Three Sources of Vapor Emission
- Flash losses occur when crude is transferred from containment at a high pressure to containment at a lower pressure
- Working losses occur when crude levels change and when crude in the tank is agitated
- Breathing losses occur with daily and seasonal temperature and pressure changes
Numerous calculation methods for calculating losses/VOC emissions are available, two popular methods are:
- Vasquez-Beggs Equation (VBE)
- An equation of state (EOS) calculation program such as E&P Tank®
Vasquez-Beggs Equation (VBE)
The VBE was developed in 1980 as part of a research project at the University of Tulsa. More than 6,000 samples from oil fields worldwide were used in developing correlations to predict oil properties. The VBE can be utilized as a default method to estimate potential flashing losses/VOC emissions from hydrocarbon storage tanks.
This equation has eight input variables: stock tank API gravity, separator pressure (psig), temperature (°F), and gas specific gravity, volume of produced hydrocarbons (bbls/day), molecular weight of the stock tank gas, VOC fraction of the tank emissions and atmospheric pressure (psia). The VBE estimates the dissolved GOR of a hydrocarbon solution as a function of the separator temperature, pressure, gas specific gravity, and liquid API gravity.
Flashing losses/emissions from the VOC storage tank are then estimated by multiplying the GOR by the tank throughput, the stock tank gas molecular weight, and the weight fraction of VOC in the gases. This method was designed for gases dissolved in crude oils. The equation is available in a spreadsheet format (EXCEL®) and can be downloaded from the ODEQ website.
Equation of State (EOS) Calculation Program
E&P TANK® is a software program designed for use on personal computers developed in an effort to estimate the working, breathing and flashing components of hydrocarbon production tanks. The E&P TANK program is based on the Peng-Robinson (PR) EOS. An EOS is a mathematical equation relating to the relationships between thermodynamic variables such as pressure, temperature, and volume of a specific material in thermodynamic equilibrium
The minimum inputs needed for the model are separator oil composition, separator temperature and pressure, sales oil API gravity and Reid Vapor Pressure (RVP), and sales oil production rate and ambient temperature and pressure. The separator oil composition can be determined using an analysis of the low-pressure separator oil, high-pressure separator oil, or low-pressure separator gas, or using an analysis from the geographical database provided with the program.
The database is sorted by geographic region, sales oil physical properties, and separator pressure and separator temperature. The selected case should be selected such that the parameters are similar to your particular facility. Using these inputs, the program estimates flashing losses/emissions as well as working and breathing losses. The API 4697 program costs about $532 (for nonmembers in 2012) from the American Petroleum Institute (API).