Tolerance for Dog leg

[Sankepalli R.S.Reddy]

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

 

[Raghuram Bathula]

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:*
> **

[Sankepalli R.S.Reddy]

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

 

[Shyam Herur]

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.

 

[pii engineer]

Please refer PFI(Pipe fabrication institute) Standard
to get the details.According IS:3589-1981 the
finished pipe shall not deviate from straightness by
more than 0.2 % of the total length.

regards,
gopalsankar
DM(ES-Inspn)
CPCL
Chennai

Now you can chat without downloading messenger. Go to http://in.messenger.yahoo.com/webmessengerpromo.php

[Quality]

The dog leg is terminology normally used by European/ American pipeline inspectors for "mitre bend" in pipeline construction (cross country). These mitre bends/ deflection ( dog leg) are due to misalignment between two pipes (butt joint/ girth welds).

These type of un intentional mitre bends upto 3°  are acceptable as per ASME B 31.4 Article 406.2.2 & ASME B 31.8 Article 841.2.3.2.

 

Dogleg terminology not generally used for the straightness of a single pipe (not used for manufacturing of line pipe).

 

best regards,

 

Sanjeevan

Gulf Petrochemical Services, Muscat, Oman

 

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[M K Malhotra]

 

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

From: M K Malhotra

To: material...@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: material...@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 <sankepall...@gmail.com> wrote:

[M K Malhotra]

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: material...@googlegroups.com

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

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

[Sankepalli R.S.Reddy]

Here by attached the photo related to typical dog leg.

We are facing this problems in construction stage of plant piping. Allowing 3 Deg as tolerance appears to be more liberal and in lines like relief /blow down where slope of the line is  to be maintained process point of view. 

Thank you very much to all of you for your prompt reply.

 

Regards

 

Srs reddy

Saudi Aramco

[Quality]

It seems to be a process piping with design code ASME 31.3.

If the fabrication/ construction code is ASME B 31.3, it also permits a miter bend upto 3° is acceptable (ref ASME B 31.2 Article 304.2.3).

In other words, a deviation/ deflection upto 3° can be considered as straight pipe & does not effect the design conditions (unless otherwise this tolerance is over ruled by designers with a strict requirement for maintaining a slope or aesthetic appearance is important than the allowable tolerance). Then do not look further for acceptance tolerance. Rectify the joint & make it straight.

best regards,

Sanjeevan 

Gulf Petrochemical Services, Muscat, Oman 

 

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

From: Sankepalli R.S.Reddy

To: material...@googlegroups.com

---

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[Ashish Bandyopadhyay]

The relevnt tolerances have been illustrated in PFI ES3

 

Thanks,

Ash