Nozzle Elevation and Orientation for Distillation Towers


Nozzles must be elevated to meet the internal requirements of the tower and oriented for maintenance and operational needs. Their position must also facilitate economic and orderly interconnection of piping between the tower and related equipment.

A maintenance access is usually located in the bottom; top and intermediate sections of the tower and is used to gain entry to the tower during shut downs for internal inspection and component removal.

Maintenance accesses must not be located at downcomer sections of the tower. Care must be taken at sections of the tower that contain internal piping to avoid blocking the maintenance access entrance.

Following diagram shows typical elevation and orientation requirements for maintenance accesses.

Feed connections to trayed towers usually must be located in a specific area on the tray by means of internal piping, which can restrict nozzle orientation options.

The restrictions are minimized by optional routing of the internal piping to facilitate the most economic exterior arrangement.

Internal feed piping to packed towers is piped directly to the distributor and can be oriented at any angle.

Following figures illustrate several options for internal feed piping.

If specified, reboiler connections are usually located at the bottom section of the tower.

For the horizontally mounted thermosyphon reboiler, the draw off nozzle is located just below the bottom tray. For the vertically mounted recirculating thermosyphon reboiler, the draw off
nozzle is located at the bottom head. For both systems, the return nozzles are located just above the liquid level.

Following diagram shows both of these arrangements.

The vapor outlet is usually a vertical nozzle located on the top head of the tower. It is usually a single nozzle, but in certain cases (e.g. towers with very large diameters) more than one nozzle is specified.

On large diameter vapor lines, the vessel connection could be butt  welded instead of flanged. In addition, the vent and relief valve could be located on top head instead of attached to the overhead piping.

Following figure shows the typical top head arrangement.

The liquid outlet is located on the bottom head of the tower. If the tower is supported by a skirt, the nozzle is routed outside the skirt.

As with the vapor outlet,more than one nozzle may be specified.
Elevation of the nozzle is dictated by the constraints previously discussed. The orientation can be at any angle, but generally it is dictated by pump suction piping flexibility.

Following figure shows a typical bottom head arrangements. Temperature and pressure instrument connections are located throughout the tower.

The temperature probe must be located in liquid space and the pressure connection in vapor space.

Following diagram shows preferred location for both connections.

Level instruments are located in the liquid section of the tower, usually at the bottom.

The elevations of the nozzle is dictated by the amount of liquid being controlled or measured and by standard controller and gauge glass lengths. This information is furnished on instrument vessel sketch.

Following diagram illustrates level instrument requirements. When nozzles, especially those with internal piping, are positioned, the plant layout designer must allow adequate clearance at tray support steel.

Following diagram shows approximate tray suport beam sizes. Standouts shown below, are measured from the internal diameter of the vessel to the face of the flange.

To set top and bottom head nozzle elevations, the type of head must be specified. This information is highlighted in process vessel data.

Two most commonly used are flanged and dished and 2:1 elliptical heads. Following drawing shows approximate dimensions for these heads.

As an example, the nozzle elevations shown below have been set using the following guidelines.

  1. Process vessel sketch
  2. Try details
  3. Type of heads – 2:1 elliptical
  4. Bottom tangent line elevations
  5. Nozzle summary
  6. Instrument vessel sketch
  7. PID
  8. Plant layout specification
  9. Insulation.
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