Re: Vent line

On Jul 24, 2007, at 1:34 PM, Ben Nottingham wrote:

> 1) Organ pipe beveling has a small effect on standing wave
> frequency and this is figured into the tuning process of the pipe/
> organ.

It isn't an organ pipe so whether it's in tune is probably not at issue
>
> 2) The lateral load component at the tail pipe exit is equal to
> the normal thrust force multiplied by the sine of one half of the
> bevel
> angle.

That's what B31.1 says.
>
> 3) The lateral load component at the end of a relief tail pipe is
> often introduced to reduce the bending moment on the header. The bevel
> angle is adjusted so that the exhaust thrust force acts along a line
> that passes through the centerline of the manifold or header.
If you don't bevel the end you won't have a bending moment to begin with
>
> 4) Beveled ends break the flow symmetry at the exit, disturbing
> the periodic shedding of stable torus type vortices that contribute to
> noise strength. Rotational symmetry in the diamond shaped shock
> wave/expansion pattern downstream of the tail pipe, in combination
> with
> the convection of vortices is considered the origin of the screeching
> noise in jet exhausts.

This might work, but there must be something else to it, since you never see vent stacks of any size at all which are bevelled. I don't think noise is enough of a problem that the piping codes need to address it.

> I hope you can also explain the additional effect of having an
> elbow at or
> near the vent outlet.

Same effect as I mentioned. Turn the flow and you build in a transverse reaction. If course an elbow will keep out some of the rain--all of the rain if it's a 180 deg return.

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

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