Metallic - Special

FCCU is one of the most important units in the oil refinery as it converts heavy distillates into lighter ones (gasoline and diesel), which increases the yield and efficiency of the refining process. The feedstock is mainly vacuum gas oil mixed with refinery residues heated to extremely high temperatures (around +700°C to 800°C), making this application very critical. Consequently, the FCCU expansion joints utilised in this application are the most technically advanced among all the existing types. The design and manufacturing of them requires great expertise and capability.

Types of FCCU Expansion Joint

Typical types of expansion joints used in FCCU applications are: Restrained Universal, Gimbal, Hinged and Pressure Balanced (in-line and elbow type) and can be categorized in three major groups:

• Hot Wall
• Cold Wall
• Unlined

The Cold Wall units are refractory lined to ensure the shell wall temperature does not exceed the allowed parameters. The lining consists of stainless steel anchors and a high-density self-levelled refractory material. They incorporate internal insulation/packing made of bio-soluble ceramic fibre or silica blankets, a liner seal that keeps in place the internal insulation blankets while keeping fluid particles out of the bellows/liner cavity.

An important advantage of cold wall design Expansion Joints is that the pipe is insulated so. It operates below the media temperature reducing the pipe growth and consequently the expansion joint movement.

An insulation pillow is included to reduce the temperature. An external insulation is incorporated to prevent the bellows element from dropping below the acid dew point, the main reason for premature bellows failure.

The Hot Wall units incorporate an abrasion resistant lining, including hex-mesh and castable material or refractory (a multi-purpose abrasion resistant castable which can be hand-packed, vibration cast and gunned) such as RESCO AA-22. The lining is not intended to be used as a thermal fence and requires a specific and controlled drying.

The Unlined FCCU Expansion Joints can be exposed to very high temperatures but usually do not convey catalysts so they do not require abrasion resistant lining. This type is generally used for inlet and outlet air and transferring gases from the reactor.

The Bellows

The bellows, as the most critical part of the Expansion Joint, can be single ply, multiply, redundant ply or reinforced and generally incorporate an early warning system (active or passive monitoring).

FCCU expansion joints generally incorporate 2-ply testable bellows where each ply is designed for the full operating conditions. If a hole or stress crack develops in the inner ply during service, the outer ply takes over without exposing operators to increased risk or creating the need for an unscheduled shutdown.

2 ply testable bellows system improves reliability and makes the expansion joint more maintenance friendly.

• Monitored Bellows
  The annular space between plies can be monitored for leakage to detect a ply failure. This will serve as a warning of an imminent problem. A pressure device in the outer ply alerts about the inner ply failure. The 2-ply testable bellows also allows inspectors to pressure test the inner and outer ply during shutdowns. There are several types of devices used for monitoring the 2 ply testable bellows from simple pressure gauges to electronic devices and can be categorized as Active and Passive Monitors.
  • Passive Monitor
    When the inner ply fails the monitor is activated by the pressure between the plies.
  • Active Monitor
    The active monitor can detect inner and outer ply failures. A vacuum is created between the inner and outer ply before installing the monitoring device. If the inner ply fails, the pressure between the plies will activate the monitoring device and if the outer ply fails the vacuum will be lost and the monitoring device will be activated.
• Packed Bellows

  FCCU bellows are generally internally and externally packed with ceramic blankets and the gap between the sleeve and body is filled with a metal braided hose connected to the sleeve by pins. The thickness of the blankets is determined following heat transfer calculations to assure bellows temperature 200 < T < 500°C. The reason to specify the temperature of the bellows at a temperature higher than 200 °C, is to prevent dew point corrosion. The upper limit of 500 °C was to prevent high temperature embrittlement of lnconel 625.

  To avoid the dust entering into the bellows cavity as the catalyst can solidify and damage the bellows or restrain the movement capability packed or purged bellows are used. The most frequent is the packed bellows. Purged bellows are not as commonly used today.

Self-equalizing Rings

Are commonly used on FCCU Expansion Joints to prevent the convolutions from contacting each other, ensuring a uniform compression distribution over the convolutions.

Equalizing rings prevent bellows from an excessive deflection or stress concentration in one or a few convolutions by spreading the movements over all the corrugations.

They also provide a pressure reinforcement capacity when necessary.

Materials selection

• Bellows Element

  The material for bellows for most FCCU applications is Inconel 625LCF. INCONEL® alloy 625LCF (UNS N06626 / W. Nr. 2.4856) developed as a fatigue- resistant, bellows-quality version of INCONEL alloy 625.
  
  The fatigue life of alloy 625LCF can be up to 100 times that of conventional alloy 625. This can result in greatly improved service life over that possible with conventional alloy 625 products.

  Inconel 625LCF is the best commercially available material for FCCU bellows in an external expansion joint. This material provides excellent stress corrosion cracking resistance, very good general high and low temperature corrosion resistance and very good mechanical properties including fatigue properties.

  Some specifications require for the bellows to be annealed, or solution annealed, after forming.

• Pipe Body

  The pipe body for the bellows in hot wall piping systems where operating temperatures are higher than 600 °C, is manufactured from stainless steel Type 304 or 321 H SS. For both materials the carbon content is often restricted to 0.4 < C < 0.6%.

  In hot wall piping systems where the maximum operating temperatures are 500 < 600°C, the low Alloy steels like 1.25 Cr - 0.5 Mo are used.

  In cold wall piping systems where the maximum operating temperature is 343 °C, fine grain carbon steel like ASTM A 516 Gr. 70 is used. No special requirements apply.

• Refractory Materials & Installation

  Refractory is a major element of all FCC components. Without proper refractory installation the process unit is risking a costly unscheduled unit shutdown. FCC Expansion Joints are designed with a hot wall (external insulation) or cold wall (internal insulating refractory). The internals that are exposed to catalyst erosion are protected with abrasion resistant refractory even in hot wall designs.

  Generally, all these critical linings are designed and installed according to the most up-to-date and demanding refractory licensors specifications as UOP 3-series, ExxonMobil Research and Engineering EMRE Gp, KBR and API 936 or proprietary own specifications.

  The insulating or abrasion resistant refractory linings can be installed by casting, gunning (pneumatic application), hand-placing or pneumatic ramming. The abrasion resistant refractory installation involves several critical activities like anchor system (hexmesh or equivalent), QC of materials, prequalification, application and heat dry outs. It is very important that all refractory installation activities are, during all the process, carefully controlled to ensure a good quality control through a good method of statement with drawings, materials selection, testing frequency, installation systems and dry out curve.

• External hardware

  FCCU joints usually fit in different types of external hardware. The most widely used are:

  • Control Rods

    Used to control and limit the movement of the bellows. Devices, usually in the form of rods ( 30 mm diameter minimum ), attached to the expansion joints assembly whose primary function is to restrain the bellows movements (axial, lateral and angular) during normal operation. Four control rods are normally provided at each bellows element. The rods are equally spaced circumferentially. Control rods are not designed to restrain pressure thrust.

  • Pantographic linkages

    In universal expansion joints the pantographic linkages ensure that each bellows absorbs exactly half of the total bellows movements.

    In case of vertical or inclined installation, the function of the pantograph linkage is also to support the weight of the connecting pipe of the two bellows, including refractory and medium weight. Pantograph linkage is not designed to restrain pressure thrust.

  • Slotted Hinges and pantograph central bar

    Slotted hinges installed in a universal expansion joint ensure that each bellows absorbs the same axial movement only. In case of horizontal installation, slotted hinges also support the weight of the connecting pipe of the two bellows, including refractory and medium weight.

    A pantograph central bar other than the same function of the slotted hinge, in case of vertical or inclined installation, can support one component of the weight of the connecting pipe of the two bellows, including refractory and medium weight. A pantograph central bar is always assembled with the pantograph linkage.

  • Central Gimbal

    A central gimbal system must be installed in universal untied expansion joints, where a pantograph linkage is mounted and if the required out of plane lateral movement is more than 10 mm.

    The primary function of the central gimbal is, in presence of the pantograph linkage, to permit large lateral movements out of plane. The central gimbal is also designed to take the weight of the connecting pipe of the two bellows, including refractory and medium weight.