B31.8 Location Design Factor Change

[bware50]

Hello,

I was hoping someone may have any experience with attempting to change the original design location factor for an existing natural gas facility. This facility was built to B31.8 with a location factor of .5 . The facility used to be considered a compression station, but all compression equipment has been removed. The facility is now used for gas transfer to/from another compression station.

Is it possible to change the design location factor from .5 to .6 or greater? The facility is located far away from the public with only 1 building designed for human occupancy (control room). If it is possible, what requirements would need to be considered?

 

[LittleInch]

Your description is not strictly correct.

Compressor stations in table 841.1.6-2 give the DESIGN FACTOR of pipework as 0.5 in compressor stations regardless of where they are in terms of general pipeline Location class ( which is 1 to 4). Please don't mix and match defined terms, i.e. "design location factor" is a mix of two defined terms in B31.8

If what you now have is a pressure / flow control / metering / valve station and it is located in location class 1 or 2 then its design factor to that table increases to 0.6. In location class 3 or 4 it stays at 0.5.

There is a requirement in 31.8 to do a reclassification of location class every 5 years to account for new developments or in this case removal of developments. You simply write a report complying with the code and re assessing the location class or design factor along the pipeline.

But 0.5 to 0.6 isn't exactly a big difference. Why are you bothering?

 

[bware50]

Thank you for your response. Sorry for my errors.

The reason I was reaching out for information is because when calculating the pressure minimum thickness for a section of pipe, the current wall thickness is below the required thickness when using the original design factor of .5. The customer had installed a 20" API 5L X60 pipe with a .500" nominal thickness (more like .480-.490" due to under tolerance) MAWP of 1440 PSI @ 100 F. When I run pressure T-min calcs, .480" is the minimum thickness for this line.

If it was possible to increase the design factor to .6, the pressure T-min would much lower (.400")

I am not an engineer, just a contract API inspector hoping to learn and help this customer.

 

[1503-44]

Does B31.8 actually require that you subtract undertolerence from the wall thickness? I don't have a new version of B31.8, but old versions do not require that you consider undertolerence. That is already considered in the DF.

 

[LittleInch]

B 31 4 and B 31 8 just use nominal thickness and work on the basis that the under tolerance is included in the design factor.

So it looks to me like you're OK.

Read the code!

 

[GD2]

OP,
1st Part:
Littleinch has taken you in the right path.
Just as a refresher Location Factors are in whole nos (1,2,3,4) and Design Factors in decimals (0.4, 0.5, 0.6 etc).

I am not sure how it works where the facility is located but in many countries, the design is registered with the local safety Regulator.

As the facility is now no more a compressor station, rather works something like a gathering /metering station, the Code allows you to use design factor of 0.60 up to Location Class 3. You need to first define the location factor for the facility.

Check with your Regulator, you might have to submit a new design registration.

2nd Part:
As my colleagues have pointed out, ASME Code B31.8 provides the pressure/thickness equation for nominal thickness. The under tolerance has been already factored in the design factor, F.

What you calculated 0.480 in is the nominal wall thickness. You can straightway order this pipe without thinking for under tolerance.

The problem is that you won't get this pipe available in the market readily unless you order a bulk quantity with the mills. The next higher available pipe is a XS (Sch 30) pipe with 0.500 in wall thickness. With this you are back to the original pipe (WT =0.500 in) without having to go through the hassles of reviewing and reprocessing the entire design approval by the Regulator.

 

[bware50]

OP,
1st Part:
Littleinch has taken you in the right path.
Just as a refresher Location Factors are in whole nos (1,2,3,4) and Design Factors in decimals (0.4, 0.5, 0.6 etc).

I am not sure how it works where the facility is located but in many countries, the design is registered with the local safety Regulator.

As the facility is now no more a compressor station, rather works something like a gathering /metering station, the Code allows you to use design factor of 0.60 up to Location Class 3. You need to first define the location factor for the facility.

Check with your Regulator, you might have to submit a new design registration.

2nd Part:
As my colleagues have pointed out, ASME Code B31.8 provides the pressure/thickness equation for nominal thickness. The under tolerance has been already factored in the design factor, F.

What you calculated 0.480 in is the nominal wall thickness. You can straightway order this pipe without thinking for under tolerance.

The problem is that you won't get this pipe available in the market readily unless you order a bulk quantity with the mills. The next higher available pipe is a XS (Sch 30) pipe with 0.500 in wall thickness. With this you are back to the original pipe (WT =0.500 in) without having to go through the hassles of reviewing and reprocessing the entire design approval by the Regulator.

Thank you for your help here! I appreciate all of your input.

I was not sure once a facility is built that the original design factor could be altered. With this information, I should be able to point the owner/user in the right direction.

If the required thickness of .480" at 1440 psi for this line is already so close to piping with a nominal of .500", wouldn't most design personnel typically jump up another schedule to account for corrosion allowance?

 

[bware50]

Does B31.8 actually require that you subtract undertolerence from the wall thickness? I don't have a new version of B31.8, but old versions do not require that you consider undertolerence. That is already considered in the DF.

Yes, you are correct. That has not changed.

 

[LittleInch]

Thank you for your help here! I appreciate all of your input.

I was not sure once a facility is built that the original design factor could be altered. With this information, I should be able to point the owner/user in the right direction.

If the required thickness of .480" at 1440 psi for this line is already so close to piping with a nominal of .500", wouldn't most design personnel typically jump up another schedule to account for corrosion allowance?

The required thickness should include whatever corrosion allowance you think is required. This could very easily be zero or 1mm or 3mm or 6mm - up to you, but for pipelines an extra 1mm could cost many tens of thousands of dollars. Hence once you get your required thickness you choose the next size up only.

If you're only buying 50m then its a different story - you buy whatever the stockist has on his shelf so long as its not ridiculously thick.

 

[GD2]

Thank you for your help here! I appreciate all of your input.

I was not sure once a facility is built that the original design factor could be altered. With this information, I should be able to point the owner/user in the right direction.

If the required thickness of .480" at 1440 psi for this line is already so close to piping with a nominal of .500", wouldn't most design personnel typically jump up another schedule to account for corrosion allowance?

Unlike piping codes ASME B31.1/31.3, B31.8 don't consider corrosion allowance (CA) in it's thickness equation. Instead, the following preventive one or more measures are taken to protect the pipeline against internal corrosion:

• Internal protective coating,
• Use of corrosion inhibitor
• Use of pigging
• Sweetening of upstream gas
• Use of corrosion resistance materials