Unsupported Pipeline Spans Found On Operating Pipelines

[mbpipe]

In determining the maximum acceptable length of existing pipeline spans found on operating pipelines, what is typically assumed for installation temperature and operating temperature when both are unknown? Is it more conservative to assume the pipe is in compression or tension for doing the calculation?

 

[cingold]

I'd actually like to know the answer to this one too. I assume you mean the operating and design temperatures are "ambient" correct?

I've heard different senior stress engineers suggest using anything from 100F to 160F as an ambient, blocked-in temperature for analysis. The most common suggestions seem to be around 120F and 130F.

For installation, I've always used 70F, but that's because no one has ever given me any details on the installation. This number could have a large range based on the location of installation and the time of year.

I'm tagging along on your thread to see if anyone else has suggestions.

 

[mbpipe]

Yes, I mean for the operating temperature to be ambient or actual pipe temperature at the span. Let me ask this a different way. I am trying to establish a standardized conservative method to evaluate pipe spans found on cross country pipelines. I have been told that it is conservative to assume "winter construction" for such calculations. For winter construction cases, the pipelines are operated at a higher temperature than when they were installed, and are therefore in compression.

I am calculating longer acceptable spans when I assume "summer construction" where the pipeline is operated at a lower temperature than when it was installed (tension).

What should the conservative assumption be for the calculation to give you the shorter acceptable span, summer or winter construction?

 

[gfdoug]

In my experience, typical installation temperature is 70 deg. F while the operating temperatures depend on your aparatus. In Ethanol lines I've seen weather temperatures applied for the area which were something along the lines of 0-120F, but for a heated butane vapor line I've seen up to 220F used.

If you are only worried about lines under ambient temperatures, I would check weather patterns for the area and find something that encompasses the average yearly low/high values. Although, if you want to be more conservative, you can increase that range.

 

[BigInch]

Compression is the most conservative assumption. Remember to check for buckling length and stress limits as well. It would be most conservative to assume that the line was constructed at the lowest ambient temperature. If the line is not artificially heated, you might use highest ambient temperature, unless exposed to direct sunlight, when temp can reach 75C at some latitudes. If the line is artificially heated, get accurate temperature profiles and refrain from doing such generalist calculations if the proper data is not available.

you must get smarter than the software you're using.

 

[KevinNZ]

Maximum pipe span depends on the gross bending stress which will be highest mid span and local stress at the supports.

Pressure stress adds tension to the bending stress, but not no pressure may leave the compression stresses in buckling range.

Not sure why you are asking about temperature unless the pipe is clamped at each support.

Kevin K

 

[LittleInch]

Maybe it's me, but I'm not clear if this pipeline is

on land or subsea?
buried or on the surface?
On a set of supports or following the ground profile?
where in the world is this pipeline?
Are the spans an over bend or sag bend?

Often you need to look at the max stress in compression and expansion on the bend to find your worst case.

If you don't know operating temperature you won't get very far unless you take the design temperature as a worst case condition. There is no such thing as "typical" for this - it could be -150C or +400C - all depends on the contents, flow rate, insulation, conductivities.

In reality a very vague question so you won't get specific responses.


My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[cingold]

If the OP's workplace is anything like mine then the OP will get one of two answers when asking for temperatures for both above ground and below ground pipe. It's either ambient or 100F. No one takes the time to see what temperatures the line will actually experience. I know that if the pipe fluid is not moving and the pipe is sitting in the sun, the temperature will be above 100F.

That's the pipeline business lol.

I've honestly thought about posting the same question here because I've had various senior stress engineers give me ranges from 120F to 160F. For long runs or pipe, that's a huge difference...

 

[mbpipe]

Thanks for your input folks. After further investigation and calculations by a number of different methods, here are my conclusions:
1. Generally, calculated acceptable span lengths using beam equations decrease as tension due to temperature difference increases. In other words, the summer construction temperature cases result in more conservative (shorter) acceptable span lengths than the winter construction cases.
2. Although this is true generally, you also need to check that winter construction cases (compression cases) are within stability acceptability limits for buckling. CSA Z662-03 recommends that the longitudinal stress due to pressure and temperature be less than 80% of the elastic critical buckling load.
3. Although generalized tables can be produced using conservative temperature assumptions, they should only be used as a first-pass screening tool for the operations guys. If the spans found in the field exceed the lengths published in the tables, a more detailed engineering analysis of the individual case is required.

 

[BigInch]

"Generally, calculated acceptable span lengths using beam equations decrease as tension due to temperature difference increases. "

I wouldn't have thought that to be the case.

you must get smarter than the software you're using.