Continued from Part 1 of this article…..Click here to go to Part-1.………………….Purge reduction seals (Fig. 6): To reduce the purge rate purge reduction seals are used.
The liquid seal drum SHALL [PS] be designed as a pressure vessel with a design pressure of at least 7 barg (100 psig) to maintain containment against an internal deflagration.
Where there is a risk of an obstruction in the flare due to process flows creating an ice plug with the liquid seal, alternate sealing fluids such as a glycol/water mixture (60% EG & H2O freezing point –45 deg C) or other means to prevent freezing SHALL [PS] be implemented.
For LNG facilities, liquid seal drums SHALL [PS] not be used, since in the event of a cold release this may form an obstruction in the flare relief system.
Allowable Liquid droplet size (to avoid burning rain): Burning rain occurs when rate of burning (depends on type of flare) of liquid droplets is lower than rate of settling of droplet (depends on droplet size).
Drift distances of burning liquid droplets from an inadequately designed flare system can be considerably greater than 200 ft (60 m).
If liquid is not drained from flare gas, at gas velocity of 3-4 m/s – liquid droplets of 1000 micron can be entrained which can cause burning rain in flare.
Liquid droplet size allowed without burning rain
- Unassisted flares : <600 micrometer
- Steam or air assisted : <600 micrometer (less than 1% mass)
- High pressure (if operated at atleast 200 kPag) : <1000 micrometer (less than 1% mass)
Design pressure of KODs:
- 5 barg (50 psig) when a liquid seal drum is located between the KO drum and flare stack.
- 7 barg (100 psig), if there is no liquid seal drum in the system.
For a multi-process unit facility (e.g., refinery) based flare KO drum where it may not be immediately clear which unit is sending liquid to the flare , liquid space on top of LA (HH) SHALL [PS] be designed to contain the maximum emergency liquid relief rate from the largest single contingency for a period of at least 15 minutes for the unit KO drum and at least 20 minutes for the flare KO drum, without taking credit for pump out capacity.
Flare height (Fig. 7):
Height of the flare is established based on allowable thermal radiation levels. Flare height depends on available plot, distance of nearby equipments from flare stack.
- More plot area : Low flare stack height
- Less plot area : Higher flare stack height
Effect of thermal radiation on person at grade or at elevated platform shall be checked by radiation calculation.
Thermal Radiation affects the human skin (skin burn).
Exposure Times Necessary to Reach the Pain Threshold
- 31 kW/m2 – Up to 20 s
- 15 kW/m2 – Up to 1 hour
- 58 kW/m2 – Continuous
If personnel exposure to radiant heat exceeds the guidelines provided, then shielding should be considered.
Depending on the location the thermal radiation limit is provided in Fig. 8
- The solar radiation need not be added to calculated thermal radiation values (0.79 to 1.04 kW/m2) from the Flare. For OXY projects, solar radiation to be considered.
- A wind velocity of 10 m/s (22 mph) at the elevation of the flare tip, blowing towards the receiver, is a typical assumption for flame tilt assessment.
- When two flares are located in close vicinity, combined radiation effects shall be calculated.
Dispersion: To ensure safe operation during periods when the flame might have extinguished, concentration of hazardous components should be determined using dispersion analyses, assuming the flare is functioning as a vent only.
|Level of Concern||Hydrogen Sulphide
|8 Hour TWA (Threshold Limit Value)||5 ppm, 8 hours||2 ppm, 8 hours|
|15 Minute STEL (Short Term Exposure Limit)||10 ppm, 15 minutes||5 ppm, 15 minutes|
Short-term exposure limits (STELs) are set to help prevent effects, such as eye irritation, which may occur following exposure for a few minutes
Local rules and regulations shall be followed. Typically flare combustion quality shall meet Ringelmann Index 1 criteria (Fig. 9).
Smokeless flowrate shall be the normal flow which is expected in day to day operation. Do not specify design capacity for smokeless operation.
A scale used to define levels of white, gray and black i.e. intensity of smoke
- Ringelmann No. 0 is clear smoke
- Ringelmann No. 5 is 100 percent black.
- Ringelmann No. 1 is equivalent to 20 percent black
- Noise : For normal flowrate (including starting-up and shutting-down): 85 dB(A) at sterile radius. For emergency conditions: 115 dB(A) at sterile radius
- Combustion efficiency : greater than 98%
- Number of pilots (Fig. 10): The number of pilots required is a function of the flare burner diameter. For very small flares, a single pilot will reliably light the flare gas. However, it should be noted that if only a single pilot is used, a single pilot failure would represent a complete failure of the ignition system. Recommended to install atleast 2 pilots for tip size of up to 8″ to increase reliability. As the flare burner diameter increases, the number of pilots required to reliably light the flare, regardless of wind direction, increases.
Flare gas recovery system Safety considerations:
- Path to flare : PRVs, depressuring systems, etc., shall always have flow paths to the flare available at all times.
- Reverse flow :Because flare gas recovery systems usually involve compressors that take their suction directly from the flare header, the potential for reverse flow of air from the flare into the compressors at low flare gas loads shall be considered.
- Monitor oxygen content in the flare header and provide recovery system trip. Provide low low pressure trip on the recovery system suction to avoid air ingress. Liquid seal drum (not practical in AP flare systems)