A gas flare, alternatively known as a flare stack, is an elevated vertical conveyance that is part of installations such as oil and gas wells, oil rigs, refineries, chemical, petrochemical and natural gas plants, and other facilities (Fig. 1). On oil- production rigs and in refineries and petrochemical plants, the flare stacks primarily serve to protect vessels or pipes from over-pressuring during unexpected plant upsets.

Fig. 1. Flare stack.

Safety system.

Whenever plant equipment is over-pressured, the pressure, relief valves on the equipment automatically release gases (and sometimes liquids as well) that are routed through large piping runs called flare headers after liquid hydrocarbons are completely vaporized and then send to the flare stack. The released gases are burned as they exit the flare stack. The size and brightness of the resulting flame depends on how much flammable material was released.

Steam can be injected near the end of the flare tip to reduce formation of black smoke. The injected steam does, however, increase the noise level of the burning gas. To keep the flare system functional and instantly useable, a small amount of purge gas is continuously burned. It thus resembles a pilot light, maintaining the system ready for its primary purpose as an over-pressure safety system. The continuous gas source also helps to prevent oxygen ingress into the system.

As mentioned earlier, flare systems enhance plant safety by dependably disposing of all hydrocarbons discharged during plant upsets. All safety valve releases go to the flare system. There are, however, two types of flare feeder systems in ISBL:

• A wet flare header is used to handle flare gases that contain moisture but are not “cold” gases.

• An intermediate flare header, which could contain some moisture and normally handles some cold vapors (up to –45°C).

• A dry flare header designed to handle dry flare gases. These will also be cold, with normal temperatures below –45°C.

• A low-pressure acetylene flare header, exclusively provided to handle acetylene-rich gases.

At Reliance’s Nagothane facility, a large flare stack with a design load of 1,000 metric tph is located on the north-east side of a gas-cracker plant. Flare headers from individual plants—polypropylene, low-density polyethylene, linear-low density polyethylene, gas cracker-OSBL and gas cracked-ISBL) join the main flare header, which routes to the flare stack (Fig. 2). The main flare header leads to a knockout drum in the flare area.

Fig. 2. Flare system schematic at the   Nagothane plant in India.

The molecular seal consists of a gas “lead pipe” and an inverted cylinder over the pipe. Gas flows in an upward direction, turns through 180° and flows downward for a short length before being redirected again through 180° and back to the original flow direction. In the static condition, gas lighter than air will tend to collect in the upper bend and heavier gases will tend to settle at the lower bend, sealing off the stack against backflow of air. The flare tip is mounted on top of the molecular seal and contains three pilot burners. Damage to the flare tip due to flame burn back near the tip is avoided through the use of refractory lining on all exposed anchor and mesh surfaces.

Exploring the failure history.

The flare stack at Nagothane was commissioned in 1989. Since then and at every plant shutdown, the flare tip was being replaced because it experienced damage during flaring operation. Until 2010, the flare stack structure had never been repaired and neither had it been repainted after plant commissioning because no time was available during annual or major shutdowns. However, inspections of the flare stack structure, ladders, grating, clamps and associated piping was conducted before a major turnaround scheduled for early 2010.

The support structure of the flare stack was found damaged, and loss of thickness was observed and measured at various locations, mainly under the support plates (Figs. 3 and 4). Fuel gas and steam piping were found damaged as well and some grating had been totally eaten away by corrosion. Replacement or repair of the entire structure during a projected 17- day maintenance shutdown was contemplated but judged very difficult. It was also realized that working on a flare stack structure during normal plant operation involves high risks; needless to say, flaring can occur at any time due to plant upsets.

Fig. 3. Damaged structure.

Fig. 4. Extreme corrosion on a flare stack in   India.

Repair approach.

The conventional mode of replacing or repairing a flare structure, piping and subsequent painting would take more than 100 days. It was, therefore, judged impossible to do the entire job during a planned shutdown of only a 17-day duration. With that in mind, initial discussions were aimed at completing the job in discrete phases; specifically up to 44 m elevation, in steps dubbed non-shutdown or pre-shutdown tasks. The remaining work from 44 m to 100 m elevation was to be done during the scheduled major shutdown.

Scaffolding and crane arrangements were implemented as pre-shutdown work (Fig. 5). That left about 24 days as the time required for work conducted while the facility was shut down. Therefore, and after further deliberations, it was agreed to plan additional pre-shutdown work to a height of 65 m during non-shutdown and carry out the remaining jobs from 65 m to 100 m elevation with the facility shut down.

Fig. 5.  Pre-shutdown scaffolding work.

Risk assessment and safety.

 Listed among the special risks and risk mitigation steps were:
• Heat radiation due to flaring
• Exposure to work at heights above grade
• Descending “fire balls” during heavy flaring
• Stinging insect attack or bites.

Among the major work items were protective metal shields of 1 mm thickness. These were installed at elevations 26 m, 44 m and 65 m (Fig. 6). The sheet-metal guards were affixed to the grating of all scaffolding grating. In addition, ceramic blankets were fastened to the sheet metal to substantially reduce the intensity of the radiant heat and avoid burning risks.

Fig. 6.  Lift arrangement and protective   metallic shields.

Flood lights were provided for job execution at night, and water shields were installed at the 66-m elevation to effect cooling during flare events. As part of the water-shield system, two water-curtain nozzles were affixed horizontally to the structure. Their effectiveness was demonstrated before they were mounted at the jobsite.

Two lifts and a crane (hoist) were deemed appropriate for worker rescue and to facilitate the lifting of both personnel and materials. One crane was designated for emergency rescue of workers; and a suitable cage was fabricated and load tested before usage. Of course, the crane was also used for lifting and lowering of materials. The rack and pinion lifts were rated at 1 ton and 0.4 ton capacities, respectively. They too could be used for rescue purposes and up to 22 persons could be evacuated in case of an emergency.

Nomex coveralls were mandatory for all workforce members and their supervisors. Whenever heavy flaring was to take place, warnings would be issued to the workforce in the flare area through redundant means, including mobile handsets and a plant-wide loudspeaker (audio) system activated from the control room. The water-spray curtain would commence immediately so as to proactively cool the working area. All persons could immediately retreat safely to the protective area below the metallic shield (Fig. 7).

Fig. 7.  Water shield at 66 m.

In essence, the non-shutdown and shutdown work encompassed:
• Erection and dismantling of crane and lifts
• Scaffolding erection and removal up to 100-m elevation
• Affixing of metallic shields on the scaffolding gratings at elevations of 26 m, 44 m and 65 m
• Water-shield system installation at the 66 m elevation
• Replacement or repair of 20 metric tons of structure and replacement of 2.3 metric tons of grating (Figs. 8 and 9)
• High-pressure water blasting of structure and flare stack; power tool cleaning instead of manual wire brush; all followed by painting
• Insulation and cladding replacement of 3-in. and 8-in. steam lines up to 100 m elevation
• Damaged fuel gas and steam line replacement.

Fig. 8.  Platform and clamps after   replacement.

Fig. 9.  Piping, insulation and cladding   replacement.

Goals.

Initially, the non-shutdown tasks were planned to be done during daylight hours. With unscheduled flaring on some days, the daytime work had to be suspended. Lost time was recovered and work execution scheduled on a round-the-clock basis using floodlights at night to make up for lost time and to complete the non-shutdown part of the repair job in time.

All repair work on the flare stack structure was successfully done without incident within the scheduled period—-85 days for non-shutdown work and 16 days for shutdown work. This was the first time in the history of Reliance that repair work on a rather massive flare stack structure (Fig. 10) has been done online without any safety incident. HP

Fig. 10A.  Massive flare stack at full usage.

Fig. 10B.  Flare stack structure after repair   and painting.

Abbreviations

NMD- Nagothane Manufacturing Division

PP- Poly propylene

LDPE- Linear density polyethylene

LLDPE- Linear-low density polyethylene

GC- Gas cracker

OSBL- Outside battery limit

ISBL- Inside battery limit

The author
	Surinder Singh is vice president (mechanical) with Reliance Industries Ltd. at the Nagothane Manufacturing Division in Maharashtra, India. He has over 30 years of petrochemical industry experience. At present, he is assigned as head of plant mechanical maintenance for the entire complex. He has had wide experience in plant downtime reduction and major turnaround planning. He is credited with filling lead roles and involvement in the development of various safety procedures. Mr. Singh graduated with a BSc degree in mechanical engineering from Regional Engineering College, Kurukshetra, India.