Hysys Depressurization Utility Calculation basis/assumptions and rule of thumb for Process Engineers

INTRODUCTION

This article summarizes the basis, assumptions, and rule of thumb for blowdown using hysys depressurization utility. The following blowdown cases are picked from traditional blowdown requirements in oil and gas plants (or similar configurations). The fire case blowdown mostly governs the peak load and determines the blowdown orifice calculation basis. However, isochoric blowdown is critical for vessel/ piping MDMT (minimum design metal temperature) adequacy check.

This article is more beneficial to the process engineers/ leads who are familiar with Hysys B/D (blowdown) utility. The below point-wise summary can be used as a rule of thumb to design Hysys blowdown/ depressurization utility for calculating peak blowdown loads and minimum temperature derived in this dynamics.

BLOWDOWN CASES

1.???FIRE CASE:

For fire case depressurization following stepwise inputs will be required to achieve the blowdown results through hysys depressurization utility.

a)??Create a new hysys stream-1 which shall be same as the inlet stream of the Vessel / System and rename it accordingly for depressurization calculation.

b)??Pressure & Temperature:

Change the pressure of the new stream-1 equal to PZI HH

PZI HH = 90% of the design pressure. (This will be also dependent on company B/D ? philosophy.)

No change in Temperature

c)??Create Depressurization Utility for this stream.

d)??Select orientation as per the datasheet of the vessel.

e)??Flat End Vessel Volume:

Input the calculated system total volume (Vessel + Piping) from the in-house spreadsheet based on the inputs received from the piping.

Input the calculated Total Initial Liquid volume (Vessel + Piping) from the in-house spreadsheet based on the Normal Liquid Level. (NLL shall be considered from the datasheet. If it’s not available, then it is to be assumed at the center between LLL & HLL). If no liquid is available, then mention 0.

f)????Diameter & Height:

If the liquid is available in the system, then the Wetted surface area within the fire circle [(20 m diameter) Cross-sectional area of 300m2 as per philosophy] shall be calculated using an in-house spreadsheet. NLL is to be considered for this calculation.

Enter the hypothetical diameter & Height of the vessel calculated in the in-house spreadsheet.

?If the system is gas-filled, then the Un-Wetted surface area within the fire circle [(20 m diameter) Cross-sectional area of 300m2 as per philosophy] shall be calculated using an in-house spreadsheet. Adjust the Diameter in Hysys Utility to match the calculated unwetted area from the spreadsheet to the heat transfer area in Hysys. Height will be adjusted automatically to match the total volume of the system.

Top & Bottom head areas to be considered as 0.

g)??Nothing to input in Correction Factors. If the system is having internals like in the column, then the same is to be discussed and agreed upon based on the company philosophy.?

h)??Sampling Interval - Consider 0.1 seconds (If the system is critical then consider 0.01 seconds)

i)?????Heat Flux ##:

For Liquid Filled / Two-phase System:

Operating Mode: FireAPI521

C1: 70.9 (No proper drainage is considered as same as feed)

C2: 0.82

C3: 1

For Only gas system:

Operating Mode: Fire

C1: 100 * Unwetted Surface Area calculated from an in-house spreadsheet in kW.

C2 to C5: 0

## For Heat Flux please use the option to calculate metal wall temperature/UTS considering local peak heat flux and stress/depressurization load considering surface average heat flux based on below API 521 Recommended Values for heat flux equation. The formulas can be inserted in the spreadsheet option of Hysys, and the result shall be exported to calculate the max. heat flux.

j)?????Heat Loss Model – None

k)???Valve Parameters:

Back Pressure – 101.3 kPa

Vapor flow equation - Universal Gas sizing

Cv – From feed for the initial run

% Opening – 100

Liquid Flow equation – No flow

l)?????PV Work Term Contribution – 0% or as per or as per company Philosophy.

m)?Operating Conditions:

Time Step Size - Consider 0.1 (If system is critical then consider 0.01 seconds)

Depressuring Time – 15 minutes (900 Seconds)

Select Calculate Cv

Initial Cv estimate - From feed for the initial run

Max Cv Step Size – 1% or as per company guideline

Pressure Tolerance – 5e-2

Maximum Number of iterations – 100

Final Pressure – 690 kpag or 50% of design pressure whichever is lower.

2.???LOW-TEMPERATURE DEPRESSURIZATION CASE (ISOCHORIC):

a)??Create a new stream-2 which shall be the same as the inlet stream of the Vessel / System and rename it accordingly for depressurization calculation.

b)??Pressure & Temperature:

Change the pressure of the new stream-2 equal to PZI HH

PZI HH = 90% of the design pressure. (This will be also dependent on company B/D philosophy)

No change in Temperature

c)??Create a new stream-3 to calculate the cooled down pressure at 5 oC. Name it accordingly.

Create a Balance to match the component mole flow of Stream-2 & mention the temperature as 5 oC.

Create Adjust & Adjust the Stream-3 pressure to match Stream-2 Actual volume flow. (Only pressure is decreased while adjusting the actual volume).

d)??Create Depressurization Utility for Stream-3

e)??Select orientation as per the datasheet of the vessel.

f)????Flat End Vessel Volume:

Input the calculated system total volume (Vessel + Piping) from the spreadsheet based on the inputs received from the piping.

g)??Diameter & Height:

Adjust the diameter to match the total surface area (Vessel + Piping) calculated from the in-house spreadsheet with the heat transfer area in HYSYS.

Top & Bottom head areas to be considered as 0

h)??Input the calculated Total Initial Liquid volume (Vessel + Piping) from the spreadsheet based on Low Liquid Level. (LLL shall be considered from the datasheet. If no liquid is available, then mention 0.

i)?????Nothing to input in Correction Factors. If the system is having internals like in the column, then the same is to be discussed.?

j)?????Sampling Interval - Consider 0.1 seconds (If system is critical then consider 0.01 seconds)

k)???Heat Flux:

Operating Mode: Select Adiabatic

l)?????Heat Loss Model – Detailed

Do not select Tick Box: Apply duty stream to the Outside wall.

General:

Recycle Efficiency: 1% for gas, 100% for light & heavy liquid

Ambient Temperature: 5 oC

Conduction:

Thickness: Enter hypothetical thickness calculated from an in-house spreadsheet.

Specific Heat Capacity: 0.473 kJ/kg-C

Density: 7850 kg/m3 for CS

Conductivity: 45 W/m-K

Insulation Thickness: 0

Convection & Correlation Constant: Default HYSYS Values.

?m)?Valve Parameters:

Back Pressure – 101.3 kPa

Vapor flow equation - Universal Gas sizing

Cv – Input the Cv Calculated from Fire case depressurization

% Opening – 100

Liquid Flow equation – No flow

n)??PV Work Term Contribution – 98%

o)??Operating Conditions:

Time Step Size - Consider 0.1 Seconds (If system is critical then consider 0.01 seconds)

Depressuring Time: 1 hour (3600 Seconds)

Select Calculate Pressure

Enter Cv calculated from Fire Case Depressurization.

3.???NORMAL DEPRESSURIZATION CASE :

a)??Create a new stream-4 which shall be the same as the inlet stream of the Vessel / System and rename it accordingly for depressurization calculation.

b)??Pressure & Temperature:

Change the pressure of the new stream equal to PZI HH (As per the report this pressure is normal operating pressure. However PZIHH is considered for a conservative result). Starting depressurization from PSV set pressure to be checked.

PZI HH = 90% of the design pressure (This will be also dependent on company B/D philosophy)

No change in Temperature

c)??Create Depressurization Utility for Stream-3

d)??Select orientation as per the datasheet of the vessel.

e)??Flat End Vessel Volume:

Input the calculated system total volume (Vessel + Piping) from the spreadsheet based on the inputs received from the piping.

f)????Diameter & Height:

Adjust the diameter to match the total surface area (Vessel + Piping) calculated from the in-house spreadsheet with the heat transfer area in HYSYS.

Top & Bottom head areas to be considered as 0

g)??Input the calculated Total Initial Liquid volume (Vessel + Piping) from the spreadsheet based on Normal Liquid Level. (NLL shall be considered from the datasheet. If no liquid is available, then mention 0.

h)??Nothing to input in Correction Factors. If the system is having internals like in the column, then the same is to be discussed and agreed upon based on the company philosophy.?

i)?????Sampling Interval - Consider 0.1 seconds (If system is critical then consider 0.01 seconds)

j)?????Heat Flux:

Operating Mode: Select Adiabatic

k)???Heat Loss Model – Detailed

Do not select Tick Box: Apply duty stream to the Outside wall.

General:

Recycle Efficiency: 1% for gas, 100% for light & heavy liquid

Ambient Temperature: 5 oC

Conduction:

Thickness: Enter hypothetical thickness calculated from the spreadsheet.

Specific Heat Capacity: 0.473 kJ/kg-C

Density: 7850 kg/m3 for CS

Conductivity: 45 W/m-K

Insulation Thickness: 0

Convection & Correlation Constant: Default HYSYS Values.

l)?????Valve Parameters:

Back Pressure – 101.3 kPa

Vapor flow equation - Universal Gas sizing

Cv – Input the Cv Calculated from Fire case depressurization

% Opening – 100

Liquid Flow equation – No flow

m)?PV Work Term Contribution – 87% for gas-filled systems and 40% and liquid filled systems or as per company Philosophy.

n)??Operating Conditions:

Time Step Size - Consider 0.1 Seconds (If system is critical then consider 0.01 seconds)

Depressuring Time: 1 hour (3600 Seconds)

Select Calculate Pressure

Enter Cv calculated from Fire Case Depressurization.

CONCLUSION AND RECOMMENDATIONS

The total vapor load for a system to be depressurized is the sum of the individual occurrences for all equipment involved at the same time. The depressurization load calculation is carried out using the depressurizing tool/utility available in HYSYS simulation software. The calculation shall consider the volume of the main equipment and associated pipework based on plot plan during the initial stage of engineering, however same depressurization calculation to be re-performed based on as-built isometrics as well.

With the above useful points, process engineers will be able to successfully calculate the peak load achieved due to rapid depressurization during a fire case based on heat flux calculation during pool and Jet fire. Jet fire applicability will be based on the HSE safety study and dependent on the placement of equipment on the plot plan. Use local peak heat flux recommended values in heat flux calculation to establish the temperature profile of the metal surface (Avg. Mean Metal Wall Temperature –Vapor) and to generate the UTS (ultimate tensile strength) profile of the material.?

Adiabatic and Isochoric depressurization is applicable to the systems which are not exposed to fire scenarios. This depressurization case may also govern the size of the depressurization orifice. Isochoric depressurization cases govern the MDMT of the system. Starting condition for this type of depressurization is the lowest operating temperature and equipment relief valve set pressure/PAHH/Design pressure/Project specific criteria. The system is considered to be cooled down to minimum ambient temperature with corresponding pressure corrected for the reduction in temperature at constant volume (Isochoric).

Process Engineers are further requested to follow their company flare and blowdown philosophy and specification and any assumptions mentioned in this article shall be replaced accordingly.