## the TGB partnership

May 4, 2006 1325 Farmview Road Hillsborough, NC 27278 Phone: 919-644-8250 FAX: 919-644-8252 e-mail: Rob.Ferry@TGBpartnership.com 

### Emission Factors for a Ladder/Guidepole Combination prepared by Rob Ferry

Synopsis The American Petroleum Institute (API), in cooperation with the U.S. Environmental Protection Agency (EPA), has developed and published an algorithm for det ermining emission factors for the deck fittings of internal floating-roof tanks (IFRTs). 1 The API algorithm was derived from the results of a series of emissions tests performed on full-sized floating-roof deck fittings. These tests accurately accounted for emissions variables such as wind, temperature, and barom etric pressure. The API algorithm predicts the emission rate for IFR T deck fittings that have open pathways for vapor loss. Any reduction in emissions achieved by gaskets and seals is not accounted for in the algorithm. A deck fitting equipped with effective gaskets or seals, then, will have lower actual emissions than would be predicted by the algorithm. Thus the algorithm represe nts a conservative, worst-case scenario for a gasketed deck fitting. This algorithm may be used to estimate emission facto rs for ladder/guidepole combinations, and EPA has used this algorithm in the determination and approval of equivalency for ladder sleeves as an alternative control device for emissions from the IFRT deck fit ting that accommodates a ladder/guidepole combination.

Development of the Algorithm for IFRT Deck Fittings Testing was performed on a range of full-sized IFRT d eck fittings, and the emission rate was measured for each. It was determined from this testing that the emission factor for a given IFRT deck fitting is a function of the area of the liquid surface within the deck fitting, as long as sufficient pathways for vapo r loss are present. The relationship of liquid surface a rea to the emission factor may be expressed in the following form:


### K fai 

### = C

f 

### ( A fi 

### ) p Equation 1
Where:


### K fai = emission factor for a particular type of IFRT deck f itting, in pound-moles per year.


### C f = a correlation coefficient, in pound-moles per (square inches)

p

- year, 

### A fi = liquid surface area within a particular type of deck f itting, in square inches, and



### p = a correlation exponent, dimensionless.

The deck fittings with open pathways for vapor loss, for which emission factors were developed by fullsized testing of the given deck fitting configuratio n, are summarized in the following table. The liquid surface area shown is the area inside the deck fitti ng well or leg sleeve, less any area occupied by an obstruction in the deck fitting well or leg sleeve (s uch as a fixed-roof support column, unslotted guidepole, guidepole float, or deck support leg).

1 American Petroleum Institute, Evaporative Loss from Floating-Roof Tanks , Manual of Petroleum Measurement Standards chapter 19.2, Second Edition, Washington, D.C., Se ptember 2003.Background on the ladder sleeve emissions control device 5/4/06 page 2 of 6

API Test No. and Year of Report Deck Fitting

Description Control Status Liquid Surface Area A f (sq.in.) Vapor Space

Height in the well Emission

Factor, K

fa (lb-mol/yr) 13 (1982) Access hatch 1/8 in. gap all around lid 398 12 in. 54.2 8 (1982) Column, built-up 1/2 in. gap all around column 391 12 in. 52.3 4 (1982) Slotted guidepole (8") no gasket, float, sleeve or wiper 389 12 in. 45.3 1 (1993) Slotted guidepole (8") no gasket, float, sleeve or wiper 346 18 in. 45.5 25 (1993) Slotted guidepole (8") no gasket, float, sleeve or wiper 346 18 in. 40.6 3 (1993) Slotted guidepole (8") float, no pole sleeve or w iper 288 18 in. 35.9 26 (1993) Slotted guidepole (8") float, no pole sleeve or wipe r 288 18 in. 25.6 18 (1993) Unslotted guidepole (8") no gasket or wiper 288 18 in. 31. 2 2 (1993) Slotted guidepole (8") pole sleeve & gasket, no f loat 58 18 in. 16.4 31 (1995) Slotted guidepole (8") pole sleeve & gasket, no flo at 58 18 in. 4.9 5 (1993) Deck leg, pontoon (3") no gasket or sock 10 18 in. 2.0

Equation 1 was fit to the test data by using a standard least-squares regression routine on a log-log scale, which required that the equation first be trans formed from an exponential to a linear form. Taking the log of each side of the equation yields t he following:



### log( K fai 

### ) = log( C

f 

### ( A fi 

### ) p 

### ) Equation 2 This may be expressed as:



### log( K fai 

### ) = log( C

f 

### ) + p log(

A fi 

### ) Equation 3 A least-squares regression was used to fit the linear curve of Equation 3 to the log-transformed emission factor and liquid surface area data. The sl ope of the resulting line was taken as the exponent
p , and the y-intercept was taken as the value of log( C f ). The coefficient C f was then obtained from Equation 4.



### C f 

### = 10 [log( C f )] Equation 4 The regression of the log-transformed data yielded a value for the correlation coefficient C f of 0.27 and a value for the correlation exponent p of 0.86, resulting in the following equation:



### K fai 

### = 0.27 (

A fi 

### ) 0.86 Equation 5 The suitability of the model represented by Equation 5 wa s evaluated by the R-squared coefficient of determination, which is a measure of the fit of the linear regression to the data. The R-squared coeffi cient represents the proportion of variation in the data tha t is explained by the model. The regression of the lo gtransformed data had an R-squared correlation of 0.92, which indicates that the model explains 92% of the variation in the data. In other words, the model f it the data extraordinarily well. The emission factors from the full-sized testing of IFRT deck fittings are shown in the graph below, alon g with a plot of the API algorithm. The results of the tests are shown as diamonds, and the algorithm is represented by a curve.Background on the ladder sleeve emissions control device 5/4/06 page 3 of 6



### IFRT Deck Fittings with open vapor loss pathways

0 10 20 30 40 50 60

0 100 200 300 400 500 Liquid Surface Area (in^2)Emission factor (lb-mole/yr)

test results plot of algorithm

Control of IFRT Deck Fitting Emissions One strategy for the control of emissions from IFRT deck fittings with open vapor-loss pathways is to use gaskets or seals to close off the pathways. This is illustrated in the graph below by the green downward arrows, which indicate the reductions achieved for t he slotted guidepole and the ladder well.



### IFRT Deck Fitting Emission Factors effect of gaskets to reduce emissions

0 20 40 60 80 100 120

0 100 200 300 400 500 600 700 800 900 1000 Liquid Surface Area (in^2)Emission factor (lb-mole/yr) emission factor curve for open vapor-loss pathways

ladder well

, gasketed cover

slotted gaugepole

, gasket, float & wiper ladder well

,

ungasketed cover

slotted gaugepole

, no controls

The reductions achieved by the controls illustrated in the graph are summarized in the following table: Deck Fitting Open Pathways Controlled Control Description Slotted guidepole 43 lb-mole/yr 21 lb-mole/yr Pole float sy stem Ladder 98 lb-mole/yr 56 lb-mole/yr Gasketed sliding cover The value of 43 lb-mole/yr is from the test data (the a verage of tests 1 and 25). The value of 98 lb-mole/yr is estimated from the algorit hm.Background on the ladder sleeve emissions control device 5/4/06 page 4 of 6

The Ladder Sleeve Control Device The ladder/guidepole combination is a ladder which uses an 8-inch diameter slotted pipe for at least one of the vertical legs of the ladder. This 8-inch ladde r leg is then used for gauging and/or obtaining samples of the stored liquid. The well for this ladder/guidepole combination is typically the same size as the well f or a standard ladder. In the uncontrolled condition, the n, this fitting would have the same emission factor as for an ungasketed ladder well. When equipped with a sleeve around the ladder, however, the liquid surface area surrounding the ladder is significantly reduced. A ladder sleeve is illustrated in the figure below:
Slotted guidepole as one leg of a vertical ladder

Ladder sleeve Well cover

Ladder well cutaway to show the sleeve

Ladder well Unslotted leg (shown smaller than the slotted leg)

Section Through Ladder Well and Sleeve ladder well is nominally 20x51 free liquid surface within the ladder sleeve is sho wn in white C

D Slotted leg (shown by dotted line) Unslotted leg (shown same size as slotted leg) Opening for

ladder rungs

Area within the ladder well, outside the sleeve Ladder well perimeter A

B E

When equipped with a ladder sleeve, the liquid surface a rea contributing vapors to potentially escape through the guidepole and past the ladder is limited to the liquid surface area within the sleeve, rather th an the entire liquid surface area within the ladder well

. Ladder sleeve dimensions vary with the size of the ladder components and the manufacturer of the sleeve, but one design uses the dimensions shown below. Net area around unslotted leg Area (in 2 ) A B 5.1875 4.3125 26.1 Area of slot for ladder rungs

C D 5 23.625 118.1 Area for slotted leg

E 5.1875 84.5 Total net area (sq. inches) = 229Background on the ladder sleeve emissions control device 5/4/06 page 5 of 6

The resulting emission factor, from the algorithm (equation 5), is e stimated as:

K fai = 0.27 (229) 0.86

K fai = 29 lb-mole/yr
The reduction in the emission factor, as illustrated in the graph below, is from nearly 100 lb-mol/yr to just under 30 lb-mol/yr.


### IFRT Deck Fitting Emission Factors
effect of ladder sleeve on emission reduction

0 20 40 60 80 100 120

0 100 200 300 400 500 600 700 800 900 1000 Liquid Surface Area (in^2)Emission factor (lb-mole/yr)

emission factor curve for open vapor-loss pathways ladder well

, ungasketed cover

ladder/guidepole combination

, ladder sleeve reduction achieved by the ladder sleeve

The reduction in emissions illustrated for the ladder /guidepole combination is based strictly on the reduction in contributing liquid surface area, and does not take any credit for the further reduction that m ay be achieved by the use of wiper gaskets around the ladde r. This, then, would be a conservative estimate of the emission factor for an IFRT ladder/guidepole combi nation equipped with a ladder sleeve. The effectiveness of the ladder sleeve in reducing e missions is compared to the conventional control measure of a gasketed cover in the following table:

Emission Factors for an IFRT Ladder Well

Control Status Emission Factor Ungasketed cover, no sleeve 98 lb-mole/yr Gasketed cover, no sleeve 56 lb-mole/yr Ladder sleeve (& gasketed cover) 29 lb-mole/yr

The ladder/guidepole combination further reduces emissio ns by eliminating the need for a separate deck fitting for gauging or sampling. An IFRT with a stan dard ladder ( i.e. , one that does not have one leg serving as a guidepole) would need either a separate gui depole/gaugepole or a separate sample well. The table below compares the emission factor for a ladder/ guidepole combination, equipped with a ladder sleeve, to the sum of the emission factors for a sta ndard ladder and a separate deck fitting for gauging or sampling. The case of the ladder/guidepole combination shows the factor for the ladder sleeve (29 lb-mole/yr), plus the factor for an access hatch with a gasketed, unbol ted cover (31 lb-mole/yr). This access hatch emission factor is used to model emissions from the ladder we ll area that is outside the ladder sleeve.Background on the ladder sleeve emissions control device 5/4/06 page 6 of 6

IFRT Emission Factor Comparison for a Ladder/Guid epole Combination

Deck Fittings Emission Factor Ladder + Guidepole 56 + 21 = 77 lb-mole/yr Ladder + Sample Well 56 + 12 = 68 lb-mole/yr Ladder/Guidepole combination 29 + 31 = 60 lb-mole/yr

It is evident that it is more effective to use a lad der/guidepole combination, equipped with a ladder sleeve, than to use a standard ladder with a separate deck fi tting for gauging or sampling, each equipped with gasketed covers.

Conclusion The use of a ladder/guidepole combination, equipped with a gasketed cover over the well and a ladder sleeve around the ladder, should be approved as an alter native to the conventional configuration of a ladder and a separate deck fitting for gauging or sampling in that:

- it meets the regulatory requirement of having a gaske ted cover [as specified in NSPS Subpart Kb at 60.112b(a)(1)(ix) – IFRT ladders are expressly exempt from th e “no visible gap” clause of 60.112b(a)(1)(iv)], and

- it achieves further emissions reductions by means of t he ladder sleeve. The emission factor for a given ladder sleeve is de pendent upon the dimensions of the sleeve, and is calculated from the algorithm: 

### K fai 

### = 0.27 (

A fi 

### ) 0.86
Where:


### K fai = emission factor for a particular IFRT deck fitting, in pound-moles
per year. 

### A fi = liquid surface area within a particular type of deck fitting, in square inches,