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Scallops form an outer slit screen in radial flow reactors. These devices are located along the inner vessel surface. Scallops are installed inside vertical reactors and distribute or collect the flow. Scallops hold the packing, collect and direct the flow. The outer surface of the scallop contacts the catalyst. Depending on the project’s requirements, it can be made of triangular wire or of perforated metal. It can be used to move the flow towards the center or outwards the center. Expanding rings are used to mount the scallops in the radial direction during installation. The space between the scallops and the central pipe in the upper part is closed with a blind plate. The flow area of the scallops is calculated to ensure stable operation in a wide range of flow rates.

Design parameters

  • Working and design pressure and temperature
  • Flow parameters
  • Physical characteristics of the working environment
  • Pressure drop
  • Packing parameters
  • Dimensions of the reactor and top hatch

Design features

  • Large screen area, minimal hydraulic resistance
  • Gaps between scallops to compensate the thermal expansion
  • The usage of a slit screen stops catalyst leakage and excludes clogging of slots due to the usage of triangular wire
  • Strict control of verticality, straightness and torsion tolerances


  • Gas treatment
  • Platforming
  • Reforming
  • Ammonia synthesis
  • Styrene production


Pictures: 1, 6 – flow direction; 2 – clearance between scallop and central pipe; 3 – scallops in UOP reactor; 4 – scallops made of perforated sheet metal; 5 – scallops of slit grate.