Flare System Study Epa

7/25/2019 Flare System Study Epa

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FLARE

EPA-600/2-76-079

March

1976

SYSTEMS

STUDY

by

M.G.

Klett

and J B. Galeski

Lockheed Missiles

and

Space Co.

Inc.

4800

Bradford

Drive

Huntsville Alabama

35807

Contract

No. 68-02-1331 Task 3

ROAP No. 21AXM-030

Program

Element

No. 1AB015

EPA

Task Officer:

Max

Samfield

Indus trial Environmental Research Laboratory

Office of Energy Minerals

and

Industry

Research Triangle

Park

NC 277

Prepared

for

U.S. ENVIRONMENTAL

PROTECTION

AGENCY

Office of

Research

and

Development

Washington

DC 20460


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LMSC-HREC

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FOREWORD

This study

of

industr ial

flare

technology

was conducted

under Task 3 of Contract EPA-68-02-1331 by personnel

of

Lockheed Miss i les Space Company

Inc. Huntsville

Research

Engineering

Center

Huntsville

Alabama for the Controls

System

Laboratory

of

the Environmental Protect ion

Agency

Research

Park T:r -iangle North Carolina .

Dr

Max M. Samfield

was

the EPA

Task

Officer. In

addit ion to the authors

Dr.

S.

V.

Bourgeois

participated

in

the

study as

Lockheed

Project

Manager.

The authors are grateful for

the cooperation

and t ime

of

the staffs of the equipment manufacturers f lare users

and

the Air Pollution Control Distr ic ts

who

provided

much

of

the

information

upon which this study is

based


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TABLE OF CONTENTS

Section

Page

FOREWORD

I

INTRODUCTION AND SUMMARY

1.1

Introduction

1.2

Summary

1.2.1 Elevated Fla res

2

1.2.2

Low-Level Enclosed F la r e s

2

1.2.3 Auxil iary

Equipment

2

1.2.4

Costs

3

1.2.5

Fla re

Per formance and Emiss ions

3

1.2.6

Proposed Research and

Development

Programs 4

BACKGROUND

5

2.1

Applications

of Flar ing

for Waste

Gas

Disposal 5

2.2 Flar ing Methods 7

COMMERCIALLY AVAILABLE F L A R E

SYSTEMS

9

3.1

Elevated

Fla res

9

3.1.1

Fla re Tips

9

3.1.2

Gas Traps

12

3.1.3

Pilo t and Ignition

System

18

3.1.4 The Stack and Its

Support

18

3.1.5

Water Seals Flame r r es to r s and

Knockout

D rum s 18

3.2 Ground Fla res

22

3.3

Forced Draf t

Fla res

22

3.4

Comparat ive

Costs

of

Fla re Systems

27

IV

FLARE DESIGN CRITERIA 29

4.1 Selection of Applicable Fla re System

29

4.2

Flammabi l i ty Limi ts and Flame

Stabil i ty

30

4.3 Fla re Emiss ions 32

i i i


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TABLE

O F CONTENTS

Continued)

Section

e

IV

4.3.1 Thermal Emiss ions and

Luminosi ty

32

4.3.2 Noise Emiss ion

33

4.3.3 and Par t icula te Emissions

36

4.3.4

Chemical

Emiss ions

39

4.3.5 Oxidation Products

40

4.3.6

Other Gaseous Emiss ion

Sources

41

4.3.7

Dispers ion Chemical

Emissions

Flammable Gases 41

4.3.8 Air Pollution Rules

and

Regulat ions

Affecting Fla r es

51

4.3.9

Fla re Emiss ion Factor s

53

4.4

Flare

54

4.4.1 Explosion

Potential

54

4.4.2 Vapor u

55

4.3 Molecular Seals

4.4 .4

4.4.5 Explosion

Suppression System.s

56

4.4.6 Water Seals and Flame Arres to r s

56

4.4.7

External Fi res

and

Emissions

4.4.8 Knockout Drum

and Design

Cr i te r ia

58

4.4.9

Thermal Radiat ion

Hazards

60

RECOMMENDED

DESIGN

METHOD 71

5

. l

Elevated Flare System 71

5.1.1 red

Design

Information 71

5.1.2

Fla r e

Burner

Diamete r

72

5.1.3

Requirements

75

5.1.4 Fla r e Height

5.1.5 Support ing Struc tures

79

5.1.6

Auxil iary

and

Control Components

79

5.1.

7 rmic

L o w

Btu Gas

St reams

8

5.2

Low-

Flare 8

VI SAMPLING AND

ANALYSIS

TECHNIQUES

8

6.1

Pr esen t

Sampling

Prac t ices and Prob lems

8

6.2 Analyt ical

s

6.2.1

Hydrocarbons

84

6.2.2

Oxidized Hydrocarbons, Carbon

Monoxide,

Carbon Dioxide

84

iv


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Section

VI

VII

VIII

LMSC-HREC

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TABLE OF CONTENTS Continued)

6.2.3

Nitrogen

Oxides

6.2.4

Sulfur

Dioxide

6.3

Long

Path

Remote

Sensing Techniques

FLARE LOADINGS

AND

EMISSIONS FOR VARIOUS

INDUSTRIES

7.1

Questionnaire ormat and Circulation

7.2 Refinery

Quest ionnaire

Results

7.3 Impact of l a res on

Refinery

Emissions

7.4

Iron

and

Steel Mil ls Quest ionnaire

Resul ts

7. 5 Impact of l a res on Iron and Steel Mill

Emiss ions

7.6 Manufactur ing Chemists Quest ionnaire

Results

7. 7 Summary of l a re

Loadings

RECOMMENDED

RESEARCH PROGRAM

8.1

Theoret ical

Analysis

of Combust ion

Page

84

87

87

90

90

90

97

99

100

104

106

107

Modifications Applicable to Flar ing

107

8.1.1 Summary

and

Object ives

107

8.1. 2 Background

107

8.

1. 3 Validation

of

the Analytical Model

109

8.1.4

Evaluat ion

of

l a re

Design Modifications 109

8.1.

5 Prio r i ty

109

8.2

Evaluat ion of Remote Sampling Methods 109

8.2.1 Summary and Object ives

8.2.2 Background

8.2.3 Summary

of

Remote Sampling

Technology

8.2.4 Remo te Sampling

Field

Studies

8.2. 5 Prio r i ty

8.3

Application

of Flar ing

to Control

of Gaseous

109

109

109

110

110

Emiss ions

110

8.3.1

Summary and

Object ives

8.3.2 Background

8.3.3

Theoret ical

Analysis

8.3.4

Exper imental

Analysis

v

110

110

110

3


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TABLE OF

CONTENTS

Continued)

Section

Page

VIII

8.3.4

Experimental

Analysis

113

8.3.5

Prior i ty

113

8.4 Economic Analysis of Waste Stream

Recovery

and

Alternate

Disposal Methods

113

8.4.1

Summary

and

Objectives

113

8.4.2 Background

114

8.4.3 Identify Economic Considerat ions Now

Used to Determine

Whether

Given

Fla red Stream has

Sufficient

By-Product

Value

for Recovery

114

8.4.4 Identify Alternative Uses

of

Low

Pre s

sure

Flammable

Hydrocarbon

Gases

115

8.4.5 Evaluation of Alternat ive D isposal

Methods

115

8.4.6

Prior i ty

115

8.5

Emiss ion Factors for

Elevated

Fla re Systems

115

8.5.1 Summary and Objectives

115

8.5.2

Background

115

8.5.3

Site Selection

and

Evaluation of

Sampling

Methods and

Hardware

116

8.5.4

Field Testing of Elevated Fla re Systems 116

8.5.5

Prior i ty

116

X

REFERENCES

117

vi


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1.1 Introduction

LMSC-HREC TR

D390190

SECTION

INTRODUCTION

AND

SUMMARY

This

repor t p resen ts

the

resul ts

of

a

study

of

emiss ions

f rom

f lare

sys

tems. Flares

a re used for the control

of

gaseous

combustible

emissions

f rom sta t ionary

sources .

The

scope

of

the

study includes an evaluation

of

existing

f lare sys tems an

examinat ion of f lare des ign and sizing c r i

t er ia recommended design methods and features an assessment

of

pre

sent

emiss ion problems

and

a recommended research program for flare

emissions control . Information was

obtained

f rom

the

published l i t era

ture equipment manufac ture r s equipment user s

a i r pollution

control

agencies

and

univers i t ies .

Visi ts were

made to

many of these

sources

of

information

in

order

to hold

detailed

technical

discuss ions

about

the de

sign

and performance of f lare sys tems.

Flaring is

intended

pr imar i ly as a safety measu re for disposing of la rge

quanti t ies

of

gases

during

plant

emergenc ies .

Flows

are

typically

in ter

mittent with

flow r t ~ s

of several mill ion cubic

feet an

hour

during

major

upsets .

Continuous

f lar ing is general ly l imited to

flows

not greater than

a few

hundred

cubic feet

an

hour. Since f laring is re la t ively

inexpensive

this technique has been suggested

for

the

control of gaseous

combust ible

emissions

f rom

sta t ionary

sources .

However emiss ions f rom f lares

could

also

create a

potential

problem. This

study

was car r ied

out with

two

objectives

in mind. One was to

determine the

potential

of

f lares

as

a control

sys tem and the

second was

to

assess the emiss ion hazards of

present

industr ia l f lares .

Section l l

of this repor t explains the different applications of flaring waste

gases .

Section

i l l

descr ibes

the

commerc ia l ly

avai lable

f lare

systems

and

gives

comparative cost data.

Section

IV discusses

f lare des ign

cr i ter ia

including in some deta i l the two main

problem

areas

of

f lare emissions

and

safety.

Section

V

presents

recommended design

methods; Section VII

discusses presen t

f lare

loadings for

var ious industr ies

and their

impac t on

emissions; Section Vll i contains

a

recommended

f lare r esea rch program.

1.2 Summary

Commercial ly avai lable f lare sys tems a re

of

two basic t y p e s -

elevated

and

ground

f lares. Present ly

these

serve separate

functions; elevated

flares are

used

pr imar i ly for disposal of gaseous

wastes

generated

during

plant

emergencies such as during power fa i lure plant f i res component

fai lure

and

other

overp ressu re

situations

in

which

discharge

direct ly

to

the a tmosphere could resu l t in explosion hazards . Elevated

f lares

are

therefore used pr imar i ly in

conjunction with

vapor re l ief collect ion

-

terns

in

l a rge- sca le chemical manufactur ing

or

pet ro leum refining

opera

t ions.

Other

l imited applications include

venting

of s torage

tanks

and

loading platforms.

Although s team wate r

and

air a re frequently injected into the elevated

flare burner

to reduce

smoke

and

luminosi ty

expedient vapor disposal

ra ther than pollut ion control has been the design emphasis .

Recently


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LMSC-HREC

TR 039019

developed low-level f lare

sys tems

represent a

depar ture f rom

conven

t ional

design.

With recent emphas i s reducing noise , chemical

emissions

heat

and

luminosi ty , low-level f lares have

become increasing popular

for disposing

of

rout ine discharges . These

inc lude

disposal of

f lam

mable gases

leaking

f rom

process and re l ie f

valves,

process waste

s t r eams and excess or off-speci f icat ion product .

1.2.1

Elevated Flares

Elevated flare sys t ems provide a means for disposal

of

gaseous w as te

s t reams with an

a lmos t unl imited

range of flows and a minimal

pr e s su r e

drop of 0

to 60

inches H

2

0 .

As

such,

elevated f lares provide a

unique

function which

cannot be

duplicated by other types of

combust ion equip

ment .

Design cr i t e r i a for e levated f la re sys tems

are

or ien ted a lmost exclusivel

toward

safe

ra ther than efficient combust ion

of

gaseous wastes .

Accord

ingly, s iz ing

calculat ions

present ly

available

a r e

based

on

allowable

pres

sure drop

(Section 5.1.2) and

dispers ion

of the rmal

radiat ion (Section

5.1.

or the dispersion of toxic gases

when

a

f la re -out

occurs

(Section 4.1.

7).

Discharge of l iquids into the f la re sys tem can cause problems and knoc

out 1 or l iquid disent ra inment drum s

are

required for l iquid removal .

1.2.2

Low-Level

Enclosed Flares

Low-level f lares with enclosed

combust ion

a r e

being

used

in

conjunction

with

the

elevated f lare in response

to

recent emphasis on pollut ion. These

a r e descr ibed

in

detai l

in Sect ion III. The study indicates

that

low-level

f lares although relat ively expensive to

build

and maintain ,

are

effective

in

reducing noise and

the rmal

emiss ions .

Relatively

l i t t le

informat ion has

been found on s1z1ng

and

design

of

low

level f lares . The norma l

configurat ion for construct ion of a low-level

f lare involves

a s tee l

outer

shell ,

l ined with

re f rac tory m ater ia l . The

outer

shel l se rves

to conceal

the

f lame

and prevent the rmal and luminous

radiat ion. As

in other types

of combust ion

equipment , the re f rac tory

also protects the s tee l

shel l f rom direct

exposure to the effects

of high

t empera tures and

cor ros ive

ma te r ia l s and to

improve

combust ion effi

ciency by

minimizing heat losses . Refractory thicknesses typically var ie

from about 4 to 8

inches .

The

re f rac tory

used

results in

a sluggish

re

sponse to

abrupt

changes

in gas

flow

and adds

considerably to the con

struct ion and maintenance costs of a low-level f lare .

Because

of

the

slow

heatup assoc ia ted with re f rac tory const ruct ion, the low-level flare is

normal ly

used

only

for

low

or

continuous

flow

r a t es

with

an

elevated

flare

of

conventional design used

to

accommodate sudden upsets . An e le

vated

f la re

mus t be assoc ia ted

with low-level f la re

applicat ions in most

convent ional

designs .

1.2.3

Auxi l iary

Equipment

Auxi l iary equipment

for

the f lare system

includes ign i te rs pilots

and

safety-oriented

equipment descr ibed

n Sect ions

3.1,

4.4 and 5.1.6.

2


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LMSC-HREC TR D390190

Knockout

drum s

are normal ly

provided

for removal of l iquids

f rom

the

f lared

s t ream. Water

sea ls and, l ess frequently, f lame ar r es to r s

are

used to

isolate

the

f lare

s tack

f rom

the vent

collection sys tem.

Purge

gas

generators

and vapor t raps se rve to

prevent

the

formation of ex

plosive mixtures

within

the f lare

s tack. Maintenance

of the

liquid

level

in

water sea ls

and

disent rainment drums

is

cri t ical : l iquid

level control

and

a l a r m sys tems are available for

these

sys tems . Pi lo t burners are

also

frequent ly equipped

with f lame

detect ion

and

a la rm

sys tems.

1.2

.4

Costs

Capital costs for

low-level

f lares and

various

types of

elevated f lares

are

given

in

Section 3.4.

This information is based on discuss ions with

f lare vendors

and

user s .

Elevated

f lare

equipment

cos ts vary

considerably

because of

the dis -

proport ionate costs for auxi l iary and

control

equipment and

the relatively

low cost of

the

f la re s tack

and

burner . As a resu l t , equipment

costs

are

rarely diameter-dependent . Typical instal led

costs

range f rom 30,000

to about 100,000.

Low-level

f lares

a re

approximately ten

t imes more

expensive

for similat : capaci ty ranges .

Operating costs a r e

determined

chiefly by

fuel cos ts

for

purge

gas

and

pilot

burners ,

and by

s team requi red

for

smokeless f laring.

Steam

and

other requi rements

are

discussed in Sect ions 5 .1.3 and 5.1. 7. On the

basis

of

30 cents per mil l ion Btu's fuel requirements . typical elevated

f lare

stack

operat ing

costs

(2-foot-diameter s tack) a re about

1,500

per

year .

1.2.5

Flare Per formance and Emiss ions

Since

flaring

has t radi t ional ly

been

used for the

safe

disposal

of

gases

discharged under emergency

condit ions,

performance s tandards relating

to

combustion efficiency

and gaseous emissions a re l imited. Probable

air polh; tants f rom elevated f lares include CO, unburned

hydrocarbons,

aldehydes, and par t icula tes as expected f rom

any

combust ion process

involving

large, turbulent diffusion

f lames . These emiss ions

result

f rom

flame quenching. Relat ively

low f lame t emperatures are typical ly

observed for

both

elevated

and low-level

f lares ,

probably

result ing in

low NOx emiss ion fac tors compared to other types of industr ial

combus

tion equipment.

Results

of a

survey

to

determine

flare loadings and es t imated

flare

emissions

are discussed in Sect ion VII. I t was

found that

the

average

year ly emiss ions f rom

f lares

consti tute

just

a

smal l

fraction,

less

than

lo/o of the average year ly plant* emiss ions . Tota l f lare emiss ions

over

a

year s t ime

the re fore probably only

have

a smal l

impact on

total plant

*Representa t ive plants include U.S. per t ro leum re f iner ies , i ron and steel

mil ls and chemical manufactur ing faci l i t ies .


10/130

LMSC-HREC

TR D 3 9

emiss ions .

However , because of the intermit tent nature of flaring, th

majori ty of f lare emissions

are

concentra ted

into

just a few

minutes

actual f lar ing. During

this

t ime five

or

mo r e

t imes

the normal plant

emiss ions a re re leased into the atmosphere .

1.2.6 Proposed Research

and Development Programs

Programs

have been developed to

provide

technology

where

deficienci

exist , to generate the data requi:red

to evaluate

combust ion modificatio

and

extend the application of

flaririg to

a i r pollut ion

control.

Since l i t t le quantitative

performance data were found in

this

study,

fie

test ing

of

elevated and

enclosed

ground

level

f lare

systems is r e c o n : u n

Testing

should

be

done

to

determine the

concentra t ion

and character is

of f lare

combustion products

as

well

as the

mass r a te

of emiss ions in

order to evaluate the efficiency

of

f lare

sys tems as

a control device.

A combust ion

r esea rch

program

is recommended to

fi l l

the

gaps exis

in

the technology of

l arge diffusion f lames.

or

this study, construct

of a l arge scale f lare burner

and

combust ion chamber

i s

recon:unende

Par t

of

the ra t ionale and incentive

for this

prog ram is that many

indu

f lames

are

of the turbulent-diffusion-flame

type.


11/130

SECTION

II

BACKGROUND

LMSC-HREC TR

D39019

In many industr ia l operat ions , and

part icular ly

in chemical plants and

petroleum

ref iner ies large vohimes of

combust ible waste

gases a re

produced.

These gases

resul t

f rom undetected leaks

in

the

operating

equipment, from upset condit ions

in

the

normal

operation of a plant

where

gases

must be

vented to

avoid

dangerously high

pressures

in

operating equipment,

f rom

plant s tar t ups and f rom emergency shut

downs.

Large quantit ies of

gases

may also resul t f rom

off-spec

product

or excess

product which cannot

be

sold. Flows are typically

in termit tent with flow ra tes during

major

upsets of several million

cubic feet per hour .

The

prefer red

control method for

excess

gases and vapors is to recover

them in a blowdown recovery sys tem. However, l arge quantit ies of gas,

especially those produced

during upset

and

emergency

condit ions, are

difficult

to

contain and

reprocess .

In

the pas t

al l

waste gases were

vented direct ly

into

the a tmosphere . However , widespread venting

caused

safety

and environmental problems. In

pract ice

therefore, i t

is

now

customary to collec t

such

gases in a closed f lare system and to

burn

these gases

as

they a re

discharged f rom an elevated

flare

stack or

alternate ly

the

gases may

be

discharged

and

burned

at ground level

usuall

with

shielding

for the

f lame.

The f lare sys tem is used pr imar i ly as a

safe

method for disposing of

excess waste gases . However,

the

f lare

sys tem

i tself

can present

addi

t ional safety

problems.

These

include

the

explosion potential of

a flare,

thermal radiat ion hazards f rom the f lame, and the

problem

of toxic

asphyxiat ion during f lame -out . Aside f rom safety there are several othe

problems

associated

with

f lar ing

which

must

be

dealt

with

during

the

de

sign

and operation

of

a f lare system.

These

problems fall into

the

genera

area of

emiss ions

f rom f lares

and

include the

formation of smoke,

the

luminosity of the f lame, noise during f lar ing and the possible emission of

ai r pollutants during flaring.

2.1 Applications

of Flaring for

Waste

Gas

Disposal

There

a re

th ree main considera t ions

in deciding

whether to flare a

waste

gas.

These are: 1) the

variability

of the flow

of

the waste

s tream,

2) the

expected maximum volume of

the

s t r eam to be flared, and 3)

the heat co

tent

of the

waste

s t ream.

A

high

variability

of

flow of

the waste

s t r eam is

probably

the

most

im

portant factor . A f lare

is des igned

to operate for pract ical ly

an infinite

turndown range of

flows.

Alternate

waste gas

disposal sys tems

such as

incinerators or af terburners need an adequate control

on

the flow of waste

gases and

can

only be used for

continuous

or at least

fairly

continuous

ga

flows.

5


12/130

LMSC-HREC TR 0390

The

volume of the waste

s t ream

to

be disposed

is also

an important

factor .

With very large

volumes

of

gas,

direc t

f lame

combustion

by

incinerat ion o r a f lame afterburner device becomes impract ical due t

the

s ize of

equipment needed.

However, capaci ty

for an elevated

flare

can be

inc

rea sed

easi ly by

inc rea sing the diameter of the stack.

A

typical

small f lare

with

a

four- inch

diameter

s tack

has

a

capacity

of

30,000

scfh.

A normal ref inery f lare with a capaci ty

of

5,000,000

scf

would need only a 36-inch

diameter f lare stack.

The heat

content of

a waste

gas fal ls

into two

c lasses

The

gases

can

ei ther maintain the i r own combust ion o r they

cannot

maintain their ow

combust ion.

n general , a waste

gas with

a

heating

value greater than

200 Btu/f t3 can

be f lared successful ly .

The heat ing

value is based on

the lower heating

value of

the waste gas

a t

the f lare

Below

200

Btu/f

enriching the waste gas

by

injecting

a

gas with

a high

heating

value

m

be

necessary

The addition of such

a r ich

gas

is

called endothermic

flaring. Gases with a heating value as low as 60 Btu/ft3 have been f la

but

a t a

significant

fuel

demand (Ref. 1). I t j s usual ly not feasible to f

a

gas with

a

heating value below

100

Btu/ f t

(Ref.

2).

f

the

flow of lo

BTU gas

is

continuous, incineration can

be

used to dispose

of

the gas.

For intermit tent f lows,

endothermic

flaring

is

the

only possibil i ty.

Flares

are

well

suited for disposing of intermit tent

flows

of large and

smal l

volumes

of waste gases that have an adequate heat

value

to sust

combust ion.

For in termit tent flows of low

heating

value waste gases ,

additional

fuel

mus t

be

added

to

the

waste

s t r eam in

order

to flare.

S

the

value

of

the additional fuel can

become

considerable and is comple

lost during flaring, endothermic flaring

can become expensive. Howev

i f intermit tent flows of low heat waste gases

are

in large volumes, th

only pract ical al ternat ive to flaring is to

vent the

gases direct ly to the

a tmosphere .

This

is

usually

unacceptable

for

envirorunental

reasons

Most f la res are used to dispose of the intermit tent flow of

waste

gase

There are some continuous f lares but they

are

used generally for sma

volumes

of

gases

on the order of 500 cfm o r l ess The heating value

la rger continuous flows of a

high heat

waste s t r eam is

usually

too valu

to waste in a f lare Vapor recovery

or the

use of

the vapor

as

fuel in

process hea te r

is

prefe r red over

f lar ing.

For

large continuous flows

a

low heating value gas, aux i l iary

fuel mus t be added to

the

gas in

ord

to f lare It

is

much

more efficient to burn the gas

in an

enclosed inci

ator ra ther than

in the

f lame

of a f lare F o r smal l continuous flow

of

gases , f lares

are sometimes

used even

though

fuel or heat is ei ther lo

o r wasted. n these cases the

equipment

costs are somet imes more i

portant than

fuel

savings and

a

f lare i s mor e

economical

to use.

Flares a re most ly

used for the

disposal

of hydrocarbons. Waste gase

composed of natural

gas,

propane, ethylene, propylene,

butadiene and

butane probably

consti tute

over 95 of the mater ia l flared. Flares

ha

been used successful ly to control malodorous gases such as mercapta

and amines (Ref. 3). However,

ca r e mus t

be

taken

when

f laring these

gases . Unless the f lare is very

efficient and

gives good combustion,

obnoxious

fumes can

escape

unburned

and cause a

nuisance (Ref.

4).

6


13/130

LMSC-HREC

TR D390190

Flaring of hydrogen sulfide should be

avoided

because of i ts toxicity and

low odor threshold. In addition, burning re la t ively smal l

amounts

of

hydrogen

sulfide can crea te

enough sulfur

dioxi 1e

to cause crop damage

or local nuisance

(Ref. 5). n recent years gases

whose

combustion

products

m y cause problems,

such

as

those containing hydrogen sulfide

or

chlorinated hydrocarbons, have not been recommended for flaring.

2.

Flaring

Methods

The elevated f lare is the most common type of f lare system

in

use today.

In

this f lare, gas is

discharged

without substant ia l

premixing,

and

ignited

and burned

at the

point of

discharge.

Combust ion

of

the

discharged gases

takes place

in the

ambient atmospher ic a i r

by

means of a

diffusion flame.

This

type of combust ion often results in

an insufficient

supply of ai r and

thus

a

smoky

f lame. A

smokeless

flame

can

be

obtained

when

an

adequate

amount

of combust ion ai r is

mixed

sufficient ly

with the

gas so that it

burns

completely. Smokeless burning is usually accomplished by injecting

s team

into

the flame.

The modern

elevated

f lare allows la rge

volumes

of waste

gases

to

be

burned safely and inexpensively. However, the

elevated flare

can also present other emiss ion pr


14/130

LMSC-HREC

TR

0390190

The

use

of a ir

- ins

pirat ing burners for

premixed

a ir has

also been

at tempted with

f lares .

This

type

c:r

operation

r equi res

the gas to be

suppl ied

at substantially

constant

r a te

and

pressure

of

the

order of

to

4 psig.

In

many

cases

such pressure cannot

be

made available because

limitat ions of the vent gas col lect ing sys tem.

For ai r - inspi rat ing ins tal

lations

i t is

also

genera l ly

necessary

to provide

a

number

of

burners

of

different

capaci t ies

to

handle

the

wide

range of

venting ra tes normally

encountered. Flare

sys tems

based on this

pr incip le

have been largely

unsuccessful .

Usually,

i

there is

a continuous

flow of gas

a vapor

recovery

sys tem

is considered. While the

collect ion,

s torage and

r e turn

of gas

is

ex

pensive, the cont inuous wast ing of gas m y

be

much more expensive.

The capi ta l expendi tures

to

s tore and recompress

immense

volumes

released in termit tent ly and i r r egular ly

usually exceeds the operat ing

expense of flaring the

gas .

Many

plants are

now using their flare

sys

tem in

conjunction with a vapor recovery

sys tem.

They

have

a t r iad

sys tem

for

contro l of waste gases

which

consis ts

of

a

vapor

recovery

system,

a

low-level

f lare

for

most

of

the

f lare

occurrences

which

over

load the vapor

r ecovery sys tem

and an elevated

f lare for

large releases

which

over load the

low-level f lare.

Horton et al . , Ref. 6) have

discussed what

they feel

is

the future answer

to

reducing the possible load to

a

f lare.

The

nuclear power industry has

installed highly rel iable ins t rumented sys tems to eliminate

the

need for

rel ief valves and st i l l

protect

a sys tem f rom over

pres

sure Ref. 7). How

ever,

these

sys tems

have

not

achieved wide

use in the chemical

or

petro leum

industry .

The rea l

source

of most pressure in

gas- l iquid

systems is heat . Fired

heaters and

heat exchangers crea te

l a rge

volumes of gas

which must be

relieved.

A

highly

rel iable

means

for

automat ical ly

cutting off

heat,

when

the

pressure reaches

a

specif ied

value,

would

decrease

or eliminate

the need for

a

safety rel ief valve.

It

would

therefore

decrease the quantity

of gas sent to

the

f lare. Reliabil i ty is usually assured by independent and

redundant ins t rumenta t ion

Ref. 7).

The

high in tegri ty protec t ion sys tem

can

never

total ly el iminate

all safety

rel ief

valves

in

a

plant and

thus

the need for

a

f lare.

However, the

load

to

the

flare

would

be great ly reduced

with

the flare being

used

only in

major emergency situations.

8


15/130

LMSC -HREC TR D390l90

SECTION III

COMMERCIALLY AVAILAJ3LE FLARE SYSTEMS

In

general there

a re th ree types of f lare sys tems in

use

today, the

ele

vated, ground and forced draf t f lare . This section will

describe

the

equipment

avai lable

for f laring

waste gases by these sys tems and

will

also

present

relat ive

cost

data

for

the

different

sys tems .

3

.l

Elevated

Flares

The

modern

elevated

flare

sys tem

is made up of several

components

including

the

f lare t ip , some type

of

gas

t rap direct ly below

the t ip, a

pilot and ignition sys tem a t the top of

the

f lare t ip, and the stack and

i ts support. When

smokeless

burning

is

required, a s team injection

sys tem m us t a lso be provided at the

top

of the

flare.

Water

seals

and

knockout drums a re a lso usually required for safety reasons. Figure

3-1 shows

a

schematic of

a

typical

elevated

f lare sys tem

3

.1.1

Flare

Tips

A

f lare t ip mus t be capable of operating over

a

wide range of turndown

ra t ios .

To achieve this , the f lare must

have

excellent f lame holding

ability and mixing character i s t ics . Flameholding is ensured

by

pro-

viding mult iple continuous

pilots

around the combust ion t ip and

by

pro-

viding

a f lame stabi l izat ion r ing on the combust ion t ip.

Figure

3-2 shows

the standard f lare

t ips

available f rom John Zink

Company.

The flare t ip

is

usually

made of s ta inless

steel

or som e other high tempera ture and

corros ion-res is tant

alloy.

Smokeless burning can be achieved with special f lare t ips which inject

water , natura l gas or s team

into

the f lame thereby increasing ai r -gas

mixing

to

ensure

complete

comb ustion. Water injection

has

many dis-

advantages including ice format ion n the winter , a mis t in the

summer ,

the t remendous pressure head needed for an elevated

flare

and a tu rn-

down ra t io much l ess than

s team,

making control v ry difficult with the

possibil i ty of quenching the

flame.

Natural gas has a lso been used

to

inject into the f lame

for

smokeless burning but only in the case where the

gas i tse lf has no

value

since

t

is also burned

during

flaring. For these

reasons

s team

is

the most common

utili ty

used for smokeless burning.

There are two

bas ic

s team injection

techniques used

in elevated flares.

In one

method

s team is injected from nozzles

on

an external ring

around

the top

of the t ip . In

the

second

method the

s team

is

injected by

a

si lgle

nozzle located concentr ical ly within

the

burner t ip .

Vendors

use various

types

of

nozzles to

crea te

a

circular

swirl ,

fan,

jet

or

C

oanda

effect.

In recent years environmental regulations have required f lares

to

be

smokeless for l arge

turndown

rat ios.

To

ensure sa t is factory operation

under varied flow condit ions, the two

types

of s team injection have been

combined into one

t ip.

T'he in ternal

nozzle

provides s team

at

low

flow

9


16/130


D

l a r e Burner

and Location

of

Fluidic

Seal

Gas

Trap

G

Riser Sections

@

Entry

Dis entrainment

or

Water

Seal

Ladders and

Platforms

Fig. 3-1 Integrated l are Stack Components

10


17/130

Util i ty

Field Fla r e Tip

Smokeless F ie ld Fla r e Tip

LMSC HREC TR

D390190

Endothermic

Field Flare T1p

Steam

Distr i

.11 1 ll l

Endotherm

ion

Ring

ss is t

Ga

Supply

Fig.

3 2

Flare Tips f rom John Zink Company

11


18/130

LMSC-HREC

TR

039

ra tes

while the external je ts are avai lable at large flow

ra tes .

Figure

3-3 shows a schemat ic

of

National AirOil

f lare

t ips i l lustrat ing the

different s team in ject ion methods.

While these a re the mos t common types of

t ips

there a r e

several

oth

mainly

special purpose t ips comm ercia lly avai lable . fur ther

modif

t ion of the s team injection t ip is

shown

in Fig. 3-4. Here , an internal

nozzle

is

used to

inject

both s team and a i r

into the t ip. The major

di

advantage

of

this sys tem is that a la rger

t ip

is

needed

because

of

the

inc reased pr e s s u r e drop.

Under

some ci rcumstances the gases may

actually burn inside the t ip. Figure 3-5 shows a t ip

using

a

Coanda e

of s t eam

in ject ion to achieve

the

required ai r

gas

mixture.

While thi

method provides

efficient mixing,

the burning of the gas takes place

inside the

f lare

t ip instead

of

outside

or above a s

with the

other

t ips.

Burning ins ide the t ip can dras t ica l ly shor ten the

life

of the t ip. Figur

3-6 shows National Air Oil s jet

mix

vortex t ip. Thes e can be

used

wi

relat ively high pressure waste gases

with

l i t t le or

no

s team needed fo

smokles

s

opera t ions . Figure

3-7

shows

the

specia l

purpose

Indair

f l

t ip

which burns

gases smokelessly

without s team.

It has l imited use

since i t requi res both high pressures

and

low

pr e s su r e gas in

the

rat i

of

about

th ree

to

bne. Also i ts ma.Ximum

turndown

ra t io is only

about

two. Other special

purpose t ips a re available including endothermic t

that

inject

gas to ra ise the

heat value of

the waste s t r eam

and t ips wit

added

muffling

for

quiter flaring.

The ra te of s team injection to the f lare t ip can be control led manually

automatical ly. While automatic

control

is usually

not

mandatory,

i t i

prefer red because i t reduces s team usage, great ly reduces the amoun

of smoking and minimizes noise . Automatic

sys teme

use flow measu

ment devices

with

rat io

control

on s team. Since the flow r a t e measu

ment

cannot

include the

var iables

of

degree

of

saturat ion

and

molecul

weight , the

rat io control

is usually

set

for some average hydrocarbon

composit ion.

It

is usually necessary to have

a

f ixed

quantity of s team

flowing at al l t imes to cool the distr ibut ion nozzles a t the t ip.

3.1.2.

Gas

Traps

To prevent

air

migra t ion into the f lare s tack

as a

resu l t

of wind

effec

or density

difference

between

air

and f lare gas

a

continuous

purge

ga

flow through the f lare sys tem is maintained. The sys tem can be purg

with natural gas , processed

gas , inert gas or ni trogen. To

reduce th

amount of purge gas

requirement and to

keep air out

of

a f lare system

gas t rap

devices

are

normal ly located

in

the s tack

direct ly

under the

f lare t ip.

One

type

of

gas

t rap

comm ercia l ly avai lable i s

the

molecul

seal

(Fig.

3-8). This

type

t rap may not prevent

a i r

f rom getting in th

s tack as

a

resu l t of gas cooling in the f lare headers . Instrumentation

sys tems a re

available

to automatically i nc rease the purge ra te to

prev

air

f rom enter ing the s tack during

rapid

gas

cooling.

new developm

in gas t raps

is

National

Air

Oil s Fluidic Seal (Fig. 3-9). This

seal

w

much

less

than

a

molecular seal and

thus can

be placed

much

closer t

f lare

t ip.

12


19/130

HREC

TR

D 130

a.

hemat ic

of

or

enter Unit

for Steam

NAO 48

and

Center Unit for

Steam

3 3 Fla re

and enter Steam Injection Units

13


20/130

LMSC -HREC TR D3

Plan

El-evation

Fig. 3-4

Deta i l

of

Internal

Steam Injection System from John

Zin

Company

Fig.

3-5 Coanda-Type Flare Tip

f rom Flargas

Engineering Ltd.

14


21/130

LMSC

T

13

6 Je t Mix Vortex Flare with

Assis t

f rom National rOil

lomt

ronl

7ft

O I

. 3- The Indair

Fla re

and

Gas

15


22/130

Outlet to

la re Burner

Clean-Out

Inlet

from lare Riser

National

Air Oil

NDS Double

Seal

Patent applied for)

LMSC-HREC

TR

D3

Outlet to lare Burner

I

Inlet from.

lare Riser

John Zink Molecular

Seal

U.S.

3,055,417)

Fig. 3-8 -

Air

Reentry Seals

16


23/130

pper

Section

Lower

Section

L_ .........___..___.

Entering

Air

LMSC HREC

TR

D390190

Purge

Gas

Flow

Velocity Grad

o

Waste Gas

Flare Riser

Flare Burner with

Seal

Baffles

Velocity Profi les

Fig. 3 9 National Air Oil Fluidic Seal

17


24/130

LMSC-HREC

TR

3

.1.

3

Pilot and Ignition

System

The ignition mechanism for a flare instal lat ion usually consis t s of

pilot

burners

and

the pilot burner igni ters .

The

pilot burners ser

ignite

the

outilowing gases and

to

keep the

gas

burning. These

pil

Inust

provide a

stable

f lame

to ignite

the

flare gases

and

in

many

to keep

them

burning. To

accompl ish

th is

mor e

than one

and

usua

th ree or

four

pilot burners

are

always used. The pilot

burners

ar

somet imes provided

with

separa te wind shields as

sho.wn in

Fig. 3

A separate sys tem must be

provided

for the ignit ion of the pilot b

to

safeguard

against f lare fai lure. The

usual

method

used

is to ig

a

gas /a i r

mixture in an ignit ion

chamber by

a spark. The

flame f

t ravels through an igniter

tube

to the

pilot

burner

a t the top of the

This

sys tem permi ts the igni ter

to be set

up a t a safe

distance

fro

flare,

up to 100 feet , and s t i l l ignite the pi lots sat isfactori ly. Figu

shows one ar rangement for the

ignition

sys tem. The whole device

mounted

on

an ignit ion

panel

and set up in

an

access ible spot on

th

The

ignit ion panel mus t

be

explosion proof,

have an unlimited

l i fe

insensi t ive

to

al l

weather

conditions.

On

elevated

f lares ,

the

pilo

is usual ly

not

visible and an a la r m sys tem

to

indicate

pilot

flame

is desirable . This is usually

done

by a thermocouple

in

the

pilot

f lame. n the

event of

flame fai lure, the t empera ture drops and a

sounds.

3 .1.4

The Stack and Its Support

Fig.ure

3-12

shows

the methods used to support the complete

flare

These

towers

must be

provided

with a climbing ladder with a cage

landing

on top for repa ir and maintenance purposes . These tower

ref iner ies

can

range

from

200 to

400 feet high. Flare towers with

proport ion of length- to-diameter ra t io less than 30 are usually

co

s t ructed as

sel f-support ing

s tacks; towers

with

a

proport ion L/D

are supported with

a set

of

guys ,

and when

the

proport ion is

L/D

the

towers a r e made with two or more se ts of guys

Ref.

2). Self

supporting

s tacks

are usually not

built

over 50

feet high

because

o

l a rge

and

expensive foundation requi red

Ref.

4).

The guys need a l a rge area for high s tacks; that is why it is

often

fe r red to build steel supports to which the s tack is fastened. The

usually steel framework

with

a square

cross sect ion widened

at th

A t r iangular

cross sect ion,

adopted from

the modern television an

is mor e

economical

and has been

used

in several ref iner ies

Ref.

f lare s tack will expand because

of the

hot

gas

flow, and the suppor

s t ruc ture must

be able to accommodate

this

expansion.

3

.1 .

5

Water Seals ,

Flame Arres tors

and

Knockout

Drums

Water

seals and f lame ar res to r s

are

used to prevent a f lame

fron

entering the flare sys tem. Flame

ar r es to r s

have a tendency to pl

obstruct flow and

are not

capable of stopping a flame front

in

mixt

a i r

with hydrogen, acetylene, ethylene oxide and

carbon

disulfide;

they

are

of l i t t le

value Ref.

1).

8


25/130

Igniter

Inlet

Gas

Inlet

LMSC HREC TR D390190

Shielded Pilot Nozzle

2

in.

Pilot

Tube

1

in. Igniter Tube

Inspirator

Air

Adjuster

Thermocouple

Explosion Proof-

Weather Proof

Junction Box

Fig. 3 10 lare Pilot Burner System

19


26/130

LMSC-HREC TR D3901

Descript ion

D Mounting Plate - 18 x

36 in.

W Air Control Valve (1/2 in.)

G

Gas

Control

Valve

(1/2 in.)

G Gas Pres su r e

Gage

G Air Pres su r e

Gage

@ Spark Sight

Por t

])

Spark Plug

Explosionproof Button (Push)

Transfo rmer in

Explosion-Proof

Weather-Proof Housing

Three

ay Valves

NOTE: Quantity

of

I tem

10

will

vary with

number of pi lots

on flare.

Air

Inlet

1 2

NPT

r------

I I

I I

I I

L ~

Pilot Ignitor

Ou

Ignitor Ou

lgnitox Ou

I I

l i

TT

I I

t : ~ b

Outlet f l

8

Dia Mount n

Holes

IT

I I

I I

c . . : : : ~

Fig. 3-11

- Flare Ignition

System

from National

Air

Oil

20


27/130

LMSC HREC TR D390190

a Self Support ing

b. Flare with upport

Tower

c . Flare with Guys

Fig. 3 12

Flare

Stack Supports

21


28/130

LMSC-HREC TR

Water sea ls a re used to prevent a flame front and

ai r

from enter

f lare

gas

collect ion sys tem. The weight

of

the water seal cause

be located a t

or near

grade and

the re fore the sea l cannot

be

used

vent a i r f rom entering the s tack.

Knockout drums

are

located at or

near

the base of elevated f lare

arate

l iquid f rom gases being burned. f the la rge l iquid droplet

removed,

they

could burn al l

the way

to the ground.

Designed fo

f lare l ines can

contain l iquids

f rom l iquid

expansion

re l iefs l iqu

over f rom gas

re l ie f s

and condensed

vapors .

The knockout dru

to remove these l iquids before the gases a re flared. Water

seal

knockout drums a re found on mos t

f lare

sys tems

for

safety reaso

3.2

Ground

Flares

A

ground f lare

consis t s of a burner

and

auxilar ies

such

as a se

burner and igniter .

Two

types

are

found. One consis ts of conve

burners discharging horizontally with no

enc losures . This

f lare

ins ta l led in a la rge open area

for

safe operat ion and fire protecti

the ignition

sys tem

fai ls

th is

is

not

as

capable

in

dispers ing

the

g

an

elevated f lare . For these reasons this

type

of

ground

f lare h

only l imited

applications.

Ground f lares m y

also consist of

mult ip le

burners

enclosed with

f ractory shel l

as

in the recent ly developed

low

level f lares

(Fi

and

3-14) . The

essentia l purpose

of

a low level f lare is complet

cea lment of the

f lare

flame as wel l as

smokeless

burning a t a low

level . The

flared

gases a re connected

by a

manifold to

a ser ies

heads

which discharge

the gas into a

re f rac tory

enclosure .

Mixi

gas

and

a i r is

a..::complished by

a se r i es of mult i - jet nozzles . Co

air

is provided by

the natural

draf t of the enclosure . Smokeless

is

obtained with

l i t t le

or no s team because of the

turbulence

and

tu re

of the

burning zone due

to

the

natural draf t

and the

enclosur

s ize of the enclosure

depends

upon the capacity of the flare but c

come qui te la rge . n enclosed ground f lare with

a

capacity

of 25

lb /hr

has

an enclosure

100

feet high and 20 feet in diameter (Ref

same

capaci ty

could

be handlec

by

an 8-inch diameter elevated

f l

The

init ial costs of

an

enclosed

ground

f lare usually l imits their

to just a port ion of a plant's emergency

dump

ra tes . However, th

flare can be designed

to

handle

mos t

f lare

occurrences and

the

r

la rge re leases can be diverted'

to

an

elevated

flare.

Figure

3-15

schemat ic

showing

how

such a sys tem

might work. This type of

gra ted flare sys tem is

now becoming

common

especial ly

in

popul

areas .

3.3

Forced Draft Flares

The forced draft

f lare

uses

air

provided by a

blower

to supply pr

air

and

turbulence

necessary

to provide smokeless burning

of

re

gases without

the use

of s team. F igure

3-16

shows two

common

22


29/130

LMSC-HREC

TR

D390190

r

n)

.

I

r

I

0

w

0

0 0

0

I

0

0

I tem

Description

1

Patented

Jet

Mix Tips

z

Flare Gas

Risers

3

Flare

Gas

Header(s)

(s

4

Flare Gas

Connection(

5

Combustion

Chamber

r-

II

6

Refractory Lining and

Anchors

7

Safety

Fence (Collapsi

8

NSFP (Pilots

with

Igni

Tubes

9

Sight Ports

H

I

> V

------

_. .--------

~ { 9 )

,

(i)

/

.

,.

.

,,

,,

- ,_, 3)

I

1 I-@

I

I -

4

I

I

i

_ j

Fig.

3 1 3

Ground Flare

(from

National AirOil Burner Company)

23


30/130

LMSC HREC TR D3

Thermal

Oxidizer

. 3 14 Ground

lare ZTOF f rom

ohn

Zink Company

4


31/130

Pilot

Gas

i n e ~

LMSC HREC TR

D390190

Elevated

Flare

Burner

Diversion

Seals

lare \

Control

System

Line

Fig. 3 1 5 Ground F l a re and Elevated Flare Connected by Double

Stage Water Seal

25


32/130

l a r e ~

Gas

Inlet

Combustion

Air

Inlet

la re

s-

Inlet

LMSC HREC

TR

a. Biaxial

Forced

Draf t

Unit

b.

Coaxial

Forced

Draft

U

Fig. 3 16 Two

Designs

for Forced Draft lare Systems

26


33/130


of forced

draf t

f lares. This type of

f lare

combines smokeless burning

with

low

operat ing cost and rel iabi l i ty because

only

pilot

gas and electr ici ty

a re

required. The f lame

is

also st i ffer

and, because of

the forced

draft ,

is les

affected

by the wind.

However,

this f lare

also has a high in i t ial cost .

The

cost can

run two

to

three t imes

the cost of

a

conventional f lare , mainly since two s tacks

a re necessary

to keep the

ai r

and gas

separated

unti l they

a re

mixed

and

ignited

at

the t ip . blower f lare

should

have

an

automat ic ai r

turndown

device

to

prevent

excess a i r from quenching

the

f lame and creating smoke

i the

f lare gas

ra te is

reduced. Variable

speed blowers or baffles

couple

to flow sensing devices

have

been used on

these

f lares to

extend

their

turn

down

rat io

B eca use of

costs

and

turndown rat io

l imi tat ions,

this flare

has

been

used

most ly

in special

applications.

It

has been

used

mainly

to

provide

smokeless burning where

s team

is not avai lable .

It has

also

been

used

in tankage t r ansfe r and venting and in

conjunction

with

a

smoking

elevated f lare to provide

smokeless

burning for

day-to-day

flaring.

3.4

Comparat ive Costs of Fla re Systems

The capi tal and operat ing costs for

a given

f lare sys tem depend on many

factors such as the availabil i ty

of s team, the s ize of

the

f lare , the com

posi t ion

of the waste gas and

the frequency

of flaring.

Each

installat ion

is

a

specia l problem,

the economics

of

which must

be

solved

for the

spe-

cific

case

Vanderl inde

Ref.

9

est imated

the

re la t ive cost

of

equipment used

in

the

smokeless f lare sys tems Equipment costs include a

guyed

stack, ignition

piping, pilot piping,

the

burner r ing

and accessor i e s

As

shown

in Table

3-1 he

found that the re la t ive cost

of

smokeless

f lare

sys tems

was

not

stack

diameter

dependent . On the other hand,

relat ive

cost

of

the

equip

ment for a

forced

a i r

sys tem

is d iameter dependent , because a

s tack

Table

3-1

RELATIVE

COSTS OF

FLARE SYSTEMS

Type

of

Fla re

Smoking

Standard Tip

Smokeless

Steam Tip

Gas Tip

Water

Tip

Forced Draft

Equipment Costs

12-in. Diam. 24-in. Diam.

1.00

1.00

1.25

1.25

1.30

1.

30

1.20

1.20

2.80 3.38

27


34/130

LMSC-HREC

TR

within a s tack is

actual ly

being purchased.

Low

level enclosed

f l

with an equivalent

capacity of

an

elevated f lare can

be as much

as

t imes

mor e cost ly (Ref.

10

. or this reason the enclosed f lare

designed

to

handle

the smal le r day-to-day f la re occurrences .

Typical

costs for

the

f lare system of a 350,000 bbl/day refinery w

be

of

the order

of 750,000.

This cos t

includes

500,000

for

equ

for two

elevated and one enclosed

low

level

f lare.

Of the

500,00

equipment,

300,000

would be

for the low level

f lare.

Another

would be needed

for

the waste gas collection sys tem (Ref.

11).

8


35/130

SECTION IV

FLARE DESIGN CRITERIA

LMSC-HREC TR

D39019

The complete design specification of given f lare sys tem for use in

safety

re l ief is highly specia l ized

and

requires

close cooperat ion

be

tween the

buyer

and

manufacturer .

In

addition,

some

factor

affecting

design

are

de te rmined

y

the type

of

equipment used;

in these cases

in

which the equipment

is

propr ie ta ry design information

is

not readily

available.

Nevertheless, nwnber o design

guidelines have

been

published

in recent years

which

serve as genera l

guidelines

for

equip

ment sizing

and est imation

of plant space requirements . These are

given

as

Refs.

2, 4, 5, 12, 13

and

14.

The object ive of this section is to examine the available design and s1zmg

cr i ter ia

in

order to descr ibe the state of

the

a r t of f lare design.

Emphasi

is placed upon

calculat ions which affect

emissions of heat ,

l ight, noise,

smoke,

part iculates

and

chemicals

and the dispers ion of

gases

and par

t iculates.

Auxil iary

equipment

such as drums sea ls and

flame

ar res tor

are

also

discussed in this section.

As noted previously, flaring is

intended

pr imar i ly as

safety

measure

for disposing

of

la rge

quantit ies of gases pr imar i ly

during plant

emer

gencies such

as

f ires e lectr ical failure, failure of cooling water supplie

and

other

utilit ies,

equipment

overpressure compressor

fai lure,

or

problems

which may be encountered

during s tar t -up .

Leas frequent

applicat ions

during

which la rge quanti t ies of gas

may

be sent

to

flare

can

include the disposal of off-spec

product and

excess product which

cannot

be

s tored. Flows are typical ly intermit tent with very large flow

ra tes during major upsets

in

the range

of

severa l hundred thousand

pound

per

hour. Flare sys tems

a r e therefore required to accommodate very

large turndown

range

of flows.

Total

capacity

and

turndown

range

are

normally

the

deciding

factors

in select ing the

applicable f lare system.

The

type of flare used wil l depend to

l e s se r

extent upon the type

of

mater ia ls being

sent

to flare, the flare

location

and

available

uti l i t ies.

4.1 Select ion of

Applicable

Flare System

In

general ,

flare

sys tems

are divided into

two

broad categor ies , ground

flares and elevated f lares which

discharge

the waste

s t r eam

at some

distance above

ground level. Ground f lares may consis t e i ther of con

ventional flare burners

discharging horizontal ly

at or near

ground

level

or

of

distributed

burners

enclosed within refractory shell , as in the

mor e

recently

developed low-level f lares . Low-level f lares have

relatively large diameter

which

reduces discharge

velocity

and,

thus,

sonic

emiss ions .

Enclosing

the flame

reduces

l ight

and

thermal emission

Air for the low-level f lare is normal ly

provided

y natural draft; for this

reason,

and

because

of the

t ime

required

to heat

the

refractory,

the

low

lev

flare design has mor e sluggish response to sudden upsets than elevated

f lares . Low-level f lares are normal ly

used

for minor upset

or

for small

steady s ta te flows with an elevated flare of

convenctional design

used to

9


36/130

LMSC-HREC

TR D390190

accommodate ful l -scale emergency upsets . Horizontal discharge f lares

are essent ia l ly elevated f la re sys tems discharging a t ground level

and

have a

somewhat l imi ted appl ica t ion because

of

the large

open area of

a

minimum

of 1500 t2 required for

safe operat ion.

Heat and sound

emiss ions and other reasons for this requi rement will be discussed la ter

in

this

sect ion.

Flares

discharging

a t

ground

level

a r e

general ly

con

s idered unsui table for f lar ing gases which

may

be odorous, noxious, or

toxic in

nature

or for

f lar ing gases which

may

produce

compounds

having

these

proper t ies as

in te rmedia tes o r

f inal

combust ion products .

For genera l purposes , in

which

a variety

of

flow ranges

and

compositions

m y

be encoun tered, the elevated f lare is m o r e

common.

Elevated f lares

and elevated f lare burners discharging a t ground

level) provide ai r

for

combust ion

ei ther by

forced

draf t or by diffusion of a i r into

the fuel be

yond

the

point of

ignit ion and discharge to the a tmosphere . Burning

the

waste s t r eam by

means

of natural

convection

as

in a ground flare) or

by

forced convection resu l ts in a premixed

f lame,

while burning without

added ai r results

in a diffusion flame.

Typically, elevated f lares

used for

large waste sys tems a r e

diffusion

burning with s team

added

to reduce

smoking.

The

appl ica t ion

of forced

draft f lares is

l imited

to smal le r , s teady

flows

such as in tankage t ransfer

storage tanks,

and for

use

in

plant faci l i t ies where s team is

not

available.

Typical

maximum

f lare capacity ranges

are

Capaci ty 1000 lb /hr )

Low

Level

Flare 80 - 100

Elevated,

Diffusion Flame

1000

- 2000

Elevated,

Forced

Draft

I

00

The maximum

capacity ranges

were obtained

from conversat ions

with

flare vendors

and

should be used as a guideline

only.

Actual capacity

will vary

somewhat

with the

typ of

gas being f lared and other require

ments .

A number of

specia l ized

f lare burner designs

are

a lso available to

accommodate

high pressu re

side s t r eams . Endothermic f lares are also

available to

support combust ion

of gases

which

a r e too

lean or

have too

l i t t le heat content

to

support a

f lame.

Endothermic f laring m y be

accompl ished using either auxiliary

heaters or

an

ass is t fuel

gas .

4.2 Flammabi l i ty Limi ts

and Flame

Stability

Whether

o r

not

a given waste

s t ream will

support a f lame is normally

dPtermined

experimental ly , but

methods

a r e

avai lable

for

es t imat ing

flammabili ty

l imits Ref. 1 . In some

cases ,

f lammable mixtures may

not

re lease

sufficient combustion heat to maintain

the

f lame

at

a

stable

t empera ture . The lower

{net)

heating

value required

to

suppor t a f lame

30


37/130

LMSC-HREC

TR

D390190

var ies somewhat

with f lare

burner

design;

l arger

flames require a

higher

heating

value

fuel

than would be required for combust ion in a

dis t r ibuted

burner . A

lower heating value

of 200 250

Btu/sc f

is

normal ly considered

adequate

for f laring in la rge

elevated f lares.

Heating

values

for gases normal ly f lared may be calcula ted using s tan

dard

methods

or

obtain6ld

f rom

furnace handbooks such as

Ref. 1.

Endo

thermic f lare

sys tems with auxi l iary hea te r s

or ass is t

gas addition

to

increase

heat

content

may somet imes

be

used

in

flaring

low heating

value

gases . .

Flame ins tabil i ty

may

occur when

the

discharge veloci ty exceeds or

falls

below the burning velocity. In the case of ei ther

premixed

or diffusion

f lames,

an instabil i ty

m y

occur when

the

discharge veloci ty exceeds the

f lame

velocity

leading

to a l if ted flame

in

which

mixing of the

fuel

and

dilution with a i r must

precede

the re- ignit ion of the f lame. This condi

t ion is known as "blowoff"

(Ref.

12).

The

f lame i t se l f may even blow out

i f the

discharge

veloci ty grea t ly exceeds

the f lame

veloci ty. The

opposit

condition in which the

gas

veloci ty

falls

below the burning velocity

resul t a

in a condition known as "f lashback."

Maximum

discharge

veloci ty,

and

therefore f lare

burner d iameter i s

fixed

between these upper

and

lower l imi ts of "blowoff" and "f lashback" by the

burning

ra te of the

fuel. In

practice in order to minimize

capital

costs an

increase the f lare throughput , most f lares are

designed

for maximum thro

put based on

the

maximum allowable

pressure

drop. Flame holders

are

used

to mainta in flame stabil l ty and extend these stabil i ty l imits . These

are of propr ie ta ry

design, typical ly consist ing of

a perfora ted

r ing

at

the c ircumference of

the flare tip.

The gas flow

is

divided by the r ing

into smal l s t r eams thereby

increasing air

-gas

mixing

in a port ion of

the

gas s t r eam (Ref. 9). Large pilot

f lames

can also be used to s tabi l ize

the f lame. Smal l

amounts of gas having a relat ively high

burning

rate ,

such as hydrogen,

may

be added

to

the flared s t r e a m in order

to

widen

the stabil i ty l imi ts

(Ref.

12). The instabil i ty

at

the lower

velocity

l imit

can

be avoided

by the

use of a

purge

gas

which may

be either a

f lammable

or ine r t gas . The low flow instabil i ty is

not

a prob lem when

vapor

purging

is

employed,

for safety reasons to prevent the format ion of flammable

mixtures in

the f lare

stack at

low

or no flow.

Vapor purging is

discussed

further in Sect ion 4.4.2.

Flare

d iameters are

normal ly

sized,

within

the

m a x im u m allowable

pres

sure

drop, to

provide vapor velocit ies a t

m a x im u m

throughput of about 20o

of

the sonic veloci ty in the

gas

(Refs. 12 through

14

). There is evidence

that flame stabil i ty can be maintained at Mach

numbers up to

0.5

(Ref. l2) .

Exact

analysis

of

flame

stabil i ty

appears

to

be

beyond

the

state

of

the

a r t

for

f lare design. t is doubtful

whether

a model exists for

turbulent

flame

which is

satisfactory

for est imat ion

of

the burning veloci ty. It has been

determined

(Ref.

15)

that the u r ~ i n g

ra te

is severa l ordf rS of

magnitude

lower

than theoretical

even

for highly

efficient combustion equipment

31


38/130


such as

gas- turb ine

combustors . It is

probable

that mixing controls

the

burning

velocity in flare

sys tems. Recent

flare t ip designs for

smok ss

burning

have included tangential discharge

of ei ther

the f lare

s t ream or s team to s tabi l ize the f lame at high discharge veloci t ies,

but

such

developments appear

to be

based on

empir ica l

observat ion

ra ther

than

analysis .

4.3 Flare Emissions

Flare

emiss ions inc lude chemicals and par t icu la tes , thermal and visible

radiat ion and noise . It is the purpose of

th is

sect ion to discuss the

probable causes

of

emissions, the state of the a r t in

quantifying

and con

troll ing

these emiss ione,

and the

extent to which f lare

design

has been

affected.

4.3.1

Thermal Emiss ions and

Luminosi ty

Emission of heat from f lares will be discussed in detail in Section 4.4.9.

As

in the

case

of

the rmal

radiat ion,

i t

is

probable

that most

of

the visible

radiat ion is the resu l t

of

radiat ion from hot carbon par t ic les .

Electronic

t ransi t ions, such

as

in the

formation and

recombinat ion

of certain radicals:

CH,

c

2

, H

CO, NH,

and

NH

2

are also accompanied y emission in the

visible and near

ultraviolet ,

but

probably contr ibutes only

a smal l

fraction

of the total luminous radiat ion

(Ref.

16). The

distribution

of radiation f re

quencies

from hot carbon

part ic les

is predic ted f rom Planck s

radiation

law and requires a knowledge

of the f lame

tempera ture .

For pract ica l

use, a close approximat ion is given by

Wien s

law (Ref. 16) for AT< 0.2

cm-deg:

where

A

T

l

c2

radiat ion wavelength,

em

radiat ion

intensity

between A and A

(per unit surface

of the emi t te r

2

dA,

W/cm

= the emiss ivi ty a t A (for blackbody radiat ion,

=

=

=

=

=

=

EA =

l

for

all

values

of

A

the

surface

a r ea of

the

emi t te r ,

absolute

t empera ture ,

OK

f i rs t radiat ion

constant

12

2

0.588 x

10

W em

second radiat ion constant

1.438

ern-

K

2

em

4 . l

radiation maximum calculated from Wien

1

s

law allows an est imat ion

temperature

dependence

of

the fract ion

of visible l ight

emitted:


39/130

A T 0.289

em -

K

m


{4.2

thus,

the maximum

wavelength depends st rongly

on

t empera ture . Since

the

intensi ty a t this wavelength is direct ly proport ional to area i t follows

that control of the emiss ion of visible

l ight

is closely re la ted to the con

centrat ion

and sur face area.

of part iculates and the flame

temper ture .

For hot ter f lames, the radiat ion is

shifted

toward the vis ible

port ion of

the spec t rum. In

flaring

pract ice therefore inject ion of s team to reduce

carbon formation decreases both the f lame tempera ture

and

the a rea for

emissions and therefore

the emiss ion of

visible l ight . Increasing

the

s team beyond

the

amount needed to prevent soot

formation causes a furthe

reduct ion

in luminosi ty

Ref.

12). Smokeless f laring achieved y pre -mix

burning or multi je t burning should resul t

in

a higher f lame

temperature

and

a higher luminosi ty

than

would

be

observed

during s team injection.

No design m odif ication has been

developed

which wil l complete ly eliminate

luminosity, and in pract ice the tendency in

populated

areas has been to

enclose the

flame a t

ground level. This requi res a spec ia l type of ground

f lare

and

has

severa l

disadvantages

and

l imitat ions .

Such

f lares

a re

essent ial ly

ground level

dist r ibuted

burners to reduce f lame height)

en

closed within a ref rac tory shield to reduce

thermal

and l ight emissions .

Air is supplied y

a

natura l draft , therefore turndown

is l imited and

an

init ial

t ime lag between initial

fuel

firing and a i r

supply is

inevitable

Ref. 17).

Capital costs for thea e units a re higher than

those

for con

ventional f lares

of

the same

capacity y about a factor of 10, and main

tenance cos ts a re also higher. Because of the relat ively low discharge

height, such

f lares a re

not suitable for

flaring toxic or

hazardous

gases .

Because of the

l imited turndown and inability

to respond

to sudden flow

changes, low-level f lares a re more

sui table

for flaring when normal flows

a re

continuous. Elevated f lares

a re r ecommended

for use in addition to

the ground

f lare

whenever protect ion

against

sudden upsets is

required.

4.3.2

Noise Emiss ion

Sonic

emiss ions

f rom

f lares consis t of contributions f rom high frequency

jet noise and combustion

noise

which

is of relat ively

low

frequency Refs.

9 and 18). Je t

noise

is

caused

y

a fluid passing

through

a constr ict ion

and is direct ly proport ional

to the pressure

drop Ref.

9) or

equivalently)

roughly proport ional to the square of the mass flow

ra te

through a nozzle

of fixed diameter Refs. 18 and

19

according to the behavior expected for

highly turbulent flow.

The

intensity of je t noise is

also

a

function of the

fluid proper t ies .

Combustion noise

is a function of

f lame turbulence and

is direct ly proport ional to the

amount

of a i r mixed with the f lare gas

Ref.

9).

Jet

noise

in f lare

sys tems

resul ts

most ly f rom

high

pressure s team

in

jection

to achieve

smokeless flaring, and t;his

is

the major

source

of the

noise problem. The major steps taken to curb high

frequency

noise

emiss ion have involved re-des igning

s team

in jec tors to reduce s team

exit

velocity and the use

of

per iphera l muff le rs shrouds) to prevent both

33


40/130

LMSC-HREC

TR D390190

the

direct

sound

radiat ion

and

ref lec t ion f rom thr f lare s tack (Refs. 19

and

20).

A

mul t ipa r t nozzle

des ign

r epor t ed

by

Chevron

(Rf'f.

19) rf ' -

sulted in a

reduct ion

(by 14 de els) in

the

sound power

radiated

to

the s t eam- a i r

in ject ion system. The major reduct ion was in the range

1000

to

2000 Hz

with

l i t t le

reduct ion

of

low

frequency

combust ion

roa r

(290Hz


41/130

where

C

=

exposure

durat ion

n

T

=

allowable

exposure

durat ion

n

LMSC-HREC TR

039019

Present and proposed regulations l imiting noise exposure

are

summar ize

below Ref.

24)

Current OSHA

Proposed

NIOSH

Exposure

per

Day

Regulation

Regulation

hr)

dB

A)

dB

A)

8.0

90 85

4.0

99

90

2.0

100

95

1.0

105

100

0.5

110

105

0.25

119

110

Very ser ious low frequency

noise problems

can

resu l t from improper ly

designed water

seals

which may vibrate a t frequency levels Ref. l8):

where

; 0.31 D /H

1

2

a

s

0.149 D /H

1

2

T

a = per iod

Documents

Flare System Study Epa