M. Knauf, Garlock, Rochester, New York
In more complex petroleum refineries,
a secondary processing step may be implemented in more complex petroleum
refineries to produce more usable end products. Either coming from atmospheric
or vacuum distillation units (VDUs)—physical separation processes—heavier gasoils (HGOs) can be converted in the
fluid catalytic cracking unit (FCCU). Since atmospheric and VDUs either use
higher-temperature steam or lower pressures to lower the processing
temperature, higher boiling point gasoils (typically < 340°C) are left over.
Unlike the preceding processes, fluid
catalytic cracking (FCC) is a chemical process that uses a catalyst to
chemically break down these longer-chain HGOs into molecules that make up
desirable and useful products like gasoline, distillate, butane and propane.
The FCCU uses a sand-like material—such as zeolite, bauxite,
silica-alumina or aluminum hydrosilicate—as catalysts. The HGO feedstock, which is heated
and pressurized, is then brought into contact with one of these powders, which
are maintained at very high temperatures. This combination facilitates the
desired breakdown of these long-chain hydrocarbons, and the resultant products
are collected as a vapor.
As with any chemical process,
equipment with piping and flange connections are used to collect and transport
the desired chemicals for further processing, remove undesirable wastes and
allow for manways used for cleaning.
When introducing these connections, a
conformable material, typically a gasket, in conjunction with a loading force,
usually bolts, is used to seal the mating surfaces. Due to the extreme
temperature of the FCC process, finding a sealing material to withstand these
conditions is very difficult.
Previously in this process, with
temperatures around 690°C–730°C and pressures around 2 bar, refineries used a
spiral-wound gasket with a graphite and asbestos filler. A spiral-wound gasket,
which is semi-metallic, contains at least a wire-wrapped soft filler (graphite
and asbestos in this case) and sometimes uses an outer and/or inner metal
compression stop. These are commonly used for higher pressure and temperature
applications due to the robust nature and great thermal resistance provided.
While graphite is commonly used for
higher-temperature applications, it is limited to 454°C in environments where
oxygen may be present. Above this temperature, oxygen reacts with graphite,
more readily creating a situation where the soft filler will be oxidized and
eventually disappear over time. As the temperature increases above this 454°C,
the reaction steadily increases in rate, lowering the reliability of the joint
in question. Even if the process does not contain oxygen, the air outside the
flange and gasket may be heated enough to facilitate this failure mode, acting
from outside inward. Over time, this overheating will cause the windings to
lose load on the flange, causing the bolts to loosen and leakage to occur.
The refinery in question was hot
retorquing the connections with the spiral wound gaskets, and service life for
this application was about 1 mos–2 mos. This raised concerns about safety risks
and caused routine system shutdowns to replace the gasket, resulting in costly
downtime. Graphite was certainly not well-positioned here due to the extreme
Solutions and benefits.
In this application, higher thermally resistant/lower
oxidation materials would be preferred when the temperature exceeds the ideal
range for graphite. Mica and vermiculite products are commonly used materials
in service where extreme heat exists; however, these products tend to be
hydrophilic and can begin to degrade with moisture exposure from the process
fluid (e.g., steam) or external conditions (e.g., rain).
The challenge in the case was to
develop a product that contained inorganic components that could withstand
extreme heat from superheated steam and exhaust. The product also had to remain
hydrophobic enough to resist degradation and deformable enough to initiate and
maintain an effective seal long term. It also needed excellent thermal
stability and minimal weight loss, as these factors directly impact the load
retention properties of the flange connection.
The product developed to meet these
requirements is an inorganic fiber and filler-based material available in sheet
form and filler or facing material for metallic gaskets. FIG. 1 shows the
results of the thermogravimetric analysis test, comparing weight loss
characteristics of traditional flexible graphite gasket material to the newly
developed extreme temperature material when tested at 1,000°C. The graphite oxidizes
rapidly around 650°C
and loses nearly all its mass as the test temperature approaches 900°C. The oxidation of the
extreme-temperature material seems to exceed that of graphite initially. This
is due to the small amount of rubber binder, which provides improved compressibility,
flexibility and handling. Once the small amount of binder is thermally degraded,
the material becomes very stable and weight loss ceases. This is important for
the integrity of the windings. If too much weight is lost, you may have a
loosening of the windings and a loss of sealing quality.
In this application, the gasket was
checked after 6 mos in service, with no bolt loosening or retorque required.
This not only improved the reliability of the connection but helped reduce
concerns with the safety of the hot retorque being implemented. In this case
study, the newly developed inorganic extreme-temperature material was used as a
filler in a spiral-wound gasket. HP
KNAUF has 9 yr of experience
with gasketing and fluid sealing related to practical application and design. Knauf
earned ME and BS degrees in mechanical engineering from the Rochester Institute