Flare tip

Mak has developed an improved method of relief valve manifold design. The APT has adopted this method, which starts at the flare tip (atmospheric pressure) and calculates backwards to the relief valves, thus avoiding the trial and error of other methods. This is especially helpful when a large number of relief valves may discharge simultaneously to the same manifold. 

Starting at the flare tip calculate logical segments using Figure 1 until all relief valve outlet pressures are found. 

Mak used the same Lapple article to develop a method for designing flare headers. His method has the advantage of starting calculations from the flare tip (atmospheric) end, thus avoiding the trial and error calculations of methods starting at the inlet. 

Open flares have a flare tip with no restrietion to flow, the flare tip being the same diameter of the staek. Open flares are effeetively a burner in a tube. Combustion and mixing of air and gas take place above the flare with the flame being fully eombusted outside of the stack. 

With the flare tip and flare seal pressure drop and flare elevation fixed, the flare stack, headers and laterials are sized for the largest release, while not exceeding the maximum allowable operating pressure on the associated blowdown drums and water disengaging drums. These maximum allowable operating pressures are in turn determined by ... 

Flare stack sizing and pressure drop is included with considerations of pressure drop through the safety valve headers, blowdown drums, flare headers, seal drum, etc. Elevated flare tips incorporating various steam injection nozzle configurations are normally sized for a velocity of 120 m/s at maximum flow, as limited by excessive noise and the ability of manufacturers to design tips which will insure flame stability. This velocity is based on the inclusion of steam flow if injected internally, but the steam is not included if added through jets external to the main tip. 

Figure 1. Available pressure drop may in some cases dictate acceptance of a lower maximum velocity, but at least 75 m/s is recommended to insure good dispersion. Flare tips consisting of a simple open-ended pipe with a single pilot are subject to flame lift-off and noise problems at lower velocities, and should therefore be designed for a maximum velocity of 50 m/s.

Recommended nominal steam rates at 60 m/s exit velocity for a typical flare tip are shown in Figure 2. At lower velocities, higher steam ratios are required. Typical steam control consists of a flow ratio controller with adjustable ratio set point, related to flare gas flow. The ratio adjustment, located in the control house, provides for the higher steam ratios necessary at low flaring rates. 

Steam injection introduces an additional source of noise. An effective flare tip is one which achieves a good balance of smoke and luminosity reduction without exceeding acceptable noise levels. Low-frequency noise is encountered at relatively high steam to hydrocarbon ratios. 

A flare performance chart for the hydrocarbon being flared, should be consulted for additional guidelines on flare tip design. Figure 3 provides a provisional performance chart for propane. The chart defines the design envelop of exit velocities and steam ratios necessary to avoide smoke formation, excessive noise, flame boilover and flame lift-off. 

There are various designs of flare tips that incorporate such features as central steam injection, an annular ring of steam nozzles, internal air-inspirating steam nozzles, windshields, etc. Table 2 provides some details of suitable types from which selection may be made. 

Some final details to note on flare tips include ... 

In the consideration of elevated flare pilots and igniters, proprietary flare tips are normally provided with the manufacturer s recommended igniter and pilot system. Usually, one to four pilots are used depending on the flare tip type and diameter. The forced air supply type of igniter system (described below) is normally preferred. Controls should be located at a distance from the base of the... 

Figure 2. Nominal steam requirement for a typical flare tip.

Smokeless Center Steam Cheapest steam-injection flare tip. Steam jet emerges at high velocity and penetrates to the exit plane of the flare without mixing completely with flare gas. Results are intense steam noise (much greater than with steam ring for the same steam rate) and higher steam consumption than the steam ring. 

Non- Smokeless Utility or Field Flare Cheapest flare tip. Produces trailing smoke even with namral gas. 

Figure 11. Continued - Other typical smokeless elevated flare tip designs.

When winterizing is required, the steam tracing and insulation should include the first 7.5 m of the flare stack above the vapor inlet and also in the case of a drum seal, the vapor line from the seal drum to the flare. Where steam injection to one of the smokeless tips, as shown in Figures 11 B and C is used, then the steam ring should remain outside the top of the flare tip (i.e., not internal). Where severe ambient conditions are encountered then it is recommended that the entire seal drum and flare be insulated in addition to steam tracing and open steam injection at base of flare. 

Flare Gas - Molecular weight, lower explosive limit, density at flare tip, fraction F of heat release radiated by the flame. 

Step 4 Calculate the scaling parameter R, which accounts for the effect of flame shape of the relative thrusts of the wind and the gas jet discharging from the flare tip ... 

Height of flame center above flare tip, m h = Height of flare tip above grade, m F = Fraction of heat release radiated from the flame m = Mass flaring rate, kg/s H = Lower heating value of the flare gas, MJ/kg r = Relative humidity, percent The following are the calculation steps ... 

The flare tip assembly shall be furnished complete with smoke suppression system, pilot and igniter assemblies, windshield, and flame retention assembly. 

The flare tip assembly shall be provided with a sufficient number of adequately sized lifting attachments for proper handling of the assembly. 

Design and materials of the flare tip assembly shall ensure that burning of the tip will not occur during low relief rates. The following are minimum acceptable materials ... 

N P Refinery will be responsible for the wiring and piping materials from the ignition panel to the bottom of the flare tip assembly. 

The flare tip and associated equipment shall be piece marked and the electrical terminals shall be ceded to facilitate field erection. 

Pressure at the base = Atm pressure at the flare exit + 0.5 psi flare tip. 

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