Is really LP drum level control valve or some other drum control valve was wide open due to the low discharge pressure of the in service CEP. Is there any relation in operating the control valve with pressure rather than flow (make up).
I am giving incident report here for ur better understandig:Div Damper tripped from HRSG due to LP drums Level High HI. The root cause of this LP drum level high Hi was the huge flow of condensate during the changeover ofcondensate pump. Ops team was doing the condensate pump change over as the discharge pressure of the in service pump went down. LP drum level control valve was wide open due to the low discharge pressure of the in service CEP. So as soon as the second CEP started with normal operating pressure, huge flow of condensate went inside the LP drum through the wide open make up valve and the drum level reached to High Hi limit. As a consequence of this, diverter damper and ST also tripped as it was running with only one hrsg.