Gas motions in the hot intracluster medium (ICM) of galaxy clusters have an important effect on the mass determination of the clusters through X-ray observations. The corresponding dynamical pressure has to be accounted for in addition to the hydrostatic pressure support to achieve a precise mass measurement. An analysis of the velocity structure of the ICM for simulated cluster-size haloes, especially focusing on rotational patterns, has been performed, demonstrating them to be an intermittent phenomenon, strongly related to the internal dynamics of substructures. We find that the expected build-up of rotation due to mass assembly gets easily destroyed by passages of gas-rich substructures close to the central region. Though, if a typical rotation pattern is established, then the corresponding mass contribution is estimated to be up to ∼17 per cent of the total mass in the innermost region and one has to account for it. Extending the analysis to a larger sample of simulated haloes, we statistically observe that (i) the distribution of the rotational component of the gas velocity in the innermost region has typical values of ∼200–300 km s−1; and (ii) except for few outliers, there is no monotonic increase in the rotational velocity with decreasing redshift, as we would expect from approaching a relaxed configuration. Therefore, the hypothesis that the build-up of rotation is strongly influenced by internal dynamics is confirmed and minor events like gas-rich substructures passing close to the equatorial plane can easily destroy any ordered rotational pattern.