The internal flow of safety valves was analysed with the commercial CDF-programme CFX-TASCFlow by the company AEA Technology. The pressure reduction ratios were investigated. They are crucial for the mode of operation of safety valves and consequently constitute the characteristqics. The internal flow of typical construction types was calculated. Lifting bells play an essential factor. Special lifting bells were assigned to construction types and accordingly investigated. Additionally observed was the pressure reduction in the h-s diagram. It is confirmed that the entropy increases slightly within the acceleration section. The pressure reduction takes place in the delay section. The studies provide some good insights into the internal flow of valves and allow specific intervention in the characteristics. An example demonstrates correspondence between the calculation results and experimental analyses.