Refractory Ferrules (Boiler Tube Inserts)

Refractory ferrules provide the best protection possible for vulnerable boiler tube inlet areas and metal tube sheets in sulfur recovery units (SRUs found in oil refinery operations), methane reformers and waste heat boilers (WHB). Ceramics and refractories have high melting points and are therefore suitable for applications requiring wear resistance, high temperature strength, electrical or thermal insulation or other specialized characteristics. Alumina is commonly used in boiler tube ferrule applications. Its chemical inertness and resistance to hydrogen sulfide protects the tube sheet inlets and tube sheet from hot sulfur gas corrosion. Other materials used include silicon carbide and zircon, which may or may not be matrixed with alumina.

Hexhead refractory ferrules have an interlocking shape that provides stability and an interlocking fit that can eliminate or reduce the need for additional refractory mortar between ferrules. Fibrous ceramic rope and paper are used to form a seal.  Hexhead ferrules are designed to fit in a tubesheet with a triangular pitch or arrangement of boiler tube openings.

Square head refractory ferrules have an interlocking shape that provides stability and an interlocking fit that can eliminate or reduce the need for additional refractory mortar between ferrules. Fibrous ceramic rope and paper are used to form a seal. Square head ferrules are designed to fit in a tubesheet with a square pitch or grid-like arrangement of boiler tube openings.

Standard round ferrules with double flanges have a smooth circular shape with two raised ridges (flanges) along the ferrule's length.  Standard round ferrules require additional refractory mortar between the refractory ferrules to form a gas tight seal.  The refractory cement results in a large monolithic sheet across the tubesheet face. Monolithic designs may not withstand repeated thermal cycling as well as interlocking hexhead or squarehead ferrules.

Some types of refractory ferrules are manufactured with a cement-free design.  This means that the refractory ferrule relies on close fitting, interlocking components to provide a gas seal and protection of the tube sheet face with or without fibrous refractory rope or paper.  The close fit eliminates the need for sealing with refractory mortar or cement.  This is important because a layer of monolithic refractory cement on the tubesheet will eventually crack under repeated heating and cooling cycles. 

When determining which of the available refractory ferrules would be best for a given application, there are a number of key specifications to keep in mind.  First, the maximum operating temperature at which the ferrule will perform.  Second, the chemical environment in which the ferrule will function; sulfur, methane, hydrogen sulfide. The chemical composition of the ferrule should be selected based on these two parameters. Other important specifications include the outer diameter (O.D.) of the boiler tube, the thickness of the boiler tube wall, the pitch or distance between the boiler tube openings, and the O.D of the refractory ferrule itself.

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