If a system has a long suction line riser an oil trap should be installed:

Installing an oil trap every 15 to 20 feet of vertical rise in a suction line riser is crucial for maintaining proper lubrication in the refrigeration system. As refrigerant circulates, it can carry oil upward through the suction line. In long vertical risers, oil tends to collect and can create an oil slug that disrupts the refrigeration cycle. By placing oil traps at intervals of 15 to 20 feet, you ensure that any oil carried along with the refrigerant is adequately collected and returned to the compressor, preventing oil starvation and maintaining a balance in the system. This is essential for maximizing efficiency and protecting the compressor from potential damage caused by insufficient lubrication.

In contrast, placing oil traps every 6 inches or at the top of the riser would be impractical and not necessary, as it would add unnecessary complexity and potential points of refrigerant loss. Additionally, the recommendation to place the trap downstream of the accumulator does not address the specific need for oil management in a long riser, as oil traps should be strategically located to ensure oil is returned to the system effectively.