Replacing the GTA Shroud

Answer

This Information Applies To: Agilent GTA 120, 240/280 GTA Furnace

Issue

The shroud should be replaced annually, or when there is evidence of damage caused by mechanical shock or contamination from liquid spillage. 

Items Required

Shroud Removal Tool (p/n 9910033300)

Warning: Always switch off the unit, disconnect the power supply cable, and allow enough time for the workhead to cool down to a safe handling temperature.

Steps to Follow

• Remove the GTA Furnace Workhead before replacing the shroud.
• Remove and replace the electrodes before replacing the shroud if needed. 
• Remove the graphite tube from the shroud of the workhead if applicable.
  • Tubing can remain in place when removing and replacing the graphite shroud in the center block.

Removing the Furnace Shroud

1. Push the draw bolt through the hole in the center block from the opposite side of the ridge on top of the block. The threaded portion of the bolt will protrude through the center block (Figure 1).
   

   
   
   Figure 1. Pushing a draw bolt into the block
   
   

2. Place the spacer, collar (with shouldered face of the collar facing the draw bolt), and washer to the end of the draw bolt (Figure 2). A specified order is shown in Figure 3. 
   

   
   
   Figure 2. Placing the spacer, collar, and washer to the draw bolt 
   
   

   
   
   Figure 3. Direction of Removal
   
   

3. Place the nut onto the draw bolt. There should be no gaps between the tools to ensure it is aligned (Figure 4).
   

   
   
   Figure 4. Placing the nut onto the draw bolt 
   
   

4. Pull the shroud into the spacer by rotating the nut clockwise. The shroud opening should begin to move closer to the spacer, indicating that the shroud is being drawn out from its position (Figure 5).
   

   
   
   Figure 5. Pulling the shroud into the spacer
   
   

5. When the shroud has been removed, remove the nut on the bolt and dismantle the shroud removal tool. The used shroud can be discarded (Figure 6). 
   

   
   
   Figure 6. Removing the nut and dismantling the shroud removal tool
   
   

Replacing the Furnace Shroud

Caution: Wear either powder free nitrile or fabric gloves completing the following procedure.

1. Clean the inside shroud bore in the center block before installing the new shroud.
2. Place the replacement shroud at the opening of the center block so that the sample injection hole is facing upwards and is aligned with larger opening of the center block. The shroud should be oriented with the gas flow hole facing the side of the center block with tubing connections (Figure 7).
   

   
   
   Figure 7. Aligning the shroud
   
   

3. Insert the draw bolt through the shroud in the center block. The threaded portion of the bolt will protrude from the opposite end (Figure 8). A specified order is shown in Figure 9. 
   

   
   
   Figure 8. Inserting the draw bolt
   
   

   
   
   Figure 9. Direction of Installation
   
   

4. Place the collar (with the shouldered face of the collar facing the draw bolt) and the washer at the threaded end of the draw bolt (Figure 10).
   
   

   
   
   Figure 10. Placing the collar and washer into the draw bolt
   
   

5. Place the nut on the draw bolt. Rotate the nut until the tooling is aligned, with no gaps between tooling. Do not tighten. Ensure the shroud opening and the center block opening are aligned (Figure 11).
   

   
   
   Figure 11. Placing the nut on the draw bolt 
   
   

6. Insert the larger stop pin into the larger lower opening on the side of the center block (Figure 12).
   

   
   
   Figure 12. Inserting a stop pin
   
   

7. Insert the narrow end of the second stop pin into the opening of the draw bolt. Rotate the draw bolt and stop pin counterclockwise until it rests against the larger stop pin. These stop pins should restrict rotation when pulling the shroud inside the center block. Ensure that the shroud opening remains aligned. Carefully rotating the shroud for any minor adjustments (Figure 13).
   

   
   
   Figure 13. Inserting a second stop pin
   
   

8. Ensure that the stop pins are in place, then draw the shroud into the center block by turning the nut clockwise. The shroud opening should begin to move closer to the center block, indicating that the shroud is being placed into position (Figure 14).
   

   
   
   Figure 14. Drawing the shroud into the center block
   
   

9. Keep rotating the nut until it comes to a stop. Check the alignment of the sample entry openings; the shroud opening should be aligned in the middle of the center block opening (Figure 15). 
   

   
   
   Figure 15. Incorrect alignment of the shroud
   
   

10. If aligned correctly, disassemble the tooling (Figure 16). 
    

    
    
    Figure 16. Correct Alignment of the Shroud
    
    

11. If the sample entry holes are not aligned correctly, assemble the shroud removal tool and remove the shroud.
    1. Follow the shroud replacement instructions in this article and ensure that the openings are aligned before reassembling the GTA Furnace.
    2. The same shroud should only be replaced and removed two to three times as the graphite surface is lost when repeatedly removing and replacing the shroud.
    3. If the surface of the shroud is shiny, it should be replaced with a new shroud.
       

       Tips: To ensure that the shroud is fitted correctly, place a graphite tube into the shroud and close the workhead. The tube opening must be centered in the shroud. If necessary, remove the tube and connect the shroud removal tool and adjust the position of the shroud. After the adjustment has been made, insert a tube to check the alignment (Figure 17).

       
       
       Figure 17. Alignment of the Tube and Shroud
       1. Perfectly Aligned shroud and tube,
       2. 0.2 mm off center alignment that is within tolerance for use,
       3. 0.5 mm off center alignment, this is outside tolerance and will affect results
       
       

12. Replace the GTA Furnace once completed.