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[pinakiranjan.patra]

hello everybody,

i have been working in an petrochemical industry for the past one year....
one question that is striking me is that i have seen some control valves on the steam line and the others on the condensate line of the reboilers existing here....
can somebody please tell me the difference between a control valve on the steam line and on the condensate line...

[Art Montemayor]

Good observation work. It seems your engineering curiosity has been at work and you’ve spent some time debating what you perceive as an important piece of information or something that could be of value to know in the future. If that is the case, you are correct in expecting something of importance in what appears to be simply a "subtle" difference in applying steam condensate control.

What is customary and “usual” in designing and installing a steam supply system to a heat exchanger or a reboiler is that a standard flow control valve is employed upstream of the heat exchanger to throttle the steam admitted to the heating chamber of the same exchanger. Additionally, a steam trap is installed to “trap” and drain the condensate as fast as it is formed in the heating chamber. This is the usual way that academics introduce young chemical engineers to the art of heating with a supply of saturated steam.

What is also customary and “usual” during this time is that the young chemical engineer is never told that there is a definite downside and disadvantage in this operating technique. He/she is never told that this method – while being dependable and time-proven in many applications – has its drawbacks. One outstanding drawback is that the operation is not as steady and continuous as you might expect. The “puffs” and cycles that the steam trap makes ought to raise your suspicions at once. If you take a steam Mollier chart and plot a typical heating cycle on it you will quickly become aware of where I am going. Note that in some instances and applications, the saturated steam is expanding across the control valve trim and becoming superheated in the process prior to reaching the surface it has to heat. This is so because of the required pressure drop needed to induce the steam flow into the heating chamber. Because of this, the pressure in the chamber is not what you might guess it is. It is a pressure that is varying as the steam makes its way through the chamber. That is why many young engineers can’t understand why you can’t use the steam tables to calculate the true temperature difference in a steam heater. You have to calculate the true average temperature in the chamber because you really don’t know the saturated pressure there.

When you allow the steam chamber (or “chest”) of a heater or reboiler to “float” with the saturated steam supply header you are allowing the heat transfer surface to receive the full capacity of saturated steam – at the actual header pressure – and you can therefore know exactly what the chest conditions are, constantly and evenly. Additionally, in some cases where you want or need to vary the rate of heating it is very fortuitous to simply allow condensate to accumulate within the chest by simply throttling the condensate drain valve in accordance to a level controller. This is particularly done in vertical heaters - like thermosyphon reboilers. By doing this you are, in effect, varying the effective heat transfer area because you are covering some of it with a condensate level. This is an effective way to bring about heat transfer control. I have used it with success on reboilers in many columns in the past and I would continue to do so in the future in those applications where I have a need or where it is attractive from a process control point of view. Pay particular attention to the fact that the driving force to evacuate the formed condensate from the steam chest is the maximum achievable in any application. This is another incentive to use the method because it ensures the quick and sure evacuation of condensate into a header and far away from a boiler house. This sometimes saves you many condensate return pumps. I personally favor this type of condensate control because I have found that it is conducive to steady, constant heating – much more that steam throttling.

I hope the above has served to explain to you the advantages and difference in steam control on process heaters and reboilers and that it gives you an appreciation for still another way to “skin a cat”.