What is the periodicity of HST for ammonia storage vessels ?

For Ammonia pressure vessels, the periodicity of testing is 2 years.

For Horton spheres, it has been permitted vide Govt.of India notification that  to the first periodic inspection after 2 years of installation  with non destructive testing and second  period inspection shall be carried out after 3 years of the first inspection with  non destructive and hydrostatic  testing of Horton spheres. Subsequent period testing including non destructing testing and hydrotesting shall be carried  out every 5 years.