12 replies to this topic

[Sagar Nawander]

Dear  All,

 

What is the difference and advantages on putting Control valve on Steam side flow and/or on Condensate side in case of thermosiphon reboiler ???

[katmar]

The advantages of controlling on the steam flow are quicker response, and easier understanding for the operators.

 

The main advantages of the condensate option are that the control valve is smaller (and cheaper) and you save the cost of a steam trap.

 

In the case of over-sized thermosiphon reboilers controlling on the steam side can result in cyclical or unsteady boil up, and putting the control on the condensate outlet can overcome this because it makes the reboiler a "variable surface area" device by partially flooding the shell.  The converse of this is that there is a danger when controlling the condensate if the reboiler operates close to its maximum possible duty.  This is because the condensate valve opens when trying to increase the heat transfer and this has the risk that the condensate seal can be removed and live steam is blown out - thus lowering the pressure and temperature in the reboiler when you actually want more heat transferred.  I do not have experience with this, but I have read that the problem can be overcome by including a liquid level control on the reboiler shell side cascaded into the control scheme.

[fallah]

katmar,

 

Good point on the risk of condensate seal removal, when FV is considered at condensate line, which may lead to steam blowing out...But the protection is not cascade control scheme. Actually, the FIC to be overridden by a LIC when low liquid level is detected in relevant piping pot bottom...

[Sagar Nawander]

Katmar / Fallah,

 

Thank you so much for your valuable answer.

 

However, As far as i know putting "control valve on steam" side results in following cycle :

 

-  The heat transfer rate is too high.

-  The controller decreases the steam valve opening.

-  When the pressure in the reboiler steam chest drops below the pressure required for condensate return, the         reboiler begins to fill with condensate.

-  The heat transfer rate decreases.

-  The controller increases the steam valve opening.

-  Once the pressure in the steam chest exceeds the pressure required for condensate return, the condensate is     forced out of the reboiler.

-  The heat transfer is again too high, and the cycle repeats.

 

But my query is what will be the advantage of putting control valve on "Condensate return line".

 

 

[fallah]

Hi,

 

Some advantages of putting CV on "condensate return line" are as follows:

 

-Smaller size CV

-Higher and steadier driving force across the CV

-Higher heat transfer rate

-Constant steam pressure equal to that of the supply steam source inside the reboiler

[katmar]

For a well-designed system with the control valve in the inlet steam line I would disagree with the control cycle you have described.  I would see it more as:

 

putting "control valve on steam" side results in following cycle :

 

-  The heat transfer rate is too high.

-  The controller decreases the steam valve opening.

-  When the pressure in the reboiler steam chest drops the condensing temperature of the steam also drops.

-  The condensate continues to drain to the condensate collection pot or to the return main via a correctly designed steam trap.

-  The heat transfer coefficient remains much the same, but the temperature driving force decreases, resulting in a decreased rate of heat transfer.

-  The controller increases the steam valve opening.

-  As the pressure in the steam chest increases, the condensing temperature and the driving force increase as well.

-  The heat transfer is again too high, and the cycle repeats.

 

The steam trap (or condensate drain valve) should be sized so that the condensate is removed rapidly, even if the pressure in the reboiler fluctuates a bit around the expected point.  However, when the control valve is put on the condensate line the temperature driving force remains constant, while the condensate level (and therefore the heat transfer area) varies. 

[latexman]

katmar,

 

We have no details from sagar nawander.  There may also be some accumulation of inerts going on also.

 

The noncondensable gases present in all steam, with even higher concentrations

present in steam generated within the process rather than at the power house, normally must be vented at

the interface between the condensate and the vapor. Unless the steam side has been designed to permit

adequate venting for any possible accumulated condensate level, the noncondensable gases cannot be

appropriate vented. If the vent nozzle is placed at a location other than at the liquid-vapor interface,

venting is not effective. Excessive amounts of steam will be removed from the system without removing

appreciable quantities of the noncondensables that have been concentrated at the liquid-vapor interface.

The noncondensable gases contain oxygen and carbon dioxide and a corrosive environment is created,

especially if the material of construction is carbon steel.

 

If the noncondensable gases are not removed, they will continue to accumulate. The accumulation of

noncondensable gases has the same effect upon heat transfer as condensate flooding. Condensate is

gradually replaced with unvented gases and the condensate liquid level gradual diminishes as it is

replaced with unvented gases. Eventually, the condensate level disappears and process control results

only because of the accumulated and unvented noncondensable gases. When no condensate level exists,

the noncondensables will be vented through the condensate nozzle and removed from the system. The

result is that there now is no accumulation of either condensate or unvented gases and the condensing

pressure for the steam will decrease as a result of the process control. The condensing pressure will no

longer be sufficient to cause condensate to leave the system, and condensate again begins to accumulate

and the cycle will be repeated.

[katmar]

latexman is absolutely correct that a potential disadvantage of the "valve in condensate line" option is an increased risk of corrosion due to oxygen and carbon dioxide.  It is difficult to position an air (incondensibles) vent on the reboiler when the level of condensate inside is varying.  While some people have reported corrosion problems, others (Art Montemayor included) have reported that they had no problems.  It all comes down to the quality of the steam.

 

There was a recent discussion here (see http://www.cheresour...em-on-reboiler/) where bad corrosion was reported on a system where the supply of superheated steam was desuperheated by sparging it through the condensate drum.  I believe this situation is even worse than the "valve in condensate line" option because the incondensibles that might have been removed by being dissolved in the condensate were stripped out in the sparged condensate drum and returned to the reboiler.

 

When using the "valve in condensate line" option it is probably advisable to install an air vent near the top of the shell to remove what air it can.  Although it would not be as effective as it could be if mounted at the vapor-liquid interface it will help to some extent.

[fallah]

IMO, noncondensible gases collection inside the shell could be a problem in reboiler operation either with the CV at incoming steam line or with the CV at outcoming condensate line...What should be carefully be noted is that the balance line between the condensate pot and the reboiler itself should be arranged shch that its connection to reboiler side to be below pass partition rather than the top of the shell...

[xavio]

hello all,

 

I think the OP should tell us the steam supply pressure as well as downstream condensate recovery system pressure.

 

Control valve on condensate line is usually employed in very low (or even vacuum) pressure steam system.

The reason is clear, we want to maintain sufficient steam pressure (=temperature) so that:

1. sufficient delta T can be maintained for heat transfer

2. sufficient delta P can be maintained for condensate discharge to recovery system

 

The repeating cycle experienced by the OP is possibly caused by sufficient delta T or delta P or both.

Katmar also mentioned about faulty trap (undersized?), I think it is a very credible suspect as well.

 

xavio

[Sagar Nawander]

Thanks once again Xavio / fallah / katmar.

 

@latexman: you mentioned about non condensible gases entering the system. Considering the fact that it pure steam (and no inert substance present in it), what would be the approach then.

 

I am now much more clear about the pros and cons of putting Control valve on steam and condensate line.

 

Thank you.

 

-Sagar Nawander 

 

 

[latexman]

My response below assumes your steam has absolutely no non-condensable gases, which I have never seen in 35 years, but, if you insist.

 

From a control stand-point, the approach is the same, because "The accumulation of noncondensable gases has the same effect upon heat transfer as condensate flooding."

 

From a materials stand-point, use less expensive metallurgy.

 

I hope I have answered your question, which was not totally clear to me.  If not, please clarify.

[iplan]

Putting a control valve in the steam side would reduce the pressure in the exchanger  thus making the steam   farther from its condensing temperature which would reduce the heat transfer rate , as stated above.

 

In addition, in some services such as sour water stripper reboiler, it helps to keep the pressure in steam side more since in case of any tube leak, sour water would not contaminate the steam condensate but only steam condensate would ingress in the sour water side.

 

Please advise if the above understanding is incorrect.

 

Thanks in advance