If the anti surge valve is open, then the flow through the compressor is larger than the required flow through the compressor at the given pressure. This is done so as to fool the compressor into thinking that the flow is larger than necessary so as not to surge the compressor. This means that the compressor load will be increased because of the increased flow. The operating of the compressor with an open surge valve maybe necessary not to surge the unit, but it is a very expensive way to operate the compressor.