Hi Young2009
because of the momentum change in the steady folw,Normally this force is constant,and is small enough that it can be easily absorbed through tension in the pipe wall,to be passed on to adjacent elbow which may have equal and opposite loads,therefore zeroing the net load on the system,therefore these type of the momentum loads are usually ignore by the stress analyst.if the fluid velocity or density change quicklly with time,this momentum load can lead to a dynamic load ,so which may not be cancelled!

regards