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공학석사 학위논문
A Study on Design Procedure for Efficient Depressurization
System in High Pressure Hydrocarbon Process
고압 탄화수소 프로세스의 효율적인 감압 시스템 설계 방법론에 관한 연구
2014년 8월
서울대학교 대학원
화학생물공학부
Qiang Wang
Abstract
A Study on Design Procedure for Efficient Depressurization System
in High Pressure Hydrocarbon Process
Qiang Wang School of Chemical & Biological Engineering The Graduate School Seoul National University
High pressure hydrocarbon processing facilities accompanied by exothermic reactions such as hydrocracker plant usually handle flammable, combustible, or toxic fluids which pose severe risks with respect to fire, explosions, equipment ruptures and toxic gas releases. Loss potential from such incidents can be prevented and/or limited efficiently by providing inventory isolation and removal systems which are commonly referred to as ESD (emergency shutdown) system and emergency depressurization system respectively.
Many papers established a variety of models to predict gas flow rate through emergency depressurization system and temperature experienced by equipment and piping during depressurization, but they were all under the premise of that emergency depressurization
system had already been installed in the plant. Besides, the applications of emergency depressurization system in industries are mainly empirical, dependent on licensor’s design which includes design conditions for emergency depressurization system, not based on exact operating conditions, nonstandard and just considering consequences of equipment ruptures due to fire or other over-temperature scenarios, even many literatures provide lots of guidelines. To overcome these shortcomings, an integrated design procedure was proposed covering judgment of emergency depressurization system installation in this study.
First of all, carry out the qualitative risk assessment-HAZOP Study to find out hazards related to fire, exothermic runaway reactions or other over-temperature scenarios. In this study, the HAZOP Study was performed by using computer program TechmasNavi® series which can reduce time and efforts required, and build database for HAZOP report. Through HAZOP Study, the necessity of emergency depressurization system installation can be discussed qualitatively.
Secondly, by calculating equipment rupture stress and comparing with selected rupture criterion to judge whether equipment ruptures or not, thereby the necessity of emergency depressurization system installation can be confirmed quantitatively.
Thirdly, if the emergency depressurization system installation is confirmed, estimate and decide depressurization rate which will be used for emergency depressurization system design including
temperature variation which will support equipment and pipe material selection. Herein, dynamic simulator HYSYS v7.3 with rigorous dynamic model was used to reduce the efforts required.
This study proposed an integrated design procedure for efficient depressurization system in high pressure hydrocarbon processing facilities. Following this procedure, not only the emergency depressurization system can be designed efficiently, but also the plant safety can be improved. Otherwise, this study is expected to be more beneficial research from the point of view of CBA (cost- benefit analysis).
Keywords: Depressurization, equipment rupture, HAZOP Study Student Number: 2012-23973
TABLE OF CONTENTS
Chapter1 Introduction ... 1
1.1 Background ... 1
1.2 Research Scope and Objective ... 2
Chapter2 Background Theories ... 3
2.1 Emergency Depressurization System ... 3
2.1.1 General Descriptions ... 3
2.1.2 Objectives ... 5
2.1.3 Applications ... 6
2.2 Emergency Shutdown System ... 7
2.3 HAZOP (Hazard and Operability)Study ... 8
2.3.1 Traditional HAZOP Study ... 8
2.3.2 TechmasNavi® Series ... 12
2.4 Equipment Rupture Criterion ... 16
2.4.1 Stresses in Equipment ... 16
2.4.2 Rupture Stress Criterion ... 18
2.5 Modeling of Emergency Depressurization System ... 20
2.5.1 System Identification ... 21
2.5.2 Fluid Volume Calculation ... 22
2.5.3 Dynamic Simulation ... 24
2.5.4 Results Interpretation ... 25
Chapter3 Development of the Procedure ... 26
3.2.1 General Process Description ... 28
3.2.2 Detailed Hydrocracking Process ... 31
3.3 HAZOP Study ... 35
3.4 Equipment Rupture Confirmation ... 41
3.5 Modeling of Emergency Depressurization System ... 45
3.5.1 System Identification ... 45
3.5.2 Fluid Volume Calculation ... 46
3.5.3 Dynamic Simulation ... 48
3.5.3.1 Exothermic Runaway Reactions Scenario .. 49
3.5.3.2 Fire Scenario ... 53
3.5.3.3 Low Temperature Evaluation ... 57
3.5.4 Results ... 60
Chapter4 Conclusion ... 61
4.1 Summary ... 61
4.2 Contribution ... 63
References ... 64
Appendix ... 66
LIST OF TABLES
Table2.1 Emergency shutdown levels ... 7
Table2.2 Guide words used for the HAZOP procedure ... 9
Table2.3 Valid guide word and process parameter combinations for process lines ... 10
Table2.4 Valid guide word and process parameter combinations for process vessels ... 10
Table3.1 Identified scenarios from node 1 ... 38
Table3.2 Identified scenarios from node 2 ... 39
Table3.3 Design data of reactor C-0001 ... 41
Table3.4 Max UTS at given temperature (ASME data) ... 43
Table3.5 Fluid volume calculation results ... 47
LIST OF FIGURES
Figure1.1 Typical layers of protection ... 1
Figure2.1 General configuration of emergency depressurization system ... 4
Figure2.2 Procedure of HAZOP Study ... 11
Figure2.3 Execution screen of PlantNavi ... 13
Figure2.4 Execution screen of OpeNavi ... 14
Figure2.5 Execution screen of HazopNavi ... 15
Figure2.6 Procedure for emergency depressurization system study ... 20
Figure3.1 Overall study flow ... 27
Figure3.2 Overview of hydrocracker plant ... 29
Figure3.3 Detailed schematic flow diagram ... 32
Figure3.4 Detailed schematic flow diagram of node 1 ... 35
Figure3.5 Detailed schematic flow diagram of node 2 ... 36
Figure3.6 Gas flow rate variation with time ... 50
Figure3.7 Vapor out temperature variation with time ... 51
Figure3.8 Vessel temperature variation with time ... 52
Figure3.9 Gas flow rate variation with time ... 54
Figure3.10 Vapor out temperature variation with time ... 55
Figure3.11 Vessel temperature variation with time ... 56
Figure3.12 Vapor out temperature variation with time ... 58
Figure3.13 Vessel temperature variation with time ... 59
Chapter1. Introduction
1.1 Background
High pressure hydrocarbon processing facilities accompanied by exothermic reactions such as hydrocracker plant usually handle flammable, combustible, or toxic fluids which pose severe risks with respect to fire, explosions, equipment ruptures and toxic gas releases. Loss potential from such incidents can be prevented and/or limited efficiently by providing inventory isolation and removal systems which are commonly referred to as ESD (emergency shutdown) system and emergency depressurization system respectively [1]. These systems are so called layers of protection which is shown in Figure1.1 [2].
Figure1.1 Typical layers of protection
1.2 Research Scope and Objective
Additionally, operational depressurization system is typically used to facilitate process controls and other operations such as maintenance, etc. Since the capacity of emergency depressurization system is normally bigger than that of operational depressurization system, this study will focus on emergency depressurization system.
Many papers established a variety of models to predict gas flow rate through emergency depressurization system and temperature experienced by equipment and piping during depressurization [3, 4], but they were all under the premise of that emergency depressurization system had already been installed in the plant.
Actually, the applications of emergency depressurization system in industries are mainly empirical, dependent on licensor’s design which includes design conditions for emergency depressurization system, not based on exact operating conditions, nonstandard and just considering consequences of equipment ruptures due to fire or other over-temperature scenarios, even many literatures provide lots of guidelines. To overcome these shortcomings, an integrated design procedure will be proposed covering judgment of emergency depressurization system installation through case study on hydrocracker plant in this study. The procedure developed in this study can be used as a general guideline for designing emergency depressurization system efficiently in high pressure hydrocarbon processing facilities.
Chapter2. Background Theories
2.1 Emergency Depressurization System
2.1.1 General Descriptions
According to American Petroleum Institute (API) STANDARD 521,
“Pressure-relieving and Depressuring Systems”, emergency depressurization system is “protective arrangement of valves and piping intended to provide for rapid reduction of pressure in equipment by releasing vapours” [5]. As one kind of physical protections, emergency depressurization system is usually applied to reduce equipment internal pressure and stress in the metal walls by removing inventories immediately and rapidly whenever an emergency condition occurs. About depressurization speed, API 521 also gives typical suggestions like that reducing the equipment pressure from initial condition to 7barg or 50% of the design pressure within approximately 15 minutes [5].
Emergency depressurization system generally consists of piping connected to process vessel, depressurization valve with associated actuators, instruments, and restriction orifices, tail pipes connected to flare header. The general configuration is shown in Figure 2.1.
Figure2.1 General configuration of emergency depressurization system
2.1.2 Objectives
A pressure relief valve or pressure safety valve cannot depressurize equipment or a system. It can only limit the pressure rise to the set point during upset or emergency conditions. When an emergency condition such as fire occurs, the metal wall temperature of equipment can be raised to exceed its design temperature, and the strength of construction material of equipment can be weakened leading to equipment rupture, even below the operating pressure of the equipment. To prevent equipment rupture, the pressure must be reduced through emergency depressurization system in order to lower the stress in the metal walls.
The objectives of emergency depressurization system are (1) to reduce the risk of catastrophic equipment rupture during fire exposure, (2) to reduce the risk of equipment rupture during an internal exothermic runaway reaction or other over-temperature scenarios, (3) to reduce the amount of material released if there is a leak or rupture, and (4) to rapidly shift the facility into a safe state, if other emergency situations like loss of instrument air or power occur.
2.1.3 Applications
The emergency depressurization system is generally used for (1) a vessel operated above 690kPa [1], (2) the vessel contains volatile liquids with vapor pressure above atmospheric [1], (3) high pressure process units that process flammable, combustible and/or toxic fluids, (4) a fire condition may weaken the vessel material to cause a significant loss, (5) processes in which an exothermic reaction can lead to loss of primary containment in a short time, e.g.
hydrocracker plant, (6) pipeline systems in the event of an emergency, and (7) process sections that have the potential for developing a significant flammable or very toxic cloud if there is a leak.
Actually, the applications mentioned above are almost based on considerations of consequences of equipment ruptures due to fire or other over-temperature scenarios, and they are mainly empirical, vague and indeterminate operating conditions, e.g. API 521 states that the emergency depressurization system should be considered for equipment operating at a gauge pressure of 1700kPa or higher which is different from 690kPa mentioned above, and nonstandard.
2.2 Emergency Shutdown System
The emergency depressurization system is normally a part of the emergency shutdown system and these two systems usually work together to minimize the possibility that the pressure in the equipment or system tends to exceed allowable limits and limit the containment loss by isolating inventories. Emergency depressurization system is normally initiated following completion of emergency shutdown system. A time delay after the initiation of emergency shutdown system allows emergency shutdown valves to close before the emergency depressurization valves are opened.
Usually there are several shutdown levels which are shown in Table 2.1.
Table2.1 Emergency shutdown levels
Level Cause Initiation Effect
ESD 0 Situations that pose major hazards to the integrity of the entire installation
Operator Total facility shutdown ESD 1 Hazards that threaten
individual plant areas
Operator or Automation
Total plant shutdown ESD 2 Hazards that threaten
individual trains
Operator or Automation
Train shutdown ESD 3 Hazards that threaten
individual units
Operator or Automation
Local unit shutdown ESD 4 Hazards that threaten
individual equipment
Operator or Automation
Equipment shutdown
2.3 HAZOP (Hazard and Operability) Study
2.3.1 Traditional HAZOP Study
As a qualitative risk assessment, hazard and operability study is a structured and systematic examination of a planned or existing process or operation in order to identify and evaluate problems that may represent risks to personal or equipment, or prevent efficient operation. It is normally carried out through brainstorming by a multi-disciplinary team composed of process, piping, instrument, electrical and mechanical engineers, as well as safety specialists and management representatives. Particularly, the study leader and recorder must participate in the study.
HAZOP Study is started from assumptions of hazards and operation problems caused by deviations from design or operating intensions.
Deviations are created by combinations of guide words and process parameters. The meanings of guide words, and available combinations of guide words and process parameters are shown in Table 2.2, Table 2.3, and Table2.4 [6, 7]. The next step is trying to identify all potential causes leading to deviations which are related to equipment failure, instrument failure, or human error, and evaluate existing safeguards used to prevent or mitigate consequences from the deviations. The last step is to assess possible consequences and make recommendations for process or system operation, or further studies.
Table2.2 Guide words used for the HAZOP procedure [6]
Guide Words Meaning Comments
NO, NOT, NONE
The complete negation of the intention
No part of the design intention is achieved, but nothing else happens.
MORE, HIGHER, GREATER
Quantitative increase
Applies to quantities such as flow rate and temperature and to activities such as heating and reaction.
LESS, LOWER Quantitative decrease
Applies to quantities such as flow rate and temperature and to activities such as heating and reaction.
AS WELL AS Qualitative increase
All the design and operating intentions are achieved along with some additional activity, such as contamination of process streams.
PART OF Qualitative decrease
Only some of the design intentions are achieved, some are not.
REVERSE The logical opposite of
Most applicable to activities such as flow or chemical reaction. Also applicable to substances, for example, poison instead of antidote.
OTHER THAN Complete substitution
No part of the original intention is achieved-the original intention is replaced by something else.
SOONER THAN Too early or in
the wrong order Applies to process steps or actions.
LATER THAN Too late or in
the wrong order Applies to process steps or actions.
WHERE ELSE In additional locations
Applies to process locations, or locations in operating procedures.
Table2.3 Valid guide word and process parameter combinations for process lines [6]
Process parameters
No, Not, None
More, Higher, Greater
Less, Lower
As well
as Part
of Reverse Other than
Sooner, Faster
Later, Slower
Where else
Flow o o o o o o o o o
Temperature o o o o
Pressure o o o o o
Concentration o o o o o o o o
pH o o o o
Viscosity o o o o
State o o o
Table2.4 Valid guide word and process parameter combinations for process vessels [6]
Process parameters
No, Not, None
More, Higher, Greater
Less, Lower
As well
as Part
of Reverse Other than
Sooner, Faster
Later, Slower
Where else
Level o o o o o o o o o
Temperature o o o o
Pressure o o o o o
Concentration o o o o o o o o
pH o o o o
Viscosity o o o o
Agitation o o o o o o o
Volume o o o o o o o o
Reaction o o o o o o
State o o o o
Sample o o o o o o
After the P&IDs (Piping and Instrumentation Diagrams) or PFDs (Process Flow Diagrams) get prepared, the HAZOP Study can be performed following procedure shown in Figure 2.2.
Figure2.2 Procedure of HAZOP Study
Divide P&IDs or PFDs into study nodes
Select one study node
Select one process parameter
Check hazards or operating problems
by using guide words one by one
Need more information
Log results YES NO
Uncertain
2.3.2 TechmasNavi
®Series
The traditional HAZOP Study is commonly used as a method of hazard identification analysis in industries. But it requires much time and many efforts to carry out, and HAZOP reports from study are very difficult to be updated or reused. To solve these shortcomings, the computer program TechmasNavi® series commercially developed by Techmas Corp. in Japan will be used in this study. It is an intelligent computer program having database- functional. All the information and data relevant to HAZOP Study can be built and stored in the database. This program consists of PlantNavi, OpeNavi, and HazopNavi and each function is described as below [7, 8].
a).PlantNavi is used for facility management of process plant via PFDs or P&IDs. It can search all the information of pipes and equipment relevant to plant operation and safety issues. The execution screen of PlantNavi is shown in Figure 2.3.
Figure2.3 Execution screen of PlantNavi
b).OpeNavi is used to facilitate plant operation design and execution.
It can produce the procedures for equipment operating automatically and support operator training. The execution screen of OpeNavi is shown in Figure 2.4.
Figure2.4 Execution screen of OpeNavi
c).HazopNavi is used to perform HAZOP Study for process plant using PFDs or P&IDs. The execution screen of HazopNavi is shown in Figure 2.5.
Figure2.5 Execution screen of HazopNavi
2.4 Equipment Rupture Criterion
2.4.1 Stresses in Equipment
For equipment exposed to internal pressure, the radial, hoop, and longitudinal stresses in the equipment can be estimated based on Lame’s equations shown below [9]:
( ) / ( )
2 2
2 2 2
r 2
pd pd D D d
4r (2.1)
( ) / ( )
2 2
2 2 2
h 2
pd pd D D d
4r (2.2)
/ ( )
2 2 2
l pd D d
(2.3)
Where:
D is the outside diameter of the equipment d is the inside diameter of the equipment p is the internal pressure
r is the radial coordinate t is the wall thickness
l is the longitudinal stress due to end cap forces h is the hoop stress
r is the radial stress
For equipment exposed to external forces, the additional longitudinal stress can be calculated from equation 2.4 [9].
a a
p
F M A W
(2.4)
Where:
a is the longitudinal stress due to the external forces p is the cross section area of the equipment
W is the section modulus which can be calculated from equation 2.5
4 4
D d W 32 D
(2.5)
Fa is the axial force
M is the maximum bending moment
Consequently, the maximum axial stress, ax for equipment exposed to internal pressure and external forces is a sum of equation 2.3 and 2.4 as below:
ax l a
(2.6)
2.4.2 Rupture Stress Criterion
The equivalent stress for equipment exposed to pressure and axial stress can be estimated from von Mises stress equation shown below [9]:
( 2 2 2 )0 5.
e r h ax r h r ax h ax
(2.7) Where:
ax is the axial stress e is the equivalent stress h is the hoop stress r is the radial stress
By rearranging equations 2.1, 2.2, 2.3, 2.6, and 2.7 mentioned above, the equivalent stress e can be expressed as below:
( )
2
2 2 2
e 2 2 a
3 pD D d
(2.8)
The ultimate tensile strength for one material is a limit state of tensile stress which leads to material tensile failure in the manner of ductile failure or brittle failure. The equipment rupture can be defined when equivalent stress e equals to ATS (allowable tensile strength) shown below [9]:
e ATS UTS ks ky
(2.9)
Where:
UTS is ultimate tensile strength for material of equipment ks is material safety factor
ky is pipe safety factor
2.5 Modeling of Emergency Depressurization System
To estimate and decide depressurization rate which will be used for emergency depressurization system design including depressurization valve sizing, restriction orifice sizing, pipe sizing, and flare capacity confirmation, and check the metal wall temperature variation which will support equipment and pipe material selection, an appropriate engineering procedure is proposed for emergency depressurization system study shown in Figure 2.6.
Figure2.6 Procedure for emergency depressurization system study System Identification
Fluid Volume Calculation
Dynamic Simulation
Results Interpretation
2.5.1 System Identification
The system protected by emergency depressurization system to be identified. Single equipment or multi-equipment can be protected by emergency depressurization system. If multi-equipment is protected, the connections among equipment to be checked for open path or not. For the large system, sectionalization may be taken into account.
Sectionalization is a philosophy applied to split system into a number of smaller sections. Each section shall be isolated by remotely-actuated valves such as emergency shutdown (ESD) valves; control valves may also be used. And each section shall have its own emergency depressurization systems, so that each section can be depressurized sequentially, thereby reducing the required capacity of emergency depressurization systems.
2.5.2 Fluid Volume Calculation
Fluid volume in the system protected by emergency depressurization system shall be calculated. This includes liquid inventory and vapor volume in the equipment located in the fire area, as well as liquid inventory and vapor volume contained in the equipment outside the fire area which will keep open to the equipment in the fire area. Fluid volume includes fluid volume in vessels, heat exchangers, reactor, pipes, etc. Fluid volume in vessels can be calculated from equations 2.10 and 2.11 [10].
For vertical vessel:
3 2
D D
V H
24 4
(2.10)
For horizontal vessel:
. .
2 2
3 H H D 1
V 2 0 1309D 2 1 5 L SIN
D D 4 2
(2.11) Where:
V is liquid inventory in the vessel D is vessel internal diameter His normal liquid level Lis vessel tangent length
is angle subtended by liquid level
For reactor or fractionator, the normal liquid volume at the bottom, the volume of liquid held up on trays and draw-off tray capacity shall be taken into account. For standard shell and tube heat exchangers, assuming that one-third of the shell volume is occupied by the tube bundle. For condensers and heat exchangers in vaporizing service, 80% of the volume shall be assumed to be vapor. For heater in vaporizing service, 80% of the tube volume shall be assumed to be vapor [5].
2.5.3 Dynamic Simulation
Since process variables such as pressure, temperature and flow rate vary with time during depressurization, dynamic simulation is imperative to analyze these process variables’ dynamic behaviors.
A variety of software has been developed for dynamic simulation.
After fluid volume calculation, dynamic simulation can be preceded by running computer programs such as HYSYS, ProII, or other professional simulators. In this study, HYSYS v7.3 will be utilized with PR-EOS.
HYSYS uses lumped models for all of the unit operations. Lumped models ignore thermal or component concentration gradients in three dimensions (x, y, and z) in the system, so all physical properties are considered to be equal in space, and only consider the time gradients in the system. A “lumped” system can be characterized mathematically by using a set of ordinary differential equations (ODEs). Even there are several methods to solve ODEs;
HYSYS adopts implicit fixed step size Euler method [11, 12].
2.5.4 Results Interpretation
Gas flow rate through emergency depressurization system varying with time and temperature experienced by equipment and piping during depressurization varying with time can be observed. The maximum gas flow rate can be captured which is used for depressurization valve sizing, restriction orifice sizing, pipe sizing and flare capacity confirmation, and minimum temperature experienced by equipment and piping can be checked which is used for material selection of equipment and piping.
Chapter3. Development of the Procedure
3.1 Overall Study Flow
The objective of this study is to propose a design procedure for efficient depressurization system. It consists of qualitative discussion of necessity of emergency depressurization system installation, quantitative confirmation of necessity of emergency depressurization system installation and modeling of emergency depressurization system. The overall study flow is shown in Figure 3.1 and this procedure will be applied for hydrocracker plant as a case study.
Figure3.1 Overall study flow
HAZOP Study (TechmasNavi)
Hazard Identification -Fire
-Exothermic runaway reaction, etc.
Discussion of necessity of emergency depressurization system installation
Confirmation of emergency depressurization system
installation
Modeling of emergency
depressurization system (HYSYS) -Depressurization rate calculation -Metal wall temperature check
Equipment rupture stress
calculation
Does equipment
rupture?
YES
Emergency depressurization
system is not necessary NO
Compare with rupture criterion (Allowable Tensile Strength,
ATS) von Mises stress formulation
3.2 Hydrocracker Plant
3.2.1 General Process Description
Hydrocracker plant usually processes gas oils including atmospheric gas oil from atmospheric crude oil distillation tower, vacuum gas oil from vacuum distillation tower, delayed coking gas oil from delayed coking units and cycle oil from fluid catalytic cracker to produce a broad range of products such as gasoline, kerosene, and diesel. Since these gas oils are high molecular weight hydrocarbons, and have a high boiling range, the process typically takes place in the presence of hydrogen and a catalyst at elevated temperatures (260-425℃) and pressures (35- 200bar)[13,14,15,16]. The overview of hydrocracker plant is shown in Figure 3.2 as below:
Figure3.2 Overview of hydrocracker plant
Two main chemical reactions occur in the reaction section, cracking and hydrogenation. After catalytic cracking of high molecular weight hydrocarbons into lower molecular weight unsaturated hydrocarbons, hydrogenation process will saturate these newly formed hydrocarbons with hydrogen. Besides, any impurities such as sulfur and nitrogen present in the gas oil can be, to a large extent, eliminated by formations of hydrogen sulfide (H2S) and ammonia (NH3) which are subsequently removed.
A series of separations occur in the separation section which
Reaction section
Separation section
Fractionation section Gas oil
Makeup hydrogen
Recycle gas
Recycled gas oil
Naphtha Kerosene
Diesel
recycle gas compressors and sent to reaction section for use in the reactors, and the liquid phase is sent to fractionation section.
In the fractionation section, the desired products such as naphtha, kerosene, and diesel are produced through a series of towers like product stripper, fractionator, stabilizer, splitter, etc. The unconverted oil from the bottom of fractionator is recycled to reaction section to increase product yield.
3.2.2 Detailed Hydrocracking Process
In hydrocracker plant, the fractionation section usually operates under low pressures even below 10bar, so it is obviously not necessary to install emergency depressurization system for protection of this section. Considering exothermic reactions occurring in the reactor and severe operating conditions (high pressure, high temperature) in the reaction section and separation section, the necessity of emergency depressurization system installation for these two sections needs to be analyzed. The detailed schematic flow diagram corresponding to reaction section and separation section considered in this study is shown in Figure 3.3 as below [17]:
32
Figure3.3 Detailed schematic flow diagram
The gas oil feed pumped by feed pump is mixed with high pressure hydrogen-rich stream and then flows through a heat exchanger where it is heated by reactor effluent. The mixed reactor feed is then heated further by a fuel-fired heater before it enters the top of reactor and flows downward through the beds of catalyst. The operating conditions in the reactor depend on the specific licensed design, gas oil feed properties, the desired products, the catalyst being used, etc. In this study, the operating pressure and operating temperature in the reactor are assumed as 167barg and 427℃, respectively.
After the reactor effluent is cooled by the gas oil feed, it is cooled further by a condenser and then enters into hot high-pressure separator for separation. The hydrogen-rich gas from the hot high-pressure separator flows through a heat exchanger where it is cooled by compressed hydrogen-rich gas first and then is cooled by an air-fin cooler before it enters into cold high-pressure separator for further separation. The hydrogen-rich gas from the cold high-pressure separator is routed through a H2S absorber where it is contacted with an aqueous amine solution to absorb and remove hydrogen sulfide in the hydrogen-rich gas. The hydrogen- rich gas from the top of H2S absorber enters into a knockout drum where potential liquid is eliminated and then is compressed by recycle gas compressor and recycled for use in reactor systems.
desired products such as naphtha, kerosene, and diesel after a series of separations by low-pressure separators.
3.3 HAZOP Study
HAZOP Study was performed by using HazopNavi included in the computer program TechmasNavi® series. The detailed schematic flow diagram corresponding to reaction section and separation section considered in this study was divided into 2 nodes. Node 1 is started from gas oil feed stream to reactor as shown in Figure 3.4.
Node 2 is started from reactor effluent stream to recycle gas compressor as shown in Figure 3.5. Results of HAZOP Study are included in the Appendix.
Figure3.4 Detailed schematic flow diagram of node 1
Figure3.5 Detailed schematic flow diagram of node 2
In this study, two scenarios associated with exothermic runaway reactions and fire were identified from node 1. Scenario 1 is excess burning in feed heater F-0001 causes runaway reactions in reactor C-0001; Scenario 2 is fire causes reactor C-0001 rupture.
The results of HAZOP Study corresponding to these two scenarios are shown in Table 3.1.
And three scenarios associated with exothermic runaway reactions and fire were identified from node 2. Scenario 1 is compressor K- 0001 trip causes runaway reactions in reactor C-0001; Scenario 2 is control valve FCV-0016 failure close causes runaway reactions in reactor C-0001; Scenario 3 is fire causes separator D-0002/3/4 and absorber C-0002 rupture. The results of HAZOP Study corresponding to these three scenarios are shown in Table 3.2.
38
Table3.1 Identified scenarios from node 1 Equip Tag Failure Mode Cause Consequence Countermeasure Recommendation F-0001excess burning
Excess burning will cause reactor feed temperature to rise highly There is a potential of runaway reactions inside the reactor, the reactor temperature will rise extremely to cause reactor rupture.
1. TE-0005 2. TE-0001 3. TE-0003 4.The interlock ZC- 0001 will close the feed control valve FCV-0001. 5.C-0001 By temperature high alarm , operator can recover 1.Connect interlock system to fuel gas supply system for tripping heater 2.Install emergency quench system 3.Install emergency depressurization system C-0001ruptureFire case
The strength of C-0001 construction material will be weakened and vessel rupture will occur Install emergency depressurization system
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Table3.2 Identified scenarios from node 2 ip Failure Mode Cause Consequence Countermeasure Recommendation failure stopTripping of compressor K-0001 Recycle gas pressure is lost. There is a potential of runaway reactions inside the reactor. Reactor temperature will rise extremely to cause reactor rupture
1. FIT-0007 2. TE-0003 3. TE-0001 4. TE-0005 5.Interlock ZC-0001 will close liquid feed control valve FCV-0001. 6.Make up H2 will compensate loss of pressure to some extent 7.C-0001 by high temperature alarm, operator can cover
1.Connect interlock system to fuel gas supply system for tripping heater 2.Install emergency quench system for reactor 3.Install emergency depressurization system failure close
Controller malfunctions and closes FCV-0016 Loss of recycle gas flow to reactor. There is a potential of runaway reactions inside the reactor. Reactor temperature will rise extremely to cause reactor rupture 1.TE-0003 2.TE-0001 3.TE-0005 4.The interlock ZC-0001 will close liquid feed control valve FCV-0001 5.C-0001 by high temperature alarm, operator can cover
1. Connect interlock system to fuel gas supply system for tripping heater 2. Install emergency quench system for reactor 3.Install emergency depressurization system 02 03 04
rupture Fire case
The strength of equipment construction material will be weakened and equipment rupture will occur Install emergency depressurization system
Through the HAZOP Study, the hazards related to fire and exothermic runaway reactions were identified, and the emergency depressurization system installation is necessary and recommended.
3.4 Equipment Rupture Confirmation
Through the HAZOP Study, the reactor C-0001 has great potential of rupture caused by fire and exothermic runaway reactions. To confirm whether reactor C-0001 does rupture or not under these hazards, the equivalent stress posed on reactor C-0001 was estimated firstly according to equation 2.8 stated in chapter 2.
( )
2
2 2 2
e 2 2 a
3 pD
D d
(2.8)
The data pertaining to equivalent stress estimation are shown in Table 3.3. And the estimated equivalent stress posed on reactor C- 0001 is 516 Mpa.
Table3.3 Design data of reactor C-0001
Reactor C-0001
Design pressure, p, barg 183
Outside diameter, D, m 5.265
Inside diameter, d, m 4.725
Longitudinal stress due to
external force, a, Mpa (assumed)
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Secondly, the allowable tensile strength (ATS) of material SA 336 G F22V used for reactor C-0001 construction was calculated according to equation 2.9 stated in chapter2.
s y
ATSUTS k k (2.9)
Where:
Material safety factor ks is 1.0 (assumed) Pipe safety factor ky is 1.0 (assumed)
The maximum ultimate tensile strength (UTS) of material SA 336 G F22V used for reactor C-0001 construction at given temperature is shown in Table 3.4. Since the reactor temperature under exothermic runaway reactions and fire is obviously greater than 500℃, the maximum allowable tensile strength (ATS) of material SA 336 G F22V used for reactor C-0001 construction under exothermic runaway reactions and fire is for sure lower than 479Mpa.
Table3.4 Max UTS at given temperature (ASME data)
Reactor C-0001: SA 336 G F22V Temperature(℃) Max UTS (Mpa)
40 586
100 586
150 586
200 586
250 586
300 576
325 567
350 558
375 547
400 535
425 522
450 509
475 494
500 479
By comparing the equivalent stress posed on reactor C-0001 (516Mpa) with maximum allowable tensile strength (ATS) of material SA 336 G F22V used for reactor C-0001 construction, the reactor C-0001 rupture is confirmed under exothermic runaway reactions and fire. Therefore, the necessity of emergency depressurization system installation is confirmed.
3.5 Modeling of Emergency Depressurization System
3.5.1 System Identification
Considering fire areas and connections among equipment [5], the emergency depressurization system is decided to install on the compressor knock out drum D-0004 in hydrocracker plant in order for system protection.
3.5.2 Fluid Volume Calculation
Through the HAZOP Study and equipment rupture confirmation, the emergency depressurization system is consequently decided to install on the compressor knock out drum D-0004 in order for system protection.
The calculated vapor volume and liquid volume in the system protected by emergency depressurization system in hydrocracker plant is app. 622 cubic meters and 672 cubic meters, respectively.
The detailed calculation results are shown in Table 3.5.
Table3.5 Fluid volume calculation results
ITEM NO.
Size Length Volume Vapor Zone
Vapor Volume
Liquid Volume
(in) (m) (m3) Ratio (m3) (m3)
P-0003 20 30 6.08 0.5 3.04 3.04
P-0004 24 100 29.19 0.5 14.59 14.59
P-0005 24 80 23.35 0.5 11.67 11.67
P-0006 30 120 54.72 0.5 27.36 27.36
P-0007 30 20 9.12 0.5 4.56 4.56
P-0008 30 70 31.92 0.5 15.96 15.96
P-0009 24 100 29.19 1 29.19 0
P-0010 20 100 20.27 0.5 10.13 10.13
P-0011 20 50 10.13 0.5 5.07 5.07
P-0012 16 20 2.59 1 2.59 0
P-0013 16 20 2.59 1 2.59 0
P-0014 20 100 20.27 1 20.27 0
P-0015 16 120 15.57 1 15.57 0
P-0016 18 20 3.28 1 3.28 0
E-0001 Tube 1 0.5 0.5 0.5
F-0001 8 0.5 4 4
C-0001 700 0.3 210 490
E-0001 Shell 5 0.5 2.5 2.5
E-0002 Tube 1.5 0.5 0.75 0.75
D-0002 90 0.6 54 36
E-0003 Tube 1 0.5 0.5 0.5
E-0004 10 0.5 5 5
D-0003 45 0.7 31.5 13.5
C-0002 120 0.9 108 12
D-0004 50 0.7 35 15
E-0003 Shell 4 1 4 0
3.5.3 Dynamic Simulation
To estimate and decide depressurization rate which is used for depressurization valve sizing, restriction orifice sizing, pipe sizing and flare capacity confirmation, and check temperature variation experienced by equipment and piping during depressurization which is used for material selection of equipment and piping, dynamic simulation for emergency depressurization system was conducted by using computer program HYSYS v7.3.
3.5.3.1 Exothermic Runaway Reactions Scenario
Considering exothermic runaway reactions occurring in reactor without any other external heat sources, “Adiabatic Mode” was selected and heat loss model “detailed” including conduction, convection, and others such as correlation constants was taken into account [18]. The pressure of system was reduced to 25% of initial pressure which is 150barg within 15 minutes. The results of dynamic simulation are shown in Figure 3.6, 3.7 and 3.8. Figure 3.6 shows gas flow rate throughout emergency depressurization system varies with time, Figure 3.7 shows vapor out temperature varies with time, and Figure 3.8 shows equipment temperature varies with time.
Figure3.6 Gas flow rate variation with time
Figure3.7 Vapor out temperature variation with time
Figure3.8 Vessel temperature variation with time
3.5.3.2 Fire Scenario
The gas flow rate throughout emergency depressurization system shall take into account (1) vapor generated due to heat input from the external fire, (2) the change in density of the vapor due to the pressure reduction and temperature change, and (3) liquid vaporization due to the pressure reduction. To calculate the gas flow rate, the fire shall be assumed to be in progress during depressurization, and during the fire, all input and output streams to and from the system to be depressurized and all internal heat sources within the system shall be assumed to have ceased [5].
Herein, “FireAPI521” mode was selected to calculate heat flux which is expressed as below [18]:
C21 3
QC C wettedarea timet (3.1) Where:
3
LiqVol time t wettedarea time t wettedarea time 0 1 C 1
LiqVol time 0
(3.2) C1 is a constant (21000 in United States Customary units)
C3 is an environment factor (assumed as 1.0) C2 is 0.82
The pressure of system was reduced to 7barg within 15 minutes.
The results of dynamic simulation for fire scenario are shown in
throughout emergency depressurization system varies with time, Figure 3.10 shows vapor out temperature varies with time, and Figure 3.11 shows equipment temperature varies with time.
Figure3.9 Gas flow rate variation with time
Figure3.10 Vapor out temperature variation with time
Figure3.11 Vessel temperature variation with time
3.5.3.3 Low Temperature Evaluation
Sudden depressurization of equipment or system from high pressure to low pressure usually results in cold gas venting into the flare system. The cold gas can significantly lower the temperatures within the process equipment and its related piping, as well as the relief system piping immediately downstream of the depressurization valves. This has an impact on the material selection of equipment and piping.
To check the low temperatures experienced by equipment and piping during depressurization, “Adiabatic Mode” was selected and heat loss model “detailed” including conduction, convection, and others such as correlation constants was taken into account [18].
The pressure of system was reduced to atmospheric. The results of dynamic simulation are shown in Figure 3.12 and 3.13. Figure 3.12 shows vapor out temperature varies with time, and Figure 3.13 shows equipment temperature varies with time.
Figure3.12 Vapor out temperature variation with time
Figure3.13 Vessel temperature variation with time
3.5.4 Results
By conducting dynamic simulation for emergency depressurization system in hydrocracker plant, the maximum gas flow rate is observed at the initial stage during depressurization, and the minimum vapor out temperature and minimum equipment
