Section 8: Heater and Storage Unit
General
Sometimes the contractor will set up a heater and storage
unit for large projects. The asphalt is hauled from the source by
truck and pumped into the heater and storage unit. When it is used,
it is pumped either into another transporter or directly into the
asphalt distributor.
There is no standard configuration for a storage and heater
unit. It may be a tank with heating and circulating equipment combined,
or it may be an insulated tank with a separate heater unit with interconnected
piping. Storage capacity and size vary according to the needs of
the project.
The heater and storage unit, usually called a heater unit,
is inspected at the beginning of a project along with the other
equipment. Sometimes the contractor has manufactured the heater
unit, so there may not be any identifying data. The only numbers
on the equipment may be an engine serial number on the pump unit.
Temperature
Asphalt must be stored at specific temperatures, which are
usually somewhat higher than the temperatures at which the asphalt
is applied. The asphalts used in seal coat work are stored at the following
approximate temperatures:
Asphalt cement | 325°F to 400°F |
Cutback asphalt | 150°F to 200°F |
Emulsions | 150°F to 170°F |
The higher temperatures are the maximum allowable storage
and heating temperatures.
In order for these asphalts to be sprayed properly, with the
desired results, the temperature must be closely controlled in order
to maintain the correct viscosity for spraying. The heater unit
operator must clearly understand the importance of the viscosity-temperature
relationship.
The heating and storage unit must be equipped with a continuous
recording thermometer so that the temperature of the asphalt may
be closely monitored.
Safety
The operator must be aware of the flash point (temperature
at which ignition could occur) of whatever type of asphalt binder
is being used. It is his responsibility to ensure that all necessary safety
precautions are taken, but this can never be assumed. The flash
point is especially critical with cutback asphalts. As an example,
RC-250 has a flash point of 80°F. Standard Specifications Item
300.3 recommend that RC-250 be applied at temperatures between 125°F
and 180°F. The maximum allowable temperature for
application and storage is 200°F. With this type
of asphalt, you are well into the dangerous range any time that
you work with it.
There is also danger around the heater and storage unit, especially
if using asphalt cement. Storage tanks, pipes, and valves are extremely
hot. Adequate safety precautions should be taken to ensure that
any part that might be touched is insulated.
Extreme care must be taken when obtaining a sample of asphalt
cement. Very hot asphalt cement (350°F) can easily
splatter. Proper safety equipment and clothing should be worn.
Storage Tank
The storage tank must be inspected for cleanliness and the
presence of any condition that would permit contamination of the
asphalt. There should be a continuous-recording thermometer on the tank,
which records any fluctuations in asphalt temperature.
Heater
In some cases, the heater unit is a part of the storage tank.
In others, it is a separate unit, consisting of a smaller tank with
pump and heater. Regardless of the system used, the heater unit
must be inspected.
It should have a burner that can be regulated to alter the
intensity of heat. The burner should direct the flame into the flues,
similar to the arrangement in an asphalt distributor. The pump should circulate
the asphalt through the heater unit sufficiently to prevent the
asphalt from burning next to the flues and from sticking (from cooling)
near the outside of the tank.
Pump Unit
The pump unit should be checked for proper operation, but
the primary concern is to ensure that the pump and associated piping
protects the asphalt from contamination. It should be assembled
so that no dirt or fuel can enter the piping or pump unit.
Heater Unit Location
Although the location of the heater unit is the contractor’s
responsibility, the inspector should consider the location of the
unit from the standpoint of safety for the motorists.
The heater unit will have transporters moving to and from
it, as well as asphalt distributors or boosters pulling onto and
off the highway in the vicinity of the heater. This traffic must
be clearly visible to motorists driving through the construction
area. Therefore, it should not be situated on or near blind curves
and probably well clear of intersections.
If the heater unit is situated in the vicinity of the aggregate
stockpile, it should be separated far enough away to ensure that
no contamination of the aggregate occurs. Asphalt is often spilled around
heater units, so it is best that the heater be situated well away
from aggregate stockpiles, if practical.
Recording Thermometer
When the project begins, the inspector must pick up the record
card from the continuous reading thermometer each day. A new card
is installed when the old one is removed.
Transporter and Booster
Transporters sometimes belong to the contractor, sometimes
to the petroleum company that supplies the asphalt to the contractor,
and sometimes to independent truckers. Booster tanks are not used
on all jobs but are frequently used on projects that cover 6 to
10 miles or more. This cuts down on the amount of time the distributor
is tied up while being refilled.
Identifying Data
If the transporter and boosters belong to the contractor,
record the standard vehicle identification information and include
it in the project file. This would also apply if the contractor
has leased the equipment.
If the transporter belongs to the petroleum company or to
an independent trucker, record the company name and the truck license
or other unique number. Although this may not be required, it is
a good practice, in case there is a problem with the asphalt.
Manifest
Check the transporter’s manifest before it is unloaded to
make certain the asphalt in the truck is the right type for the
project. Unfortunately, some inspectors have found out too late
that the wrong asphalt was pumped into the heater unit. Once again,
this is the contractor’s responsibility, but it will delay the project
and can easily be prevented. A copy of the manifest of each load
of asphalt delivered on the job must be retained in the project
folder.
Cleanliness
If the transporter belongs to or is leased by the contractor,
it should be cleaned if a different type of asphalt was transported
on a previous project. It is not usually possible for the inspector
to determine exactly what type of asphalt had been hauled previously,
so a good rule of thumb is to be certain it is clean before it hauls
asphalt to the current project.
Contamination
Besides guarding against contamination from previous asphalts,
the inspector should ensure that the transporter tank and piping
protect the asphalt from contamination during off-loading to the heater
unit.
Booster Tanks
On some jobs, booster tanks are used to refill the distributors
close to where asphalt application occurs. These vehicles should
be inspected for cleanliness; to guard against contamination; and have
the identifying data recorded. Their piping and shut-off valves
should be checked for leaks to guard against puddles of asphalt
being left behind after they have refilled an asphalt distributor. This
is especially important if the distributors are to be filled while
parked on the pavement that is to be seal coated.
Sometimes a transporter is used as a booster tank on the job,
transporting asphalt from the heater unit to where the distributors
are filled directly from the transporter. If asphalt cement (AC)
is being shot on the project, it must be shot near 300°F.
This would necessitate having an insulated transporter, in order
to keep the asphalt hot between the heater unit and the distributor.
Insulation
Like the transporter, it may be necessary to ensure that booster
tanks, if used to carry AC at high temperatures, are adequately
insulated. Without proper insulation, the asphalt may cool enough
to raise the viscosity above the limits for being pumped into the
distributor, especially if there are any unexpected delays.