Water quantity and quality modelling tools are often used to understand the working and operate properly water distribution systems. In this paper, we discuss how to choose flow and pressure constraints at nodes and links in the distribution network graph for sustainable operation of these systems. Using practical and concrete examples, we show how the problem of choosing the appropriate flow and pressure constraints amounts to verifying that a certain algebraic condition of maximum rank holds (a constraint qualification condition), or equivalently that there is a corresponding spanning tree with unsaturated links and demand nodes. Some situations of non-convergence in the solution path are discussed.