Hi Igor,
Your question is a bit vague on what you specifically want to model with this. Moment of Inertia follows to the simple equation of T = Ia.
Here are some Simulink examples which model something a bit more complex with Inertia. Note that a wheel or drive shaft with inertia is going behave much in the same way as the rotating pump will.
http://www.mathworks.com/help/simulink/examples/modeling-an-anti-lock-braking-system.html http://www.mathworks.com/help/simulink/examples/modeling-an-automatic-transmission-controller.html http://www.mathworks.com/help/simulink/examples/building-a-clutch-lock-up-model.html
I'd also like to point out that a physical modeling suite of products (Simscape, Simscape Multibody, Simscape Fluids, etc.) Are designed for modeling physical systems in a much more intuitive way and much of the blocks have equations of motion built into them rather than developing the equations yourself.
~Zack
