FLARE SYSTEM DESIGN – WHAT IS IMPORTANT?
J O H N Z I N K C O M PA N Y
® A KOCH INDUSTRIES COMPANY
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Flare system design - What is important? by Robert E. Schwartz and Dr. Shin G. Kang John Zink Company Introduction The flare system fills a key role in the overall safety and environmental compliance of a hydrocarbon processing plant or production facility. Because the flare system does not directly produce revenue, its design considerations and the factors influencing the flare design are often not well understood. Our purpose here is to improve understanding of these factors and considerations so that plant operators and designers can achieve the flare system's prime objective, i.e., safe, effective disposal of gases at an affordable cost. The discussion will focus primarily on emergency flaring systems as used for refineries and petrochemical plants, but many of the points discussed can be applied to other types of flare applications. Design Factors - What Is Important? As the flare designer starts his work, he needs to be aware of certain factors which can influence the design of the flaring system. Major factors influencing flare system design are: • flow rate; • gas composition; • gas temperature; • gas pressure available; • utility costs and availability; • safety requirements; • environmental requirements; • social requirements. These factors define the flaring requirements and should be designated by the plant owner, and/or the plant designer. In reviewing the list of factors, it can be seen that the first four factors are all determined by the sources) of the gas being vented into the flare header. The next factor is related to the design of the facility itself and its location. Safety, environmental, and social requirements all relate to regulatory mandates, the owner's basic practices and the relationship between the facility and its neighbors. A discussion of each factor is provided below. Flow Rate The flare system designer will rely heavily on the flow data provided. Therefore, it must be carefully determined. Overstatement of the flows will lead to oversized equipment which increases both capital and operating costs and can lead to shorter service life. Understatement can result in an ineffective or unsafe system. Flow rate obviously affects such things as the mechanical size of flare equipment. Its influence, however, is much broader. For example, increased flow generally results in an increase in thermal radiation from an elevated flare flame which in turn will have a direct impact on the height and location of a flare stack. The maximum emergency flow rate may occur during a major plant upset such as the total loss of electrical power or cooling water. However, some processes have their maximum flow rate under less obvious emergency conditions such as partial loss of electrical power whereby, for example, pumps continue to supply feedstock to a disabled section of the plant. In the past, the maximum flow rate was sometimes determined by summing the flow rates of each of the connected relief devices. This approach resulted in an unrealistically large maximum flow rate. Modern plant design and analysis tools such as dynamic simulation allow the process designer to much better define the maximum flow rate to the flare. Careful attention to the design of control and electrical power systems can significantly reduce flare loads as well [1 ]. In addition to the maximum flow conditions, it is also important to define any flow conditions under which the flare is expected to burn without smoke. Typical conditions requiring smokeless burning are flows generated by process upsets, failures such as a compressor trip-out or from various operations of the plant including startup, shutdown and blowdown of certain equipment. Attempts to short-cut the establishment of the smokeless burning requirements by setting the smokeless flow rate as a percentage of the maximum emergency flow rate can lead to disappointment or needless expense.
Figure 1. Flare System General Arrangement
Gas Composition Gas composition can influence flare design in a number of ways. The designer should be given the gas composition for each of the flow conditions identified above. By studying the composition, the combustion characteristics of the gas and the identity of potential flue gas components can be determined. For example, John Zink Company's engineers have shown that the weight ratio of hydrogen to carbon in a gas is one of the parameters that can indicate the smoking tendency of the gas. Analysis of the composition will also reveal the presence of any non-hydrocarbons such as hydrogen sulfide or inerts. Such gases might require special metallurgies or design considerations such as ground level concentration analysis. Composition combined with the flow rate allows determination of the volume flow or mass flow of gases to be handled by the flare system. Gas Temperature In addition to the impact of gas temperature on thermal expansion, gas volume and metallurgical requirements, there is the more subtle effect of gas temperature on the potential of some components of the gas to condense. The potential for condensation or two-phase flow necessitates liquid removal equipment to avoid a greater smoking tendency and/or the possibility of a burning liquid rain. While a flare stack may appear to be free to expand, there can be serious mechanical design problems as a result of large gas temperature variations. In cases where the gas temperature at the source is significantly different from ambient it is advisable to calculate the heat loss or gain from the source to the flare stack and determine the resulting gas temperature. Such a heat exchange analysis may lead to significant stack cost savings. Gas Pressure Available The gas pressure available for the flare is determined by hydraulic analysis of the complete pressure relief system from the pressurerelieving devices to the flare burner. In most flare systems, much of the system pressure drop is due to piping losses with little pressure drop allowance for the flare burner. Such system designs may be shortsighted in that they fail to utilize the gas pressure in a constructive way to promote smokeless burning. John Zink Company engineers have shown that smokeless burning can be enhanced by converting as much of the gas pressure available as possible into gas momentum. An added benefit of higher pressure drop across the flare burner is the reduction in the gas volume, which can lead to a smaller flare header size and reduced cost. In general, a reduction in flare system size also allows a reduction in purge gas requirements. Utility Costs and Availability In many cases, the momentum of the gas stream alone is not sufficient to provide smokeless burning. In such cases, it is necessary to add an assist medium to increase the overall momentum to the smokeless burning level. The most common medium is steam which is injected through one or more groups of nozzles. An alternative to steam is the use of large volumes of low-pressure air furnished by a blower. Local energy costs, availability and reliability must be taken into account in selecting the smoke-suppression medium. For example, if fresh water is in limited supply, the use of steam may not be feasible. Purge and pilot gas must be supplied to the flare at all times. The amount of purge and pilot gas required is influenced by the size of the flare system. The purge gas requirement can also be influenced by the composition of the purge gas and/or the composition of the waste gas. Pilot gas consumption will also be influenced by the combustion characteristics of the waste gases.
Figure 2. A Refinery Flare System Environmental Requirements Because most emergency flare systems employ a flare burner at the top of a tall structure, any flame becomes a focal point of neighborhood attention. Therefore, a primary environmental requirement is the need for smokeless burning for at least the most frequent flaring events. As noted in the discussion above, in many cases it is necessary to inject an assist medium such as steam in order to achieve smokeless burning. The injection of the steam and the turbulence created by the mixing of steam, air and gas causes the emission of sound. The sound level at various points inside and outside the plant boundary is often limited by regulation.
Other environmental concerns are the combustion efficiency and flue gas emissions. Pioneering tests conducted by John Zink Company established that a properly designed and operated flare burner will have a combustion efficiency of more than 98%, and in many cases, greater than 99% [2]. Safety Requirements Safety concerns include thermal radiation from the flare flame, reliable ignition, hydraulic capacity, air infiltration and flue gas dispersion. Certain aspects of safety are dictated by the basic practices of the owner. For example, the allowable radiation from the flare flame to a given point is frequently specified by the owner based on the owner's safety practices. Therefore, it is not surprising that the allowable radiation level will vary from owner to owner. A common point of variation involves the inclusion or exclusion of solar radiation in the allowable level. As flare system designers, we find that solar radiation can usually be excluded. There are several sources for guidance on the allowable radiation level. The most widely referenced is API Recommended Practice (RP) 521 [3]. Most specifications call for a maximum radiation level of 4.73 kW/m2 (1500 Btu/hr/ft2) for emergency flaring conditions. Some specifications call for an additional radiation level limit of 1.58 kW/m2 (500 Btu/hr/ft2) for long duration flaring events. Special consideration should be given to radiation limits for flares located close to the plant boundary. Reliable ignition, hydraulics and air infiltration issues will be discussed in later sections. Social Requirements Even though the plant owner has complied with all of the environmental regulations, the resulting flare system may not meet the expectations of the plant's neighbors. On a day-to-day basis, a smokeless flame may meet the regulatory requirements, but be objected to by the neighbors due to light and noise. John Zink Company recognized the need to reduce elevated flaring 30 years ago and invented the ZTOF, the world's first successful enclosed ground flare, for the elimination of day-to-day visible flaring. Since that time many plants have adopted a good neighbor policy by including a ZTOF in their flare system. The general arrangement of a flare system which incorporates a ZTOF for day-to-day flaring rates and an elevated flare for emergency rates is shown in Figure 1. The liquid seal in the system acts to divert flow to the ZTOF until it reaches its maximum capacity. Any additional flow will pass through the liquid seal and be burned at the elevated flare tip. An installation of such a system is shown in Figure 2. Design Considerations What Is Important? Having received information on the design factors set forth above, the flare designer must now apply his expertise to the following design considerations. A given project may require inclusion of all or only a few of these points depending on the nature of the design factor information disclosed. The prime objective of safe, effective disposal of gases must never be compromised as each appropriate consideration is incorporated into the overall flare design. The design considerations are: • reliable burning; • hydraulics; . • liquid removal; • air infiltration; • flame radiation; • smoke suppression; • noise/visible flame. While the design of the flare is the province of the flare expert and often involves the use of proprietary know-how, successful selection and operation of flare equipment is greatly enhanced by an understanding of these design considerations. Insight into each consideration is set forth below. Reliable Burning Venting of waste gases can happen any time during plant operation. Therefore, an integrated ignition system is required which can immediately initiate and maintain stable burning throughout the period of waste gas flow. Stable burning must be assured at all flow conditions. Such an ignition system includes a continuous burning pilot(s), pilot ignitor(s), pilot monitor(s), and flame stabilizer. Systems without a continuous pilot should be avoided. The number of pilots required may vary depending on the size and type of the flare burner. These pilots are usually premixed burners designed such that pilot gas and air are mixed together at a point remote from the flare burner exit and delivered through a pipe to the pilot tip for combustion. This ensures that the pilot flame is not affected by conditions at the flare burner exit such as the presence o1 flue gas, inert gas or steam. Pilot gas consumption varies according to the specific flaring requirements. However, there is a lower limit to the pilot gas consumption. A pilot monitor is often used to verify the pilot flame. For safety reasons, pilot ignition is usually initiated from a position remote from the flare stack. Either a flame front generator or direct spark pilot ignition may be used depending on the system requirements. There is a complex relationship between exit velocity, gas composition, tip design and the maintenance of stable burning. There are a number o advantages in using the highest exit velocity possible such as minimum equipment size and optima flame shape. In addition, since high discharge velocity tends to improve air mixing with a resultan reduction in soot formation, one can see that maximizing discharge velocity can help improve smoke less performance. It is important to note that discharge velocity may be constrained by the gas pressure available or concerns about flame stability Early authors on flare system design suggested limiting discharge velocity to 0.2 Mach due to stability concerns. Later it was suggested that the discharge velocity of 0.5 Mach or higher could be used if proper flame stabilization techniques were employed. Exit velocities of Mach 1 or greater have been successfully used by John Zink Company. Waste gas composition can significantly affect the allowable exit velocity. For example, a properly designed flare burner can maintain stable burning of propane at Mach 1 or greater. On the other hand, if the propane is mixed with a large quantity of inert gas, the maximum exit velocity must be limited to a much lower Mach number in order to assure stable burning. Hydraulics
Most flare systems consist of multiple relief valves discharging into a common flare manifold or header system. A key item influencing the flare system design is the allowable relief valve back pressure. The system pressure drop from each relief valve discharge through the flare tip must not exceed the allowable relief valve back pressure for all system flow conditions. This value is typically limited to about 10 percent of the minimum relief valve upstream set pressure for conventional relief valves. The allowable relief valve back pressure can be increased by the use of balanced pressure relief valves. Balanced valves can accept a back pressure of about 30% of upstream set pressure in most cases. Where there is a wide variation in the allowable relief valve back pressures it may be economical to use separate high and low pressure headers. Increasing the allowable relief valve back pressure can have several effects on the flare system components, such as: - smaller manifold and header piping; - smaller knock-out and liquid seal drums; - smaller flare size giving lower purge rates and enhanced operating life; - significant reduction or elimination of utilities required for smokeless burning through the utilization of increased pressure energy at the flare tip. The capital cost savings on equipment as a result of increased pressure drop are attractive, but not the only factor to consider. Savings in reduced continuous operating costs can be very significant during the life of an average flare system. Liquid Removal Inherent in many flare systems is the potential for either liquid introduction into or the formation of hydrocarbon or water vapor condensate in the flare header. Allowing this liquid phase material to reach the combustion zone produces serious problems. If large droplets are present there is the potential for producing a burning rain of hydrocarbon from the flame envelope. This can result in flaming droplets blanketing the area or, in the worst case, flaming liquid flowing down the outside of the flare stack, posing a serious safety hazard. Even very small droplets can pose problems. Hydrocarbon droplets small enough to be entrained by waste gas and carried into the flame usually burn incompletely forming soot, and, as a result, reduce the smokeless capacity of the flare.
Figure 3. Steamizer Flare Burner Incorporation of a properly designed and operated knock-out drum into the flare system can mitigate these problems. There are three types of knock-out drums which can be incorporated into a flare system: a horizontal settling drum, a vertical settling drum, and a cyclone separator. Horizontal settling drums are large drums in which the gas stream velocity is greatly reduced and liquid droplets are allowed sufficient residence time to separate from the gas by gravity. The pressure drop across these drums is comparatively low. However, large emergency flows can require extremely large drums in order to reduce velocity and provide sufficient residence time. Vertical settling drums work in a similar fashion. In designing vertical settling drums, careful attention has to be focused on droplet terminal velocity. Any small droplets which pass through the knock-out drum can agglomerate to form larger droplets in the flare system downstream of the knock-out drum. This problem can be minimized by locating the knock-out drum very near the base of the flare stack, or incorporating it into the stack base. Although the pressure drop required for these drums is low, the required drum diameter can become impractical at large flow rates. Elimination of very small liquid droplets cannot be accomplished through a simple reduction in gas stream velocity. Cyclone separation is best for this type of application. Cyclones, utilizing centrifugal force, can be incorporated into the base of the flare stack. They are smaller in diameter than horizontal or vertical settling drums and usually provide high liquid removal efficiency at the expense of a higher pressure drop. Regardless of the knock-out drum concept, the holding capacity of the drum should be sufficient for any anticipated liquid flow. Drum pump-out capacity must be adequate to prevent overfilling of the drum. In addition, backup for the pump should be provided. Any liquid removed by the knock-out drum must be carefully disposed of or stored.
Air Infiltration Infiltration of air into a flare system may lead to flame flashback, which in turn may cause a destructive detonation in the system. Air can enter the system by one or more of the following scenarios: - through stack exit by buoyancy exchange, wind action or contraction; or - through leaks in piping connections; or
- as a component of the waste gas. Prevention measures are available to address each of the air infiltration mechanisms.
Figure 4. Schematic Diagram of a John Zink ZTOF In order to prevent air ingression through the stack exit, purge gas is injected into the flare system. The quantity of purge gas required is dependent on the size of the flare, the composition of the purge gas, and the composition of any waste gas which could be present in the system. In general, the lower the density of the gases present, the greater the quantity of the purge gas necessary for the safety of the system. The purge gas requirement can be reduced by use of a conservation device such as a John Zink AirrestorTM, or Molecular SeaITM. The cost and availability of the purge gas will determine the value of such a device. Contraction of gas in the flare system occurs due to cooling following the flaring of hot gases. The rate of contraction will accelerate if the cooling leads to condensation of components of the contained gas. Air infiltration risk can be minimized by use of a John Zink TempurgeTM system to control the introduction of extra purge gas to offset contraction. An elevated flare stack filled with lighter-than-air gas, due to molecular weight or temperature, will have a negative pressure at the base as a result of gas buoyancy. If negative pressure exists at the base of the stack, the entire flare header system will be under negative pressure. Operation of the flare system under negative pressure greatly increases the potential of air infiltration into the header system through leaks or open valves. Such leakage can occur during the servicing of relief valves. Installation of a liquid seal in the system provides a positive flare header pressure, greatly reducing the potential of air leakage into the system. The liquid seal can also create the necessary back pressure for the operation of a flare gas recovery system and can also be a barrier to air entry from the flare stack. Locating the liquid seal in the base of the stack offers maximum protection of the system, and isolates the flare ignition source from the flare header and the process units. While the advantages of a liquid seal are readily apparent, the design of a successful seal is very difficult. Conventional liquid seal designs result in a coupled effect between the seal liquid and the gas flow which produces an undamped oscillation of gas flow giving the flare flame a puffing character. This on-off or smoke signal type of burning is very annoying to the neighbors and makes efficient smokeless flaring impossible. John Zink Company engineers invented liquid seal designs which prevent undamped oscillation of gas flow yielding an even gas flow to the flare burner. Waste gases which contain oxygen present a special design challenge. Flashback in systems handling such gases can be prevented through the use of flame/detonation arrestors, special liquid seals, and the use of specialized flare burners. Oxygen containing gases should be segregated from the main flare system. Flame radiation As the waste gases are burnt, a certain portion of the heat produced is transferred to the surroundings by thermal radiation. Safe installation of a flare stack requires careful consideration of the thermal radiation. The radiation limits discussed earlier provide guidance in locating the flare stack. For a given set of flare flows, the radiation limits can usually be met by adjustment of the flare stack overall height and/or the sterile area utilized. This in turn affects the economics of the plant. For plants with limited plot area or for ships, an enclosed flare can be employed to meet radiation restrictions. Several published methods are available for preliminary estimation of flare radiation. A detailed discussion of flare radiation prediction and a critical review of published methods are presented in the article by Schwartz and White [4]. It is clear that flare designers and users alike must be cognizant of the possibility of error when using traditional methods to calculate radiant heat intensities. John Zink Company recognized the limitations and risks associated with the traditional methods many years ago and undertook the development of proprietary methods for radiation prediction. Our latest prediction methods capture the effect of flare burner design, gas quantity and composition, various momenta and smokeless burning rate on the flame shape and radiant characteristics. Smoke Suppression Smokeless burning is dependent upon achieving adequate overall momentum to educt sufficient air and promote mixing. Based on the information on the factors listed above, the flare designer will generally consider the following alternatives for achieving the necessary overall momentum: energy transformation, steam injection, low pressure air addition, or a combination of these alternatives. Briefly, energy transformation entails the conversion of the internal energy (pressure) of the waste gas to kinetic energy (velocity). Systems employing this technique have been very successful and enjoy a low operating cost. Steam injection is the most common technique for adding momentum to low pressure gases. Steam itself provides an additional smoke suppression benefit as it interacts in combustion chemistry. Low pressure air can be utilized in cases where gas pressure is low and steam is not available.
John Zink's aggressive research and development effort has produced new flare burner designs capable of increased smokeless capacities and, when necessary, lower noise. This R&D effort has also yielded new analysis methods which allow the accurate prediction of the performance of our flare burners. In the case of steam assisted flares, we have developed steam injection tips and other injection methods, which provide a significant increase in the amount of air educted by a given quantity of steam. These advanced technologies are incorporated in our new SteamizerTM flare burner. A SteamizerTM flare burner is shown in Figure 3. By combining these new tips and methods, we are able to increase the smokeless capacity of many existing installations or provide the large smokeless rate required by many new plants. Noise/Visible Flame Just as a portion of the energy released in burning waste gas goes to thermal radiation, other portions of the energy go to sound and to light. Most plants are equipped with elevated flares which by their nature broadcast flaring sound to the plant and to the surrounding neighborhood. In some cases, the sound level becomes objectionable and is considered to constitute noise. Flaring noise is generated by at least three mechanisms: - by the gas jet as it exits the flare burner and mixes with surrounding air; - by any smoke suppressant injection or mixing; - by combustion. The noise generated by the first two, especially the second, can be mitigated by use of low noise injectors, mufflers, and careful distribution of suppressant. The SteamizerTM flare burner shown in Figure 3 is of low noise design with additional noise reduction coming from a muffler concept first developed by John Zink Company for use on enclosed ground flares. In situations where noise is a severe problem, such as residential areas bordering a plant, flaring noise can be reduced by the use of an enclosed ground flare. The enclosed ground flare has the additional advantage of eliminating visible flame during day-today operations. A schematic diagram of a John Zink ZTOF type enclosed ground flare is shown in Figure 4. A ZTOF has a large, often cylindrical, combustion chamber which contains the flame rendering it invisible to the neighbors. Air enters the combustion chamber by natural draft after passing over a windfence. The stack and windfence are designed to provide acoustic dampening and to limit direct line of sight transmission of noise. Use of a ZTOF allows a plant to operate in harmony with its neighbors. Summary Everyone associated with a flare system must be focused on its prime objective of safe, effective disposal of gases. The importance of flare system design factors and their influence on system design considerations have been discussed. The fundamental knowledge of the flare design factors and considerations provided will assist owners and plant designers in specifying and evaluating flare systems. The skill and experience of the flare system designer are most effective when the designer has a clear understanding of the system definition. Early interaction between the plant owner/plant designer and the flare system designer provides the opportunity to further mutual understanding of needs and opportunities. Such interaction can lead to a system design which maximizes system capability and minimizes cost. References 1-`Improve Flare Management,' Hydrocarbon Processing, July 1997. 2-`RACT for VOC - A Burning Issue,' Pollution Engineering, July 1983. 3-`Guide for Pressure-Relieving and Depressuring Systems,' API Recommended Practice 521, 4th Ed., American Petroleum Institute, March 1997. 4-`Flare Radiation Prediction: A Critical Review,' Chemical Engineering Progress, July 1997.
