In the same way that a pressure drop in a fluid passing along a length of pipe acts to produce an axial force on the pipe, so an axial force is essentially produced by a pressure drop in a tapered section of pipe. One has only to consider sections at each end to appreciate that the differential must be balanced by an axial force acting along the pipe that must be constrained by an external support.

If it is of interest, the local reaction on a surface can be obtained by resolving the forces acting at any point. In the case of a parallel pipe the radial pressure and frictional drag will resolve to a diagonal reaction. The local reaction will tend to be more inclined in the case of a taper pipe because, in additional to the pressure drop due to surface friction there is a force difference due to the differing end diameters. The operating pressure is normal to the slope of the pipe surface, the frictional drag is parallel to the pipe and the extra unbalanced force due to the diametrical difference acts in line with the pipe axis. ■