Strong sheared toroidal flows are predicted to drive parallel velocity shear Kelvin–Helmholtz oscillations unstable in spherical tokamak plasmas. Numerical simulations that include rotation in the equilibrium suggest that future spherical tokamaks will have an upper limit in uni-directional injected neutral beam heating in order to keep the sheared toroidal rotation below the critical level for the onset of the global parallel velocity shear Kelvin–Helmholtz instability.