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DEPRESSURING STUDY AND

APPLICATION ON

BP-A PROJECT

PTSC MECHANICAL & CONSTRUCTION

Vung Tau, May 22rd 2014

Prepared Checked Approved

Full Name Truong Minh Hoang Nguyen Cong Hai -

Signature

Date 22 May 2014 May 2014 May 2014

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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

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INTRODUCTION

Depressuring system consists of one Blowdown valve (BDV) and one

Restriction Orifice (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.

Typical depressuring system with SDV and BDV-RO

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INTRODUCTION

Typical criteria for depressuring in Fire Case: vessel is required to

depressurize from design pressure to 6.9 barg (100 psig) or 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:

Simulate in HYSYS

Information about liquid level on the vessel

Calculate total piping and equipment inventory.

Determine basic composition and conditions

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APPLICATION ON BP-A PROJECT

System illustration:

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

PZA HH SET @ 7100 kPag

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APPLICATION ON BP-A PROJECT

System Inventory calculation: Line No 1 2 3 4 5 6

Description

FWS Gas Flowline

U/S choke valve

(HPW)

FWS Gas Flowline

D/S choke valve

(HPW)

FWS Gas Flowline U/S

choke valve (MPW)

FWS Gas Flowline

D/S choke valve

(MPW)

FWS Production

Header

FWG Export

Line

Service PG PG PG PG PG PG

DN, mm 150 150 150 150 300 600

Piping Spec 253470X 153470X 153470X 153470X 153470X 15WWWW

Pipe Schedule 160 80S 80S 80S 100 -

OD, mm 168 168 168 168 324 610

ID, mm 131.75 146.33 146.33 146.33 280.97 541.12

Thickness, mm 18.13 10.84 10.84 10.84 21.51 34.24

Liquid Fraction 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

Pipe Length, m 5 10 5 10 35 25

Quantity 2 2 16 16 1 1

Pipe Volume, m3 0.14 0.34 1.35 2.69 2.17 5.75

Volume margin 0% 0% 0% 0% 0% 0%

Pipe volume + margin, m3 0.14 0.34 1.35 2.69 2.17 5.75

Pipe metal volume, m3

0.04 0.05 0.03 0.05 0.72 1.56

Total Pipe Volume 12.43 m3

Total pipe metal volume 2.45 m3

Total Pipe Volume + margin 12.43 Metal density 7801 kg/m3

Gas Inventory 12.43 m3

Total metal weight 19102.86 kg

Liquid Inventory 0.00 m3

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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 Feed stream Specify the composition and

conditions of the fluid holdup

in the system right prior to

depressuring.

Case Name

Horizontal

for system in which piping is

dominant.

Volume of the system

the cylindrical portion only.

Vessel Dimensions

Metal mass in contact with

liquid and vapor

Hysys will use the heat content

of this metal when performing

the calculation (for Fire Case,

this is optional).

The cylindrical area

calculated from input vessel geometry.

Head surface area can be specified.

Initial liquid inventory

based on NLL or HLL

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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

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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

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PEAK STUDY

6. SPECIFYING OPERATING CONDITIONS PZAHH set point

This value is specified in Feed

Stream.

Depressuring time = 15 minutes

Select Calculate Area:

Orifice area/ valve Cv is

iterated to meet depressuring

requirements (final pressure

and time).

Time step size = 0.5s (default)

the integration step size

Final Pressure = 690 kPag

Initial area estimate

Run simulation after all data are filled

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PEAK STUDY

RESULTS

Vapor peak flow rate = 9402 kg/h

Valve area = 187.3 mm2

Vapor peak info

Composition and

conditions of peak flow

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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 with

liquid and vapor

This values should be

specified. If not, Hysys will

assumes no metal mass and

this definitely results in over

design.

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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.

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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

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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 sub-

critical 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

153470X

13470X

153470X

15WWWW