SAFE FLARE SYSTEM DESIGN INTRODUCTION In the safe, satisfactory operation of a process plant, the flare system is the single most important element for operational or emergency relief of flammable substances in the liquid or gaseous phases. Most people see flares as simply a fire on top of a pipe that burns gases. To the public, flares often appear as a source of odor, smoke, noise, fallout and light. This paper discusses flare-related factors including location, equipment involved, and design. It also explores the most common factors involved in flaring accidents. TYPICAL FLARE SYSTEM The typical flaring system consists of equipment that safely combusts vented hydrocarbons at a pressure drop which doesn't compromise plant relief systems. The ideal operating condition would be to eliminate the need for flares, as they waste hydrocarbons which could become products and thereby improve profits. However, facilities to recover large amounts of released hydrocarbons under emergency conditions are currently not economically justified. the equipment to recover daily hydrocarbon leakage rates is justifiable, and is in use in many plants. The typical flaring system handles the discharge of all reliefs inside a designated unit or number of units. A flaring system generally consists of the following major components and subsystems: • Collection piping within a unit (including a mix of pressure reliefs and vents) • A flare line to the site • A knockout drum to recover liquid hydrocarbon from the gas stream • A liquid seal to provide positive header pressure without surging and to protect against flashbacks (optional) • A flare stack with flare tip • An assist system to maintain smokeless burning (optional) • A fuel gas system for pilots together with ignitors • Controls and instrumentation DESIGN GUIDES Flare Location Economics dictate the amount of ground space allocated to a flare stack. Not only must the location comply with all governmental regulations affecting height, noise, smoke suppression, and allowable toxic concentrations, other important factors must also be considered. These include the relation of the flare to the following: • areas in the plant where people work • outside residential areas • commercial areas • roads • other elevated structures • guy wire location with regard to possible interference Wind Effect Wind direction and velocity affect the head radiation produced by the flare by influencing the length and angle of the flame. Normal design practices assume
the average wind speed for the area and take any possible wind direction into account. Temperature Inversions Temperature inversions and other meteorological conditions affect atmospheric stability and reduce the dispersion of odors, toxic concentrations, and smoke. These conditions should be considered on the basis of the frequency of temperature inversion occurrences and the expected effect on people in the area. Type of Waste Gas to be Flared The type of gas to be flared affects the design and size of flare tip used. It is no longer acceptable to design on only a molecular weight. Some gases burn without support gas; others, such as low BTU content gases, must be supplemented. Flare manufacturers can help determine the combustion characteristics of each gas stream. Some gases require assistance to burn smokelessly, and the assist method selection is important to flare safety. Questions such as the following must be answered: What is the reliability of the assist medium? Will it be available in the quantities needed during an emergency? Which assist method is most economical? Typically, in reviewing gas stream composition, the primary concern is assuring destruction efficiency. A good destruction efficiency is assured if stable flame can be established. Accomplishing this requires different techniques for various gas properties. Another safety concern is with inappropriate materials being routed to the flare. The watery gas oil relieved at 500°F into 1,000 feet of cold relief header may be a waxy solid at the other end of the line. The steam line attached to the relief line may generate an ice block in winter weather. Relief valve location is another factor to consider for safe flare design. If the flare is blocked, relief valve protection may be lost. Flare blockage may occur because of the waxy gas-oil mentioned above or from a poorly sized system, which puts excessive back-pressure on the relief valve. Layout and selection of relief valves and the operating instructions for the relief system need always consider these problems for safe flare system design. Heat Radiation Radiation from the flare flame generally determines flare stack height. During normal operations, design intent is to limit heat intensity at grade to levels which are low enough for both humans to safely withstand and to protect surrounding equipment from heat-related damage. Heat intensity is measured in terms of BTU/hr/sq. ft. The recommended design level for plant areas is 1,500 BTU/hr/sq. ft. This intensity level permits a properly-dressed person to perform normal functions in the area of an operating flare. Equipment can withstand up to 3,000 BTU/hr/sq. ft. Often overlooked, above-grade maintenance work performed while the flare is in service must also be considered. Without special precautions, maintenance personnel can receive more heat than recommended if working above-grade. Usually the largest release of hydrocarbon (heat release) occurs during instances of major upset, such as a power loss, loss of cooling water, or a fire in the unit. Any hydrocarbon discharge under these conditions is a true emergency situation.
A summary of the capacities of those relief valves likely to be involved in a worst case emergency will dictate the design rate that flare must handle. System volume size will usually dampen transient peaks. Note that relief valves on vessels may not all open simultaneously, or remain open. Multiple unit use of a single flare complicates the sizing due to the likelihood of multiple units relieving to the flare at the same time. If a common utilities failure causes dumping at the same time to the same flare, the flare must be sized accordingly. Radiation from more than one flare in a localized area must also be considered. Not only should the combined radiation be calculated, but maintenance work at above-grade elevations on one flare while another nearby system is in service must be considered when flare spacing is set. Unit expansions normally add to the possible heat release from a flare. When considering any major addition, always check on the heat release and radiation effects which will result with such an expansion. A new flare tip and/or a higher flare stack may be necessary to maintain safe conditions. In an expansion scenario, header pressure drop is also an especially critical function. Ground Level Concentration Some waste gases are lethal in high concentration. If toxic gases such as hydrogen sulfide or hydrogen cyanide are in the waste stream, flare height must be calculated to assure that unburned toxics do not exceed safe dispersion/ground-level concentrations. Thermal rise and exit velocity effects dilute stack gases. As the gases reach grade downwind of the stack, toxic materials concentrations must be reduced to tolerable levels. Although standard calculations exist to determine proper stack height to assure safe toxics levels at grade, these methods are theoretical and each case should be investigated individually. A typical grade concentration design target is 0.10 ppm or less, downwind of the stack. This is fine for most toxic or noxious substances; however, a few substances exist for which this small concentration is still far too great, and a sever odor or health nuisance may be created. According to Volume 2 of the Manual of Disposal of Refinery Wastes, Waste Gases and Particulate Matter, published by the American Petroleum Institute, the following substances cause odor nuisance in the concentrations shown: . Compound ppm Isobutyl Alcohol 0.0003 Isoamyl Alcohol 0.0026 Methyl Mercaptan 0.041 Ethyl Mercaptan 0.0028 n-Propyl Mercaptan 0.0016 n-Butyl Mercaptan 0.001 Isoamyl Mercaptan 0.00043 p-Thiocresol 0.0027 Methyl Sulfide 0.0037 Ethyl Sulfide 0.000056 Propyl Sulfide 0.011 n-Butyl Sulfide 0.015 Maintenance
Because maintenance questions can impact flare system design, issues such as the following must be considered: • Can the flare system be removed from service when the unit is shut down? • How many days will the flare system be permitted out of service for repairs? Steamout and unit purging reduces time available to work on a flare. • Are there other stacks discharging in the area which would make work at high elevations dangerous due to pollutant concentration or heat radiation? Elevated structures are often overlooked in design evaluations. Flashback Protection A sure method of preventing the backflow of air into the flare stack must be provided to protect against explosions. Flame arresters are satisfactory in certain situations; however, because of plugging, maintenance, and expense they must be carefully considered. Systems with large turndown, hydrogen, or other high flame-speed components, dirty gases or large capacity may warrant alternative means of flashback protection. In small systems such as terminal flares, flame arresters are valuable as a endpoint during flashback. Flame tends to "live" on the flame arrester element. When using a flame arrester, a thermocouple monitor of the flame bank with high temperature alarm is a good safety feature. This setup will immediately alert the operator to the problem. When a need exists to stop a flame front that might develop in mixtures of air and flammable gases, a suitably designed liquid seal is the best checking device. However, a typical dipleg liquid seal is by no means satisfactory for this service because, as the flow quantity through the seal increases, there is uninterrupted gas flow through the water. Experience with hydrogen/air mixtures clearly shows that a flame front will move countercurrent to velocities well in excess of 200 ft./sec. A liquid seal protects equipment from explosions resulting from the ignition of a hydrocarbon/air mixture which backflows into the flare stack. The seal accomplishes this protection with two functions. First, it provides a fail-safe backpressure control which maintains a positive pressure on the flare header, preventing air ingress. Second, it acts as a positive separation between the flare and process units. Normally located in the base of the flare stack, the gas stream bubbles through the liquid seal or displaces the water to release the gas. A small stream of water is added to the seal, and the liquid level is maintained with a continuous overflow or level switch. An undesirable operating characteristic of poorly-designed liquid seals is their tendency to surge. This occurs when relatively violent interruptions in gas stream flow create rhythmic pressure surges both upstream and downstream of the water seal. This characteristic is evident in flare system operation in which violent burning is followed by complete flame extinction at the flare tip, then the gas is reignited and the burning/extinction cycle occurs again. Each time burning begins again upon re-ignition, an annoying booming sound can be heard. While not necessarily unsafe, this is an environmental concern. Good liquid seal internal design should prevent this surging problem. Air Ingress Prevention
Gas purging issued to protect flare systems from explosions which would result from ignition of a hydrocarbon mixture with air which backflows into the stack. Most hydrocarbons are considered safe and nonflammable with 6% or less oxygen in the mixture. However, when large amounts of hydrogen are present, a lower oxygen level is required. Any gas or mixture of gases which does not contain oxygen and cannot reach dew point at any condition of ambient temperature normal to the job site can be used as a purge gas for flare systems. This gas may also be referred to as "sweep" gas. Steam as a purge gas is not recommended for two reasons. First, steam is at an elevated temperature, and the steam content of the flare shrinks at the steam cools and condenses. Second, as the steam condenses, water is left in the flare system. This presents a freezing hazard and encourages accelerated corrosion. For a corrosive waste gas, the purge gas should enter the flare system immediately downstream of the relief valve so that the entire system will be sweeped. Most systems introduce the purge in a location that provides uninterrupted flow at design volumes. For a system with a liquid seal, it should be downstream of the seal or designed to continually flow through the seal at a low pressure. It is recommended that a pressure switch be installed immediately upstream of the orifice which regulates purge volume so that an alarm sounds if the purge gas pressure upstream of the limiting orifice falls below a set point. It is also recommended that the purge gas pass through a strainer in which the mesh openings are not more than one-quarter the diameter of the limiting orifice for purge gas regulation. A purge gas volume which creates an upward velocity in the flare riser at .O1 ft/sec. is normally recommended when the John Zink Molecular Seal is used on the flare. If a John Zink Airrestor is used, the purge gas velocity would be 0.04 ft/sec. If a Molecular Seal or Airrestor is not used, the purge gas volume must provide an upward velocity at .25 to 50 ft/sec. to be considered adequate for safety, depending on gas gravity. These velocities are based on full-scale test data (30 in. outside stack diameter) and may be conservative for smaller-sized stacks. Velocity-type purge conservation devices such as the John Zink Airrestor consist of a fixed baffle mounted in a flare tip. This arrangement presents little pressure drop to the out flowing gas. An Airrestor helps reduce purge gas requirements but does not eliminate their need. For example, in a 20 in. flare stack, a purge rate of 2,500 scfh is reduced to 300 scfh with an Airrestor (oxygen level 20 ft. below tip measured at 6%). All velocity devices are rated at six to eight mole percent oxygen levels. Molecular or diffusion seals are used to further reduce the amount of purge gas required. The seal is installed below the flare tip assembly and operates on the principle of gas diffusion. Molecular seals are large because of the complexity of their internal passages. The Molecular Seal has been shown to be the most effective purge gas reduction device available. For example, in the 20 in. diameter flare stack, a purge rate of 2,500 scfh is reduced to 75 scfh with a Molecular seal (oxygen level 20 ft. below tip measured at 0%). The diffusion seal is normally rated at less than 0.1 % oxygen. The difference in the design bases between the two types of purge reduction devices can be very important for some flare gas compositions. There is a danger of explosion in the flare system if the flare pilots are ignited before the total flare system has been purged. To assure low or zero oxygen levels, a volume of non-condensable gas equal to ten or more times the volume of the flare system is used. The flare system includes all piping from the relief
valves to the stack and rising to the elevation of the flare at the burning point. The pilots should be ignited only after the system has been purged and preferably while purge gas is still being admitted. Liquid Handling "Flaming rain" is a very real hazard for flares with liquid relief potential. Knockout drums are used to prevent hazards associated with flaring gas that contains liquid droplets. The drum just be large enough to effect the desired liquid/vapor separation and have the holding capacity to contain any anticipated slug of liquid. Most flares can handle a liquid mist up to the point where the oil droplet settles to grade faster than it is consumed by the fire surrounding it. Generally, this is considered to represent 600 micron particles. However, different flare tips can handle different liquid rates. The kinetic energy flare tips can handle higher liquid loads than open pipe flares because they take a significant pressure drop at the tip. Ignition Systems Ignition of the pilots is accomplished with a flame front generator. This device mixes air and fuel and ignites it. The major safety problem associated with ignition systems is the loss of ignition through improper control of pilot fuel. The pilot is the major safety control for the flare. Any reduction in the pilot's effectiveness is a significant safety hazard. The trend to pilot monitoring and automatic relight of pilots is positive because it emphasizes pilot maintenance. SUMMARY The five most frequent causes of flare accidents are: • Internal explosion • Liquid carryover • System obstructions • Faulty maintenance procedures • Ignition loss A safe, functional flare system takes all of the design factors into consideration. Of the three primary ingredients of a disaster, two are always present in a flare system: fuel and fire. If proper control is maintained, oxygen will not be present. BIBLIOGRAPHY 1. APR RP 521. "Guide for Pressure Relief and Depressuring Systems," First Edition, (New York: American Petroleum Institute, September 1969). 2. APR RP 520. "Recommended Practice for the Design and Installation of Pressure Relieving Systems in Refineries," Fourth Edition, (New York: American Petroleum Institute, 1976). 3. John Zink Company. "Advances in Offshore Flaring Technology," Presented at a seminar on Flare Systems arranged by The Norwegian Society of Chartered Engineers, (November 1980). 4. John Zink Company. "RACT for VOC - A Burning Issue," Pollution Engineering Magazine, (July 1983).
5. John Zink Company. "Flaring in Hostile Environments," Presented at a seminar on Flare Systems arranged by The Norwegian Society of Chartered Engineers, Geillo, Norway, (September 1982). 6. Reed, R. D. "Design and Operation of Flare Systems," Chemical Engineering Progress, vol. 64, no. 6, pg. 53-57. 7. Bluhm, W. C. "Safe Operation of Refinery Flare Systems," presented at a seminar on Operating Practices, 26th Midyear Meeting: Petroleum Institute's Division of Refining, Houston, Texas, (May 1961). 8. Powell, D. P and Schwartz, Robert E. "Operating Experience with a Low LevelType Flare," API Reprint no. 59-72, 37th Midyear Meeting: Petroleum Institute's Division of Refining (1973). 9. Schmidt, T. R. "A Smokeless Flare System," Ecolibrium, Shell Oil Company, vol. C, no. 1 (1977). 10. John Zink Company. "Designing Flares for Enhanced Service Life," Presented at a seminar arranged by the Canadian Gas Processors Association (June 1983). 11. John Zink Company. "Total System Approach to Flare System Design," Presented at API Seminar: Fire Protection and Safety (October 1983). 12. John Zink Company. "Flare System Design," Presented at API seminar: Fundamental Applications of Loss Prevention, Houston, Texas (October 1986). International Headquarters P.O. Box 21220 • Tulsa, Oklahoma 74121-1220 U.S.A. 11920 East Apache • Tulsa, Oklahoma 74116-1300 U.S.A. Telephone (918) 234-1800 Fax (918) 2342700 Other offices are located in major cities around the world. © Copyrighted Company 1993 Technical Paper 54908
Flare Stack Support Structure • • •
Self-Supported Stack Derrick Stack Guyed Wire Stack
Flare Tips • MRW Air-Assist Flare Tips • MRW Steam-Assist Flare Tips •
MRW Gas-Assist Flare Tips
Pilot and Ignition System MRW pilots and ignition systems are proven to be reliable, fast, and easy to operate and maintain. Our pilots are designed for continuous and intermittent use in adverse weather conditions and extreme wind speeds. MRW pilots increase the reliability and performance of the flare system by providing flame stability in any operating conditions. We are confident that our pilots are the absolute best in the world! MRW designs extremely energy efficient pilots using as little as 50SCFH of fuel gas. The pilots are generally fabricated out of 309/310 stainless steel to extend the life of the pilot burner assembly. An air/fuel gas mixer attached to pilot burner assembly allows for a combustible mixture of pilot fuel gas at the pilot burner tip. Our local control panel powers the ignition transformers and automatically ignites/re-ignites the pilots. Ignition of the pilot can be done via: 1. Manual Ignition - the pilot is manually ignited via a push-button activation. 2. Auto Ignition - a spark igniting device and ignition transformer ignites the flame front. 3. Auto Re-Ignition - the flare tip pilot temperature switches automatically trigger the ignition transformers to re-ignite the flare tip pilots upon loss of flame. 4. Remote Ignition - the flare system to be capable of automatic startup and shutdown based on either local or remote initiation. Ignition of the pilots is accomplished using one of three methods: 1. 2. 3.
Flame Front Generator (FFG) Self-Inspirated Flame Front Generator (SI-FFG) Electric Spark Ignition (EFG) Pilot Monitoring Device
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Thermocouple - MRW uses a proprietary thermocouple design and location to safely and accurately monitor the pilot status while also extending the service life of the thermocouples • Flame Scanner • Flame Rod • Video Camera
Control and Ignition System MRW can design our control systems to meet any electrical classification. Our control systems utilize state of the art design techniques and technology to obtain optimum service potential. If you have a preference as to what type/brand of control system, we are happy to accommodate your needs.
Purge Reduction Seal (Optional) • Molecular Seal • Liquid Seal • Velocity Seal Flame or Detonation Arrestor (Optional) In some situations, a flame or detonation arrestor is preferred or necessary. If a liquid seal cannot be used, gases are in the explosive range, and/or there are extended pipe runs or bends in pipe, a detonation arrestor may be used. MRW uses detonation arrestors are approved by the US Coast Guard.
Knockout Drum (Optional) Knockout drums are provided in situations where liquid separation is likely in the waste stream. The knockout drum will collect any liquids that are present in the waste stream prior to entering the flare system. A knockout drum is especially important if substantial cooling of heavy liquids is expected. If the liquid is corrosive, MRW uses noncorrosive materials of construction. A level gauge and drain connections are built into the knockout drum.
Ladders and Platforms (Optional) Access to the flare tip may be provided upon request from grade with OSHA approved ladders and rest platform(s) and one 360° access platform located just below the flare tip for easy of access and maintenance the flare tip. Air Blower (Optional) If assist air is required, an optional air blower will be included. The combustion airflow shall be automatically controlled based on waste gas flow rate to the flare. A flanged inlet and outlet as well as access door and housing drain is standard. Air duct is usually supplied from the fan outlet to the inlet of the flare stack.
