In specifying the size of a treater, it is necessary to determine the diameter (d), length or height of the coalescing section (Leff or h), and treating temperature or fire-tube rating. As we have seen, these variables are interdependent, and it is not possible to arrive at a unique solution for each. The design engineer must trade the cost of increased geometry against the savings from reducing the treating temperature.
The equations previously presented provide tools for arriving at this trade-off. However, because of the empirical nature of some of the underlying assumptions engineering judgment must be utilized in selecting the size of treater to use. The general procedure is outlined as follows:
1. Choose a treating temperature.
2. Determine oil viscosity at treating temperature.
3. Determine the diameter of the water droplet that must be removed from the oil at treating temperature from Equation 6-13 or 6-14, along with Equation 6-15.
4. Determine the treater geometry necessary to satisfy settling criteria from Equation 6-7 for horizontal vessels, Equation 6-8 for vertical vessels, or Equation 6-9 for gunbarrels.
5. Check the geometry to assure it provides sufficient retention time as indicated by Equation 6-10 for horizontal vessels, Equation 6-11 for vertical vessels, or Equation 6-12 for gunbarrels.
6. Repeat the procedure for different assumed treating temperatures.
This procedure allows the production facility engineer to choose the major sizing parameters of heater-treaters when little or no laboratory data are available. This procedure does not give the overall dimensions of the treater, which must include inlet gas separation and free-water knockout sections. However, it does provide a method for specifying a fire-tube capacity and a minimum size for the coalescing section (where the treating actually occurs), and provides the engineer with the tools
necessary to evaluate specific vendor proposals.