GAPS Guidelines
GAP.9.5.1
A Publication of Global Asset Protection Services LLC
HEATING IN PLASTIC AND PLASTIC LINED TANKS
INTRODUCTION
Corrosive solutions for plating and cleaning operations are often contained in plastic or plastic lined
tanks. Polypropylene (PP), fire retardant polypropylene (FRPP) and Polyvinylchloride (PVC) have
very good corrosion resistant properties making them the plastics of choice. Although the solutions
are usually not flammable or combustible, the fire hazard created by the plastic construction of these
tanks is still significant. To heat solutions inside the tanks to proper temperatures, industry has
traditionally used high watt density electric immersion heaters. Failure of the automatic controller to
shut off the power to the heater will increase the rate of vaporization of the liquid in the tank. Without
the liquid to absorb the heat, the temperature at the surface of the heater will climb to a dangerous
level. Once the liquid level is below the design point of the heater, exposing all or most of the heater
to the air, the surface temperature of the heater can ignite the plastic tank wall. Numerous fires have
been caused by this sequence of events. Many have occurred when heaters were left in the on
position during idle periods or were activated by a timer several hours before startup of the operation.
POSITION
Management Programs
The administrators of management programs should report to top management through a minimum
number of levels. They should also administer comprehensive loss prevention inspection and audit
programs to communicate program effectiveness to top management. This management feedback is
a key feature of OVERVIEW, Global Asset Protection Services (GAPS) total management program
for loss prevention and control. In facilities utilizing heated plastic or plastic lined tanks, pay particular
attention to the following important areas:
Hazard Identification and Evaluation Program
Develop a program to determine the pertinent physical and chemical properties of materials
contained by or used in the construction of plastic and plastic lined heated tanks. Where basing
selections on listings, choose listings based on test conditions that best represent all possible
operating conditions.
Establish routine procedures for testing physical and chemical properties of materials handled by
heated plastic or plastic lined tanks to confirm properties required for safe operating conditions.
In process design, consider the desirability of avoiding the use of, or reducing the size of, heated
plastic or plastic lined tanks. Use plastic or plastic lined tanks only where the solution or process is
incompatible with metal tanks. Where available, use plastics specifically engineered to resist ignition
in this type of operation. Plastics tested and listed in accordance with UL 2360 and FM 4910 are
specifically designed for safer use in cleanrooms. While specifically listed for cleanroom use these
plastics are more resistant to ignition and improve fire safety in similar occupancies.
Determine the safe operating and potential upset conditions of all new or existing heated plastic or
plastic lined tanks.
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provision of any service is not meant to imply that every possible hazard has been identified at a facility or that no other hazards exist. Global Asset Protection Services LLC
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specifically disclaim any warranty or representation that compliance with any advice or recommendation in any document or other communication will make a facility or
operation safe or healthful, or put it in compliance with any law, rule or regulation. If there are any questions concerning any recommendations, or if you have alternative
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GAP.9.5.1
Design all heated plastic or plastic lined tanks with inherent safety. Wherever possible use external
heat exchangers, indirectly heated coils or low watt density heaters in lieu of electric immersion
heaters. Interlock processes to shut down heating elements automatically and safely in event of
operator error or equipment failure. It may be critical to maintain or increase fume exhaust or cooling
while de-energizing the heat to the process.
Provide redundant instrumentation for all controls whose failure can result in a fire. Do not rely on
operating controls to shut the process down. The emergency controls must be installed in series with
operating controls and require manual reset. Losses have occurred where an emergency disconnect
was wired parallel to the operating control. Failure of the operating controls in the “on” position kept
the heater energized.
When designing or specifying equipment consider all possible operating conditions, both normal and
abnormal. Give particular attention to suitability of the equipment to handle the process materials and
to withstand external environmental influences. Many of the electric immersion heaters have a fusible
element within the cover. In the harsh operating environment the element fused but the conductive
solution that flooded the cover continued to complete the operating circuit and maintain operation.
When modifying operation parameters review the adequacy of all safeties and interlocks.
Employee Training Program
Educate all operators in the hazards involved with heated plastic or plastic lined tanks and in the
functions of the safety control equipment. Forbid operators to run the process when any of the safety
equipment is out of order. Forbid deviations from written procedures.
Schedule re-education and training at least annually. Include testing to assure proper performance of
all assigned duties with particular emphasis on emergency shutdown procedures.
Maintenance Program
Inspect, test and maintain heater controls of heated plastic or plastic lined tanks quarterly, or more
often, if design and service conditions warrant. An ideal time to test the emergency controls is in
preparation for refilling of the tanks. Use the emergency controls to de-energize the process while
emptying the tank. Establish a detailed record keeping system which includes a forecast for retiring
equipment.
Management of Change
Apply all management programs to any changes which are made to the facility’s heated plastic or
plastic lined tanks or operating procedures for such tanks. Pay particular attention to the following
areas:
•
Perform the process hazards evaluations program for all new heated plastic or plastic lined tanks
or for any modification to any existing tanks. Determine the need for new or different safety
equipment or measures.
•
Whenever a heated plastic or plastic lined tank is changed from one service to another, examine
the inspection and maintenance program and modify as necessary. Monitor daily operating
changes.
•
Verify new construction materials and all maintenance parts and supplies as conforming to original
(or reengineered) design specifications.
•
Maintain a current operating procedure manual reflecting all modifications and/or changes in
operating procedures.
•
Review and follow through expeditiously on all inspection recommendations from insurance, code
enforcement and regulatory agencies.
General Protection
If heated plastic or plastic lined tanks are used, provide low liquid level and high temperature controls.
Strongly discourage the use of timers arranged to heat solutions during unattended periods. Should
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GAP.9.5.1
such timers be required, provide the appropriate controls described in this section, arranged to
provide alarms indicating unsafe conditions to a constantly attended location.
Provide one of the following heating facilities, listed in preferential order:
•
•
Use remote external heat exchangers through which solutions are pumped.
•
Use low watt density immersion heaters which are incapable of producing dangerous surface
temperatures even when the solution has dissipated and it has a greater surface exposed to air.
Use indirectly heated coils, made of corrosion resistant metals or plastics, through which a heating
medium such as hot water or steam is circulated.
•
Only use conventional high watt density electric immersion heaters with proper controls.
If plastic or plastic lined ducts are used to handle the fumes or vapors generated by the solutions
being heated, provide fire protection as outlined in GAP.2.3.2; GAP.17.1.1; GAP.17.11.0 and
GAP.17.11.1. See also GAP.9.2.5.1.
External Heat Exchangers and Indirectly Heated Coils Utilizing Steam
If a plant steam supply is capable of providing pressures higher than the design limits of the heat
exchanger or coil, install pressure relief.
Low Watt Density Immersion Heaters
Make sure that installations using these heaters are in compliance with specifications that assure a
fire safe installation when the heaters are continuously powered and not immersed in a liquid. The
specifications must take into account physical characteristics of the plastics, temperature and spacing
to the tank wall.
Figure 1. Relative Location Of Controls For Installations Using High Watt Densities.
Controls For High Watt Density Immersion Heaters
Fit high watt density electric immersion heaters with controls, installed as shown in Figure 1:
•
An automatic temperature controller with the sensor placed above the heater and below the low
liquid level.
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GAP.9.5.1
•
An optical or capacitive low liquid level sensor placed above the heating element at the lowest safe
operating liquid level.
•
•
A high coil temperature sensor located on the heating element.
A high liquid temperature sensor placed below the heater level. Set the sensor at a temperature
lower than the liquid boiling point. When sludge is expected to collect in the tank bottom, relocate
the high liquid temperature alarm between the temperature controller and the high coil temperature
sensor.
•
Ground fault protection for the heater (not shown).
Install in the power supply to the immersion heater with two solenoid relays wired in series and
arranged as follows:
•
•
Connect the temperature controller to the first relay.
•
Provide overcurrent and fault current protection for the relays. Use current limiting fuses and/or
magnetic-hydraulic circuit breakers.
•
•
Arrange the alarms and interlocks so that they can only be reset manually.
Interlock the low liquid level, coil and liquid high temperature sensors with the heater power supply
through the second relay.
Properly seal and cover the relays and switches to avoid corrosion of the control elements.
If high watt density electric immersion heaters are used in plastic workbenches located in
cleanrooms, provide sprinkler protection as outlined in GAP.17.11.1.
DISCUSSION
Management Programs
Hazard Identification and Evaluation Program
Process hazards evaluation presents the possibility of solving the problem of fires in plastic or plastic
lined tanks by avoiding the hazard altogether. If a less corrosive process can be found or developed,
it would not be necessary to use plastic with its attendant fire hazard. An alternative possibility is to
design the process so that smaller tanks are used. This reduces the total amount of combustibles.
Further into the design process it is still possible to choose a tank heating system which is inherently
as safe as possible. Heaters such as external heat exchangers, indirectly heated coils and low watt
density heaters are, if properly designed and installed, incapable of achieving a high enough
temperature to ignite plastic.
If high energy heaters are used, the design stage is the correct time to install interlocks to prevent a
heater induced fire.
Employee Training Program
Even with all appropriate safety interlocks in place, many fires still break out in plastic and plastic
lined tanks. These fires are usually caused because the operator bypassed the safety systems out of
ignorance of the potential consequences.
Maintenance Program
Even if heater controls are not bypassed, they are still subject to mechanical breakdown. If such
breakdowns occur at an inopportune time, a fire will result. To address this risk, inspection and
maintenance programs must be established for all heated tanks.
The most common protection feature to fail is the fusible element. Immersion heaters are commonly
provided with a fusible element as high temperature protection. When the outer case leaks
conductive liquids can maintain heater operation even with a fused element. Only scheduled
inspection will detect this common failure mode in the protection circuits.
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GAP.9.5.1
Management of Change
Management of change programs recognize the fact that even if heated tank hazards are perfectly
arranged and protected when they are first installed, over time, can undergo modifications that create
unsafe conditions. Management of change, therefore, requires the reapplication of all the previous
programs after any changes to a process have been made.
Materials
The handling and heating of corrosive solutions often requires equipment to be made of, or lined with,
corrosion resistant materials. While ordinary plastics are often the most economical material
available, others such as less hazardous plastics, glass or special metal alloys are preferred because
ordinary plastics are easily ignitable. The combustible loading created by the presence of ordinary
plastic warrants sprinkler protection. It is worthwhile to limit or even eliminate the use of plastics.
Design sprinkler systems at not less than Ordinary Hazard, Group II. For additional guidance
involving cleanrooms consult NFPA 318, GAP.17.1.1, GAP.17.11.0 and GAP.17.11.1. The occupancy
of the remainder of the area may also require a stronger sprinkler system because of the high energy
ignition source created by this process. Sprinklers should be of the corrosion resistant type. Care
should also be taken in selecting of plastic materials, because even a small, controlled fire in some
types (PVC for example) will generate large amounts of highly corrosive products of combustion. Fire
retardant additives used in the formulation of ordinary plastics only slow ignition. The maximum heat
release and corrosion potential of the products of combustion will often increase.
External Heat Exchangers and Indirectly Heated Coils
Heat exchangers and indirectly heated coils operate as follows:
•
Indirectly heated coils can use a heat transfer fluid, circulating through a coil located in the tank.
The heat transfer fluid is, in turn, heated by remotely located heaters.
•
In external heat exchangers, the solution is pumped from the tank through the exchanger where it
is indirectly heated and returned to the tank.
•
Infrared heating units pump the fluid to an open remote reservoir. Infrared lamps are used to heat
the solution.
Locate remote heating units in safe locations. They should not be internal to the equipment nor
expose plastic or other combustible surfaces.
The coils or piping in contact with the liquid being heated in the tank are made of either plastic or
corrosion resistant metals. Both indirectly heated coils and external heat exchangers are readily
available from a number of manufacturers and can meet the heating requirements of both small and
large tanks. Some heat transfer fluids pumped through the coils may be electrically heated. When
properly located these systems do not pose the same problem as electric immersion heaters.
When steam is used in coils or heat exchangers, controlling the steam pressure and temperature will
limit the risk of reaching excessively high liquid temperatures. Temperature control is easier when low
pressure steam or hot water is used. Manufacturers provide information describing the relationship
between the inlet steam pressure and the desired bath temperatures.
If a steam source is not readily available, a small boiler could supply the heating load. A full size
range of electric boilers is available, in addition to gas and oil fired boilers, as auxiliary sources of
steam.
Some relatively small plating or cleaning operations use stainless steel tanks with plastic liners. The
tank rests on an electric hot plate. Many of the hot plates have a rheostat controlling the plate
temperature, but often no pilot light is provided. The off and high positions are adjacent to each other
and are prone to being left in the wrong position. In addition, the temperature setting on the controller
can be improperly numbered. If the hot plate thermostat control fails, the liquid contained in the tank
will totally evaporate, causing the plastic liner to ignite. Avoid the use of hot plates. Where a suitable
alternative is not available fit hot plates with controls identical to the ones required for the electric
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immersion heaters, but since tanks can usually be removed from hot plates, such controls would be
impractical. Hot plates, therefore, should not be used.
To avoid these problems the semiconductor industry started to use quartz heater integral to the
plastic lined tanks. These units are provided with similar controls to those recommended for electric
immersion heaters. They do not have the inherent problems of rheostat or core failure, but still have a
significant failure rate.
Low Watt Density Immersion Heaters
Properly designed, installed and maintained low watt density immersion heaters will not ignite
combustible tanks. When operating and not submerged by liquid, the heaters provide surface
temperatures that will not degrade or ignite plastic materials even when continuously powered.
2
2
Most ordinary immersion heaters have high watt densities of approximately 40 W/in. (62 mW/mm ) of
2
2
immersed heater surface area. Others have watt densities as low as 12 W/in. – 15 W/in.
2
2
(19 mW/mm – 23 mW/mm ). Low watt density immersion heaters, installed at a reasonable distance
2
2
2
2
from the tank wall, will provide a watt density of 8 W/in. – 10 W/in. (12 mW/mm – 16 mW/mm ) at
the tank wall. Such a watt density will probably not ignite most plastics when the heater is
continuously energized, but this can only be verified by testing. All immersion heaters should be
tested and listed by a nationally recognized testing laboratory, but to date, no low watt density
immersion heaters are listed.
Controls For High Watt Density Immersion Heaters
An ordinary electric immersion heater (high watt density) overheating adjacent to the wall of a
combustible plastic or plastic lined tank poses a significant fire hazard. Seventy five percent of the
reported fires began in small tanks having a solution capacity of 15 gal – 75 gal (57 L – 284 L).
The GAPS recommended redundancy is intended to sharply reduce the fire potential due to the
overheating of high watt density immersion heaters. Different scenarios based on liquid level varying
from Level 1 to 4, as shown in Figure 1, can be evaluated:
•
Level 1 - Normal Operation: The temperature controller monitors the liquid temperature and
functions as a thermostat, activating or deactivating the power to the heater as required.
•
Level 2 - Low Liquid Level: The low liquid level sensor is activated and shuts off the power to the
heater.
•
Level 3 - Low Liquid Level Malfunction: The low liquid level sensor malfunctions and, therefore,
allows more product to evaporate. As the liquid level drops, the temperature controller is no longer
immersed in the liquid. Being exposed to “colder” air, the temperature controller will continuously
activate the heater, further increasing the liquid temperature and evaporation. A high liquid
temperature sensor located below the heating coil would detect the high liquid temperature and
shut off the power to the heater.
•
Level 4 - Exposed Heater: As the liquid evaporates, the heater is no longer immersed in the
liquid. A high coil temperature sensor will detect overheating of the coil and shut off the power to
the heater. A failure of the high coil temperature sensor would lead to overheating of the coil,
producing the radiant heat which will ignite the plastic lining of the tank.
Electrical resistance heaters can be an integral part of the tank bottom or walls. Such devices should
be provided with the same controls as high watt density immersion heaters.
Testing the control devices is essential in limiting the fire potential should high watt density electric
immersion heaters be chosen as the heating system. But even with a preventative maintenance
program, the success of this approach is limited. First, controls are still exposed to a corrosive
environment. Corrosion disables the safety devices without giving any signs or warnings that
corrosive disabling has occurred. Second, controls may not be needed to operate for several years.
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A Publication of Global Asset Protection Services LLC