United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA-600/S2-84-189 Jan. 1985
Project Summary
Fail-Safe Devices for the
Prevention of Hazardous
Materials Spills
David B. Heard
Many hazardous material spills can
be prevented by the use of auto-
matic container-filling procedures to
determine when maximum capacity is
reached. This project assessed current
fail-safe technology, conducted labora-
tory tests of automatic devices and
monitored performance of on-site
automatic level controllers.
In the assessment of current tech-
nology, available data on devices used
to prevent tank overflow were identi-
fied and reviewed. Fourteen types of de-
vices were investigated to determine
their potential for automatic container-
filling control. Although most of the
fourteen level detectors could measure
for either wet or dry products, some
could measure both wet and dry
products. The detectors were activated
by electrical, electric/pneumatic, pneu-
matic, or mechanical action. The
fourteen types of level detector devices
are pressure devices, float level controls,
capacitance level detectors, conductive
level measurements, thermal probes,
sonic-echo level monitors, inductive-
level detectors, rotating-paddle level
detectors, vibration-level detectors,
automatic sounding, light and sight
glass measurement, radiation-level
measurement, microwave-level mea-
surement, and pressure tape.
Three different types of industrial
plants, each having an overfilling prob-
lem with a 'different material, co-
operated in the field testing phase. The
three level control devices tested were
ultrasonic, vibrating tines, and mag-
netic-coupled-float types, respectively.
Two of the units were activated by
electricity and the third was activated
pneumatically. One unit controlled dry
powder, the second, a viscous liquid,
and the third, an aqueous liquid. Each
location required an explosion-proof
system. All three units were installed
without significant revisions to existing
containers, and all operated well for an
extended period under severe weather
conditions. The proper controller con-
figuration must be compatible with the
environment and must also incorporate
requisite safety features and demon-
strate corrosion fouling and weather
resistance.
This project summary was developed
by EPA's Hazardous Waste Engineering
Research Laboratory. Cincinnati, OH,
to announce key findings of the re-
search project that Is fully documented
in a separate report of the same title (see
Project Report ordering information at
back).
Introduction
Hazardous material spills which pollute
water resources and the atmosphere
constitute a peril to human and environ-
mental health. Prevention of accidental
spills is essential to environmental
protection. An earlier report. Hazardous
Material Spills: A Documentation and
Analysis of Historical Data (EPA-600/2-
78-066), established that the chief cause
of hazardous material spills was the
overfilling of containers (tank overflow).
Many such spills can be prevented by
using automatic container filling pro-
cedures which employ remote, automatic
level detectors as fail-safe devices.
Although numerous types of level and
volume control devices are commercially
available, caution in choosing and
operating the devices will assure that
they will function adequately and safely
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within the process to be controlled.
In this project, the current technology
in level control devices was assessed to
determine the kinds of controllers
available and their ability to control
different types of hazardous materials
spills automatically, and to evaluate their
potential for use with wet and dry
materials. From this assessment, three
different types of fail-safe level control
devices were selected for field use, one
each at three different industrial sites.
Each fail-safe level controller was to be
tested for an extended period to demon-
strate its spill-prevention properties. The
field studies were also to determine the
environmental conditions in which a
device must operate.
Technology Summary
During the technology assessment,
many methods of measuring the level of
stored material in a container were
identified, ranging from extremely simple
to very complex. Most of the available
level detector-controllers also can be
used for remote, automatic sensing to
assure safety and continuity of measure-
ments of hazardous materials in pressur-
ized vessels. The transducer mechanism
within the various detectors is generally
dependent on one of the following
conditions or devices: pressure, capaci-
tance or conductance changes; float,
displacer or paddle switches; weight and
tape sensors; ultrasonic echos; in-
ductance, thermal or light sensors;
radiation and weight sensors. Activation
of a signal from within each type of
transducer can be initiated with an
electrical, electric-pneumatic, pneumatic
or mechanical trigger. A brief description
of each of the fourteen categories of level
controlling devices follows.
Pressure Devices
Generally employed for measuring
liquid levels, pressure devices can range
from simple bubble tubes that use
atmospheric or inert gases to differential
pressure devices and pressure-dia-
phragm devices. Pressure devices can be
used with other types of mechanical or
electrical connectors to integrate elec-
trical signals to control fill pumps, valves,
etc. These systems are generally simple
and can be used in hazardous atmos-
pheres. Disadvantages can include
clogging, corrosion of, or introduction of
foreign matter into a process.
Float Level Control Devices
These indicators monitor liquid levels
and may employ either ball type floats.
surface floats or displacer floats, general-
ly mounted in a container at the top or
sides, in a standpipe (open or caged), to
activate pumps, valves, or other devices
by means of mechanical linkage, electric
switches, or pneumatic transmitters.
Capacitance Level Detectors
These indicators can be used with
liquids. A variable capacitor is formed
when an electrode is mounted in a tank
with metal walls or, if the tank is not
conductive, another electrode is mounted
at a distance from the first electrode. The
capacitance between the electrodes
varies with changes in the depth of liquid
in the tank, and these changes more or
less expose the surface of the electrodes.
Thus, as the level of material in the tank
rises, the dielectric constant varies ac-
cordingly. The capacitance is measured,
and readout meters are calibrated for the
container depth. These detectors can be
point or continuous types. The electrodes
may need to be sheathed if the liquid is
conductive. Capacitance detector systems
may occasionally be coated with adhering
material, yielding false readings.
Conductive Level Measurement
Detectors
In these detectors, two probes a
conductive flu id with a device to measure
the flow of current can sense the point
level of the contents of a tank. Usually,
the tank serves as the grounded electrode,
and both a high-level and a low-level
electrode are needed. The outputs are
connected in order to control pump start-
up at the low level and shut-off at the
high level. Splashing, foaming, turbu-
lence, temperature and conductivity must
be measured to avoid false readings.
Thermal Probes
A thermal probe is a point-level
indicator using a self-heating glass bead
thermistor as a sensing element. Since
the heat dispersion is greater in a liquid
environment than in a gaseous environ-
ment, a large voltage change occurs
when a liquid level rises to envelop the
sensing element. The thermistor probe is
a negative-coefficient resistor in which
resistance increases proportionally to the
decrease in temperature; therefore, with
greater heat dispersion in a liquid, the
sensor cools, and a greater voltage signal
is sent to the amplifier, which, in turn,
operates relays that control valves and
pumps.
Sonic-Echo Level Monitors
These devices consist of a pulse
generator, transmitter, receiver, amplifi-
er, and control circuit. They are ultrasonic,
sonic and sonar echo devices which
measure the time a pulse takes to travel
from a transmitter to a reflecting surface
and back to the receiver. The measured
time is a reflection of the distance
between the surface of the material and
the transmitter/receiver. The signal
received can activate valves or pumps to
control tank filling and emptying. Ultra-
sonic and sonic systems are non-tactile,
and sonar systems are usually immersed
in the material being measured.
Inductive Level Detectors
Inductive systems can detect the level
of conducting metal objects. As the
metallic object enters an electromagnetic
field produced by an oscillator, it disrupts
the field. This disruption varies a voltage-
sensitive circuit which operates a relay
that activates a control system. Only
under special conditions are induction
systems used to monitor levels. These
systems are more often used in process
lines to detect tramp metal in order to
prevent damage to process machinery.
Rotating Paddle Level
Detectors
In these detectors, rotating paddles
detect the presence or absence of bulk
material at a specified level in a tank.
They are used for point level detection. A
small electric motor provides constant
paddle rotation while the paddles are
clear of any solid material. When the
vessel is filled to the level of the rotating
paddles, the paddles stall and a switch is
actuated, signaling a cessation of rotation
at the level caused by the material in the
tank. Paddles may be used for high-point,
low-point, or any other level detection.
Vibration Level Detector
Tuning fork, vibrating paddles, and
vibrating probes used as point-level
detectors all work on the same principle.
When the vibrating element comes in
contact with a bulk material after
exposure to gas (air) or when two bulk
materials of different densities are
encountered, the vibrating frequency
either stops or changes. All vibration
detectors must be used in dry bulk that
does not arch or bridge and is not sticky.
Any cavity formed by the vibrations or any
material adhering to the surface will
produce false readings.
Automatic Sounding Detectors
Automatic soundings are made with a
weight (for bulk materials) or float (for
liquids) and employ a cable device to
measure the level of materials in silos,
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bins, or tanks. This device has a motor-
operated drum which may be actuated
manually or automatically to obtain a
continuous record of the amount of
material in the vessel. When the motor is
actuated, a measuring cable with the
float or weight at the end runs out,
producing an electrical pulse at regular
intervals. The number of pulses counted
is an inverse indication of the quantity of
material stored in the vessel. When the
weight or float contacts the surface of the
medium, the drum rewinds the tape.
Consequently, set points for automatic
control of stored materials are easily
established.
Light and Sight Glass
Measurement Detectors
A light-beam breaker system can be
used as a method of point determination.
It consists of a light source and photo cell
connected to light conducting plastic
probes which protrude into the tank
space and are separated by a gap. When
the stored material interrupts the light
path, a relay activates control circuits.
When the material falls belowthe probes,
the relay is deactivated or another relay is
cut in and other control circuits take over.
In a similar approach, a directional light
source is focused on a phototransistor. As
a filling liquid immerses the light source,
the change in refraction inactivates the
phototransistor, triggering a signal. The
basic sight glass is a tube between a pair
of valves used for visual checks of
amounts of material in a tank.
Level Detectors Using
Radiation
When properly installed, isotope source-
level controls present no hazard to plant
personnel and are cost effective. Both
source and detector(s) are mounted
externally and are able to monitor almost
any liquid or solid. These instruments are
used for point- or continuous-level de-
tection.
For point-level detection, a radioactive
source produces a gamma ray beam
which penetrates the vessel walls and,
when there is no product intervening,
strikes a detector. The detector produces
an electrical impulse in relation to* the
gamma photons received. When the
material in the vessel rises to intercept or
scatter the photon path, a different
intensity of impulses is produced by the
detector. For full-range level indication
and control, the source has a wider angle
of emission and a continuous level
detector is used. Atomic Energy Com-
mission licensing is not required for these
systems because the radiation is minimal.
Microwave Level Detectors
Microwaves may be used when a non-
tactile control device is required. The
system consists of a transmitter, oscil-
lator and directional antenna; and a
receiver, directional antenna, amplifier,
pulse coding network, voltage comparator
circuit and a relay driving circuit. Micro-
waves emitted from the directional
antenna are picked up by the directional
receiver.
The conductivity of the material being
measured determines the signal attenua-
tion. Metal tanks and hoppers reflect
microwaves, and storage vessels using
this method of measurement must be
provided with windows of materials such
as high-density polyethylene. Air trans-
mits microwaves with little or no loss,
whereas water-based materials (or those
materials with a significant water content,
such as grain or wood) absorb them. As
the air space in the microwave beam
becomes filled with material, the trans-
mitted microwaves are either cut off from
the receiver or sharply reduced. This
method can be used for point level
indication or to drive a control system.
Pressure Tape Detectors
In these devices, a tape made up of a
loop of resistance wires or contact points,
normally held apart, is inserted in the
vessel to be measured. As the product is
introduced into the vessel, pressure on
the tape forces the wires into contact. As
the vessel fills, the wires touch or the
contact points close, shortening the
resistive path of an electric current. The
resistance measured varies inversely
with the level of product stored. This type
of instrument continually measures the
level in the vessel and can activate relays
at preset points to control process, fill,
and discharge.
Field Evaluations
After the assessment of the state of the
art technology was completed, three
different types of level control devices
were chosen for field evaluation in an
industrial environment: ultrasonic, vi-
brating tines and a magnetic-coupled
float unit. Each of the units was installed
at one of three different industrial plants,
each experiencing a problem with a
different material.
An ultrasonic controller was installed
at one plant to prevent overfilling of an
outdoor grease tank which had a history
of small spills. This unit, with its non-
tactile properties, was chosen to prevent
grease overflow. An ultrasonic transducer
was mounted on the tank and connected
to controls in the pump room. The
controls had high and low set points and a
direct meter readout showing the amount
of grease in the tank. In operation, these
controls shut off the fill pump when the
level of grease in the tank reaches a
predetermined level or if a loss/signal
occurs due to transducer clogging or
electronic failure. Although several
minor operational problems occurred,
these were corrected and the unit
operated successfully for more than three
years, after which it was removed and
installed at another plant to monitor the
levels of wood pulp in a paper pulp chest
supplying a paper machine. After two
years of successful operation in this role,
the unit was removed for laboratory
examination. After 51/2 years operating in
harsh environments, this unit was found
to function well.
At another plant, two sets of vibrating
forks were used to monitor the filling of
containers with a dry powder, PVC. A
system was designed to indicate the high
cutoff point and a low refill point. A series
of panel board lights was used to indicate
the relative fullness of the containers.
Although minor operational problems
were encountered during rapid filling, the
system worked well for plant personnel.
The unit had been in operation for five
years before it failed due to a defect in a
relay and a damaged tine assembly.
At a chemical plant, a pneumatic,
diaphragm-type level sensor was in-
stalled to control filling of a tank holding
acids or potentially explosive solvents.
The unit produce?! a pneumatic signal
that was 3 psi when the tank was empty
and 15 psi when the tank was full. Two
pressure switches in a remote control
room were activated by this pneumatic
output signal. When activated, these
switches interrupted signals to solenoid
valve's. Thus, if a tank was filled to
produce a 15 psi pressure, the fill valve
was closed and filling ceased. The second
pressure switch was designed to activate
at a pressure of 3 psi or less in order to
close the fill valve. This served as a fail-
safe device in case of a shutdown of the
pneumatic system. The device was
installed and operated over a 51/2 year
period, during which time several me-
chanical and other malfunctions of the
device occurred. However, the malfunc-
tions were not considered unusual or
excessive within the harsh operating
environment, and design changes might
well eliminate the problems.
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Conclusions
The level of material in a container, tare
weight, contents, the volume occupied by
the material, pressure exerted by a fluid,
emissions of gas or radioactivity may all
be used for monitoring a quantity of
material. The method of transferring this
information may be by mechanical
linkage, electric signal, hydraulic or
pneumatic pressure. The readout may be
continuous, representing the total range
of the container capacity, or it may be for
one or more points. Continuous signals
may be used to set limits on the contents
of the container, and the point signals
may be placed in the container to act as
limit points. In this way, low-level, high-
level, and other alarms may be obtained.
The signal activating the alarms can also
be used to start or stop pumps and to open
or close valves.
The type of level detector or controller
must be carefully chosen to be compat-
ible with the material being monitored
and the environment in which it is placed.
Chemical and physical requirements
govern the choice of equipment to be
used. Although some level detectors will
work well with either wet or dry and both
wet and dry materials, others are made
only for wet or dry materials. Dangerous,
explosive, flammable, caustic materials
in the container or in the vicinity
determine the types of level indicators
and controllers to be chosen. The
material composition of the level indi-
cator in contact with the contained
j atmo:
•——"•"•""plieie itiutil. Be caTeTttHy considered wl
~^or con
[ion of
»patible
• ••"• ll'ie iViiJKJllcll diiJ LJlih ulmuiiphere (e
explosion-proof relays). When appropri-
ate automatic level detection fail-safe
devices are chosen and installed, they
have been demonstrated to virtually
eliminate spills resulting from overfilling
of containers.
David B. Heard is with Factory Mutual Research Corporation, Norwood, MA
02062.
John E. Brugger is the EPA Project Officer (see below).
The complete report, entitled "Fail-Safe Devices for the Prevention of Hazardous
Material Spills," (Order No. PB 85-138 642; Cost: $11.50, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Release Control Branch
Hazardous Waste Engineering Research Laboratory—Cincinnati
U.S. Environmental Protection Agency
Edison, NJ 08837
*U.S GOVERNMENT PRINTING OFFICE; 1985 — 559-016/7901
United States
environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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