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

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

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.

Column System Boundary

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.

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

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

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

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

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

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

Column System Boundary

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

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

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

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

References ASME Section VIII – Division 1 ANSI/API Standard 521 5th Edition, ISO 23251, May 2008 Calculating column relief loads, Chittibabu H., Valli A., Khanna V., 2010

Questions ?

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