Reliability assessment for safety-instrumented systems (SISs) plays a vital role in improving the design of SISs. Traditional methods for SIS reliability assessment that assume constant failure rates are, however, not realistic for many final elements of SISs, e.g. electro-mechanical and hydraulic/mechanical actuators that are subject to degradation. This paper presents an approach for reliability assessment of SIS final elements with time dependent failure rates. Different operational issues, such as partial and full testing, are investigated for their effects on reliability of SISs. Approximation formulas for evaluation of average probability of failure on demand (PFDavg) involving degradation are developed within different subsequent proof testing intervals, and Weibull distributions are adopted to model the degradation processes of the final elements. The corresponding numerical results of PFDavg from the set of the derived formulations are validated by Petri nets models that are developed for different scenarios. Shutdown valves installed as part of a high integrity pressure protection system are analyzed as the case, to illustrate the feasibility of the proposed approach, and also demonstrate that the approximation can provide possibilities for testing strategies design and optimization.