April 2023


Maintenance and Reliability: Non-standard flanges for catalytic cracking units

In more complex petroleum refineries, a secondary processing step may be implemented to produce more usable end products.

Knauf, M., Garlock

In more complex petroleum refineries, a secondary processing step may be implemented 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.

Challenges faced

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 temperature.

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.

FIG. 1. Thermogravimetric analysis test results, comparing flexible graphite gasket and extreme temperature materials.

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

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