* Throughout t his s ecti on, th e work duct is meant to in clude any

conduit through which an

exhaust gas fl ows, inc ludi ng st acks and any duct work be twee n th e sour ce and s tack wher e a PM CEM S probe or dete ctor may be locat ed. V-1 V.0 STRATIFI CATION
V.1 WHAT IS STRATIF ICATI ON? Stratification of a gas stream in a duct * is a condition in which one or more characteristics of the gas stream differ significa ntly over the cr

oss-section of the duct. Stratification can be problematic for PM CEMS when establishing the initial correlation and meeting the c orrelation criteria of PS-11. In addition, changes in stratification over time can invalidate the correlation for the PM CEMS, resulting in PM CEMS responses that are not representative of PM emissions and in failure to meet the acceptance criteria for RCAs.
St ra ti fi ca ti on of

ga s s tr ea ms ca n b e d ef

in ed in te rm s o f p ol lu ta nt co nc en tr at io n, ve lo ci ty, tempera ture, or other characte

ristics. H owever , this sec tion focu ses on the stratific ation of PM concentrations, which is the characteristic of pr imary conce

rn with PM CEMS. Most of the published literatu re on str atifica tion conc erns stra

tification of ga ses, such

as SO 2 or NO x . The EPA has defined gaseous pollutant stratification as a condition in which the conc entration of a gaseous pollutant at a specific point in a duct differs fr om the average concentr

ation of that pollutant over the cross-section of the duct by more than 10 percent. Several perfor mance specifications published by EPA have incorporated a stratification test using this 10 percent criteri on. Example s include PS-2 for SO 2 and NO x CEMS and Appendix A to 40 CFR 75, Specifications and Test Procedures.
V.2 WHAT CAUSES STRATIFI CATION? The primary f

actors responsible for PM stratification are the size, mass density, and velocity of the particles; the duct configuration; and the deg ree of mixing in the gas stream. Particle shape also affects stratification, but in most cases, to a much lesser deg ree than do the other factors listed above. Stratification of PM usually results from a combination of these factors, so it can be misleading to discuss these factors independently of one another. Stratification of PM is generally more severe than gaseous stratific ation because of the inertial forces that act on PM but do not affect gase s. For a specific gas strea m velocity, the mag

nitude of the inertial forces is largely a function of particle size and mass density . For particles with a uniform ma ss density

, the small er the pa

rticles, the wea

ker the i nertial f orces a cting on thoseV-2

particles, and the more the particles behave like gases. Specifically , particles with diameters that are les s 1.0 micr

ometers ( μm) are a ssumed to b ehave a s gases
. 1 Stratific ation is mor e likely to occur in gas streams with particles that are larger than 1.0 μm, and the likelihood for stratific ation increases

with incr easing particle size. Any

factor that aff ects par ticle size can also affect the severity

of PM stratification. For example, in exhaust streams characterized by sticky particles, the likelihood of stratification downstream increases as the particles agglomerate a

nd grow in size.
Becau se the ine

rtial for ces act ing on pa rticles a re a fun ction of p article

mass dens ity, mass density also aff ects the degre e of stra

tification. Gas str

eams cha racter ized by particle s with higher mass dens ity a

re more likely to be stra tified be cause the inertia

l force s on the PM in

crease with increased particle mass density

. The velo city

of the g as strea

m can af fect PM str

atifica tion in two way s. For lower veloc ities in nonverti cal duct s, the re lative ef fect of gravit y on particle s in the g as strea

m is gre ater be cause the particles have more time to settle. As the particles settle, the PM concentr ation along the bottom or lower side of the duct increases, resulting in incre asingly stra tified flow. For vertical ducts, g ravity does not affect st

ratific ation ove

r the cr oss-section of a d

uct beca use gra vity acts in the same direction (for downward f low) or op

posite dir ection ( for upwa

rd flow)of the PM a s it flows thr ough the duct. The inertial forces on particles in an exhaust stream increase with increasing velocity . For straight ducts with no flow disturbances, the inertial forces on the pa rticle do not induce stratification. However, these inertial force s become significant following bends, obstructions, or other disturbances that cause chang es in the velocity or dire ction of the exhaust flow. In such cases, the higher the velocity of the gas stream, the gre ater the degree of PM stratifica tion. As explaine d in the pr

evious pa ragra ph, duct c onfigur

ation ca n have a signific ant eff

ect on PM stratific ation. Str

atifica tion of PM ma

y be come pro

blematic following

bends or c

hange s in direction of ducts , where

the momen

tum of the l arger

or more ma ssive par

ticles ca rries the m to the far side of the duct wall immediately following the bend. The more abrupt the directional chang e, the mor

e sever e the stra tificatio n that is lik

ely to occur . Other d

uct loca tions wher

e PM stratification is likely to occur include the areas immediately

following a junction where a secondary gas stream is introduced into the duct. I f the concentration of the secondary stream differs from the PM concentration in the duct, stratification can oc cur until a downstream location in the duct where the combined flows are re latively well mixed. For obvious reasons, the degree of mixing can significantly affect PM stratification. Stratification will not be a problem in any gas stream that is well mixed. Thus, the presence of baffles, vanes, or other devices that induce mixing will help to eliminate stratifica tion in the gas stream. V.3 WHERE I S STRATI FICA TION LI KELY TO OCCUR? Stratification is more likely to occur immediate ly downstream of a disturbance in a duct. Specific ally , stratif

ication c an be pre sent in the following

location s:V-3

# Downstream of a bend,

# Downstre am of any

obstruct ion that re sults in cha

nges in t he veloc ity o f the exha

ust stream,

# Downstre am of a ju

nction in th

e duct where an a

dditional e

xhaust or a

ir strea m is introduc ed to the d

uct,

# Along any

nonvertical section of duct where the exhaust gas ve locity is relatively low, and

# Along long sections of nonvertic al ducts. This is not meant to imply that stratification alway s is present downstream of a disturbance in the duct. As noted above, there are sever al factors that affect the deg ree of stratification. For example, if the PM in an exhaust stream consists of very fine pa rticles (< 1.0 μm), the likelihood and seve rity of strati fication

are re duced. T

he distan ce downstream of

the distur bance is also important. With the exception of relatively long horizontal ducts, the furthe r downstream, the less severe the stratification is likely to be. The deg ree of s tratific ation is al so highly depende nt on part icle size a nd mass de

nsity ; exhaust gas streams with larger or mor e dense particles are more likely to experience stratification than exhau st str

eamswith s malle

r or l ess dense

part icle s. For this

reas on, kn

owle dge of pa rtic le siz e distribution and other characteristics may be helpful in determining the likelihood of stratific ation in a

ny p articula

r duct.
V.4 HOW DOES STRATIFI CATION AFFE CT PM CEMS OPERATION? If the PM concentration in a gas stream is stratified, the location of the PM CEMS probe (for extr activ e systems ) or det ector ( for in situ systems ) may not b e repre senta tive o f the av erage concentration in the cross-section of the duct. For e xample, in a stratified stack, the concen tration of PM at the PM CE

MS probe or detector may be 80 per cent of t he aver age PM concentration across the stack at that point. I n such cases, the correlation equation will account for such a difference if the stra tification does not change with change s in PM concentrations. That is, if the PM concentration at the probe or detector re mains at or above 80 percent of the average PM concentration ac ross the stack, the stratification will have no adverse eff ect on the PM CEMS correlation. Although the correlation equation is likely to be different, the correlation coefficient, confidence interval ha lf range, and tolerance inter val half range do not chang e. In g eneral , the cor

relation will acc ount for t

he strati fication

if there is a numer

ical rel ationship between the PM CEMS response in the stratified duct and the PM CEMS response for the corresponding unstratified duct (i.e., what the PM CEMS responses would have been if the duct had not been stratified and the PM concentration at the PM CEMS probe or detector equaled the average PM concentration ac ross the duct). Appendix A includes several examples that demonstrate this principle. In eac h of the examples, the correlation equation change s, but theV-4

correlation coefficient, confidence interval half range, and tolera nce interval half range do not ch an ge si gn if

ic an tl y. If there is no such numeric al relationship between the concentration at the probe or detector of the PM CEMS a

nd the ave

rage P M concen

tration a cross the duct, it ma

y not be possib le to develop a correlation equation that satisfies the require ments of PS-11. In such cases, developing multiple correlations may solve the pr oblem. Otherwise, the only options ar e to eliminate the stratification or move the PM CEMS probe or detector to a location where the PM conc entrations are not stratified. The possible presence of stra tification reinforces the need to perform the correlation test over the widest practical range of PM concentrations. The wider the rang e of concentrations, the more representative the c orrelation equation is likely to be. Problems a

lso can a rise whe n the stra tification chang es with time as a res ult of cha nges in P M characteristics, such as particle size distribution, or chang es in the exhaust and emission control sys tem. For

example, r

eplacin g an ind uced dra ft fan wi th anothe r fan tha t chang es the exha

ust flow rate can give rise to cha nges in stratification. In c ases where the stratification underg oes significant change, the original correlation equation for the PM CEMS may no longer be a reliable means of quantify ing emissions, and it may be ne cessary to conduct a correlation test and develop a new PM CEMS correlation.
V.5 HOW DO I TEST FOR STRATI FICATI ON? If PM stratification is suspected, a stratifica tion test should be performed. Performance Specification 2 of Appendix B to 40 CFR 60 and Appendix A to 40 CFR 75 include procedures for evaluating gaseous pollutant stratifica tion. Determining the presence of PM stratification in a duct can be carried out by following the same general steps spe cified for gaseous pollutants. For gaseo us polluta nts, sampli

ng times as short a s 2 minutes a

t each po int may

be adeq uate. To test for PM stra

tification, much lon

ger sa mpling times at ea

ch point ma

y be necessa ry . The bas ic procedure is to sample over the cross-section of the duc t and compare the concentrations at eac h sampling point to the average conce ntration for the cross-section. 2 For rectangular ducts, a minimum of nine sampling points is recommended, with each point located at the centroid of similarly shaped, equal a

rea divisions of the duct cross-section. For circular ducts, a minimum of 12 sampling points is recommended, with 6 sampling points along each of two tr averses, as specified in 40 CFR 60, Appendix A, Method 1. Isok inetic sa mpling should be use d at all sa mpling po ints during

a stratif ication te st becau se sampling nonisokine

tically can disr upt strati fication

pattern s in the duc

t. 3 The minimum sampling time at each point is a function of the PM concentration and the sampling method. For example, if EPA Method 5 is used, sampling times of at least 15 minutes to 1 hour may be required at each point to collect a quantifiable PM sample on the filter. L onger sampling times help to ensure that the mass of PM collected can be acc urately mea sured. On the other hand, the longer the sampling times, the more likely there will be variations in PM concentrations over the duration of the str atifica tion test. R

egard less of du ration, the sampling

times must b e the same for each point.V-5

Rather than using

a manual refer ence me thod to sam

ple PM, it ma

y be possible to

use a PM CEMS. Even if a correlation between the PM CEMS and PM concentrations has not been established, the responses of the PM CEMS at the various sampling points across the stack can be compared to determine if there is a sig nificant difference in PM concentrations acr oss the stack. A n advanta ge to usi ng a PM CEMS instead

of a manu

al method to test for

stratific ation is that PM CEMS provide immediate results. In addition, much shorter sampling times may be adequate if a PM CEMS is used. As noted above, regar dless of duration, the sampling times must be the same at each point when testing for stratifica tion. It is c ritical that the sou

rce is op erated

consiste ntly (e.g. , at const

ant load or proce ss throug hput) through out the en tire stra tification test to en sure tha t PM conce

ntrations do not var

y si gnific antly during the test. Control device operation also should be monitored care fully.
To ensur e that the re are no tempor

al varia tions in str

atifica tion, it is a

dvisable to take se

parate independent PM concentration measurements at a stationary location throughout the entire stratification test, particularly if the test is scheduled to last several hours. To accomplish this, a second stationary pr

obe connected to an independent sampling train or me asurement system should be located at or near the centroid of the duct. One option is to use a PM CEMS as the stationary probe. I

f a manual method sampling train (e.g., Method 5) is used for the stationary measure ments, the

sampling conducte d concur

rently with eac h run of the trave

rsing pr obe must also be treated as a separate test run f or the stationary pr

obe/sampling train. For example, upon the completion of sampling at each traverse point, the Me thod 5 stationary probe is re moved and the sample recovery

procedures specified in Section 4.2 of Method 5 are f ollowed. Fol lo win g co mp let io n of

th e te st , t he P M co nce nt rat io n i s de ter mi ned for eac h sa mp li ng p oi nt . If a second, stationary

probe was used, the PM concentrations also should be determined for e ach traverse point sampling period (e.g., if the sampling time was 15 minutes, the concentrations measure d using the stationa ry probe sho

uld be det ermined for eac h corre sponding

15-minute period). If the data

from the stationary pr

obe indicate that PM concentrations did not vary significantly ove

r the test, the PM concentrations at each traverse point are then compared to the average of the PM concentr ations for all sampling points to determine the percent stratification ( S ). Thi s can b e exp ressed by the fol lowi ng: S = | c i - c ave | × 100% where
S = percent stratification c i = PM conce

ntration a t sampling

point i c ave = average PM conc entration for all s ampling points. If the perce

nt stratification is more than 10 percent for any traverse point, the duct is considered to be stratified at that location. However, as noted above, the pr esence of stratification does not necessa rily have an adverse effect on the correlati on. I f the dat a from the

second probe indi cate signific ant vari ations in PM concen tration over the c ourse of the strat ification test, the

resultsV-6

from the traversing probe are more difficult to interpret, and it may be necessary

to repeat the stratific ation test

. An alternative method is to conduct a series of test runs using two sampling trains, one of which traverses the stack and the other is placed a t the proposed location of the PM CEMS throughout the test. F or each

test run, the aver

age PM c oncentr

ation mea sured by

the trav ersing

probe is compare d to the concentra tion measu

red by the stationary

probe. If the con

centra tions are in good ag reemen t, the PM CEM S location can be c onsidere d repre sentative . Howeve r, the te st should be repeated under a rang e of process operating conditions to provide some assura nce that the location remains representative under normal ope rating conditions. As explained previously, stratification per se may not be a pr

oblem because the correlation equation can account for differe nces in PM concentrations, provided there is a numerical relationship between the average concentration and the concentration at the PM CEMS probe or detect or loc ation (e.g., the P M conc entrat ion at the pr obe or detect or rem ains a fixe d perce ntage of the av erage

PM conce

ntration a cross the duct). Exa

mples of th

is situation are pr

ovided in Appendix A. If there is no such relationship, it may not be possible to develop a correlation equation that satisfies PS-11 at that location. To determine if this is the case, the stratification test desc ribed ab ove can

be repe ated for

a rang e of PM con

centra tions. This

can be acostprohibitive undertaking and other options may be preferred (e.g ., select a different location for the PM CEMS).
V.6 WHAT CA

N I D O IF

I HA VE A STRAT IFI

ED GAS STRE

AM? If y ou suspect that PM stratification is occurring at a proposed PM CEMS sampling point, the primary options ar

e as follows:

# Loca te the PM CEMS probe or detector as pr oposed, a ssume tha

t the cor relation will accoun t for the stratific ation, an

d verify the perf ormance of the PM CEMS over time through RCAs;

# Eliminate the cause of the stratification;

# Select a

location for the PM

CEMS probe

or detector wher

e stratif ication is less likely; or

# If available, use a

PM CEMS with a multipoint probe. If y ou perform a stratification test and determine that the duct is stratified a t the proposed location, you may

decide to proceed with installing the PM CEMS at that location. In tha t case, it is recommended t

hat y ou locate the probe or detector at a p oint wher e the PM concentra

tion is comparable to the average PM concentration over the cross-section of the duct, which c an be determined through a stratification test. Eliminating the cause of the stratification may

not be practical, but it may be

possible to modify or eliminate the obstruction or disturbance that causes the stratification. If y

ou decide to select a different location for the PM CEMS probe or detector, you should take into consideration the va rious factors that affect stratification, as desc ribed aboveV-7

(see Section V.2). It is important to find a location that satisfies the c riteria of Method 1 (i.e., at least eight duct diameters downstream and at lea st two duct diameters upstream of a disturbance). Generally , the farther away from a disturbance, the less severe the stratifica tion is likely to be, although with relatively

long horizontal ducts, stratification can increase with increa sing distance downstream of the disturbance. If the stratification appea

rs to change with different proce ss or control device operating conditions, it may be necessa ry to develop multiple corre lations. Emissions could then be determined using the correlation equation that matche s the operating condition.
V.7 REFERENCES
1. U.S. Environmental Protection Agency . 1997. Handbook: Continuous Emission Monitoring

Systems f or Non-cr iteria Poll

utants . EPA/625/R-97/001. U.S. Environmental Protection Agenc

y, O ffice o f Resear

ch and De

velopmen t. 2. Bivins, D. 1994. Determination of the Presence of Stratification of Gaseous Pollutant and Diluent Emissions for Continuous Emission Monitor or Reference Method Relative Accuracy Locations . EMC GD-025. U.S. Environmental Protection Agency, Emission Measurement Center.
3. U.S. Environmental Protection Agency . 1994. An Operat or’s Guide

to Eliminat

ing Bias in CEM Systems . EPA/430/R -94-016. U.S. Envir

onmental Protection Agenc

y, Acid Rain Division.A-1 

### Append ix A Examples of Correlation with Stratified P M ConcentrationsA-2

Example 1 For this e xample, as sume that t

he PM CEMS is l ocated i n an unstr atified stack, an

d the PM concentration at the probe or detector is the same as the average PM concentr ation across the stack. The results of the initial correlation test are summarized in Table A-1, a nd Table A-2 shows the correlation equation developed from the data a nd the values for the relevant linear correl ation par

ameters develope d from the

regre ssion ana

lys is.
Table A-1. Summary of Correlation Test Data

Run no. PM CEMS

respon se, mA ( x ) PM concentration,

mg/acm ( y ) 1 4.9 2.1 2 7.6 2.9 3 7.8 5.1 4 9.2 4.8 5 10.1 2.9 6 11.1 6.3 7 18.2 8.4 8 15.7 4.9 9 24.1 8.7 10 24.7 12.6 11 29.1 16.2 12 29.3 13.5 13 30.5 15.9 14 27.1 14.4 15 19.8 12.1

Table A-2.Results of Linear Reg ression A nalysis

Parameter Criterion Results Correlation equation í = -0.633 + 0.521 x Correla tion coe fficient

(r) $ 0.85 0.94 Conf ide nce i nte rval half

range (CI) # 10% 4.7% Tol era nce i nte rval half

range (TI) # 25% 14.8%A-3

Now assume the stack is stratified, and the probe for the PM CEMS is placed in a location whe re each PM CEMS response (x N ) is 80 percent of the corresponding r esponse (x) for unstratified conditions (i.e., 80 percent of each of the x values listed in Table A-1). Ta ble A-3 shows the equivalent data for the correlation test, and Table A-4 shows the parameters for the linear correlation equation developed from the data with and w ithout the stratification.

Table A-3.Summa ry of Corr elation Te st Data a t 80% Respons e

Run no. PM CEMS response,

mA (x N = 0.80 x ) PM concentration,

mg/acm ( y ) 1 3.9 2.1 2 6.1 2.9 3 6.2 5.1 4 7.4 4.8 5 8.1 2.9 6 8.9 6.3 7 14.6 8.4 8 12.6 4.9 9 19.3 8.7 10 19.8 12.6 11 23.3 16.2 12 23.4 13.5 13 24.4 15.9 14 21.7 14.4 15 15.8 12.1

Table A-4.Results of Linear Reg ression A nalysis for 8 0% Response

Parameter Criterion Results No stratification x N = 0.80 x Correlation equation í = -0.633 + 0.521 x í = -0.633 + 0.652 x Correla tion coe fficient

(r) $ 0.85 0.94 0.94 Conf ide nce i nte rval half

range (CI) # 10% 4.7% 4.7% Tol era nce i nte rval half

range (TI) # 25% 14.8% 14.8%A-4

Although the slope of the correlation equation is differe nt, the correlation coefficient, confidence interval half range, and tolera nce interval half range a

re unchanged. Example 2 In the previous example, the PM concentration at the PM CEMS probe var ied linearly with the average PM conc entration across the stack at any

given ti me such that the PM CEMS respons e was a constant 80 percent of what the response would have been if the probe were loca ted where the PM conc entration was equ al to the a verag e PM conc entration across the stack. In

this example, assume that a nonlinear relationship exists between the PM CEMS response (x N ) and the resp onse (x) f

or the correspon

ding uns tratifie d stack. Sp ecifica lly, the PM CEMS r

esponse varies accord ing to the fo llow ing:

x N = x 0.8
where

x N = PM CEMS resp onse valu es during the corr elation te st x = PM CEMS resp onse valu es if the p robe had been loc ated whe

re the PM concentration equaled the average PM concentration across the stack. Table A-5 shows the equivalent data for the corr elation test, and Table A-6 shows the parameters for the correlation equation developed from the da ta with and without the stratification. Although the correlation equation is quite different for the stratified stack, the other correlation parameters are essentially unchanged.
Table A-5. Summary of Correlation Test Data for Example 2

Run no. PM CEMS

respon se, mA (x N = x 0.8 ) PM concentration, mg/acm ( y ) 1 3.6 2.1 2 5.1 2.9 3 5.2 5.1 4 5.9 4.8 5 6.4 2.9 6 6.8 6.3 7 10.2 8.4 8 9.1 4.9 9 12.8 8.7Table A-5.(continue d)

Run no. PM CEMS

respon se, mA (x N = x 0.8 ) PM concentration, mg/acm ( y )

A-5 (continued) 10 13 12.6 11 14.8 16.2 12 14.9 13.5 13 15.4 15.9 14 14 14.4 15 10.9 12.1
Table A-6. Results of Linear Regression Analysis for Example 2

Parameter Criterion Results No stratification x N = x 0.8 Correlation equation í = -0.633 + 0.521 x í = -2.51 + 1.14 x Correla tion coe fficient

(r) $ 0.85 0.94 0.94 Conf ide nce i nte rval half

range (CI) # 10% 4.7% 4.8% Tol era nce i nte rval half

range (TI) # 25% 14.8% 15.2%

Example 3 This example is similar to Example 2 except that in this example, the relationship between the PM CEMS response and the response for an unstratified stack is expressed as

x 

### N = x 1.2
where

x N = PM CEMS resp onse valu es during the corr elation te st x = PM CEMS resp onse valu es if the p robe had been loc ated whe

re the PM concentration equaled the average PM concentration across the stack. The correlation test data are summarized in Table A-7. T he results of the linear regression analysis are

shown in Table A-8. As indicated in Table A-8, the corre lation equation differsA-6

significantly f

rom the equation developed from the unstratified stack data. However, there are only minor differe

nces in the two sets of results for the correlation coef ficient, confidence interval half range, and tolera nce interval half range.
Table A-7. Summary of Correlation Test Data for Example 3

Run no. PM CEMS response,

mA (x N = x 1.2 ) PM concentration, mg/acm ( y ) 1 6.7 2.1 2 11.4 2.9 3 11.8 5.1 4 14.3 4.8 5 16 2.9 6 17.9 6.3 7 32.5 8.4 8 27.2 4.9 9 45.5 8.7 10 46.9 12.6 11 57.1 16.2 12 57.6 13.5 13 60.4 15.9 14 52.4 14.4 15 36 12.1
Table A-8. Results of Linear Regression Analysis for Example 3

Parameter Criterion Results No stratification x N = x 1.2 Correlation equation í = -0.633 + 0.521 x í = 0.609 + 0.24 6 x Correla tion coe fficient

(r) $ 0.85 0.94 0.94 Conf ide nce i nte rval half

range (CI) # 10% 4.7% 4.6% Tol era nce i nte rval half

range (TI) # 25% 14.8% 14.5%