Tank storage of volatile organic compounds in above ground tanks is associated with evaporation losses. To reduce product loss from the annular space between the floating roof and the tank shell, a rim seal, which usually includes primary and secondary seals, is used. Earlier studies have shown that secondary seal increases efficiency of reducing losses by mitigating wind flow effect. However, arising of gaps between the secondary seal and the wall during operation reduces the effectiveness and increases the loss. The U.S. EPA documents show that gap increasing between the secondary seal and the tank shell leads to evaporation losses growth up to three to six times. The causes of seal gaps emergences, as well as factors influencing gap increase, however, in previous studies are not given.
In the present paper the influence of operational factors and design features on the gap size between the secondary seal and the tank shell for external floating roof tank with a capacity of 50,000 m³ was evaluated, the causes of gaps were identified, and the method for secondary seal efficiency improving was proposed.
The studies took place with two existing tanks equipped with a primary mechanical shoe seal and shoe-mounted secondary seal. The following parameters were identified: the width of the gap between the floating roof and the tank shell, the width and the length of the gaps between the secondary seal and the tank shell at several liquid levels with subsequent determination of their area, the height of the excess weld metal of the vertical butt welds of the tank shell. It was established that rising of liquid level results in increasing the area of gaps between the secondary seal and the tank shell.
As indicated, the cause of formation of gaps is the insufficient ability of a secondary seal to change its length when filling and emptying the tank. In this article the principle of compatibility of secondary seal and the tank shell operation is formulated: to ensure continued contact between the secondary seal and the shell, the change in the diameter of the shell when filling and emptying the tank should lead to a corresponding change in the length of secondary seal.
References
1. Karavaychenko M.G., Babin L.A., Usmanov R.M., Rezervuary s plavayushchimi kryshami (External floating-roof tanks), Moscow: Nedra Publ., 1992, 236 p.
2. Gadel'shin R.Z., Luk'yanova I.E., Povyshenie nadezhnosti plavayushchikh pokrytiy rezervuarov (Improving the reliability of floating roof tanks), Ufa: Publ. of USPTU, 1999, 239 p.
3. Gadel'shin R.Z., Gadel'shina A.R., The influence of operating factors on tank floating roof peripheral sealant functionality (In Russ.), Nauka i tehnologii truboprovodnogo transporta nefti i nefteproduktov = Science & Technologies: Oil and Oil Products Pipeline Transportation, 2013, no. 3, pp. 80–84.
4. Myers P.E., Above ground storage tanks, New York: McGraw-Hill, 1997.
5. European Parliament and Council Directive 94/63/EC of 20 December 1994. On the control of volatile organic compound (VOC) emissions resulting from the storage of petrol and its distribution from terminals to service stations, 1994, 24 p.
6. Emission factor documentation for AP-42. Section 7.1 Organic Liquid Storage Tanks. Final Report, 2006.
7. Konstantinov N.N., Bor'ba s poteryami ot ispareniya nefti i nefteproduktov (Losses from evaporation of oil and oil products suppression), Moscow: Gostoptekhizdat Publ., 1961, 259 p.
8. Rzhavskiy E.L., Glushkov E.I., Investigation of rim seal in external floating roof tanks (In Russ.), Transport i khranenie nefti i nefteproduktov, 1970, no. 7, pp. 16–21.
9. Khafizov F.M., Sokrashchenie poter' ot ispareniya benzinov iz rezervuarov umen'sheniem vzaimodeystviya vozdukha s isparyayushcheysya poverkhnost'yu (Reduction of losses from the evaporation of gasoline from tanks by reducing the interaction of air with the evaporating surface): thesis of candidate of technical science, Ufa, 1988.
10. Evtikhin V.F., Operation of the Wiggins system seal on a tank with a capacity of 10,000 m3 with a floating roof (In Russ.), Transport i khranenie nefteproduktov i uglevodorodnogo syr'ya, 1971, no. 10, pp. 26–28.
11. Evtikhin V.F., Ivanyukov Yu.D., Kochko E.F., Fedorov V.K., Experimental tanks with floating roofs and the need to improve their designs (In Russ.), Transport i khranenie nefteproduktov i uglevodorodnogo syr'ya, 1973, no. 9, pp. 1–6.
12. Korshak A.A., Resurso- i energosberezhenie pri transportirovke i khranenii uglevodorodov (Resource and energy saving during transportation and storage of hydrocarbons), Rostov-on-Don: Feniks Publ., 2016, 411 p.
13. Runchal A.K., Hydrocarbon vapor emissions from floating roof tanks and the role of aerodynamic modifications, Journal of the Air Pollution Control Association, 1978, V. 28, no. 5, pp. 498–501.
14. U.S. Code of Federal Regulations, URL: https://www.govregs.com/regulations/expand/title40_chapterI_part60_subpartKa_section60.112a#title40_...
15. RULE 1178. Further reduction of VOC emissions from storage Tanks at petroleum facilities, California Environmental Protection Agency, 2018.
