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Whenever the flare system is on-line, the flare tip, riser and main header must be maintained in a safe condition at all times.

GBA recommend the provision of a continuous positive hydrocarbon or inert gas (nitrogen) purge dedicated to the flare under all conditions.

Extensive development work by GBA has resulted in equipment designed to reduce purge rates and to provide a positive safeguard to the flare system in the unlikely event that purge gas is lost.

Even though a flare system is not necessarily constantly flowing, the flare stack and relief header must be kept in a safe working condition at all times.

This is achieved by the use of a continuous minimum flow of gas designed to prevent air being drawn into the flare system via the flare tip, or otherwise. This is known as the purge gas flow.

Without a special flare seal device fitted the purge gas flow would need to have a velocity of between 0.3 to 0.6 m/sec to be effective.

For a large diameter stack this can represent a significant amount of gas. To minimise this requirement, it is customary to use a proprietary seal device in the flare system located within, or close to, the flare tips.

There are two main types of gas seal currently available:

i) The labyrinth type – (Molecular Seal) ii)The fluidic type – (Air Lock Seal)

Both are installed immediately below the flare tip and both will prevent air ingress into the flare system provided a continuous purge gas is available.

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3.2.1. MOLECULAR SEAL.

The Molecular Seal works by relying on the density difference between the purge gas and air. When the purge gas is lighter than air it forms a gas rich zone at the top of the seal that air cannot penetrate, conversely when the purge gas is heavier than air the seal is formed at the base of the device.

In this way only a very low continuous purge flow is necessary to maintain conditions within the seal.

A unique advantage of the molecular seal is that it will maintain safe conditions in the upstream riser for several hours in the event of a loss of purge gas.

3.2.2. AIR LOCK SEAL.

The Air Lock Seal (ALS) is a frustro-conical device, which is located as an integral part of the flare tip, welded within the main body of the tip just above the main flange.

With all flare tip operations, under low relief conditions, air will slowly diffuse down the inside walls of the tip. The Air Lock Seal design acts to locally increase the velocity of purge gas through the seal, thereby moving any air back out of the tip. The Air Lock Seal is of simple rugged construction and has no moving parts, requiring the absolute minimum of maintenance.

Drain

Inlet

Outlet

Hand Hole

Air Lock Seal

3.2.3. COMPARISION OF MOLECULAR AND AIR LOCK SEALS.

1)The Molecular Seal prevents the ingress of air into the main flare system for a period of 2-4 hours (in the event of purge gas failure) due to the buoyancy effect discussed earlier. The Air Lock Seal has no hold-up capacity, therefore if purge fails, then the system is rapidly exposed to air ingress.

2)The Molecular Seal requires a purge rate of 0.003 m/sec.

The Air Lock Seal requires a purge rate of approximately 0.012 m/sec (these are both based on flare tip exit area).

Whilst the Molecular Seal requires a lower rate, the decrease could result in the flame burning within the flare tip reducing life time.

3)The Air Lock Seal has the following advantages:

simple, open free path to atmosphere;

no plugging;

easy to install;

offers no wind loading to the support structure. The Molecular Seal is heavy and adds considerably to the overall system weight increasing structural loads and increasing costs of the riser;

no maintenance. If the Molecular Seal corrodes or is blocked, it has to be replaced requiring complete system shutdown;

no drainage or corrosion problems. The Molecular Seal has the potential to corrode at its base and within its drain line, especially with sour gas relief;

very low capital and installation costs. The Molecular Seal is expensive due to its size and complicated fabrication of the internal baffle arrangements. An extra drain line is required to grade. A full 360° inspection platform is also required for access to the drain and hand holes at the base of the Molecular Seal;

can be used in a horizontal position i.e. burn pits and angled flaring for offshore applications. The Molecular Seal can only be used vertically.

Summary.

The Air Lock Seal is a simple low cost device with significant technical and commercial advantages over the Molecular Seal as described above. The use of Molecular Seals is quite uncommon now, as industry has recognised that they create more problems than they solve. Indeed the offshore oil production industry (North Sea – offshore UK/Norway/ Denmark) without exception uses Air Lock type seals instead of Molecular Seals due to structural and weight saving advantages of great significance in the design of offshore production facilities where weight and cost is at a premium.

COMPANY PRODUCT MANUAL

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GBA PRODUCT MANUAL Rev 2004