1、Evaporative Loss from Closed-vent Internal Floating-roof Storage TanksTECHNICAL REPORT 2569AUGUST 2008Evaporative Loss from Closed-vent Internal Floating-roof Storage TanksMeasurement Coordination DepartmentTECHNICAL REPORT 2569AUGUST 2008Special NotesAPI publications necessarily address problems of
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12、g.iiiTABLE OF CONTENTS Section Page 0. SUMMARY 1 1. INTRODUCTION . 1 2. CLOSED-VENT INTERNAL FLOATING-ROOF STORAGE TANKS 1 2.1 Venting . 1 2.2 Vacuum 1 2.3 Pressure . 1 2.4 European Practice 2 3. EMISSION REDUCTIONS FROM FLOATING ROOFS VS EMISSION REDUCTIONS FROM CLOSED VENTS 2 3.1 Emission Reductio
13、ns from Internal Floating Roofs 2 3.2 Emission Reductions from Closed Vents . 3 4. THE ITERATIVE METHOD FOR ESTIMATING EMISSIONS FOR CLOSED-VENT IFRTS . 3 4.1 Daily Gain and Loss of Vapors in the Vapor Space . 3 4.2 Saturation of the Vapor Space for Closed-vent IFRTs . 4 4.3 Daily Gain and Loss Equa
14、tions for Closed-vent IFRTs 7 4.4 Example of Closed-vent IFRT Emissions for a 100-day Period . 8 5. COMPARING CLOSED-VENT IFRT AND OPEN-VENT IFRT EMISSIONS 11 5.1 Base Case 11 5.2 Effect of Product Volatility 12 5.3 Effect of Type of Floating Roof . 13 5.4 Effect of Tank Height 13 5.5 Effect of Aver
15、age Liquid Surface Temperature 14 5.6 Effect of Vent Settings 14 6. THE EQUIVALENT-DIAMETER METHOD . 14 6.1 Development . 14 6.2 Comparing the Iterative and Equivalent-diameter Methods . 16 7. THE GERMAN METHOD 22 8. FLAMMABLE MIXTURES IN THE VAPOR SPACE . 23 9. ADVANTAGES AND DISADVANTAGES OF OPEN-
16、VENT IFRTS AND CLOSED-VENT IFRTS . 23 10. SUMMARY . 24 11. CONCLUSION 24 12. REFERENCES 25 APPENDIX ANOMENCLATURE . 26 v Tables Table 1. Evaporative Loss (lb/yr) for Open-vent Tanks with and without an Internal Floating Roof . 2 Table 2. Evaporative Loss (lb/yr) for Fixed-roof Tanks with Various Pre
17、ssure/Vacuum Settings . 3 Table 3. Average Saturation 5 Table 4. Effect of Tank Diameter and Time Between Turnovers on Emissions 12 Table 5. Effect of Product Volatility on Emissions . 12 Table 6. Effect of Floating Roof Type on Emissions . 13 Table 7. Effect of Tank Height on Emissions 13 Table 8.
18、Effect of Average Liquid Surface Temperature on Emissions . 14 Table 9. Effect of Vent Settings on Emissions 14 Table 10. Estimated Losses (lb/yr) for Open-vent IFRTs Storing RVP 13 Gasoline . 16 Table 11. Estimated Losses (lb/yr) for Closed-vent IFRTs Storing RVP 13 Gasoline . 16 Table 12. Paramete
19、rs for 7 Closed-vent IFRT Emission Calculation Cases . 16 Table 13. Iterative Method vs Equivalent-diameter Method Evaporative Loss (lb/yr) . 17 Table 14. Effect of Various Parameters on Closed-vent IFRT Emissions vs Open-vent IFRT Emissions . 24 Figures Figure 1. KsSaturation Factor 6 Figure 2. Gai
20、n and Loss of Vapors from the Vapor Space . 10 Figure 3. The Effect on Equivalent of Adding Closed Vents to an IFRT . 11 Figure 4. Model of the Equivalent-diameter Tank . 15 Figure 5. RVP 13 Gasoline Equivalent-diameter vs Iterative Method 5 Days Between Turnovers . 18 Figure 6. RVP 13 Gasoline Equi
21、valent-diameter vs Iterative Method 15 Days Between Turnovers . 18 Figure 7. RVP 13 Gasoline Equivalent-diameter vs Iterative Method 90 Days Between Turnovers . 19 Figure 8. RVP 7 Gasoline Equivalent-diameter vs Iterative Method 5 Days Between Turnovers . 19 Figure 9. RVP 7 Gasoline Equivalent-diame
22、ter vs Iterative Method 15 Days Between Turnovers . 20 Figure 10. RVP 7 Gasoline Equivalent-diameter vs Iterative Method 90 Days Between Turnovers . 20 Figure 11. Diesel Equivalent-diameter vs Iterative Method 5 Days Between Turnovers 21 Figure 12. Diesel Equivalent-diameter vs Iterative Method 15 D
23、ays Between Turnovers 21 Figure 13. Diesel Equivalent-diameter vs Iterative Method 90 Days Between Turnovers 22 Evaporative Loss from Closed-vent Internal Floating-roof Storage Tanks 0. SUMMARY There is presently no recognized methodology for estimating the impact of closed tank vents on emissions f
24、rom an internal floating-roof tank (IFRT). When the vents in the fixed roof of an IFRT are closed, rather than open, estimation of emissions is shown to be highly complex. Emissions reductions from adding closed vents to IFRTs were found to be significant only for small diameter tanks storing volati
25、le liquids with infrequent turnovers. For low volatility stocks such as diesel, the emission reductions due to adding closed vents are generally less than 10% regardless of the tank diameter or frequency of turnovers. For IFRTs 60 ft in diameter and larger, experiencing 18 or more turnovers per year
26、, the emission reductions due to adding closed vents are generally less than 10%, regardless of the liquid stored or the vent settings on the tank (assuming that the pressure setting is not so high as to require the tank to be anchored). Given the high uncertainty associated with the methods evaluat
27、ed, an assumption of a 5% reduction in emissions from an IFRT due to use of closed vents would be a reasonable approach for emissions estimating. 1. INTRODUCTION This report addresses evaporative loss from internal floating-roof tanks (IFRTs) with closed vents, a subject not currently addressed by A
28、PI. Nomenclature is provided in Appendix A. The API Manual of Petroleum Measurement Standards Chapter 19, Section 1 (19.1)1addresses evaporative loss from fixed-roof tanks, and specifically excludes fixed-roof tanks that have an internal floating roof (19.1.1.1). The API Manual of Petroleum Measurem
29、ent Standards Chapter 19, Section 2 (19.2)2addresses evaporative loss from freely-vented internal floating-roof tanks, and specifically excludes “closed internal floating-roof tanks (that is, tanks vented only through a pressure-vacuum relief vent, blanketed with an inert gas, vented to a vapor proc
30、essing unit, or otherwise restricted from being freely vented)” (1d). 2. CLOSED-VENT INTERNAL FLOATING-ROOF STORAGE TANKS 2.1 Venting API 650, Welded Steel Tanks for Oil Storage3, H.5.2.2 addresses venting for internal floating-roof tanks. Two options are allowed: open circulation vents or closed pr
31、essure-vacuum vents. For closed pressure-vacuum vents, gas blanketing or another method to prevent the development of a combustible gas mixture within the tank is required. 2.2 Vacuum Until the December 2005 Addendum, API 650 limited the design vacuum to 1 in. water column, which is 0.036 psi (API 6
32、50, 5.2.1b). (API 650 now allows up to 1.0 psi design vacuum, but the vast majority of existing storage tanks are not designed to withstand more than 0.036 psi vacuum.) 2.3 Pressure API 650 limits the design pressure for tanks to 2.5 psi (API 650, 5.2.1c). Cone-roof tanks with pressure exceeding abo
33、ut 0.053 psi (the weight per unit area of typical 3/16 in. thick roof plates) require special design (Appendix F), and anchoring the tank is required if the pressure exceeds the weight of the roof and the shell divided by the tanks cross-sectional area. Also, if the design pressure exceeds a certain
34、 threshold, the shell-to-roof joint required to resist the pressure becomes too large to be considered frangible (i.e. a weak roof-to-shell joint as specified in API 650, 5.10.2.6), and the tank requires emergency vents. These pressure thresholds are shown in the API 650 Tank Design Pressures Table
35、for 48 ft tall cone-roof tanks. Shell thicknesses are taken as the greatest of those required for the stored liquid (0.7 specific gravity), the hydrotest, and minimum thicknesses allowed in API 650. (Tanks are often designed with thicker shells in order to avoid 1 2 API TECHNICAL REPORT 2569 the nee
36、d for an intermediate wind girder. A thicker tank shell would increase the maximum pressures shown below.) API 650 Tank Design Pressures Tank Diameter (ft) Maximum Pressure with Frangible Joint (psi) Maximum Pressure without Anchors (psi) 48 0.199 0.285 60 0.205 0.29490 0.178 0.251 120 0.175 0.247 1
37、50 0.169 0.238 The internal floating roof must also be capable of withstanding the internal pressure. API 650 describes several different types of floating roofs in H.2.2, including internal floating roofs that have their deck above the liquid and are supported by closed pontoons for buoyancy (H.2.2
38、.e). These pontoons are typically 10 in. diameter, 0.050 in. thick aluminum and cannot withstand pressures above about 0.07 psi (unless special fabrication measures are taken to pressurize the pontoons). 2.4 European Practice In Europe, pressure-vacuum vents are commonly used without gas blanketing
39、the vapor space above the floating roof. The German standard DIN 4119 specifies that new tanks must be designed for a 0.29 psi relieving pressure (20 mbar) and a 0.145 psi (10 mbar) relieving vacuum6. The German design pressure is approximately the maximum pressure tanks can withstand without anchor
40、s. 3. EMISSION REDUCTIONS FROM FLOATING ROOFS VS EMISSION REDUCTIONS FROM CLOSED VENTS Both internal floating roofs and closed vents reduce emissions from storage tanks. Lets first quantify the reduction each of these controls achieves separately before considering their combined effect. 3.1 Emissio
41、n Reductions from Internal Floating Roofs First, consider the emission reduction achieved by adding an internal floating roof to a tank with open vents. Consider tanks 48 ft tall storing RVP 10 gasoline or diesel at 14.5 psi atmospheric pressure, 60oF average liquid surface temperature, 20oF daily t
42、emperature range, and 25 turnovers per year. Their internal floating roof is welded steel with a vapor mounted primary and rim mounted secondary seal. The evaporative loss without the floating roof is determined using API MPMS Ch. 19.1 with zero vent pressure/vacuum settings. The evaporative loss wi
43、th the floating roof is determined using API MPMS Ch. 19.2. Table 1Evaporative Loss (lb/yr) for Open-vent Tanks with and without an Internal Floating Roof DIESEL GASOLINE (RVP 10) Tank Diameter D (ft) Loss without a Floating Roof LT19.1Loss with a Floating Roof LT19.2% Reduction Loss without a Float
44、ing Roof LT19.1Loss with a Floating Roof LT19.2% Reduction 30 139 7 95.0% 51,154 1,567 96.9% 60 558 12 97.8% 204,614 2,410 98.8%90 1,255 20 98.4% 460,382 4,168 99.1% 120 2,231 27 98.8% 818,457 5,481 99.3%For the cases shown in the table above, adding a floating roof to an open-vent tank reduces emis
45、sions by approximately 95% to 99%, a fairly substantial reduction. EVAPORATIVE LOSS FROM CLOSED-VENT INTERNAL FLOATING-ROOF STORAGE TANKS 3 3.2 Emission Reductions from Closed Vents Next, consider the emission reduction achieved by adding closed vents to a tank without a floating roof. The tank has
46、the same parameters as in 3.1 above, except that only RVP 10 gasoline is stored, and the pressure/vacuum settings are as given in Table 2. The P/V settings range from the lowest to the highest usually encountered in unanchored storage tanks. The lowest non-zero range used in the example is for +/ 1
47、in. of water column (+/ 0.036 psi), which is slightly greater than the typical breather vent setting of +/ oz/in.2(+/ 0.031 psi). The pressure for the highest range is based on the approximate weight of the tank roof and shell, which is the limit above which anchorage is required. This is equivalent
48、 to approximately 0.3 psi for a 48-ft diameter tank and less for larger tanks. The API 650 tank design standard3has historically limited the design vacuum to 1 in. water column (0.036 psi) as noted in Section 2 above. This limitation has been removed in the most recent 650 edition, however, so large
49、r vacuum settings are considered in this investigation. For the cases in which the pressure is greater than the minimum case, the vacuum setting is arbitrarily taken as one half of the pressure setting. The Table 1 loss without a floating roof is the same as the Table 2 loss for a P/V setting of zero, since these are for the same case: an open-vent fixed-roof tank without an internal floating roof. Table 2Evaporative Loss (lb/yr) for Fixed-roof Tanks with Various Pressure/Vacuum Settings GASOLINE (RVP 10) Tank Diameter D (ft) P/V +0 0 P/V +0.036 0.036 %
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