Sufe Design and Optvation ofpi-oc.ess Vents and Emission Contid $wteins by Center for Chemical Process Safety Copyright 0 2006 John Wiley & Sons, Tnc. APPENDIX G WORKED EXAMPLES This appendix provides examples of vent systems for facilities where vent gases are collected and routed to a treatment unit before being discharged to atmosphere. For new projects, or locations undergoing sigruficant changes, the design should be evaluated to ensure it is as inherently safe as practical; for example, by minimizing inventories of hazardous or environmentally sensitive materials. This is generally most successful if it begins during the preliminary design phase of the project when there is still the opportunity to re-evaluate the process and to make changes without incurring excessive cost. As the project becomes more defined, the specific requirements for the normal process and emergency venting should be identified and addressed: Normal process vent requirements should be developed as part of the process design and take into consideration all non-emergency situations, such as vent flows from: The process chemistry and other process requirements Filling or emptying (vacuum relief) of tanks and vessels Solar radiation and rapid cooling due to rain, etc. Maintenance activities, such as steaming out vessels in preparation for vessel entry, and emptying wash water from storage tanks 241
Safe Design and Operation of Process Vents and Emission Control Systems Emergency vent requirements should be based on worst credible upset conditions, typically assuming multiple failures. For example it could involve a combination of equipment failure, loss of utilities, and human errors. Emergency venting requirements should consider, but not be limited to the following: External fire exposure Runaway reactions Explosion protection As the specific requirements for the vent headers are identified it can be helpful to incorporate them into a "design basis". The design basis provides a useful reference document for the design team, and subsequently it can be a valuable source of information for the plant's operating personnel. The design basis will typically include information such as; the worst case incident scenario(s), normal process vent flows and conditions, deflagration and reactivity testing information. Figure G-1 provides a flow path for developing the vent headers for a facility. In many instances this process is iterative in nature. For example adding an This appendix provides illustrations of typical vent header systems for the following: Flammable liquid handling processes with separate examples of systems that operate, inerted, fuel lean, and fuel rich Loading road tankers with flammable liquids Refinery vent system (including combined process and emergency vents) A facility handling reactive chemicals G1. INERTED FLAMMABLE LIQUID STORAGE When mixed with air the vapor from a flammable liquid can form an explosive mixture. If this mixture ignites the resulting explosion may propagate throughout a facility's vent system potentially damaging both the vent header and the equipment connected to it. Explosion prevention methods include operating, inerted, fuel rich, or fuel lean. 248
Appendix G - Worked Examples Section 3.1.1 Chapter 5 Understand Normal Process Vent Requirements t----. 3".2 Chapter Develop Emergency Venting Scenarios Chapter 5 Section 3.2.2 Chapter Investigate Determine if Vent Headers Should be Combined H Preliminary Hazard Assessments and Design Reviews Chapter 5 Treatment is Required Chapter 6 Define Vent Header System Preliminary Design Chapter 8 Implement Improvements Process Hazards Identified in Hazard Analysis and Final Reviews Design Reviews I Chapter Finalize Design of Vent - ' Chapter 8 Figure G-1. Steps in Vent Header System Design 249
Safe Design and Operation of Process Vents and Emission Control Systems This example illustrates a flammable liquid storage tank operating inerted, i.e., the tank operates with an inert gas purge that maintains the oxygen concentration below the limiting oxygen concentration for combustion. The section describes the steps for defining the design requirements for this approach. Included is an assessment of methods by which air could enter the system, potential ignition sources, and selects treatment options. G1.l Facility Description Figure G-2 illustrates the vent system for a low pressure feed tank handling a Class 1B flammable liquid. The tank receives batches from mix tanks and provides a continuous feed of this material to reactors in an adjacent production facility. G1.2 Identify Normal Vent Process Requirements Flammable vent gases are displaced from the feed tank when batches are transferred to it from the mix tanks. Requirements for the vent header system include: Addressing potential flammability hazards Treatment of the vent gases to meet environmental requirements Providing a means to recover raw materials from the process vent gases G1.2.1 Flammabilitv Considerations Flammable liquids storage tanks typically contain vapor above the LFL and if air is also present the mixture may bum explosively. During normal operations, when the liquid level drops, or the head space cools, gas must enter to prevent the pressure going sub-atmospheric and the tank collapsing. If air is used for this purpose a flammable mixture may form creating a potential explosion hazard. This can be avoided by the use of an inert or fuel gas purge to exclude air. G1.2.2 Liquid Build-up Considerations The tank is an atmospheric pressure tank designed to operate up to a maximum of 8 W.G. positive pressure, or 4 W.G. sub atmospheric pressure. If liquid collects in the vent header it may restrict vent gas flow, increasing the pressure or vacuum that occurs as liquid is transferred into or out of the tank. Due to the tanks low pressure rating a relatively small flow restriction in the header could cause it to fail. 250
G1.3 Develop Emergency Venting Scenarios G1.3.1 Emeraencv Overpressure Protection Appendix G - Worked Examples External fire exposure is the worst credible case scenario identified for emergency overpressure in the feed tank. G1.3.2 Emeraencv Vacuum Protection Sub-atmospheric pressure can develop in the tank by the following events, both of which could occur at the same time: G1.4 A failure of the nitrogen blanketing system while the tank is being emptied 0 Sudden cooling by a thundershower on a hot day Specify Vent System Design and Treatment Options Explosion protection for the normal process vent header should include, minimizing potential ignition sources, and operating outside the flammable limits. Measures to minimize potential ignition sources include: Installing a dip pipe for liquid entering the feed tank to minimize electrostatic charge generation Grounding and bonding vent pipework to prevent electrostatic charge accumulation The feed tank has a nitrogen blanketing system that maintains the tank pressure slightly positive (approximately 2-4" WG) while the tank is emptying. A conservation vent is provided set to open at 6" WG, to relieve pressure as the tank is filled. A small continuous nitrogen purge is provided at the closed end of the normal process vent header to prevent air migrating in from the "open" end, or through any leaks that may exist. The normal process vent gas is scrubbed with water before it is released to atmosphere. Gases leaving the scrubber meet environmental requirements, and are too lean to bum. Rich water exiting the scrubber is sent to recovery to separate and recycle the raw materials. A scrubber failure could result in vapor, above its LFL being vented to atmosphere. A flame arrester is installed at the open end of the vent header to protect against flash back if air is also present in with the vapors. 25 1
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Appendix G - Worked Examples Controls and alarms associated with the vent system include: Low flow alarm on water flow to the scrubber Low pressure alarm in the nitrogen line supplying the feed tank High liquid level alarms in the vent scrubber sump Liquid level instrumentation on the feed tank, with high alarm, and high level interlock to stop liquid transfers from the blend tanks The design basis for the emergency pressure relief scenario is an external fire. The facility has good drainage and is provided with deluge fire protection. External fire is considered to be a low probability event; further, vapor from the emergency vent would not constitute a hazard to personnel either on or off site. As a result the emergency vent discharges to atmosphere without treatment. The design basis for vacuum protection is the combined effects of a failure of the nitrogen blanketing system and sudden cooling of the tank by a thundershower on a hot day. To protect against ths the tank s conservation vent has a vacuum breaker to allow air to enter if the pressure drops below 3 WG vacuum. G1.5 Determine if Process and Emergency Vent Headers Should be Combined The emergency vent header discharges directly to atmosphere without treatment. In addition the emergency vent flow rate is considerably hgher than the normal process vent flow rate. Combining both systems would require a significantly larger vent scrubber to handle the total flow, and would not provide measurable benefits. As a result there is no merit in combining the systems. G1.6 Determine if Intermediate Treatment Is Required Inerting and scrubbing the normal process vents provides an effective system for handling the vent gases. No benefits were identified that would justify installing intermediate treatment. G1.7 Specify Vent Header System Preliminary Design The preliminary design should be provided for the hazard review and include the most up to date information on items such as: 253
Safe Design and Operation of Process Vents and Emission Control Systems The design basis including the "design case" scenarios for vent flow rates and compositions in the normal process vent header Header layout drawings Specifications for the vent header equipment, such as the vent scrubber specifications Materials of construction requirements G1.8 Implement Improvements Identified in Hazard Reviews Changes to the facility that arise from concerns identified during hazard reviews or as a result of new information as the project design develops should be addressed during the design of the vent system. fis may require the design basis for the vent system to be modified as the project progresses. G2. FLAMMABLE LIQUID PROCESS OPERATING FUEL LEAN As indicated in Section G1.O, the vapor from a flammable liquid can produce an explosive mixture when mixed with air. Explosion prevention methods include operating, inerted, fuel rich, or fuel lean. This example illustrates a vent system that operates fuel lean, i.e., the composition of flammable vapor is maintained below the concentration required for a deflagration to propagate. Figure G-3 shows a multi tank flammable liquid storage facility in which individual tanks operate air blanketed. As a result from time to time they will contain flammable vapor/air mixtures. Explosion protection for the vent header is provided by maintaining a continuous air flow that rapidly dilutes the vent streams from the source vessels to substantially below the LFL. Features that should be considered in the design include minimizing potential igrution sources in the tanks, installing detonation arresters in the lines between the tanks and the header, and monitoring the air flow in the vent header. G2.1 Facility Description A flammable liquid, flash point 87'F (31 C) is stored in air blanketed atmospheric storage tanks. The tanks are equipped with pressure - vacuum conservation vents for normal venting, and have reliving manways for emergency over pressure protection (see Figure G-3). 254
G2.2 Identify Normal Vent Process Requirements Appendix G - Worked Examples Vent gases from the tanks are a mixture of air and organic vapor from the product. Requirements for the vent header system include: Addressing potential flammability hazards Routing the vent gases to a treatment system to remove, or reduce the concentration of organic vapors to an acceptable level for release to atmosphere G2.2.1 Flammabilitv Considerations The liquid has a flash point of 87 F (31 C). If the tank's head space temperature is lower than 87 F (31 "C), it will not contain sufficient organics to support combustion. When exposed to solar radiation the head space temperature can reach 140 F (60 C). Consequently during normal weather patterns the flash point can readily be exceeded. Explosion protection, e.g., inerting or operating fuel rich is employed extensively in the petrochemical industry; however, many flammable storage tanks are operated air blanketed. In these cases it is important to ensure potential iption sources at one point in a vent header system will not become a cause of iption elsewhere. G2.2.2 Environmental Considerations To meet environmental control requirements the normal process vents must be treated before they are released to atmosphere; in addition, even at very low concentrations the vapors have an unpleasant odor that would be unacceptable to the local community. G2.3 Develop Emergency Venting Scenarios G2.3.1 Emergencv Overpressure Protection The design case for emergency overpressure protection is external fire exposure. G2.3.2 Emergencv Vacuum Relief The design case for emergency vacuum relief is a failure of the pressure - vacuum conservation vent to open during conditions that cause subatmospheric pressure in the tank. For example when the liquid level is being lowered, or the tank cools as a result of a thunderstorm on a hot day. 255
Safe Design and Operation of Process Vents and Emission Control Systems G2.4 Specify Vent System Design and Treatment Options Explosion protection for the normal process vent header should include, minimizing potential ignition sources, and operating outside the flammable limits. Measures to minimize potential igrution sources include: Installing dip pipes in tanks to minimize electrostatic charge generation Grounding and bonding vent pipework to prevent electrostatic charge accumulation Installing a flame arrester at the thermal oxidizer The tanks are air blanketed and will, at times, contain flammable vaporiair mixtures. To provide an additional layer of protection beyond minimizing ignition sources the vent gases are diluted with air forming a non-igrutable, lean mixture. The facility has a thermal oxidizer to dispose of liquid waste from the manufacturing process. The combustion air for the thermal oxidizer is also used as a source of air to: Dilute the vent gases, to significantly less than 25% of the LFL Dispose of the resulting lean air/vapor mixture Design features for this system include: Ensuring the tank vents enter the combustion air header at a point where the air is turbulent Providing adequate distance for mixing between the addition point and any potential ignition source (such as the blower) Instrumentation to monitor the combustion air flow with an interlock to stop the transfer pump feeding the tanks if the air flow is below the required value An interlock to close the combustion air exhaust damper, and stop the product transfer pump if the combustion air blower stops Flame arresters below the conservation vents on the storage tanks 256
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Safe Design and Operation of Process Vents and Emission Control Systems Locating the combustion air inlet away from potential sources of flammable vapors, and ignition points Confirming the thermal oxidizer burner can operate satisfactorily over the maximum range of flammable concentrations it could be exposed to when liquid is transferred to the storage tanks When operating fuel lean it is particularly important to ensure tanks do not overflow flammable liquid into the vent header. This is prevented by the follows: Each storage tank has a high level sensor separate from the basic process controls interlocked to closes an XV valve in the tank's inlet line The vent header is located several feet above the storage tanks. If a tank is overfilled liquid will preferentially overflow through its pressure relieving manway rather than overcoming the vertical height required to reach the vent header The tanks have pressure - vacuum relieving manways for emergency pressure and va,cuum protection. These manways vent directly to atmosphere and do not have vent headers G2.5 Determine if Process and Emergency Vent Headers Should be Combined In this application the emergency vents discharge directly to atmosphere. It would not be practical or beneficial to combine them with the normal process vent. G2.6 Intermediate treatment is not applicable. G2.7 Determine if Intermediate Treatment Is Required Specify Vent Header System Preliminary Design The preliminary design should be provided for the hazard review and should include the most up to date information on items such as the following: The design basis used for determining the vent flows from the tanks A review of the thermal oxidizer vendor information on the effects of introducing low concentrations of flammable vapor into the combustion air 258
Details of proposed interlocks Header layout drawings Appendix G - Worked Examples Specifications for the vent header equipment, e.g., the blower, the flame/detonation arrester requirements G2.8 Implement Improvements Identified in Hazard Reviews Changes to the facility that arise from concerns identified during hazard reviews or as a result of new information as the project design develops should be addressed during the design of the vent system. This may require the design basis for the vent system to be modified as the project progresses. G3. FLAMMABLE LIQUID PROCESS OPERATING FUEL RICH The explosion hazards from flammable liquids can be addressed by maintaining the vapor composition, below its lower oxygen concentration, fuel lean, or fuel rich. This example illustrates a reactor and its associated headers that operate fuel rich,.i.e., the vapor concentration is maintained above the upper flammable limit. G3.1 Facility Description Figure G-4 illustrates the vent system for a batch reaction process involving two raw materials. The first step for each batch is to rapidly feed a complete charge for one of the raw materials to the reactor. Following this the second raw material is fed at a predetermined rate, based on maintaining the reaction temperature in specification. Both materials are flammable, and can form solid polymers The process is part of a larger facility with an emergency vent header that operates fuel rich and leads to a flare. G3.2 Identify Normal Vent Process Requirements The maximum process vent gas flow from the reactor occurs while the initial raw material is being charged. The maximum blanketing gas requirements occur while the completed batch is being transferred to a hold tank. Requirements for the normal process vent header system include: Addressing potential flammability hazards To route the vent gases to treatment before they are discharged to atmosphere 259
Safe Design and Operation of Process Vents and Emission Control Systems G3.2.1 Flammabilitv Considerations The reactor feeds are Class 1B flammable liquids. At the end of each batch the product is transferred to storage. As tlus transfer is made gas must enter to replace the liquid product. If air enters, either intentionally or as a result of leaks, a flammable air/fuel mixture could form. G3.2.2 Liauid Build-up Considerations The liquid raw materials can polymerize. Consequently if they collect in the header solids may form restricting the flow of vent gases through it. Measures should be taken to minimize condensation in the vent header, to prevent entrainment or liquid overflowing from the reactor, and to provide adequate drainage from the header. G3.3 Develop Emergency Venting Scenarios G3.3.1 Emeraencv Overtxessure Protection The design case for emergency venting is a runaway reaction initiated by external fire exposure. G3.3.2 Emeraencv Vacuum Protection The reactor is rated for full vacuum and consequently does not require emergency vacuum protection. G3.4 Specify Vent System Design and Treatment Options Explosion protection for the headers includes, minimizing potential ignition sources, and operating outside the flammable limits. Potential ignition sources are controlled by: Installing dip pipes in the reactor to prevent electrostatic charge generation by free falling liquid Grounding and bonding the vent pipework Installing a flame arrester at the thermal oxidizer Providing a flare drum, designed to prevent flashback, at the base of the flare Maintaining the headers fuel rich by providing continuous fuel gas purges at the end of each header, furthest away from the open end 260
Safe Design and Operation of Process Vents and Emission Control Systems The reactor and vent headers operate fuel rich. Fuel gas is supplied to the reactor from a pressure regulator that controls at 4 WG and has a conservation vent which opens at 8 WG. Vent gases from the conservation vent discharge into the normal process vent header which is routed to the thermal oxidizer. The emergency vent is equipped with a rupture disk. It discharges to a knockout tank designed to handle two phase venting resulting from a runaway reaction initiated by an external fire. G3.5 Controls and alarms on the vent system include: Low pressure alarm on fuel gas blanketing the reactor Local flow indication for the fuel gas purges at ends of header branches High liquid level alarms in thermal oxidizer knockout pot and the emergency header knockout tank High differential pressure indicator and high temperature alarm for thermal oxidizer s flame arrester High and low level alarms on the flare drum Determine if Process and Emergency Vent Headers Should be Combined The flare is designed to provide a reliable treatment system for handling the hgh vent gas flows associated with very infrequent emergency incidents. Additional fuel gas is not required to augment the calorific value of these gases. The normal process vent flow rates are low and intermittent. If they are fed to the flare additional fuel gas will be needed to acheve a stable flame. 17us would add sigruficant operating cost malung the approach uneconomical. Conversely the thermal oxidizer is designed to operate efficiently with the low and variable process vent gas flows. They are, however, vulnerable to flame failures, which can occur as a result of a sudden changes in the feed rate or a loss of utilities during an emergency. Further it can take a considerable time to purge and relight the thermal oxidizer. Based on the above it is not practical to combine the normal and emergency vents. G3.6 Determine if Intermediate Treatment Is Required The design case emergency venting scenario for the reactor predicts a two phase gas/liquid flow. Two phase flow could result in an unacceptably high pressure drop in the emergency vent header, which could potentially 262
Appendix C - Worked Examples overpressure the reactor. To address this, the emergency vent is directed to a large knockout tank where liquid can separate and provide a vent stream that is substantially 100% gas. G3.7 Specify Vent Header System Preliminary Design The preliminary design should be provided for the hazard review, including the most up to date information on items including: The design basis including the reactivity and flammability "design case" used to specify the vent system requirements Header layout drawings Specifications for the vent header equipment, e.g., detonation or deflagration arrester requirements, and the design basis for the emergency knockout tank Materials of construction requirements G3.8 Implement Improvements Identified in Hazard Reviews Changes to the facility that arise from concerns identified during hazard reviews or as a result of new information as the project design develops should be addressed during the design of the vent system. This may require the design basis for the vent system to be modified as the project progresses. G4. ROAD TANKERS, FLAMMABLE LIQUID LOADING Terminals for loading road tankers typically receive "empty" tankers filled with vapor from the previous shipment. There may, however, be occasions when the returned tankers contain air with little or no volatile organic materials present. Explosion protection for these facilities typically involves eliminating ignition sources, and installing flame arresters or detonation arresters, as appropriate. G4.1 Facility Description A terminal operation loads several different flammable liquids into road tankers (see Figure G-5). These liquids are volatile with equilibrium vapor concentrations above their upper flammable limits. Empty road tankers return to the site air blanketed and typically are not purged or cleaned before being refilled. As a result the vent gases displaced by the incoming liquid can be saturated with flammable vapors. In order to meet environmental requirements these vent gases must be treated before they can be discharged to atmosphere during the loading operation. 263
Safe Design and Operation of Process Vents and Emission Control Systems G4.2 Normal Process Vent Requirements Vent gases from several loading spots feed to a vent header and are then sent to a thermal oxidizer. The gases are flammable; consequently, the vent system design must take into consideration the potential for vapor/air mixtures to form that could burn explosively. G4.2.1 Flammabilitv Considerations Road tankers arrive on site containing air saturated with vapor from the previous cargo. In most cases the tankers are refilled without being purged or cleaned since the liquids loaded have similar properties. As a result vapors entering the vent header are typically fuel rich and are not an immediate deflagration (explosion) hazard. A variety of scenarios could result in air entering the header, potentially creating a flammable airhapor mixture. Explosion protection is achieved by taking measures to minimize the time the headers contain flammable air/vapor mixtures, and eliminating ignition sources. Specifically this includes: 0 Electrically grounding the road tanker Ensuring all conductive parts of the vent header are grounded and bonded 0 Installing a detonation arrester in the header close to the thermal oxidizer Installing flame arresters at connections to road tankers Provide lightning protection and cease operations during thunder storms Establishing administrative procedures to control hot work, and other activities that could create an ignition source These measures, along with operating to minimize instances when the header contains flammable mixtures have historically provided effective explosion protection for tank car loading facilities. G4.2.2 Liauid Build-up Considerations The following measures are incorporated to prevent liquid buildup in the header: The header is sloped towards the knockout tank at the thermal oxidizer Liquid transfers to the tankers are controlled by flow totalizers 264
Appendix G - Worked Examples Redundant high level detectors are provided in the vent lines exiting the tankers to prevent liquid overflowing the tanker if a flow totalizer malfunctions or if it is improperly set The knockout tank at the inlet to the thermal oxidizer has a high level alarm G4.3 Develop Emergency Venting Scenarios Road tankers have emergency pressure and vacuum vents installed on the tanks. These vents are designed to discharge directly to atmosphere. There is no emergency vent header provided. G4.4 Specify Vent System Design and Treatment Options The vent gases are treated in a thermal oxidizer. Alternative end-of-line treatment systems considered include: a vapor recovery unit, carbon beds, and a flare. The thermal oxidizer was selected as a compromise considering environmental, economic and community nuisance issues. The header typically operates fuel rich; however, the potential exists for air to enter creating a flammable mixture. To protect against tlus a detonation arrester is provided between the thermal oxidizer and its knockout pot. If the facility is to routinely handle liquids with vapor compositions within their flammable limits at ambient temperatures, e.g., ethanol, additional protection such as inerting may be appropriate. Combining vents from loading spots that are fuel rich, with vents that are fuel lean may create a flammable air/vapor mixture. In this case it may be inherently safer to provide separate vent headers to transfer the fuel rich and fuel lean streams to the thermal oxidizer. Controls and alarms for the vent system include: High temperature alarms to detect flashback to flame and detonation arresters Interlocks to stop feed to the tanker if liquid level is detected in the vent line exiting the tanker, or in the knockout tank at the thermal oxidizer 265
The preliminary design should be provided for the hazard review and should include the most up to date information on items such as the following: 0 0 0 0 Appendix G - Worked Examples G4.5 Determine if Process and Emergency Vent Headers Should be Combined Not applicable. Emergency vents on the tankers are not designed to be directed to a header, see Item G4.3. G4.6 A knockout tank is provided immediately upstream of the thermal oxidizer s flame arrester. G4.7 Determine if Intermediate Treatment Is Required Specify Vent Header System Preliminary Design The design basis including the maximum unloading rates, and the external fire design case Header layout drawings Requirements for the vent header equipment, e.g., detonation or deflagration arrester requirements, and knockout tank specifications Materials of construction and flexible hose requirements G4.8 Implement Improvements Identified in Hazard Reviews Hazards identified during hazard analysis reviews or as a result of new information as the project design develops should be documented and resolved before the design is considered final. G5. REFINERY EXAMPLE: CRUDE AND VACUUM UNITS Tlus example presents a case where all of the normal process and emergency vent streams from the two involved columns contain reasonably similar flammable vapors and would appear candidates to be combined. However, the vacuum column presents a problem in that it requires a very low normal process vent discharge pressure below that of a typical vent header system. The case illustrates the need to examine all characteristics of a vent stream to determine an appropriate disposition. 267
Safe Design and Operation of Process Vents and Emission Control Systems G5.1 Facility Description An expansion is planned for a refinery that will add the following additional units: 0 A crude oil process unit with a low pressure (atmospheric) distillation column A vacuum unit with its vacuum distillation column The Crude Unit will process a heavy crude oil feedstock by heating, removing sediments and water-soluble components, distilling and collecting usable products including diesel, kerosine and naphtha. The atmospheric column operates at about 35 psig (2.4 bar) and 270 F (132C) with overhead gases compressed and fed to a fuel gas system and the bottom liquids fed to the vacuum unit. The Vacuum Unit further separates heavier liquid gasoil products. The vacuum column operates at about 20 mmhg and 2200F (1040C) with 3- stage steam ejectors with inter-stage condensing to collect additional hydrocarbons and separate sour water (containing hydrogen sulfide). G5.2 Normal Process Vent Requirements All of the normal process vent streams from these unit operations contain flammable gases and vapors. Most of the normal vent streams will also contain hydrogen sulfide (HzS) and water vapor. The Crude Unit s major normal process vent stream is from the atmospheric column overheads pressure control vent which operates at about 35 psig (2.4 bar) and 2700F (132C). The Vacuum Unit s noimal vent stream is from the discharge of the column vacuum system. To achieve the desired vacuum column operating pressure, it has been determined that the back-pressure on the vacuum system from its liquid receiver or hot-well must be less than 1 psig (0.07 bar). G5.3 Emergency Venting Scenarios The vented materials for all identified emergency venting scenarios are expected to be flammable and may contain significant concentrations of H2S and water vapor. 268
Appendix G - Worked Examples For the Crude Unit, loss of cooling of the overhead stream from the atmospheric column can cause catastrophic overpressure and is considered to be the primary emergency venting scenario. The atmospheric column is provided with multiple pressure safety valves (PSVs) set at 50 psig (3.45 bar). For the Vacuum Unit, overpressure of the vacuum column is the primary concern. Further, these columns are typically designed with lower maximum allowable design pressure ratings, in this case 25 psig (1.72 bar). Primary overpressure protection is a PSV venting directly to atmosphere to eliminate back-pressure and ensure maximum venting capacity. The overpressure scenario occurs as follows: loss of cooling and condensing capability on the column overhead vapor stream in the vacuum jet results in loss of vacuum and with column liquid temperatures and continued heating from the column reboiler there is a rapid boil-up of material with a consequent rapid pressure rise. G5.4 Investigate Vent System Design and Treatment Options Essentially all of the process vent streams that can be collected will be "rich; a few minor streams may be inerted. The large total quantity of vapors and gases anticipated to be released and the environmental considerations indicate the need for end-of-pipe treatment by a flare system to combust the hydrocarbons and destroy the H2S. The likely wide range in flowrate within the vent header system favors the use of a flare over a thermal oxidizer, particularly, if a combined normal and emergency vent header system is considered. The basic flare system design must include: Knockout Tank and Seal Drum or similar device(s) to: - Prevent reverse flow of air into the header from the flare stack - Provide flashback prevention - Establish the minimum internal vent header operating pressure The vent header design must include at the most remote upstream end(s) of the header an uninterruptible flow of a flammable purge gas to ensure that the header remains under positive pressure and free of air. 269
Safe Design and Operation of Process Vents and Emission Control Systems G5.5 Determine If Normal and Emergency Vent Headers Should Be Combined Based on the common materials vented and the reasonably compatible range of vent pressures and temperatures, it would be practical to combine most of the above described normal process and emergency vent streams. However, some streams will require intermediate treatment. G5.6 Determine If Intermediate Treatment Is Required The Crude Unit s normal vent is from the atmospheric column overhead stream which is cooled and condensed and therefore requires liquid knockout. This knockout receiver requires level control to prevent liquid over flow into the header. For the emergency vent, due to the need to ensure an open vent path and the anticipated infrequency of emergency venting, no liquid knockout is provided on the emergency vent stream. The Vacuum Unit presents a different treatment situation for the normal process vent. The required low (< 1 psig, 4.07 bar) venting backpressure makes it impossible to put this stream into the vent header which will most probably operate at 2 to 4 psig (0.138 to 0.276 bar). However, a process heater is located within the unit that can (with necessary air permits) be used to achieve the environmentally required combustion efficiency. The vent stream from the vacuum column hot-well should be routed to a knockout tank then through a flame/detonation arrester to an eductor burner installed in the process heater. Alternatively, a small thermal oxidizer could be used to treat ths normal vent stream. The vacuum column emergency vent is directly to atmosphere; the quantity of flammable vapors and steam released and the height of the relief valves on top of the column will be verified to provide adequate dispersion. G5.7 Finalize Vent Header System Preliminary Design The preliminary design including layout, intermediate treatment requirements, line sizing and specification, materials of construction and the selected treatment system should be documented. This becomes the basis for a formal hazard analysis. See Figure G-5 for the basic design of a vent header system for ths example. G5.8 Implement Improvements Identified in Hazard Analysis The formal hazard analysis should be documented and the recommendations arising from it should be incorporated into the final vent header design for construction. 270
G6. REFINERY EXAMPLE: COKER UNIT AND GAS PROCESSING PLANT Appendix G - Worked Examples This example presents a simple refinery heavy oil coker and a related gas plant and illustrates the need for careful analysis of vent streams and their intermediate treatment steps that may be required. In this case all of the normal process and the emergency vent streams can ultimately be combined for final end-of-line treatment but not before extensive intermediate separation and cooling steps are taken to make one group of streams compatible with the other. G6.1 Facility Description An expansion is planned for a refinery that will add the following additional units: A coker unit consisting of coke drums and a fractionator column to produce coke from part of the process stream from the crude/vacuum units A gas plant consisting of compressors, de-pentanizer, deethanizer and unifiner to further process gases from the coker unit to usable products and fuel gas for the process heaters The Coker Unit receives vacuum column bottoms liquids that are fed to the Fractionator Column, heated and fed to each of the coke drums sequentially in a batch-wise process. The lghly heated heavy oil rapidly solidifies within the drums to coke with the off-vapors returning to the fractionator. The drums are cooled, opened and the coke is cut out using high pressure water jets and the drums are then closed and warmed up to prepare for the next batch cycle. The coke drums experience temperature up to 850 F (454 C) and pressure to 175 psig (12.1 bar) at various points in the batch cycle; the fractionator operates at about 35 psig (2.41 bar) and 520 F (271 C). Coke is produced in each of two drums sequentially in a batch mode operation. Additional heavy gasoil liquid products are distilled in the fractionator and the overhead gases are compressed and fed to the gas plant. The Gus Plant processes the compressed coker gases to separate residual sour water and collect additional light hydrocarbon products. 271
Safe Design and Operation of Process Vents and Emission Control Systems G6.2 Normal Process Vent Requirements All of the normal process vent streams from these unit operations contain flammable gases and vapors. Most of the normal vent streams will also contain hydrogen sulfide (HzS) and water vapor. The Coker Unit has two major normal process vent streams: the fractionator column overheads pressure control vent and an open vent from the coker blowdown system s receiver that can operate against a moderate back pressure. The Gas Plant has a number of normal vent streams from process pressure control vents. 212
I CRUDE UNIT VACUUM UNIT Figure G-6. Refinery Example - Crude and Vacuum Unit
Safe Design and Operation of Process Vents and Emission Control Systems G6.3 Emergency Venting Scenarios The vented materials for all identified emergency venting scenarios are expected to be flammable and may contain significant concentrations of H2S and water vapor. The Coker Unit may require emergency venting of streams from either the coke drums or the fractionator. Emergency vent streams from both sources may contain a significant amount of solid coke, from particles to large pieces, in addition to flammable gases and hot hydrocarbon liquids. Normal and emergency vent streams may range from 500 to 800 F (260 to 427 C). The Gas Plant has a number of vessels that may experience overpressure due to excessive heating, loss of pump circulation or loss of overhead stream cooling. Overpressure protection by PRVs is provided for the affected vessels. G6.4 Investigate Vent System Design and Treatment Options Essentially all of the process vent streams that can be collected will be "rich"; a few minor streams may be inerted. The large total quantity of vapors and gases anticipated to be released and the environmental considerations indicate the need for end-of-pipe treatment by a flare system to combust the hydrocarbons and destroy the H2S. The likely wide range in flowrate witlun the vent header system tends to favor a flare over a thermal oxidizer, particularly, if a combined normal and emergency vent header system is considered. The basic Flare System design must include: Knockout Tank and Seal Drum or similar device(s) to - Prevent reverse flow of air into the header from the flare stack - Provide flashback prevention - Establish the minimum internal vent header operating pressure G6.5 Determine If Normal and Emergency Vent Headers Should Be Combined Based on the common materials vented and the reasonably compatible range of vent pressures and temperatures, it would be practical to combine 274
Appendix G - Worked Examples most of the above described normal process and emergency vent streams. However, some streams will require intermediate treatment. G6.6 The Coker Unit also presents a venting problem. Normal batch venting from the coke drums as well as emergency venting from the drums and the fractionator column requires cooling of these high temperature streams and separation of the solid coke that they may entrain. This is done in a separate quench drum system with recirculation and by cooling, condensing and liquid knockout of the gases from the quench drum before they can be vented into the header system. The gas plant includes several columns and vessels that require emergency venting through PSVs into a header system. These columns and vessels also have normal process pressure control vents that also must be collected into a header system and will require liquid knockout at the process prior to venting into the header system. G6.7 Determine If Intermediate Treatment Is Required Finalize Vent Header System Preliminary Design The preliminary design including layout, intermediate treatment requirements, line sizing and specification, materials of construction and the selected treatment system should be documented. This becomes the basis for a formal hazard analysis. See Figure G-6 for the basic design of a vent header system for this example. G6.8 Implement Improvements Identified in Hazard Analysis The formal hazard analysis should be documented and the recommendations arising from it should be incorporated into the final vent header design for construction. G7. REACTIVE SYSTEM Th~s example illustrates the importance of considering the combined affects of reaction hazards and the equipment characteristics. The worst credible case was shown to be the reactor agitator stopping and then restarted several minutes later. While the agitator is stopped raw materials collect in the reactor and subsequently will react violently when the agitator is restarted. Th~s can cause two phase venting and substantially increase the volume required for the emergency vent catch tank. 215
Safe Design and Operation of Process Vents and Emission Control Systems G7.1 Facility Description A chlorinated inorganic liquid is hydrolyzed by reacting it with water in a glass lined, agitated reactor. The reaction is strongly exothermic, however, the liquids are immiscible. Reaction occurs at the interface between the liquids. In the absence of agitation the liquids form separate layers resulting in a low overall reaction rate. During normal operations the agitator disperses the two liquids, and the reaction takes place almost instantaneously. The reactor operates at close to atmospheric pressure and the heat of reaction in removed by allowing the reaction mass to boil (See Figure G-7). G7.2 Identify Normal Process Vent Requirements Vent gases from the reactor consist primarily of water vapor and hydrogen chloride (HCl), which are condensed to form a saleable biproduct acid. The vapors are not flammable and therefore explosion protection is not needed. G7.2.1 Liquid Build-up Considerations Measures to prevent liquid buildup in the header include: Eliminating low points in the header where liquid could collect Minimizing the distance between the reactor and the condenser Designing and operating the hydrolyzer to minimize liquid entrainment in the vent gases G7.2.2 Materials of Construction Vent gases from the hydrolyzer are corrosive to carbon and stainless steels. The normal process vent system should be constructed from materials that are compatible with these conditions, such as titanium, and certain reinforced plastics, and lined steel. During normal operations a rupture disk prevents the process gases entering the emergency vent header. As a result it may be acceptable to use less costly materials that are only able to withstand the process gases for short periods of time. 276
Q....: Figure G-7. Refinery Example - Coker Unit
2 P E5 Lu> Figure G-8. Reactive System
G7.3 Develop Emergency Venting Scenarios Appendix G - Worked Examples The worst case scenario was determined to be an undetected malfunction of the hydrolyzer agitator allowing a layer of the chlorinated feed material to accumulate in the reactor, followed by the agitator starting. This could result in a violent reaction developing high pressure in the reactor, the vent header, and equipment connected to it. No combustible materials are handled in the area; as a result the vent design does not need to address external fire and there are no deflagration (explosion) hazards inside the equipment. G7.4 Specify Vent System Design and Treatment Options The normal process vent stream from the hydrolyzer is fed to the condenser where most of the water vapor and HCl are condensed. Vent gases leaving the condenser are fed to a knockout pot and then treated in a caustic scrubber to remove the remaining HCl. Controls and alarms for the normal process vent header include: Hydrolyzer high level alarm and feed interlock Hydrolyzer condenser vapor outlet high temperature alarm Tail gas scrubber, low caustic feed flow alarm Tail gas scrubber sump high level alarm Test work demonstrated that the design case emergency venting scenario could result in two phase venting, carrying approximately 70% of the reaction mass into the vent header. To handle this large volume of liquid, and to separate the phases, the emergency vent is discharge into a knockout tank. Vent gases from the knockout tank are fed to a water scrubber and discharged to atmosphere. The emergency scrubber must be available at very short notice any time the hydrolyzer is operating. To satisfy this requirement water is continuously fed to the scrubber and re-circulated back to it through a storage tank. Controls and alarms for the emergency vent system include: Rupture disk with indicator/alarm at entry to emergency vent header Liquid detector in emergency knockout tank with alarm and interlock to stop hydrolyzer feeds Low flow alarm on the water feed to the emergency scrubber High level alarm in the emergency scrubber 279
Safe Design and Operation of Process Vents and Emission Control Systems During emergency venting a two phase flow may be discharged to the normal process vent system which may become substantially liquid full. To ensure it is not damaged when emergency venting occurs it must be designed to withstand: G7.5 Its weight liquid full The maximum pressure developed during a worst case venting scenario The maximum temperature that could occur, including the temperature rise caused when the highly acidic reaction mixture mixes with the caustic scrubbing liquid Determine if Process and Emergency Vent Headers Should be Combined The design case for the emergency vent system predicts two phase venting, and very high gas flow rates. It would be costly to provide a hydrolyzer condenser to handle these conditions. In addition an emergency vent system with a rupture disk that only opens when an emergency occurs would be exposed to corrosive conditions very infrequently. As a result it could be constructed from less expensive materials of construction than are required for the normal process vent system. Consequently, it is not cost effective to combine the normal and emergency vents. G7.6 Determine if Intermediate Treatment Is Required Intermediate treatment includes: G7.7 A condenser and a knockout pot in the normal process vent header system An emergency catch tank to separate the gas and liquid phases in the emergency vent header The preliminary design should be provided for the hazard review, including the most up to date information on items such as: 280 Specify Vent Header System Preliminary Design The design basis including the emergency venting requirements caused by a delay in starting the reactor agitator 0 Header layout drawings Proposed materials of construction for both header systems Instrumentation and interlock requirements
Appendix G - Worked Examples G7.8 Implement Improvements Identified in Hazard Reviews Changes to the facility resulting from concerns identifies during hazard reviews, or as a result of new information as the project design develops, should be evaluated to determine if they will affect the requirements for the vent headers. 281