The potential for fire should be considered in the design
and operation of Thermal Fluid Systems.
Flash Point and Fire Point
The Flash Point and Fire Point of a thermal fluid are
determined through laboratory testing of new fluid. The
most common test method is the ASTM D-92 Cleveland Open
Cup (C.O.C.). The cup holds a certain amount of fluid
that permits a vapor space to exist directly above the
liquid. During the test, a small flame is passed slowly
over the fluid as the temperature of the fluid increases.
The lowest temperature at which the vapor ignites is called
the Flash Point. The temperature at which sufficient vapor
is generated to support a continuous flame is the Fire
Point. While these tests do provide data for comparison
of a number of fluids, any extrapolation of these results
into real life situations must recognize the 3 basic conditions
required for a vapor ignition to occur:
Fluid Temperature: The fluid
must be at or above the Flash or Fire Point(s) while
in contact with air for any vapor combustion to occur.
This situation may not always exist around leaks since
the leaked fluid will cool rapidly on exposure to
Vapor Concentration: There must
be enough vapor present to support combustion. Any
dissipation of the vapor may reduce the concentration
below the level required for ignition.
Source of Ignition: The source
of ignition must be located within the vapor cloud.
This is not common since good electrical installation
practice dictates that potential ignition sources
be located a distance from piping or be properly enclosed.
If any one of these three conditions is not met, then
vapor ignition cannot occur.
Normal Leaks in Thermal Fluid Systems
Normal leaks in thermal fluid systems consist of fluid
"seeping" out from threaded fittings, flange
gaskets, mechanical seals and valve stem and pump shaft
packing glands. Any droplets formed will cool rapidly
on exposure to air. Extremely low volume leaks may produce
a light gray smoke. This is an indication that the fluid
is oxidizing immediately on exposure to air. This smoke
may cause respiratory irritation if inhaled for a period
of time as can any type of smoke.
There are several conditions under which
"normal" leaks can present a risk of fire:
Insulation Fires: Certain types
of insulation such as mineral wool, fiberglass or
calcium silicate has an open or porous structure that
allows fluid to wick away from the source of a leak.
As the fluid disperses within the insulation, its
surface area increases dramatically while its temperature
remains at or close to the system operating temperature.
The danger is that a substantial percentage of the
leaked fluid will remain unreacted within the insulation
due to the limited amount of oxygen available. If
the supply of oxygen is suddenly increased, the remaining
fluid in the insulation will burst into flames. Prevent
this situation by using non-porous insulation (such
as Pittsburgh Corning Foamed Glass or equal) within
several feet of areas prone to leakage such as valves,
flanges, etc. If possible, flanges should be left
completely uninsulated or, if necessary for personnel
protection, covered with drip shields.
Confined Areas: If a low volume
leak occurs within a tightly enclosed area, such as
a cabinet, the available oxygen may be consumed allowing
unreacted vapor to accumulate. This can be prevented
by ensuring that all portions of thermal fluid systems
are located in areas with adequate ventilation.
Catastrophic Equipment Failure
A catastrophic equipment failure
may result in the rapid release of large quantities of
thermal fluid. If the total system pressure is low and
the fluid is operating below its atmospheric boiling point,
then the leak will consist of liquid that may spray a
short distance before falling to the ground. Higher system
pressure may produce a finer spray that ejects a greater
distance from the equipment; however, the relatively larger
surface area of the droplets and their velocity will result
in rapid cooling. In either case, there will be a certain
amount of smoke present due to the hot fluid reacting
Vapor leaks may occur if the fluid is operating above
its atmospheric boiling point. Condensation of the vapor
can form a potentially explosive airborne mist. Similar
to "dust" explosions, mist explosions require
that the fluid particles have a high surface area to volume
ratio and of sufficient concentration to explosively ignite
if exposed to a source of ignition. Only condensing vapor
has proven to produce this type of particle.
Proper design, operation and maintenance of equipment
are the most effective method of minimizing catastrophic
failure. Any resulting fire hazards can be minimized as
Never operate a thermal fluid above
its normal boiling point: This will eliminate the
potential for mist explosions.
Maintain good ventilation in the area
around equipment: This will provide rapid cooling
of any leaks and will also disperse any unreacted
Minimize the fuel available for a fire:
The expansion tank should be no larger than necessary
and should be equipped with a low level switch to
shut down the entire system. An automatic shut-off
valve can be installed to isolate the expansion tank
in case of a building fire.
Loss of Circulation in the Heater
Severe potential for fires can exist if
the thermal fluid flow is interrupted without causing
the heater to shutdown. Under this no-flow condition,
the temperature of the fluid inside the still energized
heater increases rapidly to well above its boiling point.
Any equipment failures may result in spontaneous ignition
of the leaking fluid. A high temperature cut-off switch
should not be the only safety device on the heater since
the loss of flow may reduce its accuracy. The most effective
protection is to install a high/low pressure switch on
the pump discharge or a low differential pressure switch
across an orifice plate or similar type flow meter. The
switch should be wired to shutdown the system immediately.
Flow sensing devices that have a component immersed in
the moving fluid are not recommended for use as low flow
switches in thermal fluid systems since they tend to fail
in the open mode.
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