Minimum Bend thickness at Extrados of Pipe bend

[m.wasif]

I am procuring Pipe bend with following specification for economizer section of Boiler:
Outer Diameter: 33.40mm
Thickness: 3.38mm
Length of pipe bend: 226mm
Material: A106 Grade B
Design Pressure: 800 PSIG
Design Temperature: 232 Centigrade = 450 Fahrenheit
Bend radius at Center-line of Bend = 40mm
Drawing: Attached

I am facing issue in calculation that what will be minimum thickness at extrados. Supplier is providing thickness of 2.8mm at extrados. I want to confirm that this thickness of 2.8mm at extrados is acceptable or not as per applicable ASME standard. Does any one if possible can share the calculation of minimum required bend thickness at extrados with respect to my conditions.

Regards,

 

[Dhurjati Sen]

Dear m.wasif,

Why do you think that the thickness at the bend extrados should be less than the pipe thickness at the first place?

Regards.

DHURJATI SEN

 

[m.wasif]

Dear Dhurjati Sen, because due to bending process, thickness at extrados will decrease.I want to confirm that what allowable minimum thickness of bend at extrados is acceptable?. If supplier is providing 2.8mm thickness at extrados . Can I accept it or not???

 

[Dhurjati Sen]

Dear wasif,

Check EN 12952-5 & EN 12952-2 to calculate the thickness at the bend extrados. You may get 2.8 mm.

However, you could have procured bends having same thickness as that of the pipe.

Regards.

DHURJATI SEN

 

[david339933]

B31.1 - 102.4.5

 

[MJCronin]

Wadid....

This is perhaps one of the oldest questions asked in piping design and engineering.

This is a very simple calculation. As stated above, you must open and become familiar with the piping code of

http://pipemill.com/pipe-wall-thickness-calcs.php

MJCronin
Sr. Process Engineer

 

[m.wasif]

Thanks.To compensate for corrosion allowance, what value of "A" is suitable during thickness calculation using ASME B31.1?

 

[GD2]

m.wasif,

I don't have the latest edition of B31.1 but B31.3 para 304.2 gives two equations to calculate the intrados and extrados thicknesses. Have a look. At the end, the finished, as formed thickness mustn't be less than the design thickness.

"A' value will depend on the fluid and the material of the bend. You will need to do a bit of research to find out A from some credible reports. NACE Corrosion Data Survey could be one source. Once you determine the Corrosion Rate and the design life (usually 30 years for power piping, you can calculate A.

 

[m.wasif]

Dear All Thanks for your co-operation, I want to sum up the discussion.

I have calculated the thickness of bend, whose details are mentioned on top (drawing attached also) ,using ASME B31.1.

Using formula mention in 104.1.2 of ASME B31.1 for straight pipe minimum wall thickness through eqn(3)= 2.26mm

Using formula for extrados mention in 102.4.5 Bending of ASME B31.1 equation 3(B) , minimum required thickness, of a bend,after bending= 2.15mm

As obvious, thickness will decrease at extrados form 2.26mm to 2.15mm. Also for thickness calculation, I considered corrosion allowance of 1.5mm as material of bend is carbon steel and intended to be used in economizer section of boiler .

Note supplier is providing bend with thickness at extrados = 2.5mm

According to ASME B31.1 The minimum wall thickness at any point on the bend shall conform to (A) or (B) below.

(A) The minimum wall thickness at any point in a completed bend shall not be less than required by eq.(3) or (3A) of para. 104.1.2(A). According to this requirement 2.5mm is greater than 2.26mm. Therefore bend with 2.5mm extrados thickness is acceptable.

(B) The minimum required thickness, tm of a bend, after bending, in its finished form, shall be determined in accordance with eq. (3B) or (3C). I have used eqn 3B and thickness calculated is 2.15mm . Since 2.5mm is greater than minimum required 2.15mm thickness at extrados. Therefore 2.5mm thickness provided by supplier is acceptable.

Supplier is making bend from 3.38mm pipe and required thickness at edge of bend is 3.38mm which is fulfilled by supplier and after bending, thickness and extrados becomes 2.5mm , I believe it is acceptable as it is fulfilling the two above mentioned conditions of standard.My question is that , does this is right approach for selecting 2.5mm extrados thickness bends using standard or I am making mistake if any??