Tank Losses Property Stencil - Types of Losses Described
May 23, 2013
As mentioned in a previous blog entry, the Tank Losses property stencil provides results for several types of emissions losses. For more specific information on how we calculate these, please see the EPA document found here:
As the process fluid enters the tank, it has its temperature and pressure set to the daily maximum surface temperature and the atmospheric pressure of the chosen city. This typically results in vaporization of a part of the liquid, resulting in the flashing losses. ProMax will calculate rigorously what the vaporization is based on its thermodynamic package selected.
As material is moved from the process tank to a tank truck, rail car, or marine vessel vapor is displaced by the liquid. In some cases all of this vapor results in emissions, while for other cases (such as with vapor balance service), up to 99% of these emissions may be collected. The calculation is based on:
- the Saturation Factor (based on the Mode of Operation - Submerged or Splash Loading; Clean, Normal or Vapor Balance Service),
- the liquid TVP,
- the gas molecular weight,
- the liquid surface temperature, and
- the Overall Reduction Efficiency (70% by default in ProMax, but this should be modified if necessary).
The addition of liquid organics or crude oils as production continues and storage is required displaces a volume of vapor in the tank, resulting in losses. The amount of losses vary by:
- The vapor pressure of the liquid,
- The turnover of the product, and
- The ambient temperature.
These occur as the tank temperature fluctuates with the day/night cycle and the summer/winter cycle.
- The temperature changes result in a different vapor phase pressure in the tank, causing an escape of material when the volume the vapor occupies increases. Fresh air is drawn in when the vapor volume decreases as the temperatures cool.
- As the temperature increases, the saturated vapor concentration of a substance increases in most cases, resulting in higher losses at higher temperatures.
Other Things to Consider
- The Working and Breathing Losses are often combined into a single value, and may correspond to several smaller losses jointly. For example, in the case of Floating Roof tanks the losses may include Deck Fitting losses, Deck Seam Losses, Withdrawal Losses, and Rim Seal Losses depending on the construction type. In the ProMax Tank Losses stencil, these are separated into their separate constituents in the Results tab.
- When using multiple tanks, ProMax assumes that the tanks are identical and that the liquid is equally separated amongst them. The flow rate is divided equally, and once losses have been calculated for an individual tank, ProMax multiplies the result by the number of tanks to give the total losses.
- Selecting the correct city is important since most factors rely on information about the temperature, pressure, and wind speed averages and variances that are given in the database.