Constructability Review Guidelines

Depressurisation using HysysPEC-BMS-EN-GDE-P-2548 0

Guide

Depressurisation using HysysPetrofac E&C Business Management System

Doc No.PEC-BMS-EN-GDE-P-2548

Revision0

Date01-Dec-2008

File Ref.PEC-BMS-EN-GDE-P-2548 0.doc

Revision history

RevDateDescription of Change

A17-July-2007Draft for Approval

B16-Sept-2007Draft for approval

01-Dec-2008Approved

Approval Authority: (Head of Project Services)OwnerDeveloperReviewerApprover

Lakshmi VenkateshBhushan ChonkarBhagyashree Lagwankar&

Jayesh GandhiLakshmi Venkatesh

Records of approval are retained in the BMS/Quality Department

Summary

The purpose of this document is to provide a guide for Blowdown & Depressurisation of system.

Contents

3Contents

41.0Introduction

42.0Input data

52.1Internal Volume (total inventory of the Section)

52.2Initial Liquid Volume (for Wet Sections)

62.3Weight of metal in contact with vapor and liquid

62.4Equivalent Wall Thickness

72.5Composition, Initial temperature and pressure

73.0External Fire blowdown

73.1Connections (Screen 1)

83.2Heat Flux (Screen 2)

93.3Valve Parameter (Screen 3)

103.4Options (Screen 4)

113.5Operating Conditions (Screen 5)

123.6Result of fire case

124.0ADiabatic Mode

124.1Heat Flux (Screen 2)

144.2Valve Parameter (Screen 3)

154.3Options (Screen 4)

164.4Operating Conditions (Screen 5)

174.5Result of Adiabatic case:

174.6Isochoric Blowdown

19Appendix 1 EXAMPLE

1.0 IntroductionHysys-Dynamic depressuring utility can be used to simulate the depressurisation of gas, gas-liquid filled vessel, pipelines and systems with several connected vessels or piping volumes depressuring through a single valve.

There are two cases where this utility is applicable

1.External Fire blowdown: Used to model vessel or pipe under fire conditions. This is used for determination of peak load in fire blowdown and for sizing of the blowdown valve.

2.Adiabatic mode: Used to model the blowdown of pressure vessel or piping with no external heat supplied. This is used to determine the minimum vessel temperature during non-fire case blowdown.The use of the Hysys utility is described in this guideline together with a spreadsheet Blowdown.xls that has been developed to estimate the various input data that is required for the calculation.2.0 Input dataPrior to carrying out the blowdown / depressurization calculations in Hysys, the following input data need to be estimated for each blowdown section.

a) Internal Volume (total inventory of the Section).

b) Initial Liquid Volume (for Wet Sections).

c) Weight of metal in contact with vapor and liquid.

d) Equivalent Wall Thickness.

e) Equivalent Internal Diameter.

Liquid Volume= /4*D2*H

Cross sectional area up to height H can be calculated using following formulae

Liquid Volume= L*A

The % of liquid filling for piping is calculated as the ratio of liquid - gas actual volumetric flow rate of the stream considered as obtained from the Material Balances.

E.g. if the liquid volume fraction is 20% as per the material balance and the volume of the pipework has been estimated as 10 m3 then the initial liquid volume for this section is 2 m3.

The % liquid filling for the tubes of exchangers is calculated as the average of the ratio of liquid - gas actual volumetric flow rate of the inlet and outlet streams obtained from the Material Balances.

2.3 Weight of metal in contact with vapor and liquid The Total Weight of metal is calculated as the product of the Volume of metal and the density of metal (carbon steel density is considered as 7850 kg/m3).

The Volume of metal for a vessel is calculated as the volume of the outer cylinder minus the volume of the inner cylinder. The inner cylinder volume uses the internal vessel diameter. The outer cylinder volume is calculated using a diameter obtained by adding 2 times the thickness of the wall to the internal diameter. If the mechanical datasheet for the equipment is available then the weight of the equipment indicated on the datasheet can be directly used.

The Weight of metal in contact with Liquid is calculated as the product of the Total Weight of metal and % liquid filling for Liquid. The Weight of metal in contact with Vapour is calculated as the product of the Total Weight of metal and (100% - % liquid filling).

No margin is required to be added to the weight of metal as this will result in a more conservative result for Blowdown requirements. The weight of internal can also be ignored for the same reason.

2.4 Equivalent Wall Thickness The Equivalent Wall Thickness is calculated as the sum of the product of the Wall Thicknesses and Internal Volumes of each item divided by the sum of the Internal Volumes. This thickness is used in Heat flux-detail-conduction-Metal thickness

Wall thicknesses for piping are obtained from the Piping Project Material Specifications.

Wall Thickness for vessels are taken from the mechanical datasheet.

2.5 Composition, Initial temperature and pressureThis section explains the step by step procedure for the External Fire case in Hysys

3.1 Connections (Screen 1)

This is used to specify the inlet stream, vessel volume and the initial liquid volume. More than one stream can be entered (maximum of 4 streams permitted). For each stream the vessel volume and the liquid volume needs to be entered.

If the depressuring zone is mainly a single vessel then the diameter and height of the vessel can be entered. If the depressuring zone consists of a number of equipment then it is a better option to enter the flat end vessel volume. The orientation (horizontal / vertical) should try to match the actual system as far as possible.

3.2 Heat Flux (Screen 2) Go to Heat flux tab and select Fire API 521 as operating mode.

C1=21000 Btu/hr/ft; C2=0.82 ; C3=1 ( Refer the figure given below)

This will use API 521 equation for calculation of heat load in case external fire Heat load = 21000*F*Aw0.82 Btu/hr/ft2

Heat Loss Model

There are three types of Heat Loss models available:

None: does not account for any heat loss

Simple: allows the user to either specify the heat loss directly or have it calculated from specified values

Detailed: allows the user to specify a more detailed set of heat loss parameters. There are four sections to be specified in this model : General, Conduction, Convention, Correlation constant

For fire case choose heat loss model as None.

3.3 Valve Parameter (Screen 3) Use Fisher with 100% valve opening or Masoneilan flow equation with critical flow factor equal to 1.

For initial sizing of the valve give some guess value of the Cv.

3.4 Options (Screen 4)

In the option tab we need to specify PV work contribution term

The PV work term contribution is a coefficient that allows the user to tune the energy balance equation to reflect the actual behavior. This factor should ideally be 100% to follow the thermodynamic equations, but sometime results did not match the expected behavior: transients were too rapid, therefore this factor was introduced in order to tune the models.

This is approximately Isentropic efficiency which is used in the blowdown calculations. It is a measure of the reversibility of the system. This takes into account the frictional loss within the blowdown system. Thus a higher isentropic efficiency leads to a lower minimum temperature on blowdown. In the absence of pertinent experimental data or publications it is recommended to use 0% (default) in fire case and 100% for gas filled and 70% for liquid containing systems in adiabatic case for this term.

3.5 Operating Conditions (Screen 5)

Specify depressurisation time and final pressure in operating conditions.

As per API 521 the depressuring system shall reduce the pressure of the equipment within the fire zone to 50% design pressure or 690 kpa whichever is lower within 15 minutes. As per DEP 80.45.10.10 the depressuring system shall reduce the pressure of the equipment within the fire zone to 50% design pressure within 15 minutes.

Use Option Calculate Cv

3.6 Result of fire caseImportant result in fire case depressurisation is peak flow through the valve. This will be available in Summary of performance tab.

4.0 ADiabatic Mode

The Adiabatic Case determines the Minimum Temperatures attained during the Blowdown of the Section.

Screen 1 is identical to that for the Fire Case. In addition to the data specified for the fire case, the metal mass in contact with liquid and vapour must be specified.4.1 Heat Flux (Screen 2)

For carrying out adiabatic blowdown select the operating mode as Adiabatic from heat flux model.

Use Detailed heat loss model.

Specify vapor recycle efficiency as 1% and Liquid recycle efficiency as 100%. This will ensure minimum metal temperature.

Specify minimum ambient temperature

The Conduction parameters allow the user to manipulate the conductive properties of the wall and insulation.

Specify thickness of metal and insulation in conduction tab. Use default value for rest.

The Convection view allows users to manipulate the heat transfer coefficient for inside and outside the vessel as well as between vapor and liquid material inside the vessel. These can be retained as default.

Correlation constant: feature gives users the opportunity to manipulate the coefficients used in the heat transfer correlation. This is generally not used.

4.2 Valve Parameter (Screen 3)Use the Cv obtained from fire case in valve parameter tab while keeping the other parameters same as in Fire case.

4.3 Options (Screen 4)

Use PV work contribution term 70% for wet sections and 100% gas section.

4.4 Operating Conditions (Screen 5)In operating condition tab, select the option calculate pressure. Increase the depressurizing time in a step wise manner so that the system pressure close to the flare back pressure is attained.

Now run the utility till the final value of the pressure reached. This final pressure will be backpressure of the flare header.

4.5 Result of Adiabatic case:

Minimum vessel metal temperature is the important value in the adiabatic case.

4.6 Isochoric Blowdown

When equipment is not depressurized immediately after shutdown, the equipment may cool down before blow down occurs. The start pressure associated with this temperature will be calculated by performing an isochoric flash (constant volume) from the normal conditions to the minimum attainable temperature. This minimum temperature is taken as per project guideline and generally it is equal to minimum ambient temperature. The pressure associated with this temperature is found out using isochoric flash. Procedure for this is as follows:

1. Note down the actual volumetric flow rate and mass flow rate of the stream (Stream-A) which we have used for the adiabatic blow down (stream with normal operating temperature and pressure). 2. Take one more stream (Stream-B) and specify the composition and mass flow rate of this stream same as Stream-A3. Specify temperature of Stream-B equal to minimum ambient temperature 4. Apply adjust block to Stream-B with following settingAadjusted variable = Pressure of stream-B

Target variable = Actual volume of stream-B Specifed target value = Actual volume of stream-A. 5. Final pressure is the start pressure for the adiabatic blow down. 6. Taking this stream as the start point for the adiabatic blow down, follow the same procedure mentioned in section 4.1 to 4.5.)

Appendix 1 EXAMPLE

Consider the system shown in the following sketch

Input Data :

UnitValue

Fire Case

Vessel diameterm4.5

Vessel Lengthm17

Vessel OrientationHorizontal

Head TypeEllipsoidal

Liquid Htm2.83

Vessel thicknessmm70

PressureBarg120

TemperatureDeg C27.74

System Volumem3331

Initial Liquid Volumem3179

PV Work term Contribution%0

Control Valve Opening%100

Depressuring TimeMin15

Final Pressure (50% of D.P)Barg60

Amb tempDeg C5

Mole fraction composition is taken as follows (Based on stream data from Material Balance).

Nitrogen2.78E-02

CO20.208927

H2S8.49E-02

Methane0.548892

Ethane5.11E-02

Propane2.46E-02

i-Butane3.04E-03

n-Butane8.31E-03

i-Pentane2.20E-03

n-Pentane3.70E-03

Benzene1.28E-04

Toluene3.06E-04

m-Xylene1.48E-04

p-Xylene1.48E-04

o-Xylene1.76E-04

cfc6_1*3.64E-03

cfc7_1*2.91E-03

cfc8_1*2.81E-03

F9_1*2.73E-03

F10_1*2.47E-03

F11_1*1.98E-03

F12_1*2.76E-03

F14_1*2.35E-03

F16_1*2.00E-03

F18_1*1.72E-03

F20_1*1.48E-03

F22_1*1.27E-03

F24_1*1.10E-03

F26_1*9.51E-04

F28_1*8.24E-04

F30_1*7.15E-04

F32_1*6.22E-04

F34_1*5.42E-04

F36_1*4.73E-04

F38_1*4.14E-04

F40_1*3.63E-04

F42_1*3.18E-04

F44_1*2.81E-04

H2O8.95E-04

TEGlycol1.00E-06

Blowdown study :

Initial Volume and mass calculation (Blowdown.xls spreadsheet developed for for this calculation )

Piping Volume Calculation

PipeDiaPipe thkLActual Liquid flow rateActual vapor flow rateFr Liquid FillingPipe IDPipe ODDiff VolVol.Total METAL MASSInitial Liquid volumeInitial Mass of Metal in contact with LiqInitial Mass of Metal in contact with VapV*t

inchmmmm3/hrm3/hrmmmmm3m3kgm3kgkg

PIPE-Feed89.521501.360013.20.0932032220.954.97,491.280.45699.256792.030.046309

PIPE-Gas outlet109.521000.013.20.0002542730.795.16,186.860.000.006186.860.048239

Pipe Liquid Outlet341001.40.01.00076.284.20.10.5791.140.46791.140.000.001824

TOTAL, m310.414,469.280.911490.4012978.880.0964

With 30% margin13.51.180.1253

Result

Pressure vs Min temperature

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