Sep-2009

New flare technology utilising low pressure steam

The American Petroleum Institute defines a flare to be a “device or system used to safely dispose of relief fluids in an environmentally compliant manner through the use of combustion1”.

Robert Cupp
UOP Callidus

Article Summary

This is a very broad definition and includes small flow rates from leaking valves as well as massive flows due to major unit shutdown conditions. This being the case, flares can come in all shapes and sizes, but the most critical feature of the flare is its ability to efficiently burn a wide variety of flows and vapour compositions without experiencing stability issues. This means that the flare must be designed to handle typical flows as low as a few hundred pounds per hour, and still be able to operate effectively at mass flows in excess of one million pounds per hour.

When designing a flare system, there are a number of factors one must consider to provide a good working system. The omission of any of the following could lead to major problems in the field which may force the plant to prematurely shutdown. As a minimum, hydraulic capacity, radiation, environmental impact, smokeless capacity, and utility consumption should all be addressed when designing a flare system.

The first and most important consideration is the hydraulic capacity of the flare. The flare should be properly sized to encompass the maximum mass flow rate of gases that could be released to the flare system at one time. The composition, temperature and pressure of these gases must also be taken into consideration so that one can determine the maximum volume of gases that must be burned. When determining the hydraulic capacity, one generally is looking a “worst case” upset condition. Typically, this will be an emergency shutdown situation that has a low probability chance of occurring. In those cases, due to the infrequent tendency, the priority of the flare operation is the effective combustion of the entire flow and not the smokeless capacity.

The second most important factor to consider is the thermal radiation levels and how they impact the surrounding area. If a flare tip is too close to the ground during a major release, the surrounding equipment may be damaged or personnel may be injured from the high radiations levels emitting from the flames. A detailed description of how to determine radiation levels may be found in API Standard 521 Appendix C 2. Through the use of these guidelines, an appropriate height for the flare as well as sterile areas around the flare may be determined.

A third consideration for flare design is the surrounding environment. Humidity, temperature and wind speed all affect the design of the flare. High winds will carry the flames from combustion horizontally from the tip altering the distance of the flame from grade. The humidity affects the amount of radiation emitting from the flame. In colder climates, it may be necessary to upgrade the flare stack material to accommodate subzero temperatures. Environmental regulations, such as noise level restrictions or dispersion limits, may also be in place, which could affect the type of flare that is being considered.

The smokeless capacity on a flare is directly related to the efficiency of the flare tip design. When the air surrounding the tip exit does not properly mix with the flare gases, the cooling uncombusted carbon particles in the combustion products can visually be seen as smoke. This generally occurs when heavier molecular weight gases are being burned in a flare at high flow rates. There are a number of industry designs utilising steam or air to help increase the momentum of the exiting gases to better mix them with the surrounding air. It is particularly in this area that flare tip design can have a direct affect on the ability to maximize the smokeless capacity.

The last consideration for the flare design is the utility consumption rates. As discussed above, most flares require some type of motive force to help encourage mixing at the tip. The most common method is by injecting medium pressure (100 to 150 psig) steam into the flare gases as they exit the flare tip. At design release the high pressure steam injection penetrates into the centre of the gas vapours while inspiriting the surrounding air into the gas stream as well. The turbulence created by the steam helps mix the oxygen with the flare gases to complete combustion. At low flare gas releases, a lower amount of steam is required to cool the flare tip and help keep the flames from burning down inside the tip. In some cases, an air blower may be used instead of steam to provide the mixing energy. For this type of flare, the utility consumption would be in the form of power which is used to operate the blower. In addition to the mixing medium, a fuel gas is required for each of the pilots on the flare tip. The pilots are in continuous operation and will require a constant flow of natural gas during operation.

Current technological advances are focused on improvements in each of these five areas. Most of the development is in the areas of improved efficiency for increased smokeless capacity and reduced utility consumption. Since a vast majority of the flares in service are elevated and utilise steam for mixing, we will focus on these types of flare for this discussion.

Typical steam assisted flare designs
The most conventional type of steam injection into a flare tip is known as upper steam injection. In this type of design, a steam manifold is placed around the top of the tip. Steam nozzles are then fed from this manifold and are placed around the circumference of the flare tip. The nozzles are designed to inject medium pressure steam (approximately 100 psig) into the flare gas stream as it exits the flare tip. As the steam penetrates the column of flare gas, the surrounding air is pulled into the flare gases as well. The corresponding turbulence created from the steam injection causes the components to rapidly mix. The air is then able to come in direct contact with the flare gas so that combustion can occur. This method of steam injection works well for smaller diameter flares. However, as the diameter of the flare increases, it becomes more difficult to draw the air to the middle of the flare tip. This is because the steam loses momentum as it travels away from the steam tips and is no longer able to penetrate the exiting flare gases. This can lead to free carbon escaping the flame envelope which causes smoke formation.

When upper steam injection is not able to meet the smokeless capacity requirements, a more aggressive technique called internal steam injection can be utilised (Figure 1). In this design, the steam manifold is placed at the base of the flare tip instead of at the top. The steam is then injected through a number of tubes up through the flare tip to exit into the core of the flare gas stream. As with the upper steam design, air is entrained into the tubes and pulled into the core of the flame. This method is designed specifically to bring air into the centre of the exiting flare gas column. This helps even more of the flare gases come in contact with the air and greatly improves the smokeless capacity.