your choice for a mounting location should
consider physical size of the sensor, accessibility, material flow, and surrounding equipment.
Take care that the sensor fits. You may need
to relocate a sensor or chose something different. Small bins require a compact sensor
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The temperature inside the
bin and the ambient conditions
surrounding the sensor outside the
bin can impact the choice of technology, brand, and version within
the technology category.
because of physical limits. Capacitive proximity switches are great for compact mounting
needs.
Top-mounted sensors that use rigid extensions require headroom for installation and
removal. Flexible extensions or side-mounting
can be solutions. Flexible extensions are available with rotary paddle, RF admittance, and
vibrating element technologies.
One final point regarding sensor mounting location is in regards to devices mounted
on the bins in proximity to where the point
level sensor will be mounted. An example is
to avoid locating the sensor near an industrial
vibrator. Vibrators fluidize the material in the
bin to promote flow. They introduce force into
the material through the bin wall. This can
be destructive to sensors mounted too close.
Consult with the sensor manufacturer.
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Bulk Density
This material attribute is defined in terms of
weight per volume. Most commonly it is referred to in units of lb/cu ft, g/cu cm, and kg/
cu dm. These terms all refer to density of the
bulk material. The bulk density of the material
to be monitored can impact technology choice.
The higher the bulk density, the more probability there is for physical damage to invasive
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• Bulk Storage
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• Process Controls
• Slide Conveyors
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© 2005 Nol-Tec, Systems, Inc.
December 2009
Mounting Location
December 2009
12
TechnicalExclusive
Powder/Bulk Solids
sensors. Side loading from shifting or flowing
material or falling material can damage some
sensors.
Vibrating element level sensors are sensitive
to falling material and side loading. The manufacture and design has improved in the last
15 years, but this technology is best used in
low-level applications when material density
is <45 lb/cu ft. Specific mounting instructions
must be followed if you use vibrating element
sensors in low-level applications, including the
Any technology with
high sensitivity should not
be used for applications with
sticky material unless the
manufacturer is consulted and
agrees. Vibrating element sensors
and diaphragm switches are the
use of a protective baffle over the sensor.
High-level applications can also be hazardous to some technologies. Side-loading from
shifting and flowing material should be considered. Use of top-mounted rotary paddle units
with extensions >2 ft should use a protective
guard. Some technologies may suffer from inconsistent operation or failure if the material
density is very low. Consider lightweight materials such as aluminum shavings, sawdust,
dust collector hoppers, corn bran, various
cereals, expanded polystyrene beads, potassium chloride, soybean hulls, oat hulls, and
untreated fumed silica. Powders usually are
aerated due to pneumatic conveying and this
lowers bulk density at the top of the pile since
the powder has not yet packed. Vibrating element sensors may prove best as some can
detect materials as light as <1 lb/cu ft. These
low bulk density materials can be problematic.
Dielectric Constant
εr
ABS Resin and Pellet
1.5
Charcoal
1.5
Calcined Coke
1.5
Ground Cork
1.5
Cottonseed
1.4
Goose Down
1.2
Porous Cereals
1.6
HDPE
1.6
Milk Powder
1.7
Popped Popcorn
Generally speaking, the larger the particle
the more destructive it can be to a level sensor. This doesn’t diminish the impact heavy
powder can have when falling directly on a
sensor. The destructive potential of falling
or shifting rock size materials is easily imagined. A not-so-obvious impact results from the
space created between large particles. A small
amount of contact between material and the
sensor can occur. This may be a problem for
some RF admittance sensors that rely on bulk
density or material dielectric for operation.
Fine micron size powder presents a different image. Imagine the difference in density
between a wood and aerated saw dust. A
powder is a mixture of micron particles and
air. The air doesn’t escape until the powder
settles. Monitoring a lightweight powder in a
high-level condition (backup protection in hopper of a dust collector) means the powder is
aerated and this must be taken into consideration. Vibrating element sensors tend to be the
best choice.
Dielectric Constant
most sensitive.
Material
Particle Size
Dielectric constant is also known as the relative static permittivity, or εr. This material
characteristic (refer to Table 2 for examples)
is important when considering any RF devices looking for circuit changes as a result of
the material being present at the sensor. The
lower the εr, the more sensitivity is required
of the sensor. Many low εr materials are also
lightweight. Vibrating element sensors are a
good choice, depending on bulk density.
Corrosion/Abrasion
Corrosion and abrasion impact the wetted
parts. These are parts that are in contact with
the material. Most sensors have good compatibility with the materials they will contact.
Most sensors use stainless steel for wetted
parts, which has good corrosion and abrasion
resistance. Standard wetted parts typical for
common sensor technologies are shown in
Table 3.
Good desk references for material compatibility should, with various chemicals and
other items, include the NACE Corrosion
Engineer’s Handbook and the ASM Handbook
of Corrosion. Both are available from Amazon
and can assist if you suspect a problem with
your material.
Regarding abrasion, if you Google this topic
Technology
Sensor
Technology
Typical Materials of
Construction
Neoprene, Teflon, or
Stainless Steel
Diaphragm
Tilt
Aluminum, Nickel
Plated Steel, or Stainless
Steel
Rotary Paddle
Bare or Painted
Aluminum, PBT (a plastic), or Stainless Steel
RF Capacitance/
Admittance
Bare or Painted
Aluminum, or Stainless
Steel
Vibrating Element
Stainless Steel
Capacitive
Proximity Switch
Thermoplastic Polyester
Table 3: Typical wetted part materials of construction
you will not see any obvious reference material. However, abrasion is important to consider because some materials are far more
abrasive than you might expect. One example
is whole corn. I have seen a ½-in.-diam stainless-steel probe sharpened to a point by the
abrasive nature of whole corn in a low-level
application. This is due to abrasion from the
grain flow around the probe. Abrasion resistance for low-level applications should be considered if your material might be even somewhat abrasive. Consult your sensor supplier
and take advantage of their experience.
Material Buildup
Possible material buildup on an invasive probe
needs to be considered. Some technologies
rely on contact of material to properly operate. If material adheres to the probe it may result in a false indication of material presence,
unless the sensor has the ability to ignore such
buildup. Table 4 provides guidance on sensor
technology sensitivity to material buildup.
Consult with your sensor supplier about material buildup issues if this is a concern. Any
technology with high sensitivity should not be
used for applications with sticky material unless the manufacturer is consulted and agrees.
Vibrating element sensors and diaphragm
switches are the most sensitive.
Point Level Sensor Review
Table 5 provides a general overview and selection guide of sensor technology based on common application parameters.
Sensitivity to Material Buildup
Diaphragm Switch High (use with dry free-flowing material only)
Tilt Switch
Low
Rotary Paddle
Low-Moderate (depends on amount of buildup; can reduce sensor life)
RF Admittance
High (Low-Moderate if buildup immunity feature)
1.2
Vibrating Element
Rice Bran
1.4
High (not tolerant of buildup; some dusting can be ignored by adjusting
sensitivity)
Sawdust
1.2
Capacitive
Proximity Switch
High (not tolerant of buildup and moderate dusting)
Table 2: Examples of low dielectric constant materials
2
Table 4: Sensitivity of sensor technology to material buildup
www.PowderBulkSolids.com
Some versions within the differing brands may provide some
improvement or special versions.
2
Actual dielectric constants may vary. This data is from commonly available industry references. Consult your chosen supplier.
3
Some tuning fork versions only.
1
Diaphragm
Tilt
Rotary
RF Vibrating Prox
Powder
Y
Y
Y
Y
Y
Y
Granular
Y
Y
Y
Y
Y
Y
Slurry
N
N
N
Y
Y
3
N
Liquid
N
N
N
Y
Y3
N
Very Low
1.5-5lbs/ft3
N
N
CF
CF
Y
CF
Low >5lbs/ft3
Material
December 2009
Joe Lewis is the managing director of BlueLevel Technologies Inc.
(Peru, IL). He has a BSEE, an MBA, and over 35 years of experience in process measurement and control instrumentation along
with decades of experience in level measurement and monitoring
of powders, bulk solids, and liquids. He is a regular contributor of
technical articles and an industry expert contributing editorial columns and blog posts within the powder and bulk solids industry. He
can be reached at [email protected]. For more information, visit www.blueleveltechnologies.com
13
Density
CF
CF
Y
CF
Y
CF
Medium
>15lbs/ft3
Y
Y
Y
Y
Y
Y
High >40lbs/ft3
Y
Y
Y
Y
CF
Y
Low
Y
Y
Y
Y
Y
Y
High
Y
Y
Y
Y
Y
Y
>200°F (93°C)
Y
Y
Y
CF
CF
N
>300°F (150°C)
CF
N
CF
CF
CF
N
Moisture
www.
Process
Temperature
.com
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Material Buildup
Minimal
Y
Y
Y
Y
CF
CF
Heavy
N
CF
CF
Y
N
N
Y = Yes, N = No, CF = Consult Factory
Table 5: Technology selection guide
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11/6/2009 8:56:18 AM