Steam control to reboiler
Steam control to reboiler
(OP)
Dear all,
There are two types of steam flow control into a thermosyphon reboiler. One is by having the control valve at the steam inlet line and the other is controlling the exchanger condensate outlet flow.
Based on your experience, which one is more realiable and robust?
Appreciate any comments
There are two types of steam flow control into a thermosyphon reboiler. One is by having the control valve at the steam inlet line and the other is controlling the exchanger condensate outlet flow.
Based on your experience, which one is more realiable and robust?
Appreciate any comments





RE: Steam control to reboiler
HX is regulated (usually shell-side).
RE: Steam control to reboiler
RE: Steam control to reboiler
RE: Steam control to reboiler
It's important to select a control valve that's tough enough to take some flashing of condensate, although most of the time on the shell & coil, the condensate was cooled to the point where it wouldn't flash anyway (about 160*F), and this unit had a 125 PSIG inlet steam pressure.
RE: Steam control to reboiler
Although not all designs are equal, it appears the most accepted approach is to have a CV on the steam side (being steam flow the primary variable thus offering rapid response) and a condensate removal pot with a balance line floating on steam pressure downstream the CV, removing the condensate with a LLCV. The level on the pot being the same as the condensate level on the reboiler. In this way control over a wide range of loads can be achieved, especially when the reboiler has been oversurfaced to satisfy contingencies such as unexpected fouling, design safety factors, varying reflux rates, etc. offering rapid response and fine control when operating conditions fluctuate. This type of setup may be also of help in horizontal units.
Only condensate control or only steam control can bring about noise and hammering when steam bubbles come into contact with cold condensate. Steam control at low loads, or under sudden drops of load or when the unit is largely oversurfaced may bring about condensate flooding as with condensate control. The main advantages of condensate control are: CV is smaller and robust, low-maintenance item, cheaper , easier to design, and always has positive pressure to discharge, a fact that is not always available with steam control.
RE: Steam control to reboiler
Particular circumstances would drive the choice the choice one way or the other. Three particular circumstances (all favouring steam control - but that's just coincidence) are:
1. If tubeside material is heat sensitive or likely to skin the inside of the H/E tubes, use a steam control valve to drop the operating pressure of the steam chest.
2. In some circumstances when there is sufficiently high pressure drop across the steam control valve, you could use a side flow of upstream steam to operate a pumping steam trap.
3. A steam control valve and steam trap combination will allow inerts(air) to quit the steam chest. I am not so sure how this is done with a condensate flow control valve.
RE: Steam control to reboiler
Other factors to be considered when selecting the control system on termosiphon reboilers are:
Operation on vertical units is sensitive to process parameters such as temperature or composition, because they interact with the pressure balance between the 'pushing' static head available in the column and the friction on the two-phase flowing loop.
These conditions may bring about oscillations in the process flow loop, poor tower control, and even low heat-transfer may result from mist flow in long tubes.
Working beyond a critical delta T between the wall and the boiling liquid would mean a reduction in HTC due to film-type boiling. The steam pressure, affecting the tube wall temperature, should then be carefully selected/ monitored.
Sometimes it may be advantageous to put the steam CV on cascade with the feed inflow to get some kind of 'feed forward' control, especially when the tower bottoms are removed on LLC.
Still, it appears that a combined control on both steam input and condensate removal may be the optimal approach.
RE: Steam control to reboiler
The best control system I have experience of is to install a Condensate controller that is Cascaded from a Level transmitter on the side of the Reboiler. (The Level Controller is the Primary loop). This confiquration has been in operation in 4 Plants I have worked in over the last 20 years.When calibrated correctly you always have accurate indication of the condensate level.(also removes the Operator fear factor about blowing thru to allow maximising Optermisation of the Towers.
RE: Steam control to reboiler
Controlling the steam flow is the way to go...but you need a condensate pot beside the reboiler with a level controller to complete the installation. The condensate level control valve should normally run wide open...and the pot will typically be full. When the steam valve opens, the level in the pot will drop. When you approach the low liquid level, the valve starts to close to prevent loss of the condensate seal.
Lots of Operators get confused by this scheme, thinking the condensate valve is incorectly sized because it will usually be wide open, yet the vessel will be chock full. They forget that the pressure inside the vessel is what gives the valve it's capacity...and this increases or decreases depending on how open the steam valve is. Need more Q...the steam valve opens...more pressure inside the receiver...more condensate flow (and yet the valve is wide open...as it should be).
By the way, if you want to use a heat-off scheme to reduce flare load, the "control steam flow" installation is also a better fit. Shutting a condensate valve does not immediately stop reboiler Q, where shutting steam off does.
I hope this helps.
Regards,
Bob