view_state_name = "pvr_boq_grd_" + (exl_boq_rot - 1);

ViewState[view_state_name] = null;
ViewState[view_state_name] = (DataTable)dt_prepare_excel;

view_state_name = "pvr_boq_grd_" + i;

if (ViewState[view_state_name] != null)
{
    dt_export_excel = (DataTable)ViewState[view_state_name];
}