Head Office # 420, 715-5th Ave SW Calgary, Alberta Canada T2P2X6 Tel: (403) Fax: (403)

Transcription

1 May 2008

2 Disclaimer This publication was prepared for the Canadian Association of Petroleum Producers, the Gas Processing Association Canada, the Alberta Department of Energy, the Alberta Energy Resources and Conservation Board, Small Explorers and Producers Association of Canada and Natural Resources Canada by CETAC-West. While it is believed that the information contained herein is reliable under the conditions and subject to the limitations set out, CETAC-West and the funding organizations do not guarantee its accuracy. The use of this report or any information contained will be at the user s sole risk, regardless of any fault or negligence of CETAC-West or the sponsors. Acknowledgements This Fuel Gas Efficiency Best Management Practice Series was developed by CETAC WEST with contributions from: Accurata Inc. Clearstone Engineering Ltd. RCL Environmental REM Technology Inc. Sensor Environmental Services Ltd. Sirius Products Inc. Sulphur Experts Inc. Amine Experts Inc. Tartan Engineering CETAC-WEST is a private sector, not-for-profit corporation with a mandate to encourage advancements in environmental and economic performance in Western Canada. The corporation has formed linkages between technology producers, industry experts, and industry associates to facilitate this process. Since 2000, CETAC-WEST has sponsored a highly successful ecoefficiency program aimed at reducing energy consumption in the Upstream Oil and Gas Industry. Head Office # 420, 715-5th Ave SW Calgary, Alberta Canada T2P2X6 Tel: (403) Fax: (403) cetac@cetacwest.com

3 Table of Contents 1. Applicability and Objectives 1 2. Basic Improvement Strategies 2 3. Fuel Consumption Associated with Flaring Pilot Gas 3.2 Purge Gas 3.3 Make-up Gas 4. Measuring Fuel Consumption Fuel Metering 4.2 Flare Metering 5. Reduction Opportunities Extinguishing Pseudo-dormant Flares 5.2 Reducing Pilot Gas Consumption 5.3 Reducing Purge Gas Consumption 5.4 Reducing Make-up Gas Consumption 6. Record-Keeping References 17 Tables 3.1 Average fuel gas consumption for energy efficient flare pilots 3.2 Typical minimum purge rates to avoid unsafe air infiltration Figures 3.1 Typical schematic of a labyrinth style purge reduction seal 3.2 Typical schematic of a baffle type purge reduction seal 3.3 Minimum fuel gas to waste gas ratio required to attain a combined net heating value of 20 MJ/m3

4 Background The issue of fuel gas consumption is increasingly important to the oil and gas industry. The development of this Best Management Practice (BMP) Module is sponsored by the Canadian Association of Petroleum Producers (CAPP), the Gas Processing Association Canada (GPAC), the Alberta Department of Energy, Small Explorers and Producers Association of Canada (SEPAC) Natural Resources Canada (NRC) and the Energy Resources and Conservation Board (ERCB) to promote the efficient use of fuel gas in flaring operations used in the upstream oil and gas sector. It is part of a series of 17 modules addressing fuel gas efficiency in a range of devices. This BMP Module: identifies the typical impediments to achieving high levels of operating efficiency with respect to fuel gas consumption, presents strategies for achieving cost effective improvements through inspection, maintenance, operating practices and the replacement of underperforming components, and identifies technical considerations and limitations. The aim is to provide practical guidance to operators for achieving fuel gas efficient operation while recognizing the specific requirements of individual flaring systems and their service requirements.

5 EFFICIENT USE OF FUEL GAS IN THE UPSTREAM OIL AND GAS INDUSTRY MODULE 4 of 17: Flaring Operations 1. Gathering Systems 2. Pumpjacks 3. Pneumatic Instruments 4. Flaring 5. Chemical Injection Pumps 6. Fired Heaters 7. Engines 8. Compression 9. Glycol Dehydrators 10. Desiccant Dehydrators 11. Fuel Gas Measurement FIELD 3. Pneumatic Instruments 4. Flaring 5. Chemical Injection Pumps 6. Fired Heaters 7. Engines 8. Compression 9. Glycol Dehydrators 10. Desiccant Dehydrators 11. Fuel Gas Measurement 12. Fractionation 13. Refrigeration 16. Tail Gas Incineration SWEET GAS PLANTS 4. Flaring 3. Pneumatic Instruments 7. Engines 6. Fired Heaters 5. Chemical Injection Pumps 9. Glycol Dehydrators 8. Compression 11. Fuel Gas Measurement 10. Desiccant Dehydrators SOUR GAS PLANTS 12. Fractionation 13. Refrigeration 14. Amine 15. Sulphur Recovery 17. Acid Gas Injection 16. Tail Gas Incineration

6 1. Applicability and Objectives This module provides guidance to operating staff to identify opportunities where gas consumption associated with flaring operations can safely be reduced. The determination of fuel gas efficiency in flaring is made by understanding the sources of fuel gas use and making periodic assessments of performance and comparing those with possible performance improvements. Flares are designed to dispose of intermittent or continuous volumes of combustible gas which cannot economically be recovered or disposed of in another manner. Although reducing the volume of waste gas which is disposed of by flaring has economic and environmental benefits, this is the topic of other BMPs 1 and will not be addressed here. This module will focus on identifying opportunities for reducing fuel gas consumption associated with operating flare systems. This includes fuel gas used for operating flare pilots, purging the flare system and enriching waste gas streams. This module outlines the basic improvement strategies for reducing fuel consumption in flaring and identifies sources of fuel consumption. Evaluation of flare performance using metering of waste gas and fuel consumption will be discussed with consideration to the identification of potential reduction opportunities. The final objective of this module is to outline suggestions and processes to develop a reduction program. Module 4 of 17 Page 1 of 17

7 2. Basic Improvement Strategy The chief elements for achieving safe, effective and lasting reductions to fuel consumption associated with flaring operations are: application of best available technology, implementation of operating and management systems, and corporate commitment. Fuel consumption is often necessary to ensure the safe and reliable operation of flare systems. However, whenever it is economically and technically feasible to do so, fuel consumption should be minimized or eliminated. Achieving fuel efficiency in flaring requires: understanding the sources of fuel consumption in flaring operations, periodic checking of fuel consumption rates to evaluate system performance and make adjustments as required, assessment of opportunities to upgrade or replace underperforming systems, maintaining adequate records to support the company s flaring fuel reduction program. Module 4 of 17 Page 2 of 17

8 3. Fuel Consumption Associated with Flaring Operations Achieving efficiency with respect to the use of fuel in flaring systems requires an understanding of where fuel consumption occurs in these systems. The sources of fuel gas consumption typically associated with flaring include pilot gas, purge gas and make-up gas. This section of the module discusses these sources and presents methodology for estimating the expected fuel consumption rates. In all cases the fuel consumption rates provided by system designers and equipment manufacturers and/or actual site measurements are preferred for identifying substandard performance and evaluating reduction opportunities. 3.1 Pilot Gas Many flares are outfitted with continuously burning gas pilots to ensure ignition of flared gases. The number and type of pilots required depends on the flare size, stream composition and wind conditions. Typical pilot requirements and fuel consumption rates are summarized in Table 3.1. These rates assume an average pilot fuel consumption rate of 1.98m 3 /h/pilot which is reasonable for energy efficient pilots fueled by sales quality natural gas 2. However, actual consumption will depend on burner design and fuel properties. Table 3.1 Average Fuel Gas Consumption for Energy Efficient Flare Pilots Flare Tip Diameter Number of Pilot Burners Average Pilot Gas Consumption Inches mm m 3 /h 10 6 m 3 /y > 60 > Adapted from [EPA CH1] 3.2 Purge Gas Typically the header of an intermittent flare system is continuously purged with fuel gas to prevent air ingress into the flare system. Additional benefits of purging include mitigating fouling and damage resulting from burn back and displaced products that have been released into the flare header. Purging the flare system is necessary to ensure safe operation. However, purge rates above those required to maintain safe reliable operation are undesirable and should be Module 4 of 17 Page 3 of 17

9 avoided. The required purge rate will depend on the type of seal used, stack diameter, properties of the gas as well as ambient and system conditions. For plain end flares the purge gas required to avoid unsafe air infiltration can be estimated using the Husa purge model. Equation 3.1 is an adaptation of the Husa purge model that can be used to estimate minimum purge gas consumption rates for flare systems. 3 Q O % KD 21 Ls [ ( MW / 28.96) ] = ln (3.1) Where: Q is the purge gas consumption in m 3 /h, K is 5.26 x 10-8 D is the internal diameter of the stack in mm, O 2 % is the acceptable oxygen concentration at Ls in % (note 6% is usually acceptable), Ls is the distance into the stack where the safe condition is met in m (note the lesser of 7.62 m or 10 stack diameters is usually acceptable), MW is the molecular weight of the purge gas (note 19.5 is typical for natural gas). Larger flares are often outfitted with seals which reduce the continuous purge rate required to avoid unsafe air infiltration into the stack. Purge reduction seals do not physically isolate the stack from the surrounding atmosphere. Instead, they utilize proprietary internals, either baffle-type or labyrinth-type, to reduce the ability for buoyant movement of air into the stack. Typical schematics of the seal internals are provided in Figures 3.1 and 3.2. Equation 3.2 can be used to estimate typical purge requirements for flare systems outfitted with baffle-type seals and Equation 3.3 can be used to estimate the typical purge gas consumption associated with labyrinth-type seals. Actual purge rates will depend on the seal design and should be obtained from the manufacturer. Q 5 2 = * D (3.2) Where: Q is the purge gas consumption in m 3 /h, D is the internal diameter of the stack in mm, Assuming: Module 4 of 17 Page 4 of 17

10 The average required purge velocity for flares outfitted with baffletype purge reduction tips is m/s (0.04 fps). Q 6 2 = * D (3.3) Where: Q is the purge gas consumption in m 3 /h, D is the internal diameter of the stack in mm, Assuming: The average required purge velocity for flares outfitted with labyrinth-type purge reduction tips is m/s (0.01 fps). Figure 3.1 Typical Schematic of a Labyrinth Style Purge Reduction Seal Module 4 of 17 Page 5 of 17

11 Figure 3.2 Typical Schematic of a Baffle Type Purge Reduction Seal Module 4 of 17 Page 6 of 17

12 Typical purge consumption rates calculated using the above formulas are presented in Table 3.2. Table 3.2 Typical Minimum Purge Rates to Avoid Unsafe Air Infiltration Flare Diameter Baffle Type (NPS) 1 Plain End 2 Seal 3 Purge Gas Consumption Rate (m3/h) Labyrinth Type Seal Standard wall pipe Calculated according to Equation 3. assuming a stack condition of 6% oxygen at the lesser of 10 stack diameters or 7.62 m from the open top and a purge gas molecular weight of 19.5 Calculated according to Equation 3.2 Calculated according to Equation 3.3 Prevention of flare tip damage resulting from burn back may necessitate purge rates greater than those required to prevent air infiltration. Burn back occurs when the flame regresses into the stack due to inadequate purge velocities. A visible flame under purge-only conditions is an indication that purge rates are sufficient to prevent burn back. Smaller flare tips do not normally experience burn back problems 3. Header sweep is another consideration which may require purge rates in addition to the minimum requirements to prevent air infiltration or burn back. This is especially important when the potential for corrosive gases to enter the header exists. Module 4 of 17 Page 7 of 17

13 3.3 Make-up Gas Make-up fuel is sometimes required to raise the calorific value of flared waste gas to levels that will support stable and efficient combustion. The ERCB 4 requires the combined net heating value (i.e. lower heating value) of flared gases and make-up fuel to meet or exceed 20 MJ/m 3 except for existing flares with a history of stable operation and emergency flare systems in sour gas plants where the heating value may be as low as 12 MJ/m 3. In all cases Alberta Ambient Air Quality Objectives must be met and flares which are subject to AENV approval may have more stringent requirements for minimum heating values. Equation 3.4 or Figure 4.1 can be used to estimate minimum make-up gas requirements. Q m LHV LHV r w = Qw (3.4) LHVm LHVr Where: Q f is the fuel gas flow rate, Q w is the waste gas flow rate, LHV r is the required combined net heating value (i.e. 20 MJ/m 3 ), LHV m is the lower heating value of the make-up gas, LHV w is the net heating value of the waste gas. Module 4 of 17 Page 8 of 17

14 Typical Natural Gas (LHV = 34.6 MJ/m3) High Heat Content Natural Gas (LHV = 38.5 MJ/m3) Low Heat Content Natural Gas (LHV = 31.9 MJ/m3) Minimum fuel gas to waste gas ratio Net heating value of the waste gas (MJ/m 3 ) Figure 3.3 Minimum Fuel Gas to Waste Gas Ratio Required to Attain a Combined Net Heating Value of 20 MJ/m3 Module 4 of 17 Page 9 of 17

15 4. Measuring Fuel Consumption When evaluating opportunities for reducing fuel consumption actual site measurements are helpful for identifying substandard performance and supporting economic evaluations. Depending on the source, fuel consumption can metered independently or as part of the total flare stream. 4.1 Fuel Metering Pilot gas, purge gas and make-up should be metered independently wherever possible. These streams are of known composition and flow rates are generally quite stable so conventional metering technologies are appropriate. 4.2 Flare Metering Flare meters are excellent diagnostic tools which can be used to identify excessive purge rates and/or leakage into the flare system that might otherwise go unnoticed. The ERCB mandates metering of continuous or routine flare sources at all oil and gas production and processing facilities 4 where average total flared and vented volumes exceed 0.5 x 10 3 m 3 /d and recommends the use of flare meters at larger oil and gas batteries, pipeline facilities and gas processing plants where there are multiple connections to the flare system even when the aforementioned average flaring rate is not exceeded 4. At a minimum, sufficient fittings should be installed to facilitate spot checking of the residual flare rate if continuous flare metering is not required or deemed necessary. Flare streams are particularly challenging to meter because of the high variability in flow and composition. In applications where metering is required by the ERCB the meter must have an accuracy of 5 percent over the entire range of flows and compositions encountered 5. Generally, flare meters should be composition independent and exhibit accuracy over a high turndown (i.e. 1:100 or better). Metering technologies which meet these requirements include: ultrasonic meters, micro-tip vane velocity probes and optical velocity probes. Thermal dispersion probes have adequate turndown but are highly composition dependant. Most traditional metering technologies used in the oil and gas industry (e.g. differential pressure, turbine, positive displacement) fail to meet one or both of these requirements. Traditional metering technologies may be applicable for flare measurement under certain circumstances where the composition and/or flow rate are adequately stable. If no flare metering is in place the residual flare rate (i.e. purge and leakage) can still be spot checked using a portable velocity probe or tracer test to establish if excessive purge rates or leakage are occurring. In order to conduct a tracer test it is necessary to have fittings on the flare line for injecting trace gas and withdrawing a sample. The injection point must be located somewhere on the flare line where there is flow and the sampling point Module 4 of 17 Page 10 of 17

16 needs to be sufficiently downstream of the injection point and all tie-ins to allow for good mixing of the entire flare stream and the tracer gas. Most portable velocity probes can be inserted into the flare piping through a NPS ¾ full port valve. Velocity measurements should be taken downstream of all tie-ins in a straight section of pipe. Module 4 of 17 Page 11 of 17

17 5. Reduction Opportunities When evaluating reduction opportunities it is important to base decisions on reliable data using a systematic and consistent approach. Safety is paramount and must be considered ahead of economics and operability when evaluating reduction opportunities. Any case where the actual fuel consumed in flaring operations exceeds required or manufacturer recommended rates represents an opportunity to conserve fuel by correcting the deficiency or replacing the underperforming equipment. Other opportunities to reduce fuel consumption include extinguishing pseudo-dormant flares and application of best available technology. This section of the module discusses these reduction opportunities. 5.1 Extinguishing Pseudo-dormant Flares Appreciable quantities of fuel gas are consumed in the operation of pilot and purge gas systems to maintain flares in a pseudo-dormant state. In situations where gas is not continuously or routinely flared and the probability of an emergency depressurization is low, an opportunity exists to conserve fuel gas by extinguishing the flare. When considering extinguishing flares operators should carefully assess the probability of all possible relief cases. At a minimum the following conditions should be satisfied prior to extinguishing a flare: There should be no continuously or routinely flared streams. The stack should be fitted with a glycol/water seal or other positive sealing device which will isolate the flare header from the atmosphere. This will prevent oxygen from entering the flare header and enable monitoring of leakage into the header. The maximum allowable working pressure of the piping and pressure vessels should be greater than the potential supply pressure from any connected sources (e.g. wells, compressors, etc.). No active injection or cycling schemes should be taking place or planned for any pools with wells connected to the facility. Pressure Safety Valves (PSV) connected to the flare system should be outfitted with upstream rupture disks and a pressure gauge should be installed between the PSV and rupture disk to enable detection of a disk rupture. Manual depressurizing valves connected to the flare should be double blocked, tagged and carsealed closed to mitigate the possibility of an accidental opening and/or leakage. Module 4 of 17 Page 12 of 17

18 Emergency shutdown valves should not be configured to depressurize equipment to the flare. ERCB approval is required prior to extinguishing sour flares. Guidelines and requirements for submitting a request to extinguish a sour flare can be found in the ERCB Directive When the flare is extinguished flammable gases remaining in the flare system are a potential hazard. To remove this hazard shutdown of the flare should be immediately followed by an inert gas purge. If the flare system is restarted another inert gas purge should be conducted to remove air from the stack and flare header prior to lighting the pilot. 5.2 Reducing Pilot Gas Consumption The use of electronic ignition devices and/or energy efficient flare pilots can minimize the amount of fuel gas used to sustain flare pilots. Electronic Ignition Devices Electronic ignition devices that ensure continuous flare ignition by systematically producing high voltage electric sparks can often be used in place of gas operated pilots. Electric consumption is low and is typically supplied by solar recharged batteries. The ERCB allows the use automatic ignition devices in place of gas pilots to ensure reliable continuous ignition of acid gas and sour flares at all facilities except sour gas plants which require the use of both devices 4. Energy Efficient Pilots In situations where pilots cannot be replaced by electronic ignition devices the fuel efficiency of the gas pilot should be evaluated and consideration given to installing a more energy efficiency design. Efficiency of pilots can be maintained by ensuring that wind shielding and pilot nozzles are in good condition. Some vendors offer designs that consume as little as 0.57m 3 /h/burner of fuel gas. 5.3 Reducing Purge Gas Consumption An opportunity may exist to reduce fuel consumed to continuously purged flare systems by installing purge reduction seals, using instrumentation to control purge rates, switching to an inert gas purge and/or reducing purge rates in response to leakage into the flare system. When evaluating purge gas reductions the purge rate required to maintain a safe stack condition (i.e. prevent air ingress) should be considered in conjunction with purge requirements to prevent burn back and provide adequate header sweep. Module 4 of 17 Page 13 of 17

19 Purge Reduction Seals Purge reduction seals use proprietary internals, either baffle-type or labyrinthtype, to reduce the ability for buoyant movement of air into the stack. This reduces the purge velocity required to avoid air infuriation and can lead to a significant reduction in purge gas consumption especially on larger diameter stacks. These devices should be considered in most situations where flare systems are continuously purged. Instrumentation to Control Purge Rate The minimum purge rate required to avoid unsafe air ingress into the stack is not only a function of the stack diameter and purge gas but is dependent on changes in ambient temperature, pressure, wind speed and temperature of products in the flare header. In order to compensate for the dynamic nature of these dependencies, continuous purge rates are often set above the minimum value required for the conditions under which the flare usually operates. An alternative to specifying an excessive purge rate is to use instrumentation to monitor critical parameters in the flare system (e.g. oxygen concentration, temperature, etc.) and automatically adjust purge rate accordingly to maintain a safe stack condition. In regards to instrumented purge rate control systems, an adequate purge rate is essential to system safety, redundancy and fail safe operation. The reliability, regular calibration and preventive maintenance are critical to the success of the instrumentation. Inert Purge Gas Inert gases can be used in place of fuel gas for purging flare systems. Inert purges have a safety advantage over enriching purges because in addition to preventing oxygen infiltration, combustible gases are also swept from the system. However, inert purges can extinguish pilots and may cause the combined heating value of flared streams to drop below required levels to maintain reliable and stable combustion. Additionally, inert gas is typically more expensive than natural gas so inert purges are not normally used. Reducing Purge Rates in Response to Leakage Leakage into the flare system can be difficult to identify and often necessitates a plant shutdown to correct. During the time it takes to find and repair a leaking component all or part of the losses can be mitigated by using the leak as a purge source and reducing the supply of purge gas up to the volume of the leak rate. 5.4 Reducing Make-up Gas Consumption The quantity of fuel gas used to raise the calorific value of waste gas streams can be reduced by using incinerators in place of flares or installing instrumentation to automatically adjust the delivery of make-up gas. Module 4 of 17 Page 14 of 17

20 Replacing Flares with Incinerators Incinerators are an alternative to flares that can be considered for disposing of steady continuous waste gas streams with low heating values. These devices maintain waste gases in the presence of oxygen at higher temperatures for longer residence times than flares. As such destruction efficiencies are higher and gases with lower calorific values can be more efficiently combusted. In many cases waste gas streams that do not meet the calorific requirements to maintain reliable and stable combustion in a flare can be disposed of using an incinerator without adding any fuel gas. Even in situations where incinerators do require fuel gas to treat a waste stream the amount of fuel consumed is minimal compared to the make-up gas that would be required to sufficiently enrich the stream for disposal using a flare. Although incinerators offer a number of benefits they are not viable alternative to flares in all situations. Incinerators have lower turn down (i.e. typically only 10:1) and higher capital cost than flares. Instrumentation to Control Make-up Gas Delivery Instrumentation including online calorimeters and flow meters may be used to regulate the delivery of make-up gas to ensure calorific requirements of the combined stream are satisfied while minimizing fuel gas consumed. This may be particularly beneficial in situations where the composition and flow of the waste gas are variable. Module 4 of 17 Page 15 of 17

21 6. Record Keeping Operators should have a record program to support the company s flaring fuel consumption reduction program. Proper record keeping should assist in ensuring that underperforming systems are identified and that appropriate followup actions are implemented. This information will also assist in establishing the checking/testing frequency for each flare, to achieve cost-effective fuel gas reductions. Although each company will define its record keeping system, consideration should be given to recording and retaining the following information: expected fuel gas consumption by each flare, records of changes/upgrades that have been performed, actual fuel consumption measurements, the economic analysis performed to evaluate reduction opportunities where flares have not been adjusted/modified on economic grounds. Module 4 of 17 Page 16 of 17

22 7. References Canadian Association of Petroleum Producers, Best Management Practices - Facility Flare Reduction, December Accessed: September 2007 Available at: United States Environmental Protection Agency, EPA Air Pollution Control Cost Manual Sixth Edition, Section 3.2 VOC Destruction Controls, Chapter 1 Flares (EPA/542/B ), September 2000: Accessed: September 2007, Available at: David Shore, Making the Flare Safe, Journal of Loss Prevention in the Process Industry, Volume 9, Number 6, November 1996, pp Alberta Energy and Utilities Board, Directive 060: Upstream Petroleum Industry Flaring, Incinerating, and Venting, November 2006, Accessed: September 2007, Available at: Alberta Energy and Utilities Board, Directive 017: Measurement Requirements for Upstream Oil and Gas Operations, May 2007, Accessed: September 2007, Available at: Module 4 of 17 Page 17 of 17