A multiphase Markov degradation model is developed for safety valves. Real empirical data from oil and gas company is used to estimate the required reliability parameters in the model. Two situations are considered. In the first case negligible repair times is assumed. When repair times are negligible, it is rather straightforward to apply a repair matrix when integrating the Markov equation over the point of times of inspection. In the second case, where repair times are considered, virtual states are introduced in parallel to distinguish between degraded states not known to the operator, and states which are known based on inspection information. The objective of the modelling is to obtain average probability of failure on demand in order to verify that the inspection and maintenance strategy is sufficient to fulfil the required safety integrity level. The developed model is supported by a case study, and relevant calculation and sensitivity results are presented.