I was trying to simulate the below given problem albeit it was globally balanced but it displayed an error when I tried simulating the model. The error that came is
** Error: Model is structurally singular, the following equations do not allow to solve for required variables:
pipe.flowModel.m_flows[1] = pump.m_flow_nominal /* Solvable for: pipe.flowModel.m_flows[1]
volume.m = 995.586 * volume.V /* Solvable for: volume.m
der(volume.m) = -pipe.flowModel.m_flows[1]
Solvable for: volume.m, pipe.flowModel.m_flows[1]
Validation of model water_mod completed with 1 error. I tried changing the mass flow rate values but still couldn't solve this error. Any help will be appreciated. Model is given below: 
