B31.4 Induction Bend Minimum Thickness

MechEngMSU · Aug 22, 2019

Hi,

For a B31.4 design, do induction bends have the same pressure rating at the straight pipe they are made from or do you have to take into account wall thinning? (Assuming the pipe was not cold worked. Cold worked pipe heated to 600 degF is derated to 75%)
I interpret B31.4 to mean the bend pressure rating is based on the thickness before bending but B31.4 references B16.49 and I interpret B16.49 to mean the bend pressure rating is based on the thickness after bending.

Excerpts:
B31.4:
404.2.3 Induction Bends. Induction bends are made
by heating the pipe and forming the bend under controlled
conditions. [highlight #FCE94F]The minimum wall thickness of pipe
before bending shall be determined as for straight pipe[/highlight]
in accordance with para. 403.2.1. Induction bends shall
comply with ASME B16.49.

B16.49:
2.2 Design of Bends
The required internal pressure design thickness at the
intrados (inside radius) of the bend shall be determined
in accordance with eq. (1) of this Standard. [highlight #FCE94F]The thickness
at the neutral axis (see Fig. 1) and on the extrados (outer
radius) of the bend shall be no less than the mating pipe
design thickness or the customer-specified minimum
wall thickness.[/highlight]

Design Details:
2000 ft of above-ground Ethane Pipeline in a pipe rack designed to B31.4
Design Pressure = 2220 psig @ 100 degF (ANSI 900 flange rating)
API 5L X60 20" SCH 60 (0.812") pipe, DSAW
The client pipe specifications have a design factor of 0.50 and no corrosion allowance
The minimum thickness of straight pipe = 0.740"
We are using 6D induction bends to snake the pipe through the rack because the line must be piggable.

1503-44 · Aug 24, 2019

Normally in any case of inconsistant requirements between codes, regulations, design specifications (including differing opinions amongst experienced engineers), the most conservative requirement is applied and the client is notified of the inconsistancy for resolution and further action as may, or not, be necessary.

saplanti · Aug 27, 2019

Induction bending requires an additional wall thickness at around %10 of the required straight pipe thickness by considering the required bending radius and diameter, and this additional wall thickness issue needs to be solved with manufacturer.
Some manufacturers give tables for the additional wall thickness for the induction bending. You need to search by Google I guess.

LittleInch · Aug 27, 2019

I had never seen that before now but looks like a discrepany between the two alright and I think your working is correct.

Normally though you have some leeway between design thickness and the nominal thickness of the pipe which you can use to your credit.

A 6D bend will thin on the outside by approx. 7 percent.

Usually people like to buy pipe at least one thickness up from the connecting main run pipe for the bend mother pipe.

You appear though to be mixing piping design with pipeline design.

Unless you really need to you don't normally set the design pressure to equal the flange rating for pipelines as this is often very conservative.

B31.4 isn't really set up for above ground pipe racks in the same way B31.3 isn't really set up for buried pipe.

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