while solving warning coming like maximal number of iterations reached but no root found, what is this ?

Turbine trip event simulation. I have got four steam generators which currently I have modelled as constant pressure temperature reservoirs and two safety relief valves for each SG in my model. Now at the other end when I am suddenly making flow rate to zero, approximating turbine trip, in 3 SGs flow rate is reducing from the instant itself but in a single SG, flow rate suddenly going to maximum negative (i.e few time the original flow in backward direction). I have kept all the four SGs identical. Why is this happening ?