CCQTA UPDATE ON TVP/RVP of CRUDE OIL FOR CONTRACTUAL PURPOSES – July 11, 2013
This is an update to original note that incorporates clarification to application of ASTM D6377 to purchase/sale
agreements and pipeline shippers’ agreements. This and future notes will be dated for tracking purposes.
Please circulate this information on application of ASTM D6377 for purchase/sale contracts within API, COQA,
CCQTA, ASTM technical contacts as appropriate. This was discussed at several ASTM subcommittees recently,
and additional information will become available as minutes from meetings are prepared and circulated.
Cameron Konecnik is chairman of the Canadian Crude Quality Technical Association (CCQTA) TVP/RVP Project
that has become the focal point of this issue in recent months. Additional project information and a list of
member companies and representatives is available at www.ccqta.com . Please contact your member
representative or secretary@ccqta.com or cameron_konecnik@transcanada.com if you want to participate in
this project.
The US EPA has granted the API request to use ASTM D6377 Vapor Pressure of Crude Oil (VPCR,x (Expansion
Method) as an alternate method for measuring vapor pressure (VP) of high vapor pressure “live” crude, and the
entire industry will be moving in this direction quickly.
However, the EPA approval is expressly for the purpose of measuring vapor pressure for the determination of
EPA air regulation applicability, and as such does not address the concerns pertaining to “bubble point” and
“boiling point” that are germane to purchase/sale contracts and shipper’s agreements. The discussion below is
specific to the production and purchase/sale issues, for which the term “true vapor pressure” has a different
meaning than it does in US EPA regulations. The expected wide-spread use of D6377 has resulted in the need for
rapid dissemination of information around the short term application of D6377 with respect to purchase/sale
contracts and shipper’s agreements, especially as it pertains to sampling method(s), test measurement
temperature (Tm), and Vapor/Liquid ratio (V/L) used in the test. The purpose is to ensure that the sampling and
test conditions used provide an accurate estimation of vapor pressure of “live” crude oil (will boil at ambient
conditions) to address the overt safety and vapor containment issues.
The main differences in test methods are that D6377 requires sampling the high VP crude in sealed pressure
cylinders vs. ordinary bottles/cans that are exposed to ambient conditions for D323, as well as no chilling or air
saturation steps for D6377. D6377 can measure VP at any V/L and Temperature, so does not require any
correlation or nomograph to estimate True Vapor Pressure (TVP, bubble point pressure or even bubble point
temperature), if it is applied properly, using a sealed sample system.
Currently D6377 only allows D3700 Floating Piston Cylinders (FPC) for sampling “live” high vapor pressure
crudes. CCQTA made presentations to the ASTM to include two other options that will give equivalent to FPC
samples:
- a modified D1265 with liquid withdrawal to establish the “80% full” for transport (technically correct
term is “maximum fill density” as defined by the TDG regulations), with recompression of vapors by
water displacement on receipt in the lab
- proposed portable CCQTA TVP Field Test apparatus, which provides a sample equivalent to D3700
FPC, but is not currently approved for transport except locally within permitted facilities
Contrary to many publications, TVP and RVP are NOT equivalent at 100F. This, in combination with the various
VP test methods and conflicting regulatory references, causes enormous confusion.
Some companies, for very good technical reasons, have chosen in the short term to use D6377 with test
conditions of V/L=4 and Tm = 37.8C (100F), which are the same test conditions of the old D323 Reid Vapor
Pressure (RVP). This should be reported as VPCR,4(37.8) as required by the test method D6377, and this
automatically implies that any “live” crude sample has been taken in a sealed sampling system as currently
required by the method. D6377 allows the use of conventional bottle/can sampling by D4057/4177 only for
low vapor pressure “dead” crude, but this imposes a limit on the ability of D6377 to measure TVP or bubble
point pressure at low V/L due to air saturation effects (discussed in more detail below).
The D6377 method suggests that Bubble Point conditions could be as low as 0.02 V/L, but recent preliminary
date by CCQTA TVP/RVP Project indicates that this exceeds the capability of some instruments due to the high
thermal expansion of some light high VP crudes. In the interim, we are suggesting that results be reported at
both V/L=4 and V/L=0.25 (which corresponds to an 80% full LPG cylinder, and the D1267 LPG VP method) to
facilitate development of alternate sampling procedures.
Work is underway by at least two vendors to automate the procedure of generating accurate V/L curves to assist
the industry in this area. Since the sample system and measurement cell are sealed, it is possible to do multiple
measurements at different V/L or temperature on the same sample. For example, users can measure vapor
pressure at TVP and RVP test conditions, or generate curves of VP response with temperature and V/L. These
capabilities are not currently recognized in the ASTM D6377 method, which deals with single determinations
only.
CCQTA has requested ASTM to establish guidelines on the proper application of D6377 covering the range
illustrated by the following two extreme cases:
1) High VP “live” crude (and “weathered” crude that contain light ends) that will boil at ambient conditions
and lose light ends prior to analysis without sealed sampling methods.
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Sealed sampling systems are required to prevent the loss of light ends, including C1-C3 if present, as
well as to prevent contact of the sample with the atmosphere. Loss of light ends can result in
dramatically under predicted VP because of the low MW (high mole vs. mass%), and high
component vapor pressures of the light components. This sampling bias can exceed 30 psia, and is
believed to be the root cause of several rail car boil-overs and tank floating roof sinking incidents in
summer 2012.
D6377 can be run at a series of low V/L to estimate the bubble point pressure, or at a series of
temperatures to estimate bubble point temperature (boiling point) for any given crude sample.
Unlikely circumstance where crude is partially air saturated inside of the production process
2) Low VP “dead” crude (and highly stabilized crude that contain no C1-C3) that has been exposed to the
atmosphere (for example while in a sample bottle), and are partially or fully air saturated.
-
Sealed sampling system not required for “dead” crude (per current practices) since light ends are
not lost during sampling or handling used in D6377.
Use of D6377 at low V/L will include the contribution of dissolved air, and will report higher than
true hydrocarbon VP (in extreme case will report barometric pressure at the time at V/L=0.0)
o D6377 cannot be used to estimate the bubble point pressure by measuring V/L at lower
values
o Bubble point pressures will have to be estimated by correlation (data yet to be developed as
part of the CCQTA TVP project)
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Use of D6377 at higher V/L=4 will NOT include the contribution of dissolved air because of the low
solubility level of air in hydrocarbons (almost all the air will be in the larger vapor space under
measurement conditions). The D6377 method can be used on low VP crude samples taken in
bottles, cans etc. per normal sampling practice at V/L=4, and can functionally replace the D323
RVP method.
The existing correlations for RVP Equivalent (RVPE) in the (non-mandatory) D6377 appendix are
suspect, as they will include any sampling bias from the original sample set used to obtain the data.
Reporting VPCR,x(Tm) without any correction or correlation will help minimize confusion in
reporting or interpreting results.
Recent preliminary CCQTA work equilibrated several “live” (boiling) crudes (predominately shale oils), to room
temperature and ambient pressure (ie bubble point of nominally 20C and 14.7 psia ambient pressure near sea
level). This is the “breakpoint” between a “live” and “dead” crude as defined in D6377. These equilibrated
crude samples had a VPCR,4(37.8) <11.0 psia. This indicates that a crude with VPCR,4(37.8) below ~10.5 psia will
not boil in floating roof tanks or upon opening of general purpose rail cars at normal atmospheric pressure at
room temperature. Several companies have established interim purchase/sale agreement specifications in the
range of VPCR,4(37.8) = 9.5 to 10.5 psia, depending upon expected ambient temperature in their system, to
address the overt safety and flammable vapor issues.
Conversely, any “dead” crude sampled in open containers that gives a VPCR,4(37.8) > 10.5 is highly suspect.
The sample may have flashed down to ambient during sampling or sample handling, and should be re-sampled
in a sealed sampling system.
Higher limits can continue to apply to crudes that are not exposed to high ambient temperatures, for example,
some crudes produced in the Arctic, and transported only short distances, or crudes handled in pressurized or
refrigerated systems. Lower limits may be required for extremely high ambient temperatures.
At the current time, these limits are for information only, and users can use D6377 Appendix 2 “Vapor Pressure
of Crude Oil vs. Temperature” to evaluate crude oils specific to their system. These limits are subject to change
as more data is generated by the industry.
It is anticipated that the same or similar requirements will eventually be adopted in various codes, standards
and regulations as we go forward. However, formal adoption is likely to take several years due to the complexity
of the situation.
The CCQTA TVP/RVP project is also working with ASTM on the development of a sealed sample GC method for
accurate determination of light ends, especially C1-C4, in crude, condensates and related materials. Current GC
and SimDist methods suffer from the same loss of light ends during sampling and sample preparation prior to
analysis when they are applied to “live” crudes.
R.J. (Bob) Falkiner
Director of Refining
Canadian Crude Quality Technical Association