Due to the critical role of workflows in organizations, their design must be assisted by automatic formal verification approaches. The aim is to prove formally, before implementation, their correctness w.r.t. the required properties such as achieving safely the expected services (soundness property). In this perspective, time workflow nets (TWF-nets for short) are proposed as a framework to specify and verify the soundness of workflows. The verification process is based on state space abstractions and takes into account the time constraints of workflows. However, it suffers from the state explosion problem due the interleaving semantics of TWF-nets. To attenuate this problem, this paper investigates the combination of a state space abstraction with a partial order reduction technique. Firstly, it shows that to verify soundness of a TWF-net, it suffices to explore its non-equivalent firing sequences. Then, it establishes a selection procedure of the subset of transitions to explore from each abstract state and proves that it covers all its non-equivalent firing sequences. Finally, the effectiveness of the proposed approach is assessed by some experimental results.