Pressure Rating Effect of Bending Steel Pipe 1

[DRWeig]

Hi group,

Please pardon an EE with a piping question. We use these little syphons (pic attached), often with an additional length of pipe and radiation shield, to protect pressure transmitters from liquid intrusion and high temperature when the fluid to be measured is steam. Have done it for a decades.

The syphon itself is 1/4" steel, seamless, continuous weld, Sch 80XS. According to ASME B 36.10 and ASTM A53 B, the working pressure is 871 psig and the burst pressure is out there in the 15,000+ psig range.


The question has come up in the office: Bent into the loop (cold bend, around a mandrel), what can I do to find or calculate a new allowable working pressure? The loop has a 2" diameter when measured at the centerline of the pipe. Just wanted to see is anyone has gone through this before I hire an engineer for analysis or a testing lab...

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[BigInch]

You would not usually derate the bend. If bending cold, you would bend into a minimum radius such that excess longitudinal stress is being utilized in a manner that does not cause the combined stresses when under pressure, in this simple case the hoop or pressure stress plus the longitudinal bending stress to exceed the allowable combined stress. For a cold bend, this minimum radius would be considerably more than the radius your picture appears to show. For a hot bend the possible minimum radius is much smaller, as in the hot condition, bending stresses are not being created by the bending action because the material is practically fluid at the bend temperature. However the act of hot bending, actually stretching the material along the longitudinal axis on the outside of the bend much more than the inside radius, reduces the original thickness of the wall in stretched areas. The thickness of the wall primarily controls the allowable pressure, so you must start with a thicker wall that, when thinned, still gives you enough of wall thickness to permit your needed pressure rating. If you took a standard fitting with a standard wall thickness sufficient for a certain pressure and hot bent that, then yes, you would have to derate that fitting's original allowed pressure according to the wall thickness remaining in the thinned areas after bending.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[DRWeig]

Thank you BigInch. I think I have a saw that can slice this thing to get good measurements of the wall thickness all around the circle.

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[BigInch]

BTW should say "does not cause the combined stresses to go over allowable combined stress when"

You can measure with ultrasonics without cutting.

If it ain't broke, don't fix it. If it's not safe ... make it that way.