Function of steam control valve in Vaporizer

[JK101]

Folks,

Does anyone knows that in a Reboiler system (e.g. stabilizer bottom, vaporizer), sometime we use a valve to control Steam pressure at inlet & sometime we use a valve to control the condensate flow at the outlet. What is the main reason behind this and which one is more effective? Moreover, what would be the relation between steam pressure and heat exchange in the system?
Actually I am optimizing the heat flow in a vertical shell and tube heat exchanger/Vaporizer. We have steam at tube side and hydrocarbons at shell side.
Your responses will be highly appreciated .

 

[TD2K]

If you use a condensate control valve with no steam valve, you are essentially matching the required duty to what the exchanger can do by flooding/unflooding some of the tubes.

A steam control valve varies the steam pressure in the exchanger and therefore the condensation temperature and dT to match the exchanger capacity to what the process needs. You still need some way to control the condensate out, either steam traps or more commonly another control valve controlling the level in a condensate pot. I don't see many steam traps on refinery steam exchangers but there are some.

Adjusting the steam pressure gives you faster control in my opinion than flooding or unflooding reboiler tubes. A vertical thermosyphon, if you are going to flood tubes, is a good choice because the same percentage change in condensate level = a similar change in heat transfer area. For a horizontal reboiler, that's no true. Near the bottom of the tube bundle you may not be covering or uncovering many tubes for a change in level. At the mid point, you are covering or uncovering proportional a much greater area for the same change in condensate level.

 

[EmmanuelTop]

See attached the article "Controlling steam heaters" by W.Driedger, first published in Hydrocarbon Processing, November 1996. It discusses all variations of control (on both steam and condensate sides) and their advantages and disadvantages, for steam heaters.

Dejan IVANOVIC
Process Engineer, MSChE

 

[JK101]

Great info.. Thanks a lot TD2K & EmmanuelTop.. I will go through it thoroughly .