6 replies to this topic

[colt16]

Dear all,

We have a Condensate Splitter Unit in our plant.
There is a first tower that separates the Naphtha- out.

In the next tower, function is to fractionate Kero, Diesel and Residue.
At the flash zone, the liquid from the tray above the flash zone exits the tower and passes through a flowmeter to allow us to measure Overflash.

However the flow is never stable. And it fluctuates quite wildly. We try as much as possible to maintain the overflash flow at 3-5% of tower charge as per convention. But the swing is wilder than this. Currently the Diesel colour is still not affected even though this sometimes goes to zero (suspect the reason is because Condensate does not discolour the Diesel as much as the Residue from Crude).

But in order to have stability, it is better to have this stable.

Could it be flowmeter is not appropriate? Or we are misoperating?

Edited by colt16, 13 December 2016 - 09:50 AM.

[colt16]

Anyone with experience on the matter above?

[gegio1960]

my experience was in topping units...a little bit different.

the instrument is critical: a very low pressure drop instrument shall be selected and the relevant piping shall be installed in accordance with a particular design.

in very critical cases I've seen a dedicated pump installed.

good luck

Edited by gegio1960, 14 December 2016 - 12:36 PM.

[pavanayi]

Colt16,

 

My first point of investigation would be to trend the related column parameters and check side by side what other parameter is varying in your column? 

 

Examples of parameters to trend will be Column pressure,  Feed flow, Feed Temperature, Overhead temperature, Reflux Flow, Reflux temperature, Reboiler outlet temperature (or delta T), reboiling media (steam flow or fuel gas flow, if it is a furnace).

 

I would expect another parameter to vary in tandem with the overflash. My first instinct is that an instrument control loop related to the column is not tuned properly. A quick way to find out would be to put the control valves in manual mode and ensure the flow of material in and out of the column is kept constant for an amount of time and observe the overflash.

 

Pavanayi

[colt16]

Colt16,

 

My first point of investigation would be to trend the related column parameters and check side by side what other parameter is varying in your column? 

 

Examples of parameters to trend will be Column pressure,  Feed flow, Feed Temperature, Overhead temperature, Reflux Flow, Reflux temperature, Reboiler outlet temperature (or delta T), reboiling media (steam flow or fuel gas flow, if it is a furnace).

 

I would expect another parameter to vary in tandem with the overflash. My first instinct is that an instrument control loop related to the column is not tuned properly. A quick way to find out would be to put the control valves in manual mode and ensure the flow of material in and out of the column is kept constant for an amount of time and observe the overflash.

 

Pavanayi

 

Hi pavanayi, 

 

Many thanks for your comment and suggestion. I can say we are able to see wild fluctuations even when the other parameters are stable but still I can check. 

 

Actually this is my first plant (and first experience) dealing with an overflash. Initially my thought was perhaps there might be some inherent flaw. In your experience is it possible to get a steady overflash that is able to be 3-5% of the feed?

[colt16]

Dear all, 

 

The flowmeter type that is used is an annubar (pitot tube) type. 

 

Perhaps this is a possible reason for constant fluctuation?

[pavanayi]

If all the inlet-outlet flows are stable, the vapour/liquid flow within the column should be stable and as a result, the overflash should be stable too. When you say that the other parameters are stable, I guess you have taken the precaution to rule out any other malfunctioning instrument.

 

I have not used/worked with an annubar before, so I'm not able to comment on its dependability. Nevertheless, if all the other parameters are stable (including the reboiling media), then your conclusion may be valid.

 

Other members with exposure to annubar may comment on their experience.