1. Calculating Required Relief Rates for Cooling Failure in Column Systems
Fadekemi Osideinde, P.E.

2. Outline Motivation for work Cooling Failure Cause(s) Effect
First pass calculation method
Shortcomings
S.S. simulation approach
Advantages
Dynamic simulation
Conclusion

3. Motivation
Division 1 provides requirements applicable to the design of pressure vessels operating at either internal or external pressures exceeding 15 psig1. ASME Section VIII UG-125 GENERAL (a) All pressure vessels within the Scope of this Division, irrespective of size or pressure, shall be provided with pressure relief devices in accordance with the requirements of UG-125 through UG- 137.

4. Column System Boundary

5. Cooling Failure API 521 4.3.7 Loss of fans
Fans on air-cooled heat exchangers or cooling towers occasionally become inoperative because of a loss of power or a mechanical breakdown2 .
Loss of cooling provided by the reflux to the column may lead to the MAWP of a vessel being exceeded. This leads to an overpressure scenario and the need for adequate relief capacity.

6. Cooling Failure Example: Power Failure Loss of Cooling
The fans of the air coolers stop – condensing duty reduces significantly
Steam continues to flow to the reboiler
The feed pumps, reflux pumps, bottoms pumps all stop
The vapor trapped in the system needs to be relieved

7. First Pass Calculation Method
Assumptions
Feed, product, reflux compositions are unaltered during relief3
Credit may be taken for air coolers without the fan motor running for cooling effect due to convection and radiation2

8. First Pass Calculation Method
R = (QR – QC*)/ LHV
Where:
R – Required relief rate
QR – Reboiler duty from manufacturer’s specification sheet
QC* – Adjusted condenser duty (Scenario/Client specific)
LHV – Latent Heat of Vaporization at relieving conditions

9. Shortcomings
Unrealistic required relieving rates leading to inadequate orifice area for existing installations
Replacing PSVs and piping modifications
Pressure drop concerns
Projects requiring new installations
Unrealistic flare header relief loads for global scenarios

10. S.S. Simulation Approach for Cooling Failure
Simulate the column in three sections (steady state)
Column
Reboiler system
Overhead system

11. S.S. Simulation Approach for Cooling Failure
Takes the following into consideration:
The higher temperature in the bottom of the column at relief conditions
The reboiler hot side may not have an increased temperature during at relief conditions
The lower reboiler duty at relief conditions

12. Column System Boundary

13. S.S. Simulation Approach for Cooling Failure
R = (QRs – QC*)/ LHV Where: R – Required relief rate QRs – Reboiler duty, at relief conditions, obtained from the simulation QC* – Adjusted condenser duty (Scenario/Client specific) LHV – Latent Heat of Vaporization at relieving conditions

14. Advantages of the S.S. Simulation Approach
More realistic/accurate required relief rates
Saves the Client Expenses on Revalidations
New Projects
More accurate flare header analysis
Future revalidations can refer to the model

15. Dynamic Simulation Approach
Takes the transient behavior during upset conditions into consideration for a more rigorous analysis
The change in composition and flows of feed reflux and product streams
The change in top tray enthalpy
The change in column pressure and temperature before, during and after relief

16. Conclusion
The difference between rates obtained from first pass calculations, steady state simulation models and dynamic simulation models (Is it worth it?)
The importance of Clients having access to models that can properly predict relief conditions and relief loads
This knowledge and approach can be applied to various overpressure scenarios and Unit Operations

17. References ASME Section VIII – Division 1
ANSI/API Standard 521 5th Edition, ISO , May 2008
Calculating column relief loads, Chittibabu H., Valli A., Khanna V., 2010

18. Questions ?

19. Reboiler Pinch Approach
QRp = QR * LMTD QRp – Pinched reboiler duty QR – Reboiler duty from manufacturer’s specification sheet LMTD – (TH,max – TC,R) / (TH,min – TC,N) TC,N - Normal cold side boiling point TC,R - Relief condition cold side boiling point TH,max - Saturation temperature at supply pressure TH,min - Saturation temperature at letdown/condensate pressure