6 replies to this topic

[kpavan]

Dear all,

Is there any possibility of Flame at Flare stack tip to shut off due to natural reasons like wind velocity or rain. If so, at what wind velocity and how much rain fall can lead to such a situation?

I have checked API 521, but didn't find any. Please let me know if any literature is available on this topic.

Any answer to this is appreciable.

Regards,
Pavan

[fallah]

Dear all,

Is there any possibility of Flame at Flare stack tip to shut off due to natural reasons like wind velocity or rain. If so, at what wind velocity and how much rain fall can lead to such a situation?

I have checked API 521, but didn't find any. Please let me know if any literature is available on this topic.

Any answer to this is appreciable.

Regards,
Pavan

"To maintain the flame in all operating conditions" and "To re-ignite fastly after accidental flame out" are the main functions of the pilot burner.

Thus,IMO,you should set the wind velocity and rain fall values and would leave the Pilot Burner Design to Flare Package Vendor.

[ankur2061]

Pavan,

Here is what API STD 537 has to state for stable burning of a flare in the section 5.2 PILOTS:

In order to ensure stable operation and ignition of the flare gas, the recommended minimum pilot heat release is 13.2 MW (45,000 BTU/hr) (LHV) when flaring hydrocarbon gases with a lower heating value of 11175 kj/Nm3 (300 BTU/scf) or greater. Pilot heat release in common practices ranges from this minimum up to 102.5 MW (350,000 BTU/hr). The pilot should remain lit and continue to ignite the flare at wind speeds up to 160.9 km/hr (100 mph) under dry conditions and 136.7 km/hr (85 mph) when combined with 2 in. of rainfall per hour.

Hope this helps.

Regards,
Ankur.

[kpavan]

Thanks for your replies,

Ankur,
This is exactly for what i am looking for.. A lot thanks again for you. And just a small information, about the one you are specifying about pilots in API STD 537, actually it is in 4.7 but not in 5.2. This as per Second edition, 2008.

Here is what API STD 537 has to state for stable burning of a flare in the section 5.2 PILOTS:

Regards,
Pavan

[cmp74]

When it comes to flare tip and pilot designs, it is best to use a well recognized vendor to do it.Industry leaders are John Zink and Callidus technologies.

Recently I was involved in selection of new tip and pilots for a 58" flare stack.Following are the issues one should keep in mind while working on this equipments.

1) API 537 requirements are a minimum.The plant location may have very extreme conditions.Make sure the vendor understands them and optimize the design to meet these conditions.for example,my work location has harsh winter conditions where it can be - 45 deg C ambient temp with high wind and heavy snow precipitation.

2) Pilots are the most critical component of the flare system.Make sure to have a very reliable monitoring and (re)ignition system.

Current industry standard is to have dual thermocouple for pilot flame indication.There are some other technologies available but may or may not be suitable depending on application and location.These are optical flame monitoring, sound check (noise signal from pilot nozzle is monitored, any disturbance indicates "trouble"), infrared flame monitoring (where flame temperature is measured based on IR signature).

For pilot ignition,high energy igniters(HEI) is a common practice , where a high electrical current generates a spark at cathode, which is installed very close to pilot nozzle.The spark ignites the combustible mixture coming out of pilot nozzle.

Even though API suggest ( I think) to have such redundancies for any one pilot, I decided to go ahead with dual thermocouple and HEI for all pilots on the tip due to very severe environmental condition.Your local government agencies might also have some minimum requirements.

3) Wind direction has some influence on the compass position of pilots around the tip , but is left to vendors as they have more experience in this matter. Make sure to understand the wind velocities and direction at plant location.

4) While comparing different pilot models, focus on the pilot venturi(where outside air is pulled in).Even though this is a closely guarded secret of each vendor design,its always good to ask as many questions to vendor as possible.

5) Finally, tip design/operation also has some influence on pilot design/installation in terms of pilot nozzle inclination , deflector etc.

[ankur2061]

cmp,

I had prepared a company standard titled "Vent & Depressurization" for miidle east O&G company wherein the section for Flares deals with the pilot ignition system. I am reproducing the paragraph verbatim:

b. Flare Tips for Elevated Flares



b.1 Refer Section 4.5.5 for design guidelines for flare tips.

b.2 Wind Shields

Reference is made to API STD 537 Section 5.1.7.9. These are generally used for unassisted pipe flares. The wind shield shall basically function as a sacrificial equipment to promote improved service life of the pipe flare tip. The design of wind shields is proprietary to the flare system vendor.

c. Pilots for Elevated Flares



A general description about flare pilots is provided in Section 4.9.1. Reference is also made to BP RP 44-3 and API STD 537 for design criteria of pilots.



c.1 The pilots provided at the flare tips shall be capable of sustaining stable combustion of the flare gas over the entire operating range of the flare and wind speeds.

c.2 The ignition lines downstream of the fuel gas filter to the pilot tip shall be stainless steel (SS 316).

c.3 The pilot tip shall be constructed of a heat resistant material, such as SS 309, SS 310, CK 20 or Incoloy 800H.

c.4 Pilot gas systems shall be designed to provide clean, dry and solids-free gas. This shall require the installation of filter and/or strainer in the pilot gas line and to ensure that the lines are arranged without pockets.

c.5 'K' type thermocouples shall be provided to monitor pilot flames. Each pilot shall have one thermocouple. Each thermocouple shall be dual element type. One element of each thermocouple shall be used, with the other terminated at the grade junction box and available as a spare.

c.6 Pilot flame-out shall be indicated if a thermocouple fails. Consider a grade mounted infra red detector for monitoring pilots on elevated flares as a backup for thermocouples.

c.7 A backup bottled gas supply for the pilots shall be provided if fuel gas supply is not available during conditions such as start-up.

c.8 The minimum number of pilots shall be in accordance with the table below. The flare vendor should specify additional pilots if required by his design. Reference is made to table 2, Section 5.2.3 of API STD 537.



Minimum Recommended Number of Pilots



Flare Tip Diameter

Number of Pilots

Upto 8 inches (200 mm)

1

10 inches (250 mm) to 24 inches (600 mm)

2

28 inches (700 mm) to 42 inches (1050 mm)

3

46 inches (1150 mm) to 60 inches (1500 mm)

4



d. Ignition Systems for Elevated Flares



A general description about flare ignition systems is provided in Section 4.9.2.



d.1 A remote operated flame front generator ignition system shall be used.

d.2 The flame front generator panel and controls shall be located outside the 1500 Btu/hr-ft² radiation zone. However, as a general rule the distance between the flame front generated and the flare shall be kept to as minimum as possible.

d.3 Each pilot shall be provided with a separate ignition line. The ignition line shall be of type 316 stainless steel.

d.4 Pilot gas and ignitor lines shall be piped separately around the flame front generator and upto the flare tips.

d.5 Piping shall be flanged at the flame front generator and the bottom of flare tip assemblies to facilitate removal and maintenance work.

d.6 The controls shall be provided to enable each pilot to be lighted individually.

d.7 Ignitor gas shall be dry and clean (with respect to liquids and solids) and shall be either fuel gas or bottled gas.

d.8 Flame front ignitor piping shall be designed to minimize low points. Low point, if required, are to
be at ignition panels and/or at the flare base. Drains and boots shall be provided and located at
all necessary low points.

This should provide some guidance. Most of it is referenced from API STD 537 with additional inputs from other company standards. The references are mentioned.

Regards,
Ankur.

[kkala]

My few words to this interseting subject:
1. It was mentioned that pilots can usually "endure" wind velocities of 85-100 miles/h. Web research on John Zink, NAO, etc, can trace "endurance" to 250 miles/h (at least according to advertisements) through special wind shields etc. Of course price is expected to increase, if design is special.
2. In specifying max wind speed to supplier, gust should be clarified (gust=instantaneous short increase of wind speed). According to my past information, gust is not detected in wind speed as usually measured and gust can increase the latter by 50%. Most probably "max wind speed" by suppliers means "including gust", so available wind data should be compatible.
3. I assume that even one lit pilot is enough to maintain the tip flame; thus if probability of a pilot to fail is p, probability of tip flame to be put out is: p if one pilot is intalled, p^2 if two pilots are installed, p^3 if three, etc. Having more pilots by code, large diameter tips have lower probability to be put out; so probability is reduced according to the seriousness of the accident (quantity of escaping unburnt). Is it so? Safety is increased by placing more pilots anyway, in the past we had specified 2 as minimum number of pilots for small flares.