emergency flare systems design features

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Emergency Flare

Design Features

System-Some Practical

A detailed discussion of the practical design features for the emergency flare

system of a plant recently brought on stream by Monsanto.

Jose A. Boix, Monsanto Company, Alvin, Texas 77511

Much has been published on the many aspects of flare de-

sign. Largely, these articles reflect the requirements of

the traditionally large petrochemical/refinery plants.

Flare systems for the specialty hydrocarbon processing

plants may demand some different approaches. Some

practical design features for the Emergency Flare Systemof a plant recently brought on stream by Monsanto will be

discussed.

MEANING OF "EMERGENCY FLARE

Refer to Figure 1 for details.

The pla nt handle s a variety of reactive raw materials and

related hyd rocarbons. For this reason, no continuous pro-

cess vents to the flare were a llowed. All vents r equired for

process operations were routed to a dedicated high relia-

bility incinerator.

Only necessary pressure relief systems requi red for ves-

sel protection were connected to the plant flare. Rupture

disks were used throughout the plant. They w ere consid-

ered to offer a more effective process sea l, i.e., no leakageto the flare, while maintaining vessel overpressure pro-

tection. Only single rupture disk installations were used.

Relief valve use was limited to just a few very specific

cases.

Appropriate selection of materials of construction, atten-

tion to the piping design and equipment installation en-

hanced t he effectiveness of rupture discs (PSE) an d relief

valves (PRV). In addition, pressure relief systems were

specifically manifolded based on chemica l compatibility.

222 October, 1985



PSE and PRV installations were designed to be inspected

and maintained safely with minimum equipment

decontamination.

Further, the “Emergency Flare” includes a high integ-

rity inert gas purge system and has a reliable flare tip pilot

flame monitoring and relighting system. Careful design of

the flare separatorkeal drum an d flare headers enhanced

troubleshooting capabilities and long-term operationswithout compromising safety.

CONCEPT OF HIGH INTEGRITY INERT GAS PURGE SYSTEM

Refer to Figures 1 and 2 for details.

oxygen-free atmosphere within the flare system, while

providing a reliable purge equivalen t to just over 0.1 fps(0.03d s ) lare tip velocity. As such, all PSE a nd PRV dis-

charges were fitted with inert gas purges through

rotameters. Specific purges; however, were equipped

with Low Flow IndicatindAlarm (FIAL) rotameters.

These specific purge locations were at the en d of headers.

Th e FIALs provide a first alert to a potential loss of purge

flow. A Low Pressure IndicatindAIarm (PIAL) locatedjust ahead of the flare seal drum section provides addi-

tional monitoring and backup to the inert gas purge system

flow alarms. The PIAL can b e made to trigger additional

inert gas in the interim while conditions are returned toThe iner t gas purge system was designed to ensure an norm;].

TO FLARE TIP

Ca, 50mm Dia.PI PE

Figure 2. Flore separotorkeol drum.

PbntiOpwations Prozpcnt (Vol. 4, No.4) Oaober,1985 223



FLARE TIP PILOT FLAME MONITORIRELIGHT RELIABILITY

Refer to Figures 3 an d 4 for details.

This feature is rather important for our flare as the “nor-mal” flare gas would be iner t gas from the header purges.

The pilots provide the necessary ignition in case of an

overpressure release. To achieve the required reliability

the following features were included:

1). Flare tip has multiple pilots ( 3 minimum).

2). Use of direct and indirect sensing dual thermocou-

ples per pilot assembly. Direct dual thermocouplesare located at the base of each pilot flame. Indirect

THERMOCOUPLE THERMOCOUPLE

Figure 3. Flare pilot thermocouple detail.

IrmARErJ n) SCANNER

PILOT Y.2ILOT w !

Ill I ‘4 ‘Y-7ilI l l - I I

& + --r- I-L A R M

LOW TEMPERATURE

1LARM

N O F L A M E 1R00!16H/ Low

TEMPERATURE

I N D . /ALARM TO C O N T R O L ROOMCONTROL ROOM

Figure 4. Flare pilot flame monitorhelight.

dual thermocouples sense each pilot metal casing

temperature.

3 ) . Positioning of the direct sensing dual thermocouples

based on flare tip field test data. A 1/4 in. to 5/16 in .(6 o 8 mm) protrusion into the base of the pilot flame

resulted in an acceptable temperature range of315-400°C. Higher temperatures ca. 640-815°C re-

sulted in very short thermocouple life.

4). All thermocouples are 5/16 in. (8 mm) type K. They

can be removed- urin g a total plant shutdown-from the access platform just below the flare tip if

required.

5). Use of redu ndan t auto pilot relight ignition panels.Delayed low temperatu re alarm signals for any pilot

(about 176°C) sets the auto pilot relight sequence.

6). Use of an independent infrared flame scanner to

verify pilot flame. The scanner is located on an ac-

cessible structure about 700 feet (210 m) from the

flare tip. The scanner provides only an “alarm”

function on loss of flame.

7). Use of dry air with bottled air backup for the auto pi-

lot ignition panels.

FEATURES OF TH E CO MM ON FLARE SEPARATORISEAL DRU M

Refer to Figure 2 for details.

1) . Dynamic but captive seal water system to provide an“alert” of any process leaks into the flare system.

Monitoring is done by daily samples of the circulat-

ing seal water.2). Addition of proper inhibitors to the captive/circu-

lating seal can be done if requi red to minimize con-ditions such as freezing, potential fouling, etc. Steam

can also be added if required to prevent freezing dur-ing winter months.

3). Mechanically constant 12 in. (300 mm) water seal,

fixed by vessel piping nozzles.4) . Redundant seal drum section level indicatodalarm

instruments.

5 ) . Mechanically constant water seal against 10 feet (3

m) of water back pre ssur e per t he API-RP 521

guidelines.

ACKNOWLEDGMENT

The author is grateful to W. R. Stone- Monsanto I & Especialist for his he lp in preparing this article.

LITERATURE CITED

1. Husa, H. W., “How to Compute Safe Purge Hates,”Hydrocar-bon Processing, 43, No. , 179 (May 1964).

2. American Petroleum Institute, “Guide For Pressure Reliefand Depressuring Systems,” API-RP 529, 1st Edition, Sept.1969.

JoseA. Boix,Senior Specia list, is currently in the

Environmental Affairs and Technolow section ofthe Monsanto Company Chocolate Bayou, Texas,

plant. Has been with the company since 1965. A

native of Cuba, he received a B.Sc.Ch.E. from the

University of SW Louisiana in 1965. During thepast 20 years with Monsantohe has been involved

primarily in manufacturing design and startupsup-

port in various domestic and foreign projects forMonsanto.

224 October, 1985 Mant/Operations Progress (Vol. 4, No.4)

emergency flare systems design features

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