7 replies to this topic

[shady othman]

Hi

 

I work in Gas plant (NGL), lately we faced problem when De-butanizer column reboiler started leaking suddenly, by investigation found that column level indication was failed ( column dryness) and due to no HC (bottom product ) passing through the reboiler, the head gasket got damaged and hot oil start leaking.

I am preparing a proposal to install new control logic, by adding new Flow Transmitter on I/L line of the reboiler and signal controls XV valve on hot oil inlet to the reboiler, as if in case of dryness, flow transmitter will read zero and send signal to XV to close and prevent hot oil to enter the reboiler.

I searched for the same logic on the references but I did not find any similarity, is my proposal correct or wrong?

 

Thanks for help and advice.

[fallah]

shady,

 

To prevent column dryness in debutanizer system, in addition to column level indication (a level transmitter with remote indication and alarm on high level) there is normally a dedicated level transmitter will trigger a SDV (or XV) at the bottom product line to be closed on low low level inside the column. Then if column level indication would fail, there is another protection layer to keep the level in debutanizer column...

Edited by fallah, 05 December 2014 - 01:07 AM.

[katmar]

You want your control system to give an alarm or somehow react to the situation before the inlet flow to the reboiler goes to zero.  By then the damage could be done.  Even if you were to put in a more sophisticated flow measurement device in the inlet line I suspect that the reboiler steam controller would counteract the effect that a drop in the boot level would have on the inlet flow rate.  As the level drops in the boot the heat transfer in the reboiler would drop off and the reboiler steam contoller would respond by opening the steam control valve further.  This would vaporize a larger fraction of the circulating liquid and you might find that the liquid flow into the reboiler does not decrease because of the stronger thermosiphon effect.

 

I would rather go with fallah's proposal and put in a second level transmitter and put in some sort of voting system to the alarm - if either transmitter sees a low level then activate the alarm.  If you want to use a different measured variable (i.e. not put another instrument on the boot level) then maybe a pressure transmitter on the steam side of the reboiler would pick up the fact that the heat transfer rate had decreased.

[Bobby Strain]

If the system had been properly designed, you wouldn't have this problem. So you should take note of this when you might have an opportunity to design a new system.

 

Bobby

[shady othman]

First of all I would thank all of you for your help, i want also to explain one point the the re boiler is working on a hot oil system so the proposal of introducing a pressure gauge will not effect on.my system while I think introducing a temperature transmitter on the hot oil return will be more effective.
Also this system has been already built from about 40 years ago that's why I should take precaution from the instrumentation failure and put more layers of protection.
I think also if I introduce only level alarm low taking from different point than the level troll will be effective also that will give alarm on low level so I can take my precautions

Thanks

[katmar]

I should have read more carefully and then I would have picked up that you were using hot oil on the hot side of the reboiler. But you have understood my intention and I agree that measuring the oil temperature differential could be a way of detecting a level problem.  Maybe the level transmitters just need regular checking and maintenance.

[Babu Prasad]

Generally the reboiler take off point from any distillation columns will be from top side of baffle plate and hot liquid return from reboiler will join at bottom side of baffle plate. So reboiler tube will be submerged always with liquid if sufficient level maintained at bottom of the column.

During normal operation, this liquid seal will ensure always level in reboiler so that tube will not be exposed to dry.

While startup time or total reflux mode, column will not receive adequate heavy liquid to take heat transfer from reboiler, in that condition there will be no HC flow to reboiler but  column bottom will read level as bottom level control valve hold the level. Generally panel operator assume that sufficient level in the column and maintains same hotoil flow to reboiler which leads the head gasket to fail.

As you mentioned, hot oil return temperature is correct indication for hot oil requirement and HC Vapour temperature from reboiler is also another good indication of excess hotoil flow.  Drop in column bottom level indication and valve opening is another good indication of feed composition/feed flow changes during that time it is better to place field operator to monitor HC liquid level in shell side of reboiler until column bottom level control gains the level.

Providing HC flow sensor to trip hot oil control valve is not sure how far it will be helpful!

Edited by Babu Prasad, 10 December 2014 - 07:10 PM.

[shady othman]

Thanks for your valuable reply and I totally agree with you