P.1 

OAR Box 1851 

Prepped by Keeia Richards 

Document Number: 

38) ll-A-12 

Docket Number: 

A-90-19 

Printed 11/19/2008 11:19:51 AM Sheet 38 of 41 
EPA-600/2-76-079 A-90-1 9 

March 1976 ll-A-12 

FLARE 

P.2 

SYSTEMS STUDY 

E H JLI1 

•j.

 !99Z 

EPA AIR DOCKET 

by 

M.G. Klett and J. B. Galeski 

Lockheed Missiles and Space Co. , Inc. 

4800 Bradford Drive 

Huntsville. Alabama 35807 

contract N>. 68-02-1331, Task 3 

ROAP No. 21AXM-030 

Program Element No. JAB015 

EPA Task Officer: Max Samfield 

Industrial Environmental Research Laboratory 

Office of Energy, Minerals, and Industry 

Research Triangle Park, NC 27711 

Prepared for 

U.S. ENVIRONMENTAL PROTECTION AGENCY 

Office of Research and Development 

Washington, DC 20460 

\<L> 
LMSC-HREC T * D390190 

Water seals are used to prevent a flame front and air from entering the 

flare gas collection system. The weight of the water seal causes it to 

be located at or near grade and therefore the seal cannot be used to ore -

vent air from entering the stack. 

Knockout drums are located at or near the base A elevated ''•*r*ea to separate liquid from gases being burned. If the larb- Viquid dr- let s are not 

removed, they could burn all the way to the ground. Designed tor gases, 

flare lines can contain liquids from liquid expansion rfUefs, liquid carried 

over from gas reliefs, and condensed vapors. The knockout drum is used 

to remove these liquids before the gase s are flared. Water »e»ls and 

knockout drums are tovmd on most flare systems for safety veasons. 

}.2 Ground Flares 

A ground flare consists of a burner and auxilaries, such a». a seal, pilot 

jurner and igniter. Two types are found. One consists of conventional 

burners discharging horizontally with no enclosures. This flare must be 

installed in a large open area for safe op • ation and , e protect: i . If 

the ignition system fails thi6 is not as capable in dispersing the ga**s as 

an elevated flare. For these reasons this type of ground flare has found 

only limited applications. 

Ground flares may also consist of multiple burners enclosed within a refractory shell as in the recently developed "low level" flares (Figs. J- 1 \ 

and 3-14). The essential purpose of a low level flare is complete concealment of the flare flame as well as smokeless burning at a low noise 

level. The Rared gase s are connected by a manifold to a serie s of burner 

heads wuich discharge the gas into a refractory enclosure. Mixing of the 

gas and air is accomplished by a serie s of multi-jet nozzles. Combustion 

air is provided by tl • natural draft of the enclosure. Smokeless burning 

is obtained with little or no steam because of the turbulence and temperature of the burning zc. c lue to the natural draft and the enclosure. The 

size of the enclosure depends upon the capacity of the flare but can become quite large. An en>:loped ground flare with a capacity of 25,000 

lb/hr has an enclosure 1 OD feet nigh and 20 feet in diameter (Ref. 91. This 

name capacity could be h.-.ndlec by an 8-inch diameter elevated flare. 

The initial costs of an enclosed ground flare usually limits their capacity 

to |ust a portion of a plants emergency dump rates. However, the ground 

flare can be designed to handl< .nost Hare occurrences and the remaining 

large release s can be diverted to an elevated flare Figure 3-1S is a 

schematic showing how such a system might work. This type of integrated Rare system is now becoming common especially in populated 

areas. 

\ . N r ore ed Dra ft i-'la res 

The tcrced draft flare uses air provided by a blow-r to supply primary 

air an'! turbulence necessary to provide smokeless .nirning of reliei 

^ases without the use of steam. Figure }-16 shows two common designs 

22 
P.4 

LMSC-HREC TR D390190 

of forced draft flares. This type of flare combines smokeless burning with 

low operating coot and reliability because only pilot gas and electricity are 

required. The flame is also suffer and, because of the forced draft, is less 

affected by the wind. 

However, this flare also has a high initial cost. Ine cost can run two 

to three time s the cost o' a conventional flare, mainly since two stacks 

are necessary to keep the air and gas separated until they ar e mixed and 

ignited at the tip. A blower flare should have an automatic air turndown 

device to er event excess air from quenching the fl*m< and creating smoke 

if the fla: f ;;as rate it reduced. Variable speed blowers or baffles coupled 

to flow s«*nsing devices have been used on these flares to extend their turndown ratio. Because of costs and turndown ratio limitations, this flare 

has b"».n used mostly in special applications. It has been used mainly to 

provide smoke)/-*• burning where steam is not available. It has also been 

used in tankage transfer and ventin« and in con function with a smoking 

elevated flare to provide smokeless burning for day-to-day flaring. 

!.4 Comparative Costs of Flare Systems 

The capital and operating costs tor a given flMrr system depend on many 

factors such as the availability of steam, the size of the flare, the composition of the wast** gas and the frequency of flaring. Each installation 

is a special problem, the economics of which must be solved for the spe -

cific case . 

VanderMnde (l<. (, '4) estimated the relative cost of equipment used in the 

smokeless flare systems. Equipment rosts include a guyed stack, ignition 

piping, pilot piping, th** burner ring and accessaries. As shown In Table 

1-1 he lound that the relative cost of •rr»^*le*« (\*rr- systems was not 

• tack diameter dependent. On the other hand, retail** cn*t tl the equip* 

ment for a forced air system is diameter dependent, because a stark 

Table j. j 

RELATIVE COSTS Or r l.AHK SVmKM* 

Type ol Flar»- Equipment Ci»»t!» 

12-in. Diim. £4*in (Ham, 

Stunk ing 

Standard I ip 

Smokeless 

Steam Tip 

('•as Tip 

Water Tip 

Forced Draft 

I OO 

I ./« N 

I. *0 

1.20 

2.80 

I go 

t *<J 

I .20 
P-5 , 

l.MSC-HREC TR D3901«>0 

SECTION V 

RECOMMENDED DESIGN METHOD 

General design criteria applicable to flare selection and sizing were 

discussed in detail in Section IV in order to establish the state-of-

• he-art foundation of existing design calculations. The purpose of 

this section is to summarize the sizing method* that are considered 

representative of methods being used at present. Along with Section 

IV, Refs.2,4 , 5, 12, 13 and 14 are recommended for detailed explanation of the siting calculations. A recent review article by Horton 

et al. (Ref. 17) provides a detailed qualitative description of auxiliary 

flare components. A review of Vanderlinde (Ref. 9) is recommended 

along with Section III of this report for a discussion of the flare stack 

components including methods of steam injection available for smoke -

less flaring. 

Th? basis of the following discussion is an integrated disposal system 

include s l>oth ** elevated and an enclosed ground (low level) fl»re. In 

the eK . !<--d flare, either air or steam injection is recommended as 

the preferred control method for smoke and particulates. Water injection is a less desirable means of control (Ref. b). 

In the integrated s . stem, the low level flare is used for handling routine 

discharges to reduce noise, smoke, luminosit-. and thermal emissions. 

The elevated flare, with air or steam for particulate control is provided 

for handling full-scale emergency upsets. A double seal" or "diversion 

seal, - >mmon to both flares is used to divert the waste gas stream from 

the elevated flare toward the low level flare (Fig. iM. Onlv during 

maior upsets do gases pass through ti e upper level ul the liquid »«al 

and burn at the elevated flare. 

Toxic, noxious or odorous gase s or gase s which vield hazardous *. om -

••ustion t>roductj> should be collected in segregated piping s\ stems and 

prcfera'K disposed of :v some means other than flaring. If flaring of 

sue", streams is required, these should be discharged directly into the 

elevated flare burner rather than into the diversion seal. Depending 

or. stream volume, these streams can often be treated according to 

usual methods tor segregation and disposal <Ref.«.4 and IM. 

y i r. levated Flare <\*te:: 

5.1.1 Required I'Jesign Information 

The tollo'Aing information is recommended (Bets. 2, 4,? , 12, 1) and 14). 

Much of this inforrnation can normall\ *»e obtained trom relief valve 

sizing calculation*. 

!v:><- <>: Material to be Flared 

•\ ve - ,i k • \ ' .':••. -:'..\ r A'-i, - I . " ' 
P.6 

LMSC-HREC TR 0390190 

). Percent Unsaturation 

4. Lower (Net) Heating Value, U Btu/scf 

5. Soecific Heat Ratio, k = C/ C 

P

 v 

6. Mass Flow Rate at Maximum Discharge, W lb/hr 

7. Average Vapor Temperature, T °F 

8. Flowing Pressure , p psig 

9. Percent Toxic, Odorous or Noxious Gases 

5.1.2 Flare Purner Diameter 

Design parameters applicable to the calculation of the flare burner 

diameter include mass flow rate and discharge conditions, the type of 

flame holder used by the manufacturer, and density and heat capacity 

ratios which «e.rve to define the sonic velocity in the fluid stream. 

Principal desigr c-msiderations Include pressure drop and flame 

stahilit\ considerations. Either may serve to define the diameter. 

Pressur e drop rather than flow velocity is usually the controlling factor. 

The flare burner is usually limited to a pressure drop of 2 psig (60 in. 

H,Ol. 

To facilitate the sizing calculations, the maximum discharge rate is 

converted to the equivalent volume of air in scfh. The following orifice 

equation is used to calculate the burner diameter. 

V^ = 1656 (K)(A) ^I p (5.1) 

wnere 

V volume equivalent flow rate, scfh 

r. orilic* factor, dimensionless 

A - area of flare burner tip, ft* 

A-.' allowable pressure drop at tip, in. "-'>& 

T-.e >>ri:iie tartor K is normally about 0.9. The orifice factor and the 

effective internal tip area are somewhat variable depending on the flare 

design and manufacturer. 

Maximum Discharge Velocity: The sonic veloi ity is determined by 

assuming perfect gas behavior (Refs. 2, 12, l), 14 and 83) according to 

the tollowmg equation 

^g KRT M (5.2) 
whe re 

P.7 

LMSC-HREC TR D390190 

c 

K 

R 

M 

-. the acoustic velocity, or velocity of sound in the fluid, 

ft/sec 

- dimensional constant, 32.17 Ib-ft/lbf-sec 

= ratio of specific heats, C / C 

v p ' r 

- gas constant, 1546 ft-lbf/°R-lb-mol e 

= average molecular weight 

The maximum discharge velocity for flame stability depends somewhat 

on the design of the flame holder and the height of the flare pilot3 above 

the flare tip. A maximum vapor velocity of Mach 0.2 is frequently 

recommended (Refs.2, 12, 13 and 14). Discussions with flare vendors 

indicate that higher design velocitie" may be acceptable in some cases, 

but this figure seems to be a safe design basis. If this velocity is 

exceeded in Eq. (5.1), the pressure drop of the tip should be reduced 

accordingly. 

Pressure losse s in the flare seal, riser, entry and drums may also 

require a reduction in vapor velocity depending upon the available 

flowing pressure of the vent stream. For convenience, the pressur e 

losse s from these components is commonly expressed in terms of cor -

responding loss in velocity head (Refs. 83 and 84). For air under turbulent 

flow conditions, the equivalent length of pipe (in pipe diameters) is 

approximately equal to the velocity head loss multiplied by n. factor of 

about 55. Pressur e losse s from the various flare components are summarized in Table 5-1. 

Water Seal and l lame Arrestors: Water seals and flame arrestor s are 

discussed in detail in Section 4.4.6 dealing with flare safety. For purpose 

of the present discussion, selection rather than sizing criteria are 

significant in determining the pressure drop. It is the result of this 

study that flame arrestor s of conventional design are unacceptable because 

of their tendency to fill with solids and become plugged. Either water seals 

or flame arrestor s are required when flammable mixtures are being flared, 

but a water seal is normally included in all flare f.ystems as a precautionary 

measure . 

Disentrainment or "Knockout" Drum: A disentrainment system is required 

whenever the dewr->int of the flared stream is higher than ambient temperature. A variety of designs have been employed for liquid disentrainrrent. 

These may be either vertical or horizontal and may be combined with the 

water seal. In some cases, a sloped piping arrangement with a drain at 

the lower end has been found adequate for removal of small amounts of liquid. 

Typically, liquid removal requirements are difficult to determine and a 

re pa rate drum is installed between the flare stack and header, located 

external to the flare stack. 
P.8 

LMSC-HREC TR D390190 

Table 5.1 

PRESSURE LOSSES IN ELEVATED FLARE SYSTEM 

Component 

P'lare Tip 

Fluidic Seal 

Flare Riser 

Molecular Seal 

Knockout Drum 

* 

Water Seal 

Equivalent Pipe Configuration 

Orifice 

Orifice 

Equivalent Length of Straight Pipe 

2 to 3 Close Return 180° Bends 

1 Close Return 180° Bend 

1 Close Return 180° Bend and 

Depth of Diptube Immersion 

Velocity^ 

Head Loss 

Equation 5.1 

Equation 5.1 

— 

3.0-4.5 

1.5 

1.5 

*»» 

If the temperature of the vent gas is significantly higher than ambient, 

a material and energy balance on the water seal is required to determine 

the final temperature and the mole fraction of water in the flared stream. 

Units in equiva'ent pipe diameters. See Rcfs.8^ and 84 for details. 

74 
P.9 

l.M.sC-HREC TR D390190 

Knockoul drum design and sizing criteria are given in Section 4.4.8. 

Reference 63 is recommended for a qualitative discussion of the knockout 

drum, as it affects operating safety. Drum location, liquid holding capacity, 

pump capacity, other equipment requirements and a discuseton of sizing 

methods are given in No. 10 of Ref. 2. For purpose of the '^•sur e drop 

calculation, the drum is approximated by a clos e return l 0u-degre e bend 

(Table 5-1). 

5.1.3 Utility Requirements 

Steam and Air Requirements: Either steam or air is required for flaring 

most gaseous hydrocarbons other « n components such as methanol, carbon 

monoxide, hydrogen sulfide and i>. cfnane or natural gas which burn smokelessly, 

Discussions with flare manufacturing firms indicat that steam requirements 

commonly cited (Refs.2, 12 and 13) may not be suitable for design. The 

following empirical guideline is suggested for use: 

where 

W_/W = a - 10/M St He w 

(5.3) 

W / W 

WSt *Hc 

M 

w 

a 

steam-to-hydrocarbon mass ratio 

average molecular weight of flared gase s 

empirically determined parameter dependent 

on the type of material being flared 

e -

For paraffins heavier than propane, a ~ 0.50. For olefins, a '• 0.60. Other 

variables include nozzle design and point of injection. Because of these 

variables, steam utilization predicted by Eq.(5.i) may vary by +25r.>. R 

quired steam pressure is at least 10 psig at the point of discharge. 

Air requirements for -onokeless rwrning are somewhat higher. For paraffin*, 

the recommended value for Q is 0.55, increasing to 0.69 for olefins. For 

forced draft flares, blower requirements are about 0.8 hp for each 1000 lb/hr 

of gas flared. 

As mentioned previously (Section 4. i. i), use of water to control particulates 

is not r'-. on: mended because of flame quenching, limited turndown, wind 

effects ami other problems. Water requirements are also fairly high; 

1.0 to 1.2 lb water are required for each poiuid of hydrocarbon ga>. 

I lit;h Pre -^ su re H yd rot a-rhon St reams: Hydrocarbon -t reams having pressure^ 

greater than <iT>out ?. jjsi may be burned smokelessly by means of special 

tlare tip designs which increase air-fuel mixing. This is a rather specialized 

application and design criteria are not generally available except as they 

may apply to specific flare tip designs. 

IT» >