Increasing need of a wide use of operator training simulators brings specific requirements on modelling of industrial plants which have to be at the same time detailed but simple enough, capable of a credible (but not necessarily precise) simulation both for small and large changes of operating point. These contraversional requirements are often enlarged by the demand of performing operator training by means of the control and monitoring system which respective device or process was equipped with. In this paper it is shown how in the case of such modelling of a thermal power unit a non-linear model based on the first principle analysis is used for building up the operator training simulators. This process instrumentation models which can be called also a reality-oriented was developed as a own-built user block library in MATLAB-SIMULINK standard simulation package. The presented process instrumentation model represents the water feeding subsystem of a steam boiler with power capacity 250 t per h of power unit 55 MW as a part of the operator training simulator developed for Opatovice Power Plants in the Czech Republic.