PETRONAS TECHNICAL STANDARDS

STEAM INJECTION SYSTEMS FOR SAFEGUARDING

THERMAL CRACKING UNIT

PTS 10.02.51.11

NOVEMBER 2012

© 2012 PETROLIAM NASIONAL BERHAD (PETRONAS) All rights reserved. No part of this document may be reproduced, stored in a retrieval system or transmitted in any form or by any means

(electronic, mechanical, photocopying, recording or otherwise) without the permission of the copyright owner.

This revision of PTS 10.02.51.11 - Steam Injection Systems for Safeguarding Thermal Cracking Units (November 2012, Revision 3) has been updated to incorporate PETRONAS Lessons Learnt, Best Practice and new information issued by relevant industry code and standards. All updates in the document are highlighted in italic font.

The previous version of this PTS 10.02.51.11 (November 2009) will be removed from PTS binder / e-repository from herein onwards.

The custodian of this PTS is: Name: Srinivasa Karthikeyan Tel. No: 03-27836154 Please direct any questions regarding this PTS to the above-named.

Document Approval

Revision History

Rev No. Reviewed by Approved by Date

Revision 0 Oct 1984

Revision 1 Dec 1994

Revision 2 Kazuo Tanaami Zainab Kayat Nov 2009

Revision 3 S Karthikayen Zainab Kayat Nov 2012

PTS Circular

2012 - 1

PTS No: 10.02.51.11

PTS Title: Steam Injection Systems for Safeguarding Thermal Cracking Units

PTS 10.02.51.11 November 2012

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PREFACE PETRONAS Technical Standards (PTS) publications reflect the views, at the time of publication, of PETRONAS OPUs/Divisions. They are based on the experience acquired during the involvement with the design, construction, operation and maintenance of processing units and facilities. Where appropriate they are based on, or reference is made to, national and international standards and codes of practice. The objective is to set the recommended standard for good technical practice to be applied by PETRONAS' OPUs in oil and gas production facilities, refineries, gas processing plants, chemical plants, marketing facilities or any other such facility, and thereby to achieve maximum technical and economic benefit from standardisation. The information set forth in these publications is provided to users for their consideration and decision to implement. This is of particular importance where PTS may not cover every requirement or diversity of condition at each locality. The system of PTS is expected to be sufficiently flexible to allow individual operating units to adapt the information set forth in PTS to their own environment and requirements. When Contractors or Manufacturers/Suppliers use PTS they shall be solely responsible for the quality of work and the attainment of the required design and engineering standards. In particular, for those requirements not specifically covered, it is expected of them to follow those design and engineering practices which will achieve the same level of integrity as reflected in the PTS. If in doubt, the Contractor or Manufacturer/Supplier shall, without detracting from his own responsibility, consult the owner. The right to use PTS rests with three categories of users:

1) PETRONAS and its affiliates. 2) Other parties who are authorized to use PTS subject to appropriate contractual

arrangements. 3) Contractors/subcontractors and Manufacturers/Suppliers under a contract with users

referred to under 1) and 2) which requires that tenders for projects, materials supplied or - generally - work performed on behalf of the said users comply with the relevant standards.

Subject to any particular terms and conditions as may be set forth in specific agreements with users, PETRONAS disclaims any liability of whatsoever nature for any damage (including injury or death) suffered by any company or person whomsoever as a result of or in connection with the use, application or implementation of any PTS, combination of PTS or any part thereof. The benefit of this disclaimer shall inure in all respects to PETRONAS and/or any company affiliated to PETRONAS that may issue PTS or require the use of PTS. Without prejudice to any specific terms in respect of confidentiality under relevant contractual arrangements, PTS shall not, without the prior written consent of PETRONAS, be disclosed by users to any company or person whomsoever and the PTS shall be used exclusively for the purpose they have been provided to the user. They shall be returned after use, including any copies which shall only be made by users with the express prior written consent of PETRONAS. The copyright of PTS vests in PETRONAS. Users shall arrange for PTS to be held in safe custody and PETRONAS may at any time require information satisfactory to PETRONAS in order to ascertain how users implement this requirement.

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TABLE OF CONTENTS

1.0 INTRODUCTION ..........................................................................................................................................6

1.1 SCOPE ............................................................................................................................................................ 6 1.2 DISTRIBUTION, INTENDED USE AND REGULATORY CONSIDERATIONS .......................................................... 6 1.3 DEFINITIONS .................................................................................................................................................. 6 1.4 CROSS-REFERENCES ...................................................................................................................................... 6 1.5 SUMMARY OF CHANGES ............................................................................................................................... 7 1.6 ABBREVIATION .............................................................................................................................................. 7

2.0 AUTOMATED STEAM-OUT SYSTEM .............................................................................................................8

2.1 WHY STEAM INJECTION / DECOKING REQUIRED? ......................................................................................... 8 2.2 PRINCIPLE OF OPERATION ............................................................................................................................. 8 2.3 LOSS OF FEED (REFER TO FIGURE 2.1) ................................................................................................................ 9

3.0 CONTROL NARRATIVE (REFER TO FIGURE 3.1) ........................................................................................... 11

3.1 GENERAL FEATURES OF STEAM-OUT SYSTEM ............................................................................................. 13 3.2 STEAM SUPPLY ............................................................................................................................................ 13 3.3 ISOLATION OF FEED GAS AND INTRODUCING STEAM ................................................................................. 13

4.0 UTILITY FAILURES ...................................................................................................................................... 14

4.1 INSTRUMENT AIR FAILURE (REFER TO APPENDIX 8, 9 AND 10) .......................................................................... 14 4.2 ELECTRICITY FAILURE (REFER TO APPENDIX 8, 9 AND 10) ................................................................................... 14

5.0 INSTRUMENTATION .................................................................................................................................. 15

5.1 RESET OF SYSTEM AFTER USE ...................................................................................................................... 15

6.0 REFERENCES .............................................................................................................................................. 16

APPENDICES

APPENDIX 1: AUTOMATED STEAM-OUT SYSTEM ...................................................................................................... 17 APPENDIX 2: STEAM SUPPLY ...................................................................................................................................... 18 APPENDIX 3: BLOCK AND CONTROL VALVES FOR STEAM INJECTION ........................................................................ 19 APPENDIX 4: FURNACE FEED CONTROL VALVES ........................................................................................................ 20 APPENDIX 5: RESIDUE CRACKING FURNACE WITHOUT BACK-PRESSURE CONTROL VALVE ...................................... 21 APPENDIX 6: RESIDUE OR DISTILLATE CRACKING FURNACE WITH BACK-PRESSURE CONTROL VALVE OR RESIDUE

CRACKING FURNACE WITH SOAKER AND BACK-PRESSURE CONTROL VALVE ................................................... 23 APPENDIX 7: BARRIER-STEAM INJECTION INTO RESIDUE FURNACE TRANSFER LINE OF SOAKER-TYPE THERMAL

CRACKING UNITS ................................................................................................................................................ 24 APPENDIX 8: STEAM INJECTION INTO RELIEF VALVE INLET/OUTLET CONNECTIONS ................................................ 25 APPENDIX 9: AUTOMATED STEAM INJECTION SYSTEM - RESIDUE OR DISTILLATE CRACKING FURNACE COILS WITH

BACK-PRESSURE CONTROL VALVE IN FURNACE OR SOAKER OUTLET LINEAUTOMATED STEAM INJECTION SYSTEM - RESIDUE OR DISTILLATE CRACKING FURNACE COILS WITH BACK-PRESSURE CONTROL VALVE IN FURNACE OR SOAKER OUTLET LINE ................................................................................................................... 26

APPENDIX 10:BARRIER-STEAM INJECTION IN TRANSFER LINE OF RESIDUE CRACKING FURNACES FOLLOWED BY SOAKERS, AND PURGE STEAM INJECTION INTO RELIEF VALVE CONNECTIONS ................................................. 27

APPENDIX 11: VALVES POSITIONS AND FAIL-SAFE POSITIONS AND CORRESPONDING SOLENOID POSITIONS ......... 28 APPENDIX 12: EXAMPLE P&ID OF CRACKER HEATER ................................................................................................. 29 APPENDIX 13: EXAMPLE P&ID OF STEAM SUPPLY SYSTEM ....................................................................................... 30

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1.0 INTRODUCTION

1.1 SCOPE

This PTS describes the minimum requirements for the design and engineering of the steam

injection facilities in various thermal cracking units, for safeguarding their integrity and

operation. The purpose of the system is to protect the furnace tubes from coking up during an

emergency situation, e.g. In the event of unexpected feed failure. After initiation, the steam-out

operation should therefore be quick, reliable and automated.

This PTS is a revision of the publication with the same number dated November 2009. This

PTS explains the principles of the steam-out system but it does not cover furnace control

systems except where certain control features are necessary for the operation of the steam-out

system.

These facilities consist of:

- Automated steam-out facilities for the radiant section coils of the hydrocarbon cracker furnace.

This PTS shall be used for all new designs and applied to existing installations if modifications are required. Some design details may need modification to suit the requirements of individual installations.

1.2 DISTRIBUTION, INTENDED USE AND REGULATORY CONSIDERATIONS

Unless otherwise authorized by PETRONAS, the distribution of this PTS is confined to companies forming part of PETRONAS group and to Contractors nominated by them. This PTS is intended for use in oil refineries.

If national and/or local regulations exist in which some of the requirements may be more stringent than in this PTS, the Contractor shall determine by careful scrutiny which of the requirements are the more stringent and which combination of requirements shall be acceptable as regards safety, economic and legal aspects. In all cases the Contractor shall inform the Principal of any deviation from the requirements of this document which is considered to be necessary in order to comply with national and/or local regulations. The Principal may then negotiate with the Authorities concerned with the object of obtaining agreement to follow this PTS as closely as possible.

1.3 DEFINITIONS

The Contractor is the party which carries out all or part of the design, engineering, procurement, construction, commissioning or management of a project or operation of a facility. The Principal may undertake all or part of the duties of the Contractor.

The Manufacturer/Supplier is the party which manufactures or supplies equipment and services to perform the duties specified by the Contractor.

The Principal is the party which initiates the project and ultimately pays for its design and construction. The Principal shall generally specify the technical requirements. The Principal may also include an agent or consultant authorized to act for, and on behalf of, the Principal.

The word shall indicate a requirement.

The word should indicate a recommendation.

1.4 CROSS-REFERENCES

Where cross-references to other parts of this PTS are made, the referenced section number is shown in brackets. Other documents referenced in this PTS are listed in (5).

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1.5 SUMMARY OF CHANGES

The previous edition of this PTS was dated November 2009. Other than editorial changes, the following are the major changes to the previous edition. Hydrocarbon cracker unit has been chosen. However certain information shall be retained in APPENDICES.

Old Section New Section Change

2.0 2.0 4 This section is completely revised with new process

“Automated steam out system : Principle of operation, loss of feed”

3.0 3.0 This section is completely revised with new process “Control narrative “

4.0 4.0 5 This section is completely revised with new process“Utility

failure”

5.0 5.0 6 This section is completely revised with new process

“Instrumentation “

1.6 ABBREVIATION

HP High Pressure

LP Low Pressure

BFW Boiler Feed Water

N2 Nitrogen Gas

B/V Block Valve

QC Calorific Controller

PC Pressure Controller

COT Coil Outlet Temperature

FC Flow Controller

HC Special Controller

O/L Outlet

TLV Transfer Line Valve

LEL Low Explosive Limit

TLE Transfer Line Exchanger

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2.0 AUTOMATED STEAM-OUT SYSTEM

Note: Refer to APPENDIX 11 & 12 for P&ID Illustration

2.1 WHY STEAM INJECTION / DECOKING REQUIRED?

Periodically, the Cracking heaters shall be taken out of service periodically to remove coke that builds up in the radiant tubes. The parameters which indicate whether a heater needs to be decoked are the maximum tube metal temperature, the Primary TLE outlet temperature and the pressure drop across the heater's radiant coil.

Decoking is accomplished when the heater is operated on steam only, and isolated from the effluent header. Air along with dilution steam is injected into the heater at a carefully controlled outlet temperature (approximately 850º C) to "burn off" the coke. Burning of the coke forms carbon oxides (CO and CO2). The steam-air decoking reaction is exothermic. Therefore, hot spots can develop on tube wall. Care shall be taken during decoking to limit the radiant coil tube metal temperatures to the maximum tube wall temperature (design) to avoid damaging the tubes. If excessive temperature occurs the air flow should be reduced. The decoking effluent shall be sent through the Primary Transfer Line Exchangers (TLE) and Secondary TLE and then back to the firebox of the same heater. Any coke fines in the effluent shall be burned by burner fire.

2.2 PRINCIPLE OF OPERATION

A hydrocarbon cracker unit (e.g hydrocarbon cracker) shall be decoked on a regular basis for operating flexibility. Usually decoking procedure is done before a shutdown. There are times when it has to be done during trips and emergency shutdowns. Lengthening the cycle run for decoking could minimize cost of utilities usage e.g steam; and also increase profitability due to longer on-stream time. Decoking however is required when one of the following abnormal situations occurs:

Tube wall metal temperature reaches maximum temperature as per licensor specification in the radiant coil. A pyrometer survey using a temperature gun should be done based on the operations Preventive Maintenance or requirements.

Excessive pressure drop across Transfer Line Exchangers (TLE). High pressure drops normally is an indication of blockage or fouling. Depending on design, an increase of certain pressure as specified by licensor could indicate fouling has occurred.

When overall pressure drop of TLE and radiant coils increase by differential pressure (specified by licensor) above the start of run.

When fouling of TLE causes outlet temperature to reach temperature specified by licensor (mechanical design limit).

During emergency shutdown, the hydrocarbon feed supply shall be cut off immediately to prevent severe coking in the furnace tube. It should be an automated control system that shall trigger the control valve to shut off hydrocarbon feed inlet to the cracking unit. Provision shall be made to shut off the feed inlet to the cracking unit by an automated control system.

The remaining reactant shall be purged off with steam to prevent coking at the tube. About 20-30 minutes of time shall be considered to purge off the gas in the tubes completely. Steam is triggered by fully opening the stream valve during emergency shutdown of the cracking unit. The steam shall be supplied from reliable source such as auxiliary reboiler or utility boiler which has sufficient pressure for purging.

The steam shall be supplied with a high selector controller from the two inputs below:

1. Minimum flow rate of steam supply to prevent coking

2. Steam to hydrocarbon gas ratio specified by licensor.

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The cracker trip shall be initiated by any of the initiators, as listed below. The automated steam injection the facility shall be activated for decoking/ cooling of the coils. All other effects shall be as per the original C&E (cause and effect) matrix.

Loss of Amine Treatment System

Loss of hydrocarbon feed

Loss of Dilution steam

Loss of fuel gas

Loss of boiler feedwater (BFW)

Loss of steam superheat temperature control

Radiant Coil Failure

Convection section hydrocarbon coil failure

BFW preheat coil Failure

Primary Transfer Line Exchanger failure

Secondary Transfer Line failure

Low Level in the Steam Drum BFW

Loss of Induced Draft Fan

Fire in fire box

2.3 LOSS OF FEED (Refer to Figure 2.1)

Loss of hydrocarbon feed to any of the heaters shall trip an interlock (via a low-low-low pressure switch (PSLLL-1) which cause the hydrocarbon feed valve to shut and force dilution steam valve (FV-5) to a minimum preset position, equivalent to 90% of normal flow.

Note: The emergency makeup dilution steam shall be free of condensation. The rapid expansion of any condensate in this line may cause damage in the heater.

The heater draft shall be reset to minus specific level (mm) H2O (depending on licensor) and the induced draft damper shall remain under normal control.

Following the emergency action, dilution steam flow shall be adjusted (by the operator) to maintain dilution steam temperatures at the crossovers (TIA-1) to a maximum temperature specified by licensor. As the refractory lining in the fire box cools down, further manual adjustment of the damper position (or burner register openings) and dilution steam rate shall be required in order to proceed with an orderly shutdown, a heater decoke or holding a high steam standby condition. Maintain the tube metal temperature as uniformly as possible by relighting some wall burners as required.

Makeup dilution steam availability shall be reliable, even during a total feed failure. Maintain at least specific ratio of dilution steam flow. Convection section shall be protected but it cannot stop the rapid cool down of the radiant coil and the formation of coke being spalled off the tube wall.

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MY

FV6

HIC1

Feed Gas

Steam

PT

PSLLL1

PALLL

PIA1

H

L

LL

FY2

FT1PT1

Any Other Signal

TIC

TIA1

P-21

FICA2

FT2

Pressure Compensation

HIC2FY1

FT3

FICA1

Heating

CoilsP-37

FICA

FT

FV1

FY TDC

Heating

Coils

MV1

MV2

FV2

FV3

FV4

FV5

SV1

Figure 2-1: Common Setup for Hydrocarbon Feed and Steam Injection

Below are the associated valves for safeguarding of thermal cracker

2-1: List of Valves for safeguarding Thermal Cracker

Valve Fluid Tag Type of Valve

Description Action

Dilution Steam (manual block valve)

Steam

MV2

Globe valve

Steam isolation from K.O drum to heater coils

Manually open at control room

Steam flow control (to header)

Steam

FV-5

Globe valve

Controlling steam to coil flow header

ATO/ FC

Individual Flow controller

Steam

FV-1/2/3/4

Globe valve

Controlling steam to respective coil

ATO/FC

Hydrocarbon Feed isolation valve

Hydrocarbon

MV1

Globe valve

Hydrocarbon isolation for feed gas during emergency

Manually close at control room

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3.0 CONTROL NARRATIVE (Refer to Figure 3.1)

The control scheme has two main transmitters: FT-1 and FT-2 and two controllers HIC-1 and HIC-2.

FT-1 is the measurement of the feed flow rate to determine the corresponding flow rate of the dilution steam.

FT-3 is the minimum flow rate set point for dilution steam supply to produce syn gas and protect the tube from coking.

Both controller output shall be fed to a HIGH SELECTOR function block FY-1 which then shall drive FV-5 control valve opening to adjust the dilution steam supply.

When FICA-1 detects higher flow rate than set point, the controller output shall shift to high (controller direct action), FY-2 select FICA-2 and drive FV-5 to open and release more dilution steam from reliable steam source such as boiler.

When PIA-1 detects lower flow rate than the set point, the controller output shall shift to high (controller reverse action), FY-2 select HIC-2 to drive FV-5 to maintain at minimum opening to prevent the flow rate from decreasing lower than minimum dilution steam flow rate required. This precaution measurement is to prevent coking in the tube. This addition of control shall remove the current human intervention of opening the FV-5.

The feed gas line shall be installed with a ESD (Emergency shut down) valve (Air to Open) which link to the furnace heating element. In the event that the furnace is tripped, the feed gas supply shall be cut off automatically and immediately through ESD (MOV). Human intervention using MOV (motorized valve) shall cause delay and hence more hydrocarbons shall be brought forth into the furnace even after the Cracking heater is tripped.

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Figure 3.1: Process flow chart for steam Injection to safeguard cracker tube from coking during emergency shutdown

Caution: Analyzer shall be taken out of operation to prevent steam condensing inside the tubings and damage the analyzers.

Some controllers should be switched from auto to manual so that operators can control steam injection rate more thoroughly.

Hot spots on coil should be close monitored with pyrometer (temperature gun). Air flow should be decrease or switch off burner close to the hot spot.

For each coil, adjust flowrate so that each coil shall have the same flow rate. If flow stable, it is advised to put FICA control in auto.

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3.1 GENERAL FEATURES OF STEAM-OUT SYSTEM

Two main important components for a steam-out /decoking procedure shall be provided.

1. Steam supply

2. Isolation of feed gas and introducing steam

3.2 STEAM SUPPLY

Steam shall be taken from normal medium pressure or high pressure supply system depending on the required pressure and design of the heaters. It is most important that the steam is dry to avoid surges in downstream equipments.

A knock out drum is required to remove condensate and supply steam to the coils for steam-out. A dedicated steam line connected to the feed gas line shall be available with flow control (FICA) to adjust the steam flowrate.

3.3 ISOLATION OF FEED GAS AND INTRODUCING STEAM

Table below indicates common valves which shall be open or isolated during the steam-out/decoking procedure:

3-1: List of Valves and Their Actions

Valve Description Open Close

Dilution Steam (manual block valve) MV-2

Steam from K.O drum to heater coils

X

Steam flow control (to header) FV-5

Controlling steam to coil flow header

X

Steam flow control (individual coils) FV-1/2/3/4

Controlling steam from header to individual coils.

X

Hydrocarbon feed to heater (manual block valve) MV-1

Feed of hydrocarbon to cracker

X

Hydrocarbon feed to heater (Solenoid) SV-1

Feed of hydrocarbon to cracker

X

Cracked gas (XV) Cracked gas outlet

X

Decoking effluent (XV) Outlet of decoking from cracker X

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4.0 UTILITY FAILURES

The furnace coil steam-out system shall be designed to remain operable in the event of either electricity or instrument air failures. The following description is for a residue furnace without back-pressure control valve but it also applies to any cracking furnace system with a back-pressure control valve.

4.1 INSTRUMENT AIR FAILURE (Refer to Appendix 8, 9 and 10)

The spring actions of valves FV-1/2/3/4, FV-6,SV-1, FV-5 are such that when local or total air failure occurs, the valves affected move to their fail-safe positions, bleed valve closes, FV-1/2/3/4 shall open fully since the pre-setting devices, which limit the steam flow, depend on instrument air. However, the first steam valve FV-5 shall remain shut, preventing steam from entering the system. The furnace feed control valves FV-6 open fully, so the feed pump shall continue to supply feed to the furnace.

If desired, it is possible to steam-out the coils in these circumstances. Switch HZA shall trip the feed pump and condensate pump (if the furnaces are still being fired, the loss of feed shall shut the fuel supply) and de-energize the solenoids, but the steam shall not flow because valve FV-5 remains shut.

When the pump is tripped, steam shall be admitted and controlled by opening of valve FV-5 using its hand wheel. The steam flow is controlled by the degree of opening of this valve. Valves E and F shall still be open, and the non-return valve downstream of the feed pump is relied upon to prevent steam backing up the feed line through the pump. Hence, the block valve on the pump discharge should be closed at the first opportunity.

4.2 ELECTRICITY FAILURE (Refer to Appendix 8, 9 and 10)

The system is based on the use of an uninterrupted instrument electricity supply. However, the possibility of instrument electricity failure has been allowed for in the design of the steam-out system, without increasing its complexity.

The solenoid-operated valves are normally energized and, upon instrument electricity failure, shall take up their de-energized positions. Furthermore, the feed pump shall be tripped by the absence of instrument electricity supply to a relay in the mains supply to the pump. The same holds for the condensate injection pump. The system then comes into operation automatically. Controllers HIC-1 and HIC-2 can be used to control the steam flow.

Interruptions of the main electricity supply shall not affect the steam-out system, only the feed pump and the condensate injection pump. The furnace feed FZAs shall therefore trip the furnace fuel shut-off valves.

In case of loss of feed flow, if the feed pump can be restarted within about 5-10 minutes (for a coil cracker) or 10-20 minutes (for a soaker cracker) there is no need to use the system at all. If the pump remains out of action for longer periods, the steam-out system shall be initiated.

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5.0 INSTRUMENTATION

The instruments for the steam-out system shall be grouped together with the relevant furnace instrumentation. The logic diagrams are shown in Standard Drawings S 31.008 and S 31.009.

5.1 RESET OF SYSTEM AFTER USE

After activation of the steam-out system, the solenoids shall remain de-energized until the electric circuit is reset manually, also from the panel room. This resetting system is divided into two groups, one for the steam supply valve solenoids (HIC-2), and the other for the furnace feed and back-pressure control valves (HIC2-1). First reset the steam supply solenoid and then the furnace feed and back-pressure control valves.

If, in case of instrument air failure, the steam valve FV-5 has been forced (partly) open, its handwheel should be used to ensure that, upon resetting, this valve is indeed returned to its original position: closed by the spring action of valve FV-5, and decoupled from its handwheel.

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6.0 REFERENCES

In this PTS reference is made to the following publications:

Unless specifically designated by date, the latest edition of each publication shall be used, together with any amendments/supplements/revisions thereto.

PETRONAS STANDARDS

Index to PTS publications and standard specification

PTS 00.00.05.05

STANDARD DRAWINGS

Automated steam injection system of distillate (thermal)

cracking furnaces

S 31.008

Automatic steam injection system of residue cracking

(visbreaker) furnaces

S 31.009

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Appendix 1: AUTOMATED STEAM-OUT SYSTEM

PRINCIPLE OF OPERATION

After manual initiation, a number of switches and control valves are activated in a controlled sequence by an electrical/pneumatic automatic system as follows:

- The furnace feed pump is automatically tripped and the feed control valves are closed. The low-flow trip device then cuts out fuel supply to the burners.

- A back-pressure control valve, if present in the furnace or soaker outlet, is opened. - After a delay to allow the above actions to be taken, a double block and bleed control valve

set is operated to admit dry, medium-pressure steam to the inlet of each coil.

The steam valve opening is preset but can be monitored and, if necessary, adjusted from the control room. This steam flushes the tube contents into a vessel (the cyclone, soaker or column) downstream of the furnace. The furnace effluent can be pumped out of the thermal cracker by the bottoms pump of the cyclone or column through the residue rundown system.

If a soaker is installed downstream of a residue cracking furnace, its contents can be pumped out from the soaker bottom to the column bottoms pump via a separate pump-out line.

Provisions are made to operate the system even in the event of failure of mains electricity or instrument air, albeit with some manual intervention. Most of the equipment of the system shall be used during routine shutdowns and steam-air decoking.

In case of loss of feed flow, if the feed pump can be restarted within about 5-10 minutes (for a coil cracker) or 10-20 minutes (for a soaker cracker) there is no need to use the system at all. If the pump remains out of action for longer periods, the steam-out system shall be initiated.

The same basic type of system may be used for all types of thermal cracking furnaces.

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Appendix 2: STEAM SUPPLY

Steam for injection shall be taken from the normal refinery medium-pressure main, usually at about 18 bar (ga). Alternatively, high-pressure steam may be used instead. The steam supply pressure is independent of the inlet pressure of the furnace. To avoid surges in downstream equipment, it is most important that this steam is dry upon injection.

To remove condensate, the following measures shall be taken:

- A knock-out vessel shall be installed as close as possible to the injection point(s), having a condensate steam trap with bypass and a high-level alarm. A permanent water level is to be avoided and therefore an automatic level control is not required.

- For thermal crackers with more than one cracking furnace, one common knock-out vessel shall be used for the steam supply to the furnace coils. However, each furnace shall have its own block valve and bleeder in the steam supply lines to different furnaces, because in certain circumstances leaking valves could let furnace feed into the steam system.

- The steam line between the knock-out vessel and the first control valve (A in Appendix 1) shall be as short as possible and slope towards the knock-out vessel.

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Appendix 3: BLOCK AND CONTROL VALVES FOR STEAM INJECTION

In the automated steam supply to each furnace coil valves shall be lined up as a double block and bleeder system.

A + B and A + C are the double block valves, and D is a common bleeder to the flash zone of the distillation column. Tie-in shall be below the gas oil draw-off, but above the flash zone to prevent coking of "stagnant" nozzles.

These valves shall be of the tight shut-off type of the cheapest class. They shall be so located that the lines between A and the furnace inlets are as short as possible.

There shall be no pocket between valve B or C and the corresponding furnace inlet. The pipe connections of valve A with valves B, C and D shall have one common lowest point. This point shall be at or as close as possible to the branch to valve D. At this lowest point a drain valve shall be installed and opened at least once per day to check whether or not the non-return valves or control valves are passing.

Occasionally, on opening the drain valve some vapours or condensate may be vented. If a control valve or non-return valve is passing, this shall be readily apparent from the composition of material draining from the valve.

The non-return valve at column inlet should prevent hot column vapours being vented via the bleed line.

All the lines between valve A and the furnace inlets and the bleeder inlet to the distillation column shall be steam-traced.

Bleed valve D shall be fitted with a restriction orifice to limit flow to approximately 25% of the total design steam flow through A. This shall prevent all the steam passing through the bleeder should it have failed to close.

Locked-open isolating valves shall be installed, downstream of the non-return valves in the steam lines close to the injection point to the furnace feed, in the supply lines from the steam knock-out drum and in the bleed line. These shall enable the non-return valves and the double block and bleeder control valves to be serviced.

The spring actions for the valves are shown in Appendix 1. Instrument air shall be supplied to each valve via solenoid-operated valves UZ-101 to UZ-104 for valves A to D respectively. The solenoid valves shall have electric manual resets and be electrically energized when plant operation is normal. Valves A to D shall then be held in position as follows:

Valve A - Vented by UZ-101 - Normally closed Valve B - Instr. air admitted by UZ-102 - Normally closed Valve C - Instr. air admitted by UZ-103 - Normally closed Valve D - Instr. air admitted by UZ-104 - Normally open

The purpose of HIC-001 and HIC-002 is to give a preset restriction to the opening of valves B and C. This shall prevent an excessive steam flow to the furnace tubes when the system is in use, and ensure that the refinery steam demand does not increase to a level where the operation of other units is endangered.

The injection point shall be positioned immediately downstream of the furnace feed flow control valves so that the convection bank, if present, shall also be protected, although the prime aim is protection of the more vulnerable radiant tubes.

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Appendix 4: FURNACE FEED CONTROL VALVES

The furnace feed control valves are marked by E and F. These valves are normally set by FRCs acting through solenoid valves UZ-105 and UZ-106. The solenoid valves are similar to UZ-101 to UZ-104, i.e. normally energized and with electric manual resets. The usual furnace fuel trips at low-low feed flow (FZA) are provided.

On reaching the low-low flow setting, these FZAs act on the fuel shut-off valves in the fuel supply lines to the furnace.

It is possible to override the furnace feed low-low flow signal to the fuel shut-off valves using panel-mounted decoking override switch HS-004, which is part of the fuel control system. This switch shall be used only during non-standard operating conditions, such as start-up and decoking operations.

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Appendix 5: RESIDUE CRACKING FURNACE WITHOUT BACK-PRESSURE CONTROL VALVE

General

Appendix H is a diagram of the system required for a residue cracking furnace having 2 coils. For coil arrangements with more than 2 coils similar equipment shall be installed, so that each coil shall be equipped with an individual steam-out line.

Sequence of operation

When the steam-out system is initiated by HZA-001, the sequence of events shall be fully automatic:

Immediately: - The feed shall be tripped (UZ-107). - The condensate injection pump, if present, shall be tripped

(UZ-108). - Solenoids UZ-104, UZ-105 and UZ-106 shall be de-energized. - De-energizing UZ-104 shall vent bleed valve D, which shall close. - De-energizing UZ-105 and UZ-106 shall switch full instrument air to

furnace feed control valves E and F, thus closing them. - Closing E and F shall cut off the feed to the furnace, and the low-

low flow FZAs shall trip the furnace fuel supply shut-off valves. As a consequence, the furnace inlet pressure as indicated by PRA-

001 and PRA-002 shall drop. After 10 seconds: - Solenoids UZ-101, UZ-102 and UZ-103 shall be de-energized - De-energizing UZ-101 shall switch instrument air to steam valve a,

opening it. - De-energizing UZ-102 and UZ-103 shall switch the air supply to

steam valve B and C, from full instrument air supply pressure to a lower pressure supply from control stations HIC-001 and HIC-002, causing valves B and C to open partly.

The 10 seconds delay between the two steps of the valve-switching sequence is provided to ensure that the furnace feed valves and the steam bleed valve shall be shut before the steam valves start to open.

The degree of opening of B and C is pre-set by HIC-001 and HIC-002, as indicated by their output indicators.

The preset values of HIC-001 and HIC-002 shall be determined during tests during commissioning of the furnace. HIC-001 and HIC-002 should set valves B and C such that about 1000 kg/h of steam flows into each coil during conditions where the coils are empty.

Steam shall then be admitted into the furnace coils initially at a rate governed by the liquid discharge rate from the coils (1-5 min) and subsequently by the degree of opening of B and C as indicated on the panel by FR-003 and FR-004. If necessary, the flow rates can be adjusted by means of HIC-001 and HIC-002 to ensure adequate protection of the tubes.

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The unit line-up assumes that the bypass block valves of the feed control valves FRC-001 and FRC-002 shall normally be closed.

A condensate injection pump to supply condensate into the furnace coils may be present for process reasons. If a condensate pump is not present but HP process steam is injected via an FRC, this steam flow can remain in use.

The steam flow should be maintained for about 5 minutes to completely flush out the furnace tubes. The steam flow may be reduced to conserve steam, but should be continued for about 30 minutes to let the tubes cool down gradually.

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Appendix 6: RESIDUE OR DISTILLATE CRACKING FURNACE WITH BACK-PRESSURE CONTROL VALVE OR RESIDUE CRACKING FURNACE WITH SOAKER AND BACK-PRESSURE CONTROL VALVE

General

The arrangements for this type of line-up are shown in Appendix H and are basically the same as those for a residue cracking furnace without a back-pressure control valve.

The only difference is that the back-pressure control valve shall be fully opened to lower the furnace inlet pressure before steam can be admitted into a furnace coil.

Hence, compared with a residue furnace system without a back-pressure control valve, an additional solenoid is installed in the air supply to the back-pressure control valve.

Valves in Appendix 2 are designated similar to Appendix 1.

Sequence of operation

When the steam-out system is initiated, the sequence is the same as for residue furnaces without back-pressure control valves.

Panel-mounted switch HZA-001 is manually operated, and the following automatic actions shall take place:

Immediately: - The feed pump shall be tripped (UZ-107). - The condensate injection pump shall be tripped (UZ-108). - Solenoids UZ-104, UZ-105, UZ-106 and UZ-110 shall be de-

energized. - De-energizing UZ-104 shall vent bleed valve D, which shall

close. - De-energizing UZ-105 and UZ-106 shall switch full instrument

air to furnace feed control valves E and F, thus closing them. - Closing E and F shall cut off the feed to the furnace, and the

low-flow FZAs shall trip the furnace fuel supply cut-off valves. - De-energizing UZ-110 shall vent furnace or soaker back-

pressure control valve K, opening it fully. As a consequence, the furnace inlet pressure as indicated by PRA-

001 and PRA-002 shall drop. After 10 seconds: - Solenoids UZ-101, UZ-102 and UZ-103 shall be de-energized. - De-energizing UZ-101 shall switch instrument air to steam valve

A, opening it. - De-energizing UZ-102 and UZ-103 switches the air supply to

steam valves B and C from full instrument air supply pressure to a lower pressure supply from control stations HIC-001 and HIC-002, causing valves B and C to open partly.

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Appendix 7: BARRIER-STEAM INJECTION INTO RESIDUE FURNACE TRANSFER LINE OF SOAKER-TYPE THERMAL CRACKING UNITS

PRINCIPLE OF OPERATION

For residue cracking furnaces equipped with a soaker vessel, the eventuality, however remote, of a furnace tube rupture and consequent backflow of the soaker contents through the rupture has to be taken into account. Such an event is to be countered primarily by depressurizing the soaker as quickly as possible and by pump-out of the soaker contents, but the latter cannot be done immediately.

To provide for additional safety, a barrier-steam injection system is installed in the transfer line from furnace to soaker. The purpose of this injection system is to create sufficient pressure build-up by steam to prevent backflow of the soaker contents as much as possible. The system is in permanent use through a small flow via a restriction orifice, whilst a control valve can immediately admit more steam once a tube rupture has been detected.

Furthermore, in order to prevent siphoning back of hydrocarbons into the furnace, the highest part of the transfer line should be at least 1 metre above the mid-height of the soaking vessel.

FEATURES OF THE BARRIER-STEAM INJECTION SYSTEM

The injection system should be located on the common furnace transfer line as close to the furnace as possible. The MP-steam supply system consists of a control valve, which can be controlled from the panel. The control valve is equipped with a bypass in which are installed a panel-mounted low-flow indicator/alarm and a restriction orifice of 3 mm minimum diameter, preferably in a vertical line with the flow upwards. The continuous steam purge flow shall be such that the purge velocity in the injection point is at least 0.3 m/s. In practice, the minimum steam flow shall be approximately 700 to 1000 kg/day for a DN 80 injection line.

The steam connection to the transfer line is equipped with a non-return valve and an isolation valve. The injection point should be located on the top part of the transfer line.

It is essential that a permanent steam purge is maintained via the bypass, since otherwise coke build-up may occur in the injection line near the injection point. The MP steam supply line should come from the emergency steam knock-out vessel.

When a tube rupture occurs, the system shall be used as follows; in addition to the actions for a normal tube leak (which is not covered by this DEP):

- Depressurize the soaker completely by opening the back-pressure control valve, and depressurize downstream cyclone or fractionators;

- Fully open the barrier-steam control valve.

Steam flow with full opening of the control valve shall be approximately 120 tonnes/day. This flow is based on a 4-inch OD schedule 80 furnace tube; for other tube diameters the maximum steam flow should be adjusted in accordance with the cross-sectional area of the ID of the tube.

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Appendix 8: STEAM INJECTION INTO RELIEF VALVE INLET/OUTLET CONNECTIONS

PRINCIPLE OF OPERATION

In thermal cracking units, there are a number of locations where relief valves are installed on process lines containing hot (above 350 °C) cracked residual liquid and/or cracked vapours where plugging by coke can take place. Such locations are:

- back-pressure control valve of residue cracking furnace; - back-pressure control valve of soaking vessel; - back-pressure control valve of distillate cracking furnace; - isolation valves of residue cracking furnaces (multi-string units).

Relief valves shall be fully reliable at all times. In the above-mentioned locations, coke build-up may occur in the dead-end connections of the relief valve which is installed across the control/isolation valve. To prevent such coke build-up, a steam purge system shall be installed on these dead-end connections, which shall be in continuous use.

FEATURES OF THE PURGE-STEAM SYSTEM

(see Appendix 3)

The injection system on each relief valve connection consists of:

- an injection point as close to the relief valve as possible; - an isolation valve and a non-return valve; - a restriction orifice (of 3 mm minimum diameter) preferably installed in a vertical line with the

flow upwards; - a panel-mounted low-flow indicator/alarm.

The steam line has a dedicated supply header, which may be the same as for the barrier-steam injection.

The continuous steam purge shall be such that the purge velocity in the relief valve connections is at least 0.3 m/s. In practice the minimum steam flow shall be approximately 1.5 to 2.0 tonnes/day for DN 150 inlet connections and 1.0 to 1.5 tonnes/day for DN 200 outlet connections.

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Appendix 9: AUTOMATED STEAM INJECTION SYSTEM - RESIDUE OR DISTILLATE CRACKING FURNACE COILS WITH BACK-

PRESSURE CONTROL VALVE IN FURNACE OR SOAKER OUTLET LINEAUTOMATED STEAM INJECTION SYSTEM - RESIDUE OR DISTILLATE CRACKING FURNACE COILS WITH BACK-PRESSURE CONTROL VALVE IN FURNACE OR SOAKER OUTLET LINE

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Appendix 10:BARRIER-STEAM INJECTION IN TRANSFER LINE OF RESIDUE CRACKING FURNACES FOLLOWED BY SOAKERS, AND PURGE STEAM INJECTION INTO RELIEF VALVE CONNECTIONS

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Appendix 11: VALVES POSITIONS AND FAIL-SAFE POSITIONS AND CORRESPONDING SOLENOID POSITIONS

Valve Valve normal position

Solenoid Solenoid normal position

Instrument air failure

Electricity failure

A 7 closed 8 UZY-101 9 Vented 10 close 11 open 12 B 13 closed 14 UZY-102 15 Instr. air admitted 16 open 17 open 18 C 19 closed 20 UZY-103 21 Instr. air admitted 22 open 23 open 24 D 25 open 26 UZY-104 27 Instr. air admitted 28 close 29 close 30 E 31 open 32 UZY-105 33 Instr. air admitted 34 open 35 close 36 F 37 open 38 UZY-106 39 Instr. air admitted 40 open 41 close 42 K 43 closed 44 UZY-110 45 Instr. air admitted 46 open 47 open

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Appendix 12: EXAMPLE P&ID OF CRACKER HEATER

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Appendix 13: EXAMPLE P&ID OF STEAM SUPPLY SYSTEM