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.

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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 & discharge pressure More complex maintenance

Sliding Vane ----- 40 - 70 Low capital cost Low flow rates

Table 1: Compressors used in vapor recovery applications

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Ejectors are often designed to piping codes such as B 31.3 and tested in accordance with ASME Power test Code PTC 24. Ejectors efficiency is not stated in the table. Efficiency involves a comparison of the energy output to energy input. This ratio is of little value in the design and selection of ejectors for vapor recovery applications. There are other accepted formulae to express ejector efficiency, which can be used in the comparison between designs of various makes of ejectors. Liquid ring compressors, both dry and wet screw compressors as well as reciprocating compressors are usually specified in accordance the applicable API specifications as shown in table 1. Reciprocating compressors have the highest efficiency, while liquid ring compressors have the lowest efficiency (PIP REEC001 2007). Although used in many flare gas recovery systems and recommended for low throughput and low pressure applications (EPA, 2006), rotary sliding vane compressor technology is not recognized by API and as such there is no applicable API standards. 1.2 Relief Devices: In order to recover the flare gas, a backpressure device or some method of ensuring a positive pressure on the flare header is needed. As per API 521, in the event of a process facilities trip, the emergency streams such as those from PRVs and depressuring systems shall always have flow paths to the flare available at all times. The design of flare gas recovery systems shall not compromise this path. This can be achieved by more than one method. The primary relieving device can be a water seal pot as shown in figure 1. The advantage of using a water seal pot or drum is that it will ensure positive pressure and prevents air ingress. A water seal is reliable in the event of an emergency relief or a trip of the vapor gas compressor. The main drawback of water seal drums is the complication associated with the water system.

Figure 2: Vapor recovery unit using a FOV Another primary relief device commonly used is a fast acting pressure control valve or simply fast opening valve (FOV) as shown in figure 2. This valve should be extremely reliable and typically can be fully open in less than 2 seconds (Sivertsen). Although not required by API 521, the loop that activates the fast opening valve (FOV) should be SIL classified. Usually, it would be SIL 3 since the consequences of failure of the valve to open means that the flare is not available in case of an emergency. There are a number of vendors who can supply a fast opening valve that meet the requirements of a SIL 3 loop.

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If the loop is SIL classified, it means it has to be activated by a classified logic solver (either a dedicated PLC or the plant ESD system) but not the DCS. To increase the reliability of the loop, a secondary relief device is installed in parallel to the FOV. This could be:

pilot operated pressure relief valve

rupture disc

buckling pin relief valve.

The next section describes some of the vapor recovery systems installed in the Company oil fields.

2. ADCO Experience In the late nineties, the Company conducted feasibility studies to eliminate flaring form the various fields (Wasfi 2004). The main oil fields are shown in Figure 3. The different reservoirs in the Company oil fields vary in type and size and are under different stages of depletion. As such, the system duty and fluid characteristics differ from one field to another, various designs and technologies were adopted in the vapor recovery systems in the different fields.

Figure 3: Location map The first gas recovery system installed was the one in Bab field (Wasfi 2004). The compressor is an oil injected screw type. The primary relief to the flare is provided by a fast opening valve (FOV) and a pilot-operated relief valve as a secondary relief device. The sources of recovered gases are flashed gas in the flow suction tanks and produced water tanks. After the initial teething problems, the system has been operating with a high degree of reliability. In Asab field, a similar system is installed. The production facilities at Sahil field include first stage gas separators and the unstabilzed crude is pumped to Asab field for further processing. The associated gas from the first stage production separators is dehydrated and re-injected in the reservoir for pressure maintenance. Hence, there are no crude oil tanks in the field. The flare gas recovered in Sahil field is from the glycol regeneration unit. Liquid ring compressors were selected. Bu Hasa Field production facilities are one of the largest in the Company. Dedicated sets of ejectors are used for compressing the low pressure flare gas. One set is dedicated to the flow suction tanks and another is dedicated for the produced water tanks.

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Atmospheric tanks are susceptible to damage from even very low back-pressure and can be affected if there small inaccuracies in the calculations. Therefore, it is a good practice to conduct a dynamic simulation study during the engineering phase of a flare gas recovery project. The scope of the dynamic simulation would include:

Verify response time of the blanket gas PCV in case of ejectors start-up to avoid low pressure shut down

Ensure that the FOV response time is fast enough to open to the flare in the event of an ejector trip. 

The implementation of the first phase of these flare gas recovery projects in the different fields resulted in 90 % reduction of the amount gas that was flared from about 2.8 MMSCFD to 0.3 MMSCFD. The experience to date with these vapor recovery systems have been very positive. More recently, ADCO developed the NEB field. The experience with the VRUs installed in this field is described in the next section.

3. NEB Field experience

An overview of the flare system and vapour recovery systems of the NEB field is shown in figure 4. There are two flare systems, namely:

The common HP/LP Flare system

The tank flare system

The two systems are linked via the vapour recovery ejectors’ outlet line connecting to the suction of the vapour recovery compressors. The HP flare system consists of three which combine and discharge into a common flare KO drum. The three headers collect normal, intermittent and emergency discharges from the production facilities.

A liquid seal vessel is located downstream of the flare KO Drum. Its primary function is to isolate the flare network from the flare stack and so enable the normal continuous discharges to the flare network to be recovered by the vapour recovery compressors.

The tank flare system collects hydrocarbon discharges from storage tanks namely the flow suction tanks disposal water tanks. During normal operation, vapour discharges from these tanks are recovered by an ejector provided at each tank outlet as shown in figure 4. The Tank Flare network, KO drum and stack are permanently swept / purged with nitrogen.

The vapour recovery compressor is a single stage dry screw compressor. Following commissioning of the facilities, a number of operating problems were experienced in the VRU and flare system. These included:

Frequent compressor trip on high discharge temperature Corrosion and deposits in the vapor recovery compressor piping system Fouling of the compressor internals and the air cooler at the compressor discharge. Corrosion problems in flare water seals drum.

These problems were due to a number of reasons. Corrosion in the carbon steel piping system was because of presence of CO2 and H2S in the wet gas streams. The suction scrubber of the VRC was undersized and contributed to liquid carry over to the compressor. The problem of high temperature trip was attributed to the variation in gas composition. A shown in figure 4, there are complex multiple streams coming into the vapor recovery compressor. The gas composition is constantly changing. This means that the specific heat ratio and the hence the discharge temperature are also changing. This can be best demonstrated by examining the basic thermodynamic relationship for isentropic compression:

⁄

A small change in gas composition will impact the discharge tempertaure as shown in table 2, which compares between the value of (k-1) / k for Methane, Propane and Ethane. In other words, a high adiabatic exponent k leads to higher discharge temperatures,

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Figure 4: Overview of the NEB field flare and VRU systems

Gas

Propane Ethane Methane

CP @ 1 atm, 15O C (kJ/kg.K)

1.625 1.706 2.204

CV @ 1 atm, 15O C (kJ/kg.K)

1.436 1.429 1.686

k = CP / CV

1.132 1.194 1.307

(k-1) / k

0.1166 0.1624 0.2348

Table 2: Comparison of CP / CV ratio of different gases at atmospheric pressure & 15O C

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The compressor performance can only be achieved if the variation in the gas composition remains within the ranges specified in the data-sheet. A seemingly small change in the gas composition may result in a large increase in the specific heat ration CP /CV , thus causing the machine to trip on high discharge temperature The measures taken to address these problems include: Installation of corrosion inhibition points in VRC cooler. Replacement of the corroded piping. Upgrading the internals of the compressor scrubber; 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

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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: Programmable Logic Controller PRV: Pressure Relief Valve SIL: Safety Integrity Level VRU: Vapor Recovery Unit References

1) Adel K. Wasfi, “The First Real Zero Gas Flaring Project in the Middle East and the Gulf Region”, SPE paper 88669 presented at the 11th Abu Dhabi International Petroleum Exhibition and Conference (ADIPEC), Abu Dhabi, UAE, 10 – 13 October 2004.

2) API Standard 521, “Pressure-relieving and Depressuring Systems”, Fifth Edition, January 2007.

3) Installing Vapor Recovery Units on Storage Tanks, US Environmental Protection Agency, October 2006.

4) Morten Sivertsen, Seminar on “Gas Conservation Effort towards “Zero Flaring” Target” ABB Norway.

5) PIP REEC001,”Compressor Selection Guidelines”, Process Industry Practices, the University of Texas at Austin, February 2007.