Hydrocarbon Processing - January 2021 - 54

Heat Transfer
sulfide injection that forms a protective
layer and prevents carbon diffusion.
Process tube materials. Tube materials in CCR fired heaters are usually 9Cr1Mo. Tube dimension design and corrosion allowance generally follow the API
530 standard. With time, these Cr-Mo
materials experience thermal degradation, and their typical normalized and
tempered structure (pearlite or bainite
depending upon normalization heat treat-

ment) and ferrite grains will degenerate to
ferrite grains with intra- and intergranular
carbides. The material's tensile and creep
strength reduce during this process.
Damage mechanism in reforming
heaters.

* Oxidation-Severe scaling due
to oxidation is expected in these
heaters due to high operating
temperatures. Above approximately
500°C (932°F), the 9Cr-1Mo

FIG. 2. Arbor-type heaters with vertical inverted " U-type " radiant tubes are typically used
for low-pressure vapor flow units, such as CCR units.

materials tend to oxidize and may
scale very rapidly as temperatures
increase. The corrosion rate based
on material and temperature is
approximately 0.25 mm/yr.
* Carburization-This is the
diffusion of carbon into the metal
matrix at high temperatures >
600°C (> 1,112°F), forming a hard
and brittle structure. The extent of
carburization can be determined
by metallography and causes a
substantial increase in hardness
and loss in ductility. Where the
operating temperatures result in
high metal temperatures of about
650°C (1,202°F) or higher, carbon
at the surface has a higher tendency
to diffuse into the metal and thicker
carburized layers tend to form. This
phenomenon initially strengthens
the tube wall, but ultimately leads
to crack initiation, through-wall
crack propagation and, therefore,
enhanced rates of life consumption.
* Metal dusting-This catastrophic
form of carburization in the
480°C-816°C (896°F-1,501°F)
temperature range generally results
in rapid metal wastage in the form
of erosion at the hotter areas of the
tubes or small pits filled with metal
oxides and carbides. In mild form,
metal dusting results in discrete
pitting, but at an extreme can
effectively produce a rapid inner
wall thinning mechanism.
* Creep-Other prominent damage
mechanisms include creep, and its
susceptibility grows with increasing
temperature and reduced thickness.
In subject heaters, (to date) only
external oxidation has been found,
and no internal carburization or
metal dusting nor signs of creep
damage have been detected.
Aromatics plants: Petrochemical
unit. The aromatics plant is an impor-

FIG. 3. Severe scaling on the external surface of the radiant tubes.

54 JANUARY 2021 | HydrocarbonProcessing.com

tant downstream petrochemical unit that
produces various aromatic hydrocarbons,
such as xylenes, benzene and toluene,
which are the basic building blocks for the
production of polyester and polymers, resulting in different consumer goods.
The heart of the aromatics plant is a
licensed high-performance naphtha reforming unit that operates at the low pressure and high temperatures required to

http://www.HydrocarbonProcessing.com

Hydrocarbon Processing - January 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - January 2021