Flare Tip Corrosion

CORROSION OF FLARE TIPS

J O H N Z I N K C O M PA N Y P R E S E N T E D AT NACE97 CHICAGO, ILLINOIS

® A KOCH INDUSTRIES COMPANY

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CORROSION OF FLARE TIPS by Dr. Wes Bussman, Jim Franklin and Robert Schwartz John Zink Company, Tulsa Oklahoma Presented at NACE97, Chicago Illinois Introduction If a flare tip fails, it may not safely or effectively dispose of the waste gas which could result in a costly plant shut-down. The premature failure of flare tips can often be directly attributed to corrosion damage. The major factors that influence the corrosion damage of a flare tip are service and operational conditions, metallurgical selection, design and/or fabrication [1]. These factors will be discussed in more detail along with specific case examples.

Factors Contributing to the Corrosion Damage of a Flare Tip Service and Operational Conditions Experience with identical flare tips has shown that service and operational conditions are the most crucial factors influencing the service life of a flare tip. A flare tip that is exposed to continuous or frequent flame impingement is doomed to fail regardless of its’ metallurgical selection, mechanical design or fabrication. The flare tip can experience flame impingement from internal and external burning which can cause high temperature corrosion damage leading to failure. Internal Burning. One of the most prominent mechanisms responsible for the failure of a flare tip is internal burning. The mechanism that creates internal burning depends on the flare tip design and operating condition. A flare tip, installed vertically, will typically experience internal burning if it operates at a low waste gas exit velocity in windy conditions. Under these operating conditions, the wind will create an internal recirculation pattern near the outlet of the flare tip causing the waste gas to migrate to the upwind side as illustrated in Figure 1a. The waste gas stream will pull into the recirculation zone and mix with the air as illustrated in Figure 1b. When the flare pilots ignite this air-fuel mixture, internal burning occurs. The potential for internal burning is much greater in horizontal flares or flares positioned at an angle between vertical and horizontal. Any flare tip orientation which gives a wind component into the mouth of the flare will develop a re-circulation region, consisting of air and fuel, inside the flare tip as illustrated in Figure 2. When the air-fuel mixture ignites, internal burning occurs. Steam-assisted flares can experience internal burning when the waste gas exit velocity is low to moderate and the steam flow rate to the perimeter steam injection nozzles is too high. Under this condition, steam jets from the upper steam nozzles can

create a capping effect over the waste gas stream. This capping effect can drive air and waste gas into the body of the flare creating internal burning as illustrated in Figure 3. External Burning. Here, external burning is defined as a condition that occurs when a flame impinges on the external surface of a flare tip and/or its appurtenances. This phenomena most commonly occurs at modest waste gas flow rates and under windy conditions. As the wind strikes the flare tip, a low pressure zone develops on the downwind side as illustrated in Figure 4. If the waste gas flow rate is high enough, the momentum of the exiting gas stream will overcome the low pressure zone and an upward projecting flame develops. However, at some lower flow rate, the waste gas is pulled into the low pressure zone on the downwind side and burns adjacent to the flare tip. The depth of flame pull down varies with tip diameter and wind velocity. Flares can also experience external burning from secondary fires. Secondary fires can occur from liquid carryover or from flame impingement from another flare in close proximity. Some flare designs can tolerate a certain quantity of micron-size liquid droplets in the gas stream, however, large quantities of liquid can cause severe damage to a flare tip when the liquid runs down the outside of the tip and burns. Flares that are supported on the same structure can experience flame impingement if the wind forces the flame from one flare onto other flare tips. When internal or external burning occurs, the flare tip is exposed to high temperatures in a predominantly reducing atmosphere. These conditions are typically cyclic due to continuous changes in wind speed and direction and gas flow rate. Flare tip life can be improved by minimizing internal and external flame impingement. Metallurgical Selection When making a material selection for a flare tip, one must consider cost, ease of fabrication and corrosion resistance. The key parameter for flare tip material selection, however, is corrosion from flame impingement. It is important to understand the application of the flare and the possible thermal exposure. A flare tip that is exposed to continuous or frequent flame impingement from internal or external burning will ultimately suffer high temperature corrosion damage regardless of the metallurgy, however, proper material selection can enhance the service life. Our preferred metallurgy for the heat affected zone of most large flare tip applications is 310 stainless steel. 310 stainless steel gives good overall protection against low intensity or occasional flame impingement, however, like any material, it will ultimately fail with repeated, intense impingement. Some flares are designed so that they very rarely experience direct flame impingement. For these applications, certain components of the flare can be made of 304, 316 or 321 stainless steel or similar cast alloys. From time to time, we use other materials including those with high nickel content. While the use of high nickel alloy materials can be intriguing, in our experience, the added cost of such material is often not justified by the gain, if any, in flare tip service life. In addition to exposures to high temperature and reducing atmospheres, the flare tip can also be subjected to chemical attack from the waste gas steam. The most common contributors to corrosion from the waste gas stream are hydrogen sulfide and sulfur oxides. The presence of sulfur in the waste gas stream can cause sever corrosion damage to a flare tip. Some material suppliers have recommended using high nickel alloys for

flares burning a waste gas with sulfur present. Field experience, however, has shown that high nickel alloys are not the best choice for sulfur applications. The combination of high temperature, reducing atmosphere and sulfur can quickly corrode high nickel alloys. Verma [2] has shown that the presence of chromium is the most important alloying element in resisting sulfidation attack because it forms a protective layer of chromium oxide over the metal surface. Howes [3] showed that high-nickel alloys generally increases susceptibility of sulfidation attack. When the nickel diffuses through the chromium oxide scale, it reacts with the sulfur environment to form nickel sulfides on top of the oxide scale. One particular nickel sulfide that forms on top of the protective chromium oxide scale is Ni-Ni3S2. This particular nickel sulfide melts a 1175 F. The molten nickel sulfide can easily destroy the chromium oxide scale and lead to catastrophic sulfidation attack. Atmospheric corrosion is typically not a predominant mechanism that leads to the failure of a flare tip, however, it can contribute to a reduced service life. Flare tip installations on offshore platforms or near the shore-side experience salty atmospheres. This atmosphere can lead to a chemical reaction between the oxide scale and the salt resulting in a breakdown in the protective scale. Material suppliers may recommend using high-nickel alloys in salty environments. Experience with flare tips exposed to salt water atmospheres, however, indicates that high-nickel alloys are not required. Exposure to heat and sulfur are usually more detrimental than salt water exposure. Flare tips installed in various plants can experience corrosion due to the gas fumes in the atmosphere. However, the concentration of these corrosive gases in the atmosphere is typically extremely low and not a major contributor to corrosion. Tip Design and Fabrication Flare operating conditions which cause internal and external burning cannot always be avoided. In recent years flare tip diameters have increased dramatically due to larger maximum flare gas flow rates and the need for greater smokeless capacities. In general, these larger tips are more susceptible to internal and external burning. However, experience has shown that some tip designs are more prone to internal or external burning than others. We have seen flare tip designs which were totally destroyed after a few weeks of operation and other tip designs which provided more than 20 years service. For example, tips which have slotted arms are prone to internal burning and have demonstrated short service lives. A flare tip should be designed so that it has a minimum number of weld joints exposed to high service temperatures. The fewer the number of weld joints, the less likely the flare tip will suffer cracking due to thermal stresses. Cracking at the weld joints can cause gas leaks and increase flame impingement that can eventually lead to high temperature corrosion damage. Thermal and mechanical stresses can affect a flare tip, therefore, proper fabrication is critical. Welds must be carefully designed and weld procedures adhered to. The flare tip design should allow the welder good access to all welds. When using insertion welds, care must be taken to minimize heat buildup during welding. For example, if there is poor fit-up of parts, excessive welding may be required leading to an increase in heat buildup in

the adjacent metal. Metal failure has been seen in such heat affected areas, and in some cases the adjacent weld itself was still intact.

Case Examples Case Example #1 - Flame Retention Segments. The purpose of a flame retention ring is to prevent the flame from lifting off and blowing out at high waste gas flow rates. A flame retention ring typically consists of a number of segments located around the inside perimeter of the flare tip at the exit. As their purpose would indicate, it is expected that the flame retention segments will be exposed to direct flame contact. Several years ago, a flare tip with 52 flame retention segments was placed in service on a waste gas stream that contained small amounts of hydrogen sulfide. John Zink Company recommended that the segments be made of 310 stainless steel. However, the user requested that the flame retention segments be made of Incoloy 800. After a period of operation, an inspection revealed that 48 of the segments were completely deteriorated while the other four segments suffered little deterioration. An analysis showed that the segments made of Incoloy 800 were completely destroyed and that the four good segments were made of 310 stainless steel. Our investigation determined that the tip was built on a rush bases and the 310 stainless steel segments had been used, with the customers knowledge, to complete the flare tip on schedule. X-ray analysis showed the presence Fe3O4 in both good and bad segments and the presence of sulfides by qualitative chemical analysis indicated sulfidation had occurred. It is believed that internal burning, due to the capping effect from the perimeter steam nozzles, may have created a corrosive sulfur atmosphere. This particular example demonstrates flare tip corrosion damage due to both improper operation and metallurgical selection. Case Example #2 - Pilot in a Enclosed Flare. A pilot operating on refinery fuel gas had difficulty maintaining a stable flame after only two months in service. An inspection of the pilot tip revealed a metal sulfide scale and loss of tip material. Upon investigation, it was determined that the refinery fuel gas contained two percent hydrogen sulfide and the pilot was operated in such a manner that the flame, at the tip, produced a reducing atmosphere. The problem was corrected by changing the operation of the pilot to produce an oxidizing flame. There was no change in the metallurgy of the pilot tip. The pilot has now been in service for approximately 2 years without any further reports of failure or damage. Case Example #3- Pilot on an Elevated Flare. Flare pilots are typically positioned around the perimeter of a flare tip and are used to ignite the waste gas stream. Most pilots operate with an air-fuel mixture near stoichiometric conditions and can last for many years without suffering substantial corrosion damage. However, the service life of a pilot can be substantially reduced if the flame from the flare engulfs the pilot. Recently, a flare tip with 3 pilots was installed at a natural gas treatment plant. The flare was burning gas continuously at a low flow rate (approximately 1 to 1.5 ft/s exit velocity). Due to a predominant wind direction, the flare experienced external burning on one side approximately 75 percent of the time over a 18 month period. When the flare tip was inspected, it was found that the pilot that was positioned on the downwind side of the flare tip suffered extreme corrosion damage to the wind shield. The other two pilots, that

were not exposed to a continuous external burning, were not damaged. The extent of damage seriously reduced the ability of the pilot to function properly. This problem was eliminated by staging to a smaller tip at low waste gas flow rates. Case Example #4 - Air-Assisted Flare. Air-assisted flares use air from a fan as a supplemental energy source to help improve the smokeless performance. At low waste gas flow rates, wind and air from the blower can induce internal burning by creating recirculation zones inside the arms (waste gas side) of the flare tip as illustrated in Figure 5. When internal burning occurs in this type of an air flare, heat causes a buildup of stresses and crack formation at or near the weld seams. In turn, these cracks allow waste gas to enter into the air-side of the flare causing burning to occur on both sides of the gas tip. When burning occurs on both sides of the tip, it can quickly deteriorate, regardless of the metallurgy. An air-assisted flare was put into service burning a typical refinery waste gas. After 12 months in service, the flare was inspected. Inspection revealed cracks in or near many of the weld seams and severe high temperature corrosion damage to the 310 stainless steel arms. Our study of industry experiences revealed that any tip of the same or similar design would be susceptible to damage from internal burning. John Zink Company has solved the air flare problem through the development of a new tip design. The new tip design eliminates the mechanical complexity and induced pressure gradients of the previous design thereby significantly reducing the chance of damage due to internal burning. Tips of the new design, which normally use 310 stainless steel, have shown greatly improved service life.

Conclusions The service and operational conditions of a flare are considered to be the most crucial factors leading to the corrosion damage of a flare tip. If a flare tip operates so that it experiences low intensity or occasional flame impingement, it will typically have a long service life. However, if the flare tip is exposed to a continuous or frequent flame impingement, it will ultimately suffer high temperature corrosion damage regardless of the metallurgical selection, design or fabrication. Therefore, the key parameter for flare tip metallurgical selection is corrosion from flame impingement. For John Zinks’ flare tip designs, the preferred metallurgy for most flare tip applications is 310 stainless steel or similar cast materials. We find that 310 stainless steel provides a good economic balance between cost and service life. Certain operational conditions cannot be avoided. For these conditions, the selection of a properly designed flare tip with the appropriate metallurgy is extremely crucial. A properly designed flare tip should minimize internal and external burning and allow good fabrication techniques. High-nickel alloys are not the best application for flares burning a waste gas stream with sulfur present. The literature shows that nickel sulfides can form and easily destroy the protective oxide scale and lead to catastrophic sulfidation attack.

References

[1]. Schwartz, Robert, et. al., “Flaring in Hostile Environments”, John Zink Company, Presented at a seminar on Flare systems, Arranged by the Norwegian Society of Chartered Engineers, 1982. [2]. S. K. Verma, Paper No. 336, presented at Corrosion/85, National Association of Corrosion Engineers, Houston. [3]. M. A. H., Howes, “High Temperature Corrosion in Coal Gasification Systems”, Final Report, GRI-8710152, Gas Research Institute, Chicago, Aug. 1987.

wind direction

wind direction

recirculation region induced by the wind

air / fuel mixture

waste gas

flare tip

(a)

(b)

Figures 1a and b. (a) Recirculation Region Induced by the Wind, (b) Waste Gas Migrating to one Side of Flare Tip and Mixing in Recirculation Region.

wind flame

fuel flare tip

Figure 2. Illustration Showing the Recirculation Region Induced by the Wind in a Flare Positioned Horizontally.

entrained air steam producing a capping effect

waste gas being pushed into the flare tip

Figures 3. Illustration Showing Steam Capping Effect.

flare tip wind direction wind direction

flare tip

low pressure region

waste gas

Figures 4. Illustration Showing Flame Pull Down Region.

flame wind

air

fuel

air

Figures 5. Illustration Showing Air Flow through the Arms.