PWHT Furnace

PWHT Furnace

Industrial Post Weld Heat Treatment furnaces are commonly used in the Pressure Equipment Fabrication and Repair Industry.  PWHT Furnaces are ultimately and almost without exception the most efficient method of Field Heat Treating.  All other Field Heat Treating methods are attempting to duplicate the furnace process.

 

PWHT Furnaces are typically designed 1 of 3 ways:

 

1). A Rail Car Bottom commonly use a flat Rail Car lined with Fire Brick and a support grid, as a rolling heart.  The rail car could also have a sealing wall on one end, or in some cases the car bottom will fit completely within the fixed furnace box.

2). Top Hat furnaces use a fixed Hearth / Bed and requires some type of lifting equipment (usually an over head crane) to lift the furnace box off of the bed.  In some cases they can have a split wall in which the box becomes 2 pieces.  A lower section of the box would stay in place with the burners, and the top section would be removed for loading and unloading.

If the Top Hat Furnace doesn’t split and is configured properly it is possible to have a dual hearth / bed for faster loading and unloading.

 

3). With a Rolling Furnace the bed remains fixed, and the Furnace rolls into position.  Sometimes the rolling furnace will have a fixed wall, and sometime they split in the middle without a fixed wall so they can be run as two independent smaller furnaces, or joined to run as one larger furnace.

Rail Car Bottom, Top Hat, or Fixed Bed Rolling Furnaces have 3 Key Components:

 
Furnace Box:

The Furnace Box itself is constructed of Structural Steel and Sheet Metal. The box could be fixed, lifting, or rolling depending on furnace type and is fully insulated.  The Insulation is usually a combination of Fire Brick, High Temperature Mineral Wool batts, and AES Blanket.  6” of total insulation thickness is used minimize heat losses and protect the structural integrity of the box.

The Drawn Arc Stud Welding process is used in conjunction with Insultwist Pins to secure the insulation to the interior of the furnace.

 

Package Burner System and Valve Train:

Package Burners are systems where the Burner and Blower are a contained in a single unit with the Flame Monitor and Air proving system in place.

Each Package Burner will also use a Gas / Valve train that includes an assembly of piping, control and safety valves, and a flame safety system which is needed to safely control the pressure and flow rates in accordance with National Fire Protection Association (NFPA) and American and Canadian Gas Association guidelines.

Safety-interlocked pressure, flow, and flame control components ensure that the system is operated in a safe manner and provide feedback to the Technician during equipment start-up and operation.

Package burners are available in a wide variety of BTU ratings, and utilize a closed-loop microprocessor temperature controller in order to achieve the proper temperature profile for the heat treatment process. Where multiple burners are used it is possible, and recommended to slave burner to a Master Controller to ensure temperature uniformity throughout the entire furnace.

 

Furnace Hearth/Bed:

A furnace Hearth / bed is the part of the furnace that where the objects to be heat treated rest.  The Hearth can be fixed, in the case of a Top Hat or Rolling Furnace, and are moveable with the Rail Car Bottom Furnaces.

For fixed bed furnaces the bed is usually compacted sand or limestone, although fire brick can be used it has a tendency to get damaged. With rolling furnaces the outer limits of the hearth should have a I-Beam box backfilled with sand.  Adding some a couple of rows of firebrick just along the inside perimeter of the I-beam can help with ensuring a tighter seal around the furnace.

 

Car bottom furnaces commonly use a rail car lined with Fire Brick and a support grid of steel as a rolling hearth.  The Furnace Box will remain stationary in this instance, and the Rail Car will be loaded and slides along a set of rails, moving the load into the furnace.

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