8 replies to this topic

[rajprocess]

Dear forum members,

 

Attached is the snap shot of the depropanizer column reboiler system. Generally for reboilers, we desuperheat the steam with desuperheater(boiler feed water injection). but where as in this system, the steam is desuperheated with the condensate, which is  coming from the same reboiler. for better understanding of the scheme see the attachment. i am wondering why this is so, do anyone came across similar sort of scheme.

kindly explain when and why we go for such sort of control scheme. please share pros,cons and any literature on such sort of system.

 

Thanks in advance.

Attached Files

•  pfd.docx   81.75KB   134 downloads

[Bobby Strain]

The designer felt that using superheated steam would create a dry zone at the reboiler steam inlet of reduced coefficient. So, she passed the steam through the condensate collection pot for desuperheating. In all my years as a designer, I never used this scheme. If I were designing this system, I would simply route a bit of condensate to the steam inlet via the pump discharge. Your arrangement requires that the reboiler elevation, and hence the column sump elevation, is sufficient to provide head for condensate to flow from the exchanger to the higher pressure pot. But you probably already knew all this.

 

Bobby

[rajprocess]

Dear Bobby,

 

to get rid of the dry zone the designer has opted this scheme, if so, is this a better scheme for for complete desuperheating the steam. how important it is to avoid dry zone. in our plant we have total two such systems (lp steam consumers,  vertical thermosiphon) and these two reboilers are the frequent leakers, and where as the other lp steam consuming exchangers are in good shape . we are thinking to change the scheme (steam direct injection to reboiler bypassing the condensate pot with BFW injection for desperheating). to get it done what are things we need to look at.

 

thanks

[katmar]

I have also never come across this arrangement.  I wonder if sparging the steam through the condensate doesn't work a bit like a deaerator? Any small amount of air would be trapped and the air levels could build up in the reboiler/condensate pot circuit, leading to the corrosion and leaks that you have experienced.  It might be worth testing the oxygen levels in the condensate coming directly from the reboiler (not the condensate in the pot).

[PingPong]

If have seen this way of desuperheating in old designs, but not in new ones.

Advantage is that it requires no special control system for adding the right amount of desuperheating water.

 

.... and these two reboilers are the frequent leakers

What exactly is leaking? The exchanger, or the condensate pot, or what?

[rajprocess]

currently the two reboiler tubes are leaking, and we are managing the operation by venting traces of hydrocarbon along with steam from the condensate pot by providing a vent silencor. 

 

we had a metallurgical analysis and report mentions that the failure may be due to O2 pitting and CO2 corrosion. we conducted a vibrational analysis on one of the reboiler as well and it mentions the failure may be due to flow induced vibration due to high steam side pressure. actually the high steam side pressure is because of the hydrocarbon leak and we are managing the operation at high chest pressure and continuous venting from the condensate pot.

 

thanks Katmar, i appreciate your suggestion for testing the O2 levels at reboiler outlet. we may or maynot find the o2 in condensate as it may not be continous skip of o2 from the deaerator of utilities section. However i will check for the possibilities of analysing. even, if at some instance there is a skip of O2 and CO2 from the utilities, the O2 and CO2 will be completely utilized in the form of O2 pitting and CO2 corrosion. where as in other types of reboilers with different control system O2 and CO2 may go along with condensate in a dissolved state, as a result chances of failure will be far less.

 

we are planning to change he control scheme as mentioned earlier. kindly let me know, what are the things we need to look at and consider for this change.

 

thanks 

Edited by rajprocess, 11 October 2014 - 03:13 PM.

[PingPong]

we had a metallurgical analysis and report mentions that the failure may be due to O2 pitting and CO2 corrosion.

may be due to .....

That is not really a definitive diagnosis.

Are you absolutely sure that the main corrosion is on the steam side, not on the process side?

we conducted a vibrational analysis on one of the reboiler as well and it mentions the failure may be due to flow induced vibration due to high steam side pressure. actually the high steam side pressure is because of the hydrocarbon leak

may be due to flow induced vibration .....

Again not really a definitive diagnosis.

Tube vibration is more likely the result of low pressure, and thus high velocity, not high pressure. Moreover: as you say, the high pressure is the result of the failure, and can therefor not be the cause.

 

It seems to me that your team does not really know what the cause of the leakage is. Everybody is just guessing.

 

Anyway, if you want to install a new desuperheater I suggest you first read following:

http://www.spiraxsar...eater-types.asp

http://www.parcol.com/docs/1-XI_gb.pdf

http://www.documenta...ts/ag365652.pdf

[xavio]

rajprocess,

 

We have such configuration in our plant, it was designed by European licensor in mid 2000's, not too old really.

The sole purpose is to desuperheat the incoming steam, hence maintaining suitable temp. difference between heat source and heat-sensitive process fluid.

The reboiler requires only MPS, but since only HPS is available near to the plant, we use HPS directly.

 

The main disadvantage of this arrangement is like Bobby's explanation above.

However, some people love to use condensate pot instead of simple trap, so the pot is already there for you to use as desuperheater.

 

I think hydrocarbon contamination cannot be attributed to the desuperheater, since your steam is clean and is not a process steam.

You should find the leaking tubes and plug them.

The pot is not located at highest point, you cannot vent properly from it.

Proper venting must be done on reboiler shell side, e.g., from top tubesheet vent.

 

Good luck!

 

xavio

[Bobby Strain]

Actually, to vent inerts one should provide a vent near the bottom of the vertical exchanger. This is where they concentrate.

 

Bobby