3 replies to this topic

[dattatray kolte]

As per DEP 80.45.10.11 following stpes are involved for column simulation for relief valve. I have some doubts as follows

The procedure for steady state simulation of a fractionating column for loss of overhead

cooling (I guess this is applicable to all scenarios, please correct if i amwrong) shall be as follows:

1. Calibrate the process model of the column using normal operating conditions.- What mean by calibrate? i understand it as taking out column from main simulation model and matching all results.

 

2. If the column has a feed-bottoms exchanger, model it by interchanging heat between

the two streams using a specified transfer coefficient and exchanger area (i.e. set “UA”

or “U” and “A” in the simple exchanger model, do not set Q or one of the outlet

temperatures).- OK

 

3. Eliminate design specifications and convergence control from the model as follows.

a) Set all column heat inputs, feed and product flows at their design case values.- What mean by design case values? many times column in the main simulation model is not at its design condition.If column have some spec control on feed, product or heat input should i have to disable it? If yes then how column will converge without spec. It can give me any solution. If no then which spec should not be changed

Set the reflux rate or ratio at the design case value -Same question here. Where will i get design case reflux ratio. Should i remove spec (if there) and put re-flux ratio manually?

c) Set the reflux temperature at the design case value- Same question here. Where will i get design case value

d) Set the side draw flows (if any) at the design case value(s)- Same question here. Where will i get design case value

e) For partial condensers, maintain constant accumulator vent rate. Set the vent rate

at the design case value.Same question here. Where will i get design case value

f) Do not specify or vary parameters to obtain any tray temperatures, flows or

compositions in the distillation model. This means turning off all design

specifications, external/internal controllers, or other composition constraints

around the column- Does this mean removing all specs. But if all specs are removed, then how i can tie column. It will give me any solution.

g) Allow condenser duty and overhead product rate to vary during convergence.-OK

4. Run the column at the normal pressure.-OK

5. Ensure that the same results are obtained as with the original model, within the

calculated accuracy or convergence tolerance.-OK

6. Rerun the simulation with the overhead pressure equal to the relieving pressure (PRV

set pressure plus allowable overpressure).-OK

 

Apologies if i have repeated same question.

Edited by dattatray kolte, 12 May 2015 - 05:36 AM.

[Bobby Strain]

DEP belongs to Shell, and is certainly no recognized authority on fractionator relief.

 

Bobby

[dattatray kolte]

Bobby,

 

Thanks for reply, however approach should be common. Can you confirm the approach and general steps mentioned.

 

Datta.

[Anup87]

Dattatray,

We can't assure a common methodology for all columns like you have referred in DEP. You can follow different approaches unless it is technically correct. This is general guidelines,which may varry case to case.

I will try to answer points highlighted in red in sequentially:-
1- Loss of overhead cooling is not necessary in all the scenarios.Eg-Loss of Feed. This doesn't mean overhead cooling will also go off.
2- Yes you're correct.
3-Ok
4- To specify values @ normal simulation model values.
5- Change or choose the specs depending on scenario requirement. This can't be decided without studying the system.

If you hAve any specific system pls share , where we can discuss the things and share the experience.

Thanks!!

Enjoy Work,
Anup.