Ovality Standard

[4fletch]

What is the permissable amount of ovality in a carbon steel boiler tube after bending operations? I've been perusing thru Sect. I and II and not finding what I'm looking for. If ovality is not addressed in Sect. I, what standard do you apply for bend boiler tubes? Thanx in advance.

 

[FIF99]

Hi,
Normally the maximum allowable ovalities/out of roundness are specified in the relevant ASTM material standard.
Regards.

 

[sjrfc2]

B31.1 puts limits on ovality of piping bends. Theilsch's book cites 8% as max american practice (limit in B31.1) and provides a reference to German practice of basing max ovality on OD/ID ratio. The reason he gets into this is to address causes and corrections of failures near the neutral axis (max stress) of boiler tubes.

The German reference is "Verein.d.Grosskesselbesitzer 1959/60 Review" but I never made any real effort to find it.

I noted EPRI also did some FEA work showing increases in stress of a return bend as the ovality was increased to 5% and 10% in NP-2944 (1983). I used some of the info once when trying to explain why we were asking for specific limits on ovality of formed tubes.

Generally I've seen limits in place which allow pipe bend ovality to 8% if the working pressure is below 600 psi and 5% if the working pressure is above 600 psi. Generally the pipe bend radius is 5 pipe diameters or greater. On boiler tubes the bend can be tighter depending on design and space limitations. I've see allowable ovality to 10% on tight-radiused boiler tube bends but the target is to set the process up to get below 8% if possible. You'd want to review whether the tubes were hot bent or cold and whether heat treatment was required.

Boiler manufacturers may have plant guidelines they may be willing to share with you.

 

[metengr]

I can tell you that with our internal specifications, the tube ovality is held to 8% or less for tube bends. The 10% ovality for tube bends that people reference is something that I believe carries over from ASTM/ASME ovality requirements for seam welded boiler tubing.

 

[ijzer]

For tubes working under internal pressure 8% is quit common value. If vacuum or external pressure 3% max.is used.