The p-value test is used to measure the stability and compatibility of visbroken pitch, asphalt and fuel oils at moderate temperatures. This test does not predict what the stability of the blended resids will be as the temperature is increased. Part of the reason that resid incompatibility fouls the upper radiant section of the heater is that is the point of initial vaporization of the resid or bubble point. That is the lowest solvent power of the resid matrix and asphaltenes precipitate at that point.
As Evan stated, the CII is an index that can provide guidance on the stability and compatibility of the blended resids. The SARA analysis is not normally available for the resids so the CII cannot be calculated. Even if you can calculate it, there is uncertainty around the value of this index where the blend becomes unstable.
Process film temperature is dependent on the heat flux and the velocity of the process fluid. The variability of these factors between different heaters is part of the uncertainty around heater fouling.
I have used the Watson K factor and the Bureau of Mines correlation index (BMCI) as guidance around resid compatibility. They can provide some insight into the potential for precipitation of asphaltenes. These values are normally easy to generate, but they are probably less accurate than the CII.
