There are given algebraic and integral identities for a pair or a triple of plane solenoidal fields. As applications, we obtain sufficient potentiality conditions for a plane vector field. The integral identities are also important for exact a priori estimates.

The main purpose of this article is to consider the smoothness control of a weak solution after some moment if there is known solution regularity until this moment. The necessary tools can be varied. It is possible to control kinetic energy dissipation to fix moment or changing of velocity square or summability of acceleration square to fix at point time.