roofs measured at 38˚C, TVP is determined for the given storage temperature. TVP is extrapolated from RVP by means of correlations that are published by both API2 and EPA3. The correlation equations are used to calculate values for vapour pressure constants A and B that are then used in a relationship based on the Clausius-Clapeyron equation to predict TVP as a function of temperature. This equation for predicting TVP is presented as equation 1-24 in EPA’s AP-42 7.1 document: PV = exp[A – B/(TLA + 459.67)] where: PV is the TVP (psia), and TLA is the average temperature at the liquid surface (degrees Rankine). Values for A and B to be used in this equation are determined for crude oils from the equations given in AP-42 7.1 Figure 7.1-16: A = 12.82 – 0.9672 ln(RVP) B = 7261 – 1216 ln(RVP) The reference given in AP-42 for these equations is Evaporative Loss From Fixed Roof Tanks, Second Edition, Bulletin 2518, American Petroleum Institute, Washington, D.C., October 1991. This reference does not give a source for these equations, but similar equations are presented in the parallel document Evaporative Loss From External Floating-Roof Tanks, Third Edition, API Publication 2517, American Petroleum Institute, Washington, D.C., February 1989. API Publication 2517 is the document cited in EPA regulations4 for the methodology to determine TVP from RVP. API Publication 2517, Third Edition, states that these equations were derived from a regression analysis of points read off a nomograph, which is the same nomograph presented in AP-42 7.1 Figure 7.1-13a. This nomograph appears in the first edition of API Bulletin 2518, dated June 1962, but no source is indicated for it. Thus the equations for extrapolating TVP from RVP are derived from a nomograph of unknown origin that dates back to at least 1962. Problems with the RVP correlations The validity of this nomograph has been questioned from time to time. The California Air Resources Board (CARB) developed a correction factor for predicting TVP from RVP5, on the basis that ‘an error was discovered in the API nomograph calculated values of TVP so that the RVP
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was not equal to TVP at 38˚C as was expected given the general definition of RVP.’6 The CARB correction factor adjusts the TVP curve such that it reasonably matches the RVP value at 38˚C. This CARB correction has not received much attention outside of California, perhaps because the adjustment is relatively inconsequential for conventional crude oils. AP-42 suggests that a typical RVP for crude oil is 5 psi. The calculated TVP for an RVP 5 crude oil at a storage temperature of 15.5˚C would be 2.2 psia using AP-42, or 2.9 psia using the CARB-corrected value. However, at higher values of RVP the difference becomes quite dramatic. The TVP at 15.5˚C for an RVP 12 crude oil is 9.6 psia using the AP-42 correlations, but only 4.1 psia using the CARB corrections. Thus the apparent inaccuracy of the AP42 nomographs becomes substantially more significant for light crude oils than it is for conventional crude oils. Comparisons of TVP predictions for an RVP 12 crude oil Temp (C) Reid
AP-42
CARB (corrected)
5 6.9 2.3 10 8.2
3.1
15 9.6
4.1
21 11.2
5.4
27 13.0
7.2
32 14.9
9.4
38
12 17.1
12.2
It appears from Table 1 that the overstatement of TVP from the AP-42 methodology would wrongly indicate that an RVP 12 crude oil stored at 21˚C exceeds the 11.1 psia cutoff for allowing a floating roof, whereas the CARB correction indicates the TVP to be only 5.4 psia. The overstatement of TVP by the AP-42 methodology is thus a significant issue for the storage of light crude oils. Problems with the ASTM D2879 alternative EPA regulations specify, as an alternative to the RVP method of ASTM D323, use of ASTM D28797 to determine TVP. This method does not involve correlation equations, in that TVP is directly measured over a range of temperature in order to establish the TVP-temperature relationship. ASTM D2879 has been found to give significantly lower TVP values than those predicted by the RVP methodology, but questions have been raised concerning the validity of ASTM D2879 for light crude oils. In order to assure that the TVP measured by ASTM D2879 excludes the contribution of dissolved gases such as air, the test
method specifies that the sample shall be ‘degassed’ by ‘gentle boiling.’ In that the relatively high TVP of these light crude oils is due to the presence of light ends that may readily boil, it would seem that the sample is no longer characteristic of the light crude oil after undergoing this degassing procedure. Disproportionate loss of the light ends would result in understating the TVP of the sample. More work needed EPA regulations specify determination of the TVP for volatile organic liquids either by measuring the RVP and then predicting the TVP from correlations given in AP-42, or by direct measurement of the TVP over a range of temperatures in accordance with ASTM D2879. The RVP method has been shown to grossly overpredict the TVP of light crude oils, and there is potential for ASTM D2879 to underpredict the TVP of light crude oils. There is, then, a need for improved methodology to determine the TVP of these light crude oils. For more information:
This article was written by Robert L. Ferry at the TGB Partnership. To hear more Ferry will be leading a Tanks Essentials Training course at this year’s LDAR/ BWON/TANKS/FLARES (LBTF) Conference on 19-21 February at the Hyatt Regency in Austin, Texas. 1 ASTM D323 – 08, “Standard Test Method for Vapor Pressure of Petroleum Products (Reid Method),” ASTM International, West Conshohocken, PA. 2 American Petroleum Institute, Evaporative Loss Reference Information and Speciation Methodology, Manual of Petroleum Measurement Standards chapter 19.4, Third Edition, Washington, D.C., October 2012. 3 U.S. Environmental Protection Agency, 7.1 “Organic Liquid Storage Tanks,” in Compilation of Air Pollutant Emission Factors, USEPA Report No. AP-42, November 2006. 4 U.S. Environmental Protection Agency, “Standards of Performance for Volatile Organic Liquid Storage Vessels (including Petroleum Liquid Storage Vessels) for Which Construction, Reconstruction, or Modification Commenced After July 23, 1984,” 40 CFR Part 60, Subpart Kb, §60.116b(e)(2)(i); also, the definition of maximum true vapor pressure in 40 CFR Part 63 Subpart G, §63.111. 5 State of California Air Resources Board, Technical Support Division, “Technical Guidance Document for the Emission Inventory Criteria and Guidelines Regulation for AB 2588 (Air Toxics “Hot Spots” Information and Assessment Act of 1987),” August 1989. http://www.arb.ca.gov/ab2588/tgd1989.pdf 6 Ibid., 103. 7 ASTM D2879 – 10, “Standard Test Method for Vapor Pressure Temperature Relationship and Initial Decomposition Temperature of Liquids by Isoteniscope,” ASTM International, West Conshohocken, PA.
Hear more about this topic at the upcoming LDAR/BWON/Tanks/Flares Conference Feb 19-21st in Austin, TX More information at www.lbtfconference.com or info@lbtfconference.com
January/February 2013 • TANK STORAGE