7 replies to this topic

[flarenuf]

Hi all

Struggling to find anything on this in my vast library of paperwork or indeed on the net.

Fired furnace upstream of crude unit , power failure scenario, furnace goes offline .

Overhead vapour line from furnace still feeds into column. What % if any of the residual heat in the furnace should be applied to the remaining liquids in the furnace to continue vaporisation , and , for how long.

Found one reference that says 40% of design and for 1 hr ...seems high to me , any thoughts from experienced members  please?

thanks

Flarenuf

[shan]

Is the crude column shut down simultaneously when the power fails?

[flarenuf]

yes it is but the furnace will obviously still feed vapour into the column for a period of time , raising the pressure and eventually, perhaps lifting the PSV.
I am modelling the whole unit dynamically

[shan]

Even if the column feed continues, the vapor flow is lower during shut down period than that during normal operation. Your vapor outlet is not blocked. Why does the column pressure rise to lift PSV?

[SSWBoy]

Fin fan overhead coolers trip perhaps?

Which is the equiv of blocked outlet

Edited by SSWBoy, 18 April 2013 - 01:42 PM.

[chemsac2]

Flarenuf,

 

Article "Applying relief load calculation methods" that appeared in PTQ Q2, 2008 mentions use of 50% of normal duty as refractory residual heat.

 

Attached article available from FW website has following:

"it was calculated that the heater duty would be reduced to approximately 45% of normal operating duty after 1 minute from the emergency heat off."

 

I remember reading somewhere (could not locate it in my database as well) that furnace thermal inerta need to be considered for 15 minutes.

 

Will post it once I get it.

 

Regards,

 

Sachin

 

Attached Files

•  69-Chiara_Gilardi_Foster_Wheeler_Italiana.pdf   325.81KB   77 downloads

[flarenuf]

sachin

 

many many thanks

flarenuf

[invssse]

Even if the column feed continues, the vapor flow is lower during shut down period than that during normal operation. Your vapor outlet is not blocked. Why does the column pressure rise to lift PSV?

 

I am also very confused exactly by this question.

 

Normally the crude is fed to the furnace by a motor driven pump and then to the crude column.

when General Power Failure scenario is considered, the pump stops (as does those pumps associated with the column itself). The firing to the Furnace should be stopped but there are residue heat in the furnace. 

 

At relief load calculation by Conventional Method or by dynamic simulation, when power fails, it seems that ONLY crude oil in the furnace coil need to be considered. Even 30-50% residue heat can be assumed to last for about 15 min, the oil in the coil is difficult to general a large mount of VAPOR and to lift the RV standalone. the lift of RV would be due to trips of the other pumps and coolers.

 

So the during determination of relief load for this scenario, I assume that the method would be:

• For static conventional method, use steady state simulation to determine the vapor flow-rate by 30-50% residue heat duty and take this flow as one of feeds to the column at relieving condition. then use unbalanced heat method to calculate relief load for the whole crude column.
• For dynamic simulation, consider a crude oil holdup (by a pipe, header or drum) whose volume is equivalent to the furnace coil. when power fails, the feed to furnace stops and 30-50% residue heat is assumed to be imposed to that holdup. the generated vapor is fed to the column. assume the residue heat last for 15 min. Sensitivity studies can be conducted to evaluate the impact of the involvement of this holdup to the calculated relief load.

 

I am not sure if these ideas can be correct and hope the experts can shed some lights on this. thank you.

 

Invssse