Depressuring Study and Application on BP-A Project

PTSC MECHANICAL & CONSTRUCTION

DEPRESSURING STUDY AND APPLICATION ON BP-A PROJECT Vung Tau, May 22rd 2014

Full Name

Prepared

Checked

Approved

Truong Minh Hoang

Nguyen Cong Hai

-

22 – May – 2014

May – 2014

May – 2014

Signature Date

1/27

CONTENT

INTRODUCTION

PEAK STUDY

LOW TEMP STUDY

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INTRODUCTION  Depressuring is a process of releasing pressure from an isolated system, it can be manually or automatically operated.  Depressuring is considered for high pressure (> 1700 kPag as recommended in API 521 – Section 5.20.1) systems or systems with large volatile liquid inventory (e.g LPG) usually when other pressure safety devices such as PSV can not satisfy requirements of releasing pressure in a given period of time in case of emergency: 

Emergency Depressuring with Fire (Fire Case): External Fire in Process Area.



Emergency Depressuring without Fire (Adiabatic Case): process malfunction (valve failure…).



System depressuring/drainage for maintenance after long shut-down (Isochoric Case).

 In wellhead platform, typically the following systems are considered for depressuring: 

Production/test manifolds



Fuel gas header



Pig launcher system



Gas Booster Compressor… 3/27

INTRODUCTION  Depressuring system consists of one Blowdown valve (BDV) and one Restriction Orifice (RO).

Typical depressuring system with SDV and BDV-RO

 System description: In case of emergency, shutdown valves (SDVs) close to isolate the system from other process area, BDV opens to release pressure from system to flare header, RO is used downstream of BDV to restrict the flow and decrease pressure of the relieving stream. 4/27

INTRODUCTION  Typical criteria for depressuring in Fire Case: vessel is required to depressurize from design pressure to 6.9 barg (100 psig) or ½ design pressure in 15 minutes.  Depressuring calculation objectives: 

PEAK STUDY: To determine peak flow rate for Vent/Flare network line sizing and RO bore sizing based on the depressuring time requirement - Fire Case is used.



LOW TEMP STUDY: To determine minimum design temperature for proper material selection - Adiabatic or Isochoric Case is used.

 Depressuring simulation can be performed by using different dedicated softwares. Among them, Hysys is the most common tool. However, if client require or a higher accuracy needed, softwares such as LNGDYN (Technip France) or BLOWDOWN (Imperial College London) are preferred for LOW TEMP STUDY.

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INTRODUCTION  Dynamic depressuring utility in HYSYS is used to simulate the depressurization of gas, gas-liquid filled vessels and systems with several connected vessels or piping volumes depressuring through a single valve.  Steps to calculate depressuring:

Information about liquid level on the vessel

Calculate total piping and equipment inventory.

Determine basic composition and conditions

Simulate in HYSYS

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APPLICATION ON BP-A PROJECT  System illustration:

PZA

HH SET @ 7100 kPag

 Pipe lengths are estimated as follows:  FWS Gas Flowline U/S choke valve (HPW/MPW): 5m  FWS Gas Flowline D/S choke valve (HPW/MPW): 10m  FWS Production Header: 35m  FWG Export Line: 25 m  Pig Launcher: 0.5 m3 7/27

APPLICATION ON BP-A PROJECT  System Inventory calculation: Line No

1

2

3

4

5

6

FWS Gas Flowline FWS Gas Flowline FWS Gas Flowline FWS Gas Flowline U/S FWS Production FWG Export U/S choke valve D/S choke valve D/S choke valve choke valve (MPW) Header Line (HPW) (HPW) (MPW)

Description Service DN, mm Piping Spec Pipe Schedule OD, mm ID, mm Thickness, mm Liquid Fraction Pipe Length, m Quantity

PG 150 253470X 160 168 131.75 18.13 0.0000 5 2

PG 150 153470X 80S 168 146.33 10.84 0.0000 10 2

PG 150 153470X 80S 168 146.33 10.84 0.0000 5 16

PG 150 153470X 80S 168 146.33 10.84 0.0000 10 16

PG 300 153470X 100 324 280.97 21.51 0.0000 35 1

PG 600 15WWWW 610 541.12 34.24 0.0000 25 1

Pipe Volume, m3 Volume margin Pipe volume + margin, m

0.14 0% 0.14

0.34 0% 0.34

1.35 0% 1.35

2.69 0% 2.69

2.17 0% 2.17

5.75 0% 5.75

Pipe metal volume, m3

0.04

0.05

0.03

0.05

0.72

1.56

Total Pipe Volume

12.43

Total Pipe Volume + margin

12.43

Gas Inventory

12.43

3

Liquid Inventory

0.00

m3 m3

Total pipe metal volume

2.45 m3

Metal density

7801 kg/m

Total metal weight

3

19102.86 kg

3

m

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APPLICATION ON BP-A PROJECT  Feed Composition and Conditions:  Composition and conditions of stream holdup in the Production Header are used as Feed Stream.

 Initial Condition:  Fire Case: design pressure or PZAHH  Adiabatic Case: design pressure or PZAHH  Isochoric Case: relevant pressure with T (minimum ambient temperature)

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DEPRESSURING USING HYSYS Tool + Utility or Ctrl + U

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PEAK STUDY

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PEAK STUDY 1. SPECIFYING CONNECTIONS Case Name

Feed stream

Specify the composition and conditions of the fluid holdup in the system right prior to depressuring. “Horizontal” for system in which piping is dominant. Volume of the system the cylindrical portion only.

Vessel Dimensions Initial liquid inventory based on NLL or HLL Metal mass in contact with liquid and vapor The cylindrical area calculated from input vessel geometry. Head surface area can be specified.

Hysys will use the heat content of this metal when performing the calculation (for Fire Case, this is optional). 12/27

PEAK STUDY 2. CONFIGURING STRIP CHART 

Strip chart is used to store all the data of the depressuring calculation. Sampling Interval = 0.5s The length of time between data samples taken from the strip chart. Smaller interval is preferred if more details needed or if the relieving flow rate is significantly larger than the volume or if vessel depressurizes in a short amount of time. Tick to active the variable

Add a new variables 13/27

PEAK STUDY 3. SPECIFYING HEAT FLUX Select: Fire API 521 models heat from a fire using an equation based on API 521: Q = 21000.F.A0.82 (Btu/hr) C1, C2 Constants from API 521 Environmental factor = 1 C3 depends on insulation method of system. 1 for bare vessel is used as conservative value. Heat loss = None None heat loss model is used for worst case.

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PEAK STUDY 4. SPECIFYING VALVE PARAMETER Back pressure = 0 kPag For initial value: Pb = 0 kPag Pb has significant effect on Subsonic valve model only. General vapor flow equation

should be used for systems that are depressurized through a fixed orifice. Cd = 0.85 for vapor relief

Estimated RO area No liquid relief

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PEAK STUDY 5. SPECIFYING OPTIONS 0% for conservative results PV Work Term Contribution is used to approximate the isentropic efficiency. 100% indicates isentropic processes while 0% means isenthalpic processes. Hysys recommends common values range from 87% to 98%.

A higher isentropic efficiency results in a lower final temperature A lower isentropic efficiency results in a higher peak flow rate 16/27

PEAK STUDY 6. SPECIFYING OPERATING CONDITIONS

PZAHH set point This value is specified in Feed Stream. Time step size = 0.5s (default) the integration step size

Depressuring time = 15 minutes Select Calculate Area: Orifice area/ valve Cv is iterated to meet depressuring requirements (final pressure and time). Initial area estimate Final Pressure = 690 kPag

Run simulation after all data are filled 17/27

PEAK STUDY RESULTS Valve area = 187.3 mm2

Vapor peak info

Composition and conditions of peak flow Vapor peak flow rate = 9402 kg/h 18/27

LOW TEMP STUDY 

LOW TEMP STUDY is based on Adiabatic or Isochoric Case, whichever results in lower temp.



Most Options are the same with those of PEAK STUDY, except the followings.

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LOW TEMP STUDY 1. SPECIFYING CONNECTIONS Case Name

Initial liquid inventory based on LLL

Metal mass in contact liquid and vapor This values should specified. If not, Hysys assumes no metal mass this definitely results in design.

with be will and over

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LOW TEMP STUDY 3. SPECIFYING HEAT FLUX Select: Adiabatic No external heat is applied

Heat loss = Detailed - Conduction The conduction parameters allow the user to manipulate the conductive properties of the wall and insulation.

It is recommended to use detailed heat loss model and specify the thickness of the metal wall. If not, Hysys assumes no metal mass and this definitely results in over design. 21/27

LOW TEMP STUDY 5. SPECIFYING OPTIONS 100% for conservative results A higher isentropic efficiency results in a lower final temperature.

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LOW TEMP STUDY 6. SPECIFYING OPERATING CONDITIONS

PZAHH set point For Adiabatic Case: design pressure or PZAHH. For Isochoric Case: relevant pressure with T (minimum ambient temperature). Depressuring time = 30 minutes Trial depressuring time to meet final pressure of 0 kPag. Select Calculate Pressure: Final pressure is calculated from specified orifice area and depressuring time. Valve Area = 187.3 mm2

Valve area is obtained from PEAK STUDY. Run simulation after all data are filled 23/27

LOW TEMP STUDY RESULTS

ADIABATIC CASE

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LOW TEMP STUDY RESULTS

ISOCHORIC CASE

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DEPRESSURING STUDY 

References:  PTSCMC-000-WI-F-0030 - WI Report.docx - “Low Temp Study” - Nguyen Cong Hai  “Depressurisation - A practical guide”, HYPROTECH.  “Aspen HYSYS 7.2 - Unit operations guide”, 14.8 Dynamic Depressuring.  API RP 521, 5th Edition, 2008.  API RP 520 Part I, 7th Edition, 2000.  BN-MLS-21-PTSC-308012_CN - Depressurisation_Full

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APPENDIX A

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APPENDIX B

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APPENDIX B-1  Heat flux: specify heat model  Fire API 521: models heat from a fire using an equation based on API 521.  Adiabatic: no external heat is applied, this is used for LOW TEMP STUDY.  Fire Mode: models heat from a fire using a general equation.  Fire - Stefan Boltzmann: models heat from a fire using a radiation equation.  Use Spreadsheet: allows the user to customize the equation used.

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APPENDIX C

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APPENDIX D

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APPENDIX E



1. Supersonic: is used when no detailed information available on the valve and supercritical flow (generally Pupstream > 2Pdownstream)



2. Subsonic: is used for subcritical flow (usually Pupstream < 2Pdownstream).



3. Manesolian: Taken from the Masoneilan catalogue, this equation can be used for general depressuring valves to flare. Often the Cv or a valve is known from vendor data.



4. No Flow: indicates there is now flow through the valve.

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APPENDIX F

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APPENDIX G 13470X 153470X 153470X

15WWWW

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