Re: Vent line

On Jul 24, 2007, at 12:47 AM, Ben Nottingham wrote:

> The question was covered last year in much detail. Of note, ASME
> B31.1,
> Appendix II, para. II-2.3.1.2 and para. II-7.1.9, indicate the
> beveled end
> of the vent pipe causes the jet to produce lower vertical and
> horizontal
> forces than with a square cut.

I think there must be a little more to this. According to the relationships all that's happening is that the total force is only being redirected to an angle equal to the bevel without reducing the net force. Without going into the math in detail, the 'reduction' just appears to be the load caused by the change in momentum resulting from diverting a flow through an angle equal to the bevel angle. You create a lateral load by redirecting the flow, you don't reduce it. Without the bevel there is no lateral load. You do reduce the axial load but at the cost of imposing a lateral load.

By imposing an horizontal load component on the exhaust pipe you create a shear and a bending moment at the base. Since the bending moment is likely to be a more severe load than an axial force, The net effect is to increase the stress on the stack. Moreover if the outflow is unsteady the lateral loading may lead to vibration.

Christopher Wright P.E. |"They couldn't hit an elephant at chrisw@skypoint.com | this distance" (last words of Gen.

.......................................| John Sedgwick, Spotsylvania