The use of generic correlation emission factors gives more precision to estimate emissions from equipment leaks than the use of average or screening range emission factors. In this case, the real measured concentration is used in a mathematical equation to determine the emission rate for all screened equipments.
The generic correlation approach requires screening measurement on the considered equipments like the screening ranges approach (obtained by using a portable monitoring instrument to sample air from potential leak interfaces on individual pieces of equipment), but in this case the real measured concentration is saved in order to be later used in the calculation equations.
For very low or high VOC concentrations (that can not be measured), the generic correlation approach uses two limit values:
The "default-zero" leak rate is the mass emission rate associated with a screening value of zero.
These "default-zero" leak rates is used only when the minimum detection limit of the portable monitoring instrument is 1 ppmv or less above background. In cases where a monitoring instrument has a minimum detection limit greater than 1 ppmv, the "default-zero" leak rates is calculated using the generic equations with a leak concentration that equals to one-half the minimum screening value of the monitoring instrument.
The "pegged" emission rate is the mass emission rate associated with a screening value that has saturated the portable screening device (i.e. the screening value is beyond the upper limit measured by the portable screening device). Two "pegged" emission rates are available:
In the case of a saturation level at 10 000 ppmv, a dilution probe should be used to extend the upper limit of the portable screening device to 100 000 ppmv. Thus, the standard correlation equation can be used to estimate the mass emissions.
The generic correlation emission factors can be use to estimate fugitive emissions for process units on following sites:
In the case of the use of the generic correlation emission factors, both the SOCMI and petroleum industry correlations predict total organic compound emission rates.
The leak concentrations have to be measured for all considered equipments, also when screening value indicates zero or is saturated (no average value should be calculated). Each screened value is then entered into the generic correlation equations to predict emissions for each equipment piece (or for a group of pieces if equipments, fluids and screened concentrations are similar).