Key Design Parameters For Smokeless Flaring of Low Pressure Gas
Authors: Obawole Adetunji B. , Onwuka Okechukwu C. , Jan Raymond J. , Martin Michael J. , McSorley Kevin P. 
Published: December 2007

The air-assisted, continuous flare at QIT (Qua Iboe Terminal), Mobil Producing Nigeria, burns low pressure, high molecular weight gas. An increase in production volume to the terminal resulted in higher gas rates to the flare system, exceeding the original flare design. The flare facilities were re-engineered and modified using key design parameters for smokeless flares. The modified facilities were successfully restarted and functioned properly at the higher flare rates.

Introduction

The air-assisted, continuous flare system at Qua Iboe Terminal (QIT), Mobil Producing Nigeria was installed in 1994 to handle approximately 18 million standard cubic feet per day (MMSCFD) of associated gas from 300 thousand barrels per day (TBD) crude production. This gas is gathered at very low pressure, mainly from separators operating at 0.3 pounds per square inch gauge (psig).

An increase in crude production volumes (up to 600 TBD) and the corresponding increase in flare gas volumes to over 40 MMSCFD led to the need to expand the flare system capacity to maintain combustion efficiency and prevent unacceptable levels of smoking. A project was initiated to evaluate the performance of the existing flare, identify problems that would cause smoking during normal operation at the higher rates, and to expand the capacity of the system. As a result, the following improvements were made:Two additional flare tips (for a total of five) were installed. Tip areas for airflow were increased to reduce pressure drop at the flare tips.Two new 72 in. outside diameter (OD) air ducts were installed. The original 5 ft square duct was left in place and remains in service.One additional air blower was installed. The air duct configuration was modified so each blower would discharge to its own air duct.The main air plenum size was enlarged to 8 ft×8 ft×64 ft. The anchoring of the plenum base was improved to allow for thermal expansion and contraction.The new air ducts were covered with one foot of sand and aggregates for heat radiation protection.

The following sections describe the key design parameters considered during the design and expansion of the QIT air-assisted flare.

Key Design Parameters

Flaring Low Pressure Gas.

Smokeless flaring of low-pressure gas is significantly more difficult than smokeless flaring of high-pressure gas. When flaring a high pressure gas, not only is the gas lighter (gas coming off of a higher pressure separator tends to have a lower molecular weight), but the gas velocity at the flare tip usually creates sufficient turbulence to ensure the proper air and gas mixing required for smokeless combustion. On the other hand, the flash gas coming off a low-pressure separator would have higher molecular weight requiring more air for complete combustion. If the low-pressure gas is allowed to burn without the introduction of sufficient outside air or steam, the flare tends to smoke excessively. In addition, the flame becomes too close to the flare tip causing flare tip damage. The typical composition of the QIT flare gas is given in Table 1.