TECHNOLOGY New equation estimates emissions from tank turnovers

Robert L. Ferry,
J. Randolph Kissell
TGB Partnership
Hillsborough, N.C.

One of the more troublesome requirements of the Clean Air Act Amendments (CAAA) of 1990 is the calculation of emissions from sources or processes for which there are no established emission factors.

An emission incident of this type sometimes occurs in petroleum facilities operating floating-roof storage tanks.

If floating roofs are landed on their legs when a tank is emptied and refilled, vapors from the space below the floating roof may be vented to atmosphere upon refilling. Petroleum refineries must calculate the associated emissions to fulfill their reporting requirements under the CAAA.

The stakes are higher for many gasoline distribution facilities, where the assumptions made in quantifying these tank-filling emissions may determine whether the facility is designated a "major source" under Title III of the CAAA. Such a designation makes the facility subject to costly maximum achievable control technology (MACT) rules.

Background

There are well-documented emission factors for most configurations of floating-roof storage tanks. These factors, however, are based on the assumption that the deck is continuously floating on the stored liquid product.

When the liquid level approaches the bottom of the tank, the floating roof lands on support legs, which prevent it from being lowered any further as the product continues to be drawn off. To prevent the floating roof from collapsing, a vacuum-breaker vent opens automatically to allow pressure equalization.

A typical detail is shown in Fig. 1 (73111 bytes).

When the tank is drained dry, a vapor space is created under the floating roof. This vapor space is freely vented to the space above the floating roof. When the tank is refilled, vapors are pushed out from under the deck until it is once again floating and the vacuum-breaker vent has closed.

The frequency of such an event, called "tank turnover," varies from virtually never for some operations to once every 5 days at other facilities.

Estimating emissions

Fig. 2 (49824 bytes) illustrates the tank turnover emission scenario for emptying a tank completely after landing the floating roof, then refilling the tank. The stages are:

• Emptying -- If emptying occurs fairly rapidly, the receding product level will create a negative pressure under the floating roof. This will cause air to be drawn below the deck from above, rather than being expelled. It can be argued, then, that there are no emissions associated with this stage.
• Standing idle -- The potential emissions from a completely empty tank (with no sludge or large puddles in the bottom) will be limited to the evaporation of the wetted surfaces under the deck (called "clingage"). If a partial heel of product is left in the bottom or there is a layer of wet sludge on the tank floor, additional vapors will be generated as the tank stands idle.
• Refilling -- The various methods of calculating filling or loading emissions can be expressed in terms of the volume of air displaced and a saturation factor indicative of the vapor content in the displaced air, as will be shown.

An assumption involving no emissions from the emptying process, clingage evaporation while standing idle, and emissions created by the displacement of air during refilling, is consistent with the method recommended by EPA on its Chief Bulletin Board.

The tank-turnover calculation procedure presented on the bulletin board is to add the emissions calculated from one turnover as a floating-roof tank (to account for clingage) and one turnover as a fixed-roof tank (to account for vapors displaced during filling).

If the tank in question is a floating-roof tank, the assumed tank capacity in these calculations (for both the clingage and filling losses) is limited to the space under the floating-roof deck while it is landed.

The emissions-estimating equations may be expressed in terms of tank diameter and deck height, as described in the following sections.

Clingage losses

Withdrawal (clingage) loss from floating-roof tanks is defined by Equation 1 (49824 bytes) (see equations and nomenclature).1 The derivation of the term "0.943Q/D" for the total area of wetted shell exposed in a year, in units of 1,000 sq ft-gal/bbl, is shown in Equation 2 (49824 bytes).

For one turnover, the term in brackets in Equation 2 (49824 bytes) is equal to 1. (In other words, the throughput equals the tank capacity.) In this case, the withdrawal loss is calculated as shown in Equation 3 (49824 bytes), where the term "pDh" represents the area of the wetted shell.

For the area of wetted surface under the floating roof, the horizontal areas may be added to the shell area, as shown in Equation 4 (49824 bytes). Equation 4 (49824 bytes) then can be reduced to Equation 5 (49824 bytes).

Filling losses

Fixed-roof tank working loss (filling loss) is calculated using Equation 6 (49824 bytes). The turnover factor, KN, defined in API Publication 2518 as a saturation factor, will be replaced here by the variable S.2

For refined stocks, the product factor, KP, is equal to 1 and may be deleted for comparison purposes. The variable Q is annual throughput in bbl/year, but, again, for one turnover, would be taken as the capacity of the vessel to be filled, in bbl.

For a tank of diameter D with a floating roof held by deck legs at height h above the floor, Q may be expressed as Equation 7 (49824 bytes). Substituting this value for Q and redefining the result as Lf (filling loss) yields Equation 8 (49824 bytes). Combining and rearranging Equations 7 and 8 yields Equation 9 (49824 bytes).

Table 1 (17807 bytes) shows a summary of results for various tank diameters and leg heights. In Table 1 (17807 bytes), Lw was calculated using Equation 5, and Lf using Equation 9 (49824 bytes). The value of S was assumed to be 1 (100% saturation) in all cases.

It is apparent from the table that clingage losses are insignificant; therefore, the total losses are approximately equal to the filling losses.

Comparison

While there is not an accepted filling-loss equation for the specific case of refilling a floating-roof storage tank, several approaches to estimating filling losses are available for various other types of vessels and operations. A comparison of these methods to the fixed-roof tank turnover equation can provide a theoretical method for calculating tank turnover losses.

Gasoline loading

The gasoline-loading equation used in AP42 (EPA's emission-estimating document) expresses emissions in terms of lb/1,000 gal loaded (Equation 10). To solve for L in terms of Lf, Equation 10 must be multiplied by the volume to be filled, in 1,000 gal (Equation 11). (49824 bytes)

Substituting for V and selecting a typical temperature of 63 degrees F. (63 + 460 = 523degrees R.) yields Equation 12. Equation 12 can be simplified to, again, produce Equation 9. (49824 bytes)

Marine vessel loading

The marine-loading emission factors given in API Publication 2514A also are expressed in terms of lb/1,000 gal loaded.3

To express marine-loading emissions in terms of a saturation factor, some typical values for Pva, Mv, and T in Equation 10 (49824 bytes) must be assumed. Using Pva = 5.4 psia, Mv = 64 lb/lb-mole, and T = 523 degrees R., the loading loss emission factor is calculated by Equation 13 (49824 bytes).

Saturation factor

The range of marine-loading emission factors, L, is given as 0.7-3.9 for gasoline, depending on the type and condition of the vessel to be loaded. This range of L translates to a range of 0.1-0.5 for S. By comparison, the range of S for loading gasoline into tank trucks is 0.6-1.45.

While Equation 9 (49824 bytes) offers a reasonable representation of filling or loading emissions, there is significant potential for variation in the value of the saturation factor, S. This factor may be a function of:

• The rate at which the tank is emptied. (If the tank is emptied very slowly, for example, evaporation of the receding product will increase the saturation of the vapor space.)
• The extent to which puddles of liquid stock or wet sludge remain in the bottom of the tank. (These residual sources of liquid will result in additional vapors, thereby increasing the saturation level.)
• The length of time the tank stands empty. (Residual puddles or sludge will be evaporating while the tank stands idle. The longer the tank stands empty, the more saturated the vapor space will become.)
• The rate at which the tank is refilled. (Very low flow rates into the tank will allow vaporization to occur while the liquid level is rising, while very fast rates can result in splashing. The optimum condition for a low saturation factor is the fastest fill rate that can be achieved without producing turbulence.)

Having isolated the saturation factor, S, as the unquantified variable in the estimation of filling losses, the tank turnover factor, KN, in API Publication 2518 for fixed-roof tanks, can be reconsidered. (Note that KN is indicative of the time required to saturate the vapor head in a storage tank after filling.)

The values for KN given in API Publication 2518 suggest that the vapor space does not become saturated until 10 days after filling, and that a liquid-air contact period of 1 day results in only about 25% saturation (S = 0.25).

API's KN values, however, may be based on an assumption of the types of stocks typically stored in fixed-roof tanks, which tend to be significantly less volatile than gasoline. Better data, therefore, are needed to make more accurate calculations of turnover emissions.

Recommendation The authors propose that filling emissions from floating-roof storage tanks can be expressed using Equation 9, and that methods for determining reasonable saturation factors for various operating scenarios are needed.

Equation 9 provides a simple and straightforward method of estimating tank turnover emissions, yet allows user evaluation of the saturation factor. Future research may result in a uniform method for determining saturation factors. At this time, however, the value of S is subject to an engineering judgment.

Equation 9 is proposed for estimating emissions associated with the refilling of storage tanks that have been drained dry, landing the floating roof on its legs, and subsequently refilled. This emission event does not occur if the deck is kept floating, nor does it occur with fixed-roof tanks not equipped with internal floating roofs.

References

1.API Publication 2517, Evaporative Loss from External Floating-Roof Tanks, Equation 5, 3rd ed., February 1989. 2.API Publication 2518, Manual of Petroleum Measurement Standards, Ch. 19, Sect. 1, Equation 7, 2nd ed., October 1991. 3.API Publication 2514A, Atmospheric Hydrocarbon Emissions from Marine Vessel Transfer Operations, 2nd ed., September 1981.

Table 1

 TANK TURNOVER EMISSIONS -------------------Emissions, lb/turnover*------------------- Tank 3 ft leg height 6 ft leg height diameter, ft Lw** Lf Total Lw** Lf Total ------------------------------------------------------------------------------------- 60 2 523 525 3 1,045 1,048 90 5 1,176 1,181 6 2,351 2,357 120 9 2,090 2,099 10 4,180 4,190 150 14 3,266 3,280 15 6,532 6,547 -------------------------------------------------------------------------------------- *Assumptions: Product = gasoline; Pva = 5.4 psia; Mv = 64 lb/lb-mole; Wl = 6.1 ppg; C = 0.0015 bbl/1,000 sq ft; S = 1.0. **Clingage losses. Filling losses 

The Authors