The screening ranges approach gives more details than the average emission factor approach. The equipments are classified as leaking or no leaking and some adjustment for individual unit conditions and operation are possible.
The screening ranges approach requires screening measurement on the considered equipments (a portable monitoring instrument to sample air from potential leak interfaces on individual pieces of equipment is used) that are an indication of leak rates.
Screening data (measured in units of parts per million by volume - ppmv) are then used to classify the equipment in two categories:
Based on those screening results, it is then assumed that the average emission rate is different for the equipments of each category.
The screening range emission factors are a better indication of the actual leak rate from individual equipment than the average emission factors. However, this approach is too general when a real good evaluation of the site emission rate is required.
The screening range emission factors (as the average emission factors) can be use for process units on following sites:
The SOCMI, marketing terminal, and oil and gas production operations screening ranges emission factors predict total organic compound emission rates, whereas the refinery average factors predict non-methane organic compound emission rates.
If a speciation is to be performed or if the stream doesn't contain only organic components (but else other products or methane for refineries), VOC concentrations data in the stream may be necessary for a good characterisation of emissions.