For official work, you should purchase the standard from the or access it through a licensed corporate library like IHS Markit or Techstreet.
API RP 556, formally titled is a critical recommended practice (RP) established by the American Petroleum Institute (API) . It provides essential guidelines for the installation and operation of measuring, control, and protective systems to ensure the safety and efficiency of gas-fired heaters in the petroleum, petrochemical, and chemical industries. Scope and Purpose of API RP 556 api rp 556 pdf
: The Second Edition , published in April 2011 , is the most widely used version . For official work, you should purchase the standard