Sand Knockouts act to remove sand and particulates left over after drilling or hydro-fracking. Sand traps may be used to remove persistent reservoir sand that would otherwise damage valuable equipment. There are two types of traps: gravity or centrifugal.
More about sand traps:
Gravity sand traps have a notably long life and lower cost. They provide good separation across a wide range of flow regimes and operating pressures.
A gravity sand separator has no moving parts and can last years with proper inspection and maintenance.
Gravity sand separators work well in dry flow and moist conditions as well as when there is water slugs and very wet flowback conditions.