[MW:563] Re: Tolerance for Dog leg

 

----- Original Message -----

From: M K Malhotra

To: materials-welding@googlegroups.com

Sent: Tuesday, February 12, 2008 9:24 AM

Subject: [MW:558] Re: Tolerance for Dog leg

Due to unconventional terminology, I understand the matter has been distracted and created a lot of confusion . Here is possible answer:-

 

1. Mr. Shyam Herur is right. The term you have mentioned is Straightness of a pipe which is measured in say 1 mm/m length of pipe. So API 5L specifies it as 2mm/m length. that gives 24 mm/12.2m pipe joint. Good pipe mills can give 15-18 mm/12.2 m pipe joint length.
2. Terms ovality and out of roundness are different and let not be confused with straightness. straightness is a function of complete pipe lenght where as ovality and out of roundness are measured as ends/pipe body and are independent of pipe length. Even a ring or cut cylinder may exhibit ovality and out or roundness but still be straight. ovality and out of roundness are function of ODmax, ODmin, ODnom. Different formulation is available to calculate ovality and OOR.
3. Now the big question is Mr. Reddy related his query to welding. Here please know that straightness, ovality, out of roundness all can still be present in non welded pipe say a seamless pipe can also be checked for straightness, ovality and out of roundness. Infact all these defects should be maximum in seamless pipe since mandrel can not support larger diameter pipe (larger pipe=>more weight=>more sagging=>more deviation from circular shape) and that is why the tolerances are more for seamless pipe.
4. Still, if the question is pertainign to welded pipe only, there is one more parameter to be checked which is peakedness. it means during pipe forming/wlding when the edge crimping is done, the edges do not straighten during press forming in O-shape and a kind of peak is formed at the weled point. This dangerous from stress point of view and also may tear the pipe coating.
5. Now, as the query states, i understand you are saying what could be the deviation from a staight line when the pipes (welded/seamless/ERW etc. any make) have been welded in a string (say 10 pipes of 10 m length welded to form a 100m long pipe string). Then in this case the deviation should not be more than 3 degree from a straight line. If the deviation is more than 3 degree then it would be regarded as miter bend. ASME B 31.4./ ASME B31.8 (liquid/gas transmission pipelines code) specify different stress levels (% of allowable stress under operating stress state) underwhich miters are permitted or prohibited. That is the reason that you will find fabricated bends (mitres) in fire water lines and low pressure piping but not in high pressure lines.

Best Regards.

 

Manoj K Malhotra

Manager - Pipelines Engineering

Engineers India Limited, New Delhi.

----- Original Message -----

From: Shyam Herur

To: materials-welding@googlegroups.com

Sent: Monday, February 11, 2008 10:40 PM

Subject: [MW:558] Re: Tolerance for Dog leg

Can you specify the pipe spec and what it is used for ? Since you are referring to "line pipe" I presume its the pipe covered in spec API 5L. However I did find an abstract which  goes" usually out of straightness of pipe usually follows a log normal distribution with diametre, and, as would be expected the larger the diametre of the pipe, the straighter is the pipe. API 5L allows  24mm in a 12m joint. usually it is possible to obtain pipe to 12mm in 12m. to reduce total cost over specification of straightness should be avoided".

 

also refer to API 5L for further details.

 

On 2/11/08, Sankepalli R.S.Reddy <sankepalli.rsreddy@gmail.com> wrote:

It is the out of straightness of line pipe. The out of straightness may be due to weld distortion at joint locations or due to improper fitup.

regards

 

On 2/11/08, Raghuram Bathula <raghurambathula@gmail.com> wrote:

Can you please explain the term Dog leg, i have no idea what it is?
Is it a term related to steel structure? type of stair case
arrangement?

On Feb 11, 7:39 pm, "Sankepalli R.S.Reddy"
<sankepalli.rsre...@gmail.com> wrote:
> *Dog leg or out of alignment of line pipe:*
> **
> Can any body gives permissible tolerance for Dog leg. Whether any standard
> is covering this tolerance.
>
> Thanks and regards
>
> S. Rama subba reddy
>
> Inspection Engineer
>
> Khursaniya Gas Project, KSA
>
> Bldg 7, Cu 1
>
> Ph no: 6770120

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