I think I found the mistake. I didn't make the annotation for the FluidPort. There's a difference between the annotation for the whole interface and for the ports themselves. Each port should have a separate annotation like the code below. I copied the annotations from the Fluid library since I don't really understand how it works.

FluidPort_a port_a annotation(Placement(transformation(extent={{-110,-10},{-90,10}}, rotation=0)));
FluidPort_b port_b annotation(Placement(transformation(extent={{110,-10},{90,10}}, rotation=0), iconTransformation(extent={{110,-10},{90,10}})));

Thanks Johan for your help anyway :)

Hi Mohamed,

I'd recommend you to take a look at the IntroductoryExamples package. It is bundled with SystemModeler. There's a guide called "Getting Started" that you can reach from the Help menu. Look at chapter 7 and especially 7.2.1. I hope that helps!

Johan