石油工程师协会技术年会论文spe166133.pdf

SPE 166133 Ten Years Experience with Flare Gas Recovery Systems in Abu Dhabi Hisham Saadawi ADCO Abu Dhabi UAE Copyright 2013 Society of Petroleum Engineers This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in New Orleans Louisiana USA 30 September 2 October 2013 This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author s Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author s The material does not necessarily reflect any position of the Society of Petroleum Engineers its officers or members Electronic reproduction distribution or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited Permission to reproduce in print is restricted to an abstract of not more than 300 words illustrations may not be copied The abstract must contain conspicuous acknowledgment of SPE copyright Abstract Abu Dhabi Company for Onshore Oil operations ADCO is one of the major oil producing companies in the Arabian Gulf The Company produces oil from a large number of reservoirs in various fields which are different in size and are under different stages of depletion In order to achieve near zero flaring and reduce emission the Company began implementing several projects for flare gas recovery in its oil fields more than ten years ago Various compressor technologies were selected to suit the specific field applications These included liquid ring sliding vane compressor both dry screw and oil wet screw compressors as well as ejectors Methods adopted to ensure positive pressure on the flare recovery system while preserving a path to the flare for emergency relief included a water seal drum a SIL rated quick opening valve and a pilot operated relief valve The paper presents comparison between the designs of the various systems compressor technologies and pressure relive methods used in flare gas recovery systems Applicable codes and standards are discussed The paper also addresses some of the lessons learnt and the guidelines on best practice for selecting and designing vapor recovery systems Contents 1 Overview of Flare Gas Recovery Systems 1 1 Compressor Type 1 2 Relief Devices 2 ADCO Experience 3 NEB Field Experience 4 Conclusions Nomenclature References 1 Overview of Flare Gas Recovery Systems Operating Companies have more than one reason to install vapor recovery units VRUs Incentives to consider such systems include compliance with local regulations providing additional revenue and eliminating emission of hydrocarbon vapors to the atmosphere The design and installation of VRUs is usually done in accordance with API RP 521standards API 2007 The main components of a standard VRU are the compressor and a relief device to ensure clear path to the flare This is shown schematically in figure 1 2 SPE 166133 Figure 1 Main Components of the standard vapor recovery unit 1 1 Compressor type There are several compression technologies that are used in VRUs Each has its advantages and limitations The selection of the right type of compressor depends on the particular field process and facilities A high level comparison between the various compression technologies used in flare gas recovery systems is given in table 1 Compressor Type Standard Adiabatic Efficiency Main Features Ejector ASME B31 3 Simple no rotating parts Low pressure ratio Require motive gas Liquid Ring API 681 25 50 Tolerant towards liquid ingestion Requires closed loop water system Less efficient than other types Oil Flooded Screw API 691 65 70 Requires lube oil circulation system Better efficiency than dry screw compressors Dry Screw API 691 55 70 Do not require a circulating water system nor a circulating lube oil system Poor cooling limitation on discharge temperature High noise Reciprocating API 618 80 90 Higher flow rates from vane pack type to cyclone separator type internals Bare carbon steel vessel replaced by Alloy 825 cladded vessel Replacement of existing carbon steel piping of the seal vessel water circulation system with Inconel 825 piping The problem of varying gas composition is a bit more complicated With a single stage dry screw compressor the options are limited One possible solution investigated is to inject demineralized waer in the suction nozzle Part or all of the injected evaporates and thereby cools the compressed gas and the compressor 4 Conclusions For small tank batteries the design of VRUs is well established in the industry However for large production facilities with multiple sources of gas to the VRU the design of such units is not straight forward It is important to look beyond flow rates suction and discharge pressures and consider the overall gas recovery configuration Based on ADCO experience with flare gas recovery units the following conclusions and guidelines are noted when designing and selecting such units Liquid ring compressors have achieved very high reliability for VRU applications Oil flooded screw compressors also achieved a high degree of reliability When motive gas is available ejectors provide a simple and reliable alternative means for compressing the recovered gas It is a good practice to conduct a dynamic simulation study during the engineering phase of the project The scope of this study should cover the complete VRU system tanks gas blanketing control system and relevant facilities downstream the VRU This will ensure that the system design can cope to a wide range of dynamically changing loads Reservoir fluids can have a wide range of composition that change with time Moreover flare gases can also have a widely varying composition with seasonal and daily variations Such variations should be thoroughly evaluated when specifying designing and selecting the compressor Care should be taken when combining multiple streams from different sources in a single VRU as the entire spectrum of gas composition variation can be difficult to predict Dry screw gas compressors in particular are sensitive to this variation in gas composition Conducting a SIL study on the VRU during the design phase will provide confidence that the emergency streams from the process facilities shall always have flow paths to the flare available at all times Nomenclature CP Specific heat of a gas at constant pressure CV Specific heat of a gas at constant temperature 8 SPE 166133 DCS Distributed Control System EPA US Environmental Protection Agency ESD Emergency Shut Down FOV Fast Opening Valve K ratio between specific heats of a gas PLC Programmab