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 determining 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 barometric pressure. 

The API algorithm predicts the emission rate for IFRT 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 represents a conservative, worst-case scenario for a 

gasketed deck fitting. 

This algorithm may be used to estimate emission factors 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 fitting that accommodates a ladder/guidepole combination.

Development of the Algorithm for IFRT Deck Fittings 

Testing was performed on a range of full-sized IFRT deck 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 vapor 

loss are present. The relationship of liquid surface area to the emission factor may be expressed in the 

following form: 

 Kfai = Cf (Afi)

 p Equation 1

 Where: 

Kfai = emission factor for a particular type of IFRT deck fitting, in 

pound-moles per year. 

Cf = a correlation coefficient, in pound-moles per (square inches)p-

year, 

Afi = liquid surface area within a particular type of deck fitting, 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 configuration, are summarized in the following table. The liquid 

surface area shown is the area inside the deck fitting well or leg sleeve, less any area occupied by an 

obstruction in the deck fitting well or leg sleeve (such 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., September 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

Af (sq.in.)

Vapor Space

Height 

in the well

Emission 

Factor, Kfa

(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 wiper 288 18 in. 35.9 

26 (1993) Slotted guidepole (8") float, no pole sleeve or wiper 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 float 58 18 in. 16.4 

31 (1995) Slotted guidepole (8") pole sleeve & gasket, no float 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 transformed from an exponential to a linear form. 

Taking the log of each side of the equation yields the following: 

 log(Kfai) = log(Cf (Afi)

 p) Equation 2

This may be expressed as: 

 log(Kfai) = log(Cf) + p log(Afi) 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 slope of the resulting line was taken as the exponent 

p, and the y-intercept was taken as the value of log(Cf). The coefficient Cf was then obtained from 

Equation 4. 

 Cf = 10 [log(C f )] Equation 4

The regression of the log-transformed data yielded a value for the correlation coefficient Cf of 0.27 

and a value for the correlation exponent p of 0.86, resulting in the following equation: 

 Kfai = 0.27 (Afi)

 0.86 Equation 5

The suitability of the model represented by Equation 5 was 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 coefficient 

represents the proportion of variation in the data that is explained by the model. The regression of the logtransformed 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 fit the data extraordinarily well. 

The emission factors from the full-sized testing of IFRT deck fittings are shown in the graph below, along 

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 the 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 system 

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 average of tests 1 and 25). 

The value of 98 lb-mole/yr is estimated from the algorithm. 
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 ladder 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 for a 

standard ladder. In the uncontrolled condition, then, 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 shown 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 area contributing vapors to potentially escape 

through the guidepole and past the ladder is limited to the liquid surface area within the sleeve, rather than 

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 (in2

)

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) = 229
Background on the ladder sleeve emissions control device 5/4/06 page 5 of 6

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

 Kfai = 0.27 (229) 0.86

 Kfai = 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 may 

be achieved by the use of wiper gaskets around the ladder. This, then, would be a conservative estimate of 

the emission factor for an IFRT ladder/guidepole combination equipped with a ladder sleeve. 

The effectiveness of the ladder sleeve in reducing emissions 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 emissions by eliminating the need for a separate deck 

fitting for gauging or sampling. An IFRT with a standard ladder (i.e., one that does not have one leg 

serving as a guidepole) would need either a separate guidepole/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 standard 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, unbolted cover (31 lb-mole/yr). This access hatch emission

factor is used to model emissions from the ladder well 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/Guidepole 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 ladder/guidepole combination, equipped with a ladder sleeve,

than to use a standard ladder with a separate deck fitting 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 alternative 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 gasketed cover [as specified in NSPS Subpart Kb 

at 60.112b(a)(1)(ix) – IFRT ladders are expressly exempt from the “no visible gap” clause of 

60.112b(a)(1)(iv)], and 

- it achieves further emissions reductions by means of the ladder sleeve. 

The emission factor for a given ladder sleeve is dependent upon the dimensions of the sleeve, and is 

calculated from the algorithm: 

 Kfai = 0.27 (Afi)

 0.86

 Where: 

Kfai = emission factor for a particular IFRT deck fitting, in pound-moles 

per year. 

Afi = liquid surface area within a particular type of deck fitting, in square 

inches,