Interpretation of "analyzed between anchors" in B31.1

[lejeunem]

Can anyone provide me with a bit of clarification? I'm new to piping design. The Analysis of Piping Components clause of ASME B31.1 (clause 104.8 in the 2004 edition) states "To validate a design under the rules in this paragraph, the complete piping system must be analyzed between anchors for the effects of thermal expansion, weight, other sustained loads, and other occasional loads." My interpretation is that where you have existing anchors (tanks, pumps, etc.) you must analyse the piping system between those anchors. I want to confirm whether or not this clause obligates me to add an anchor to the end of a line if I'm going to have it analysed. Any thoughts?

 

[BigInch]

Not necessarily, even though such an anchor might be a good idea just to validate the stress model. It might help avoid having to include the rest of the plant in the model in order to find if there is a "virtual anchor" in there somewhere.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[vpl]

How do you analyze the line if you don't have an anchor to serve as the end point in the analysis?

Or are you saying you want to ignore the loads on segments of piping that are sitting out there unsupported?

Patricia Lougheed

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[BigInch]

Perhaps its more common with buried pipe, where after 1000 feet of contact with the soil, pipe can be considered as fully restrained in all but the radial hoop stress direction. Or after 200 meters of rack piping, where there may be essentially no movement, no stress, because all forces have been resisted by the summation of friction between 20 slide plates and supports. That's a virtual anchor. Something like that.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[vpl]

Big Inch

I was going by his tagline which said Nuclear. Most nuclear plant piping is not buried in soil but rather hung or otherwise supported above ground. From an analysis standpoint, there is usually an anchor point that starts or stops a code class boundary.

Where it gets often interesting is in what is called "balance of plant" piping -- which is done to B31.1 rather than Section III. A lot of plants are going through life extensions and are now analyzing piping that was not originally analzyed when installed 40 years ago. This balance of plant piping often doesn't have good anchor points -- the piping is simply supported rather than being restrained. This increases the complexity of the modeling as, without an anchor, the model has to include everything and disconnects occur because there is no end point.

None of this

Patricia Lougheed

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[vpl]

Big Inch

I was going by his tagline which said Nuclear. Most nuclear plant piping is not buried in soil but rather hung or otherwise supported above ground. From an analysis standpoint, there is usually an anchor point that starts or stops a code class boundary.

Where it gets often interesting is in what is called "balance of plant" piping -- which is done to B31.1 rather than Section III. A lot of plants are going through life extensions and are now analyzing piping that was not originally analzyed when installed 40 years ago. This balance of plant piping often doesn't have good anchor points -- the piping is simply supported rather than being restrained. This increases the complexity of the modeling as, without an anchor, the model has to include everything and disconnects occur because there is no end point.



Patricia Lougheed

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[BigInch]

Anywhere you could remove an anchor and see no movement or deformation, you wouldn't need an anchor there for either real or imaginary purposes. That condition might occur with or without soil restraints.

I agree it would be much easier and cleaner to just put in an anchor for analysis purposes of a new installation, even if you didn't need one for stress purposes. I have done the same myself a few times. I know things can get very messy when you're in brownfields. With refineries its dangerous enough. I don't even like to think about the implications in nuclear plants.



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[lejeunem]

Great comments. The particular problem I'm having is with adding a short (<20' total) run of 3/4" line from an existing, previously analysed, 16" line. The code is B31.1 but the line does need to be analysed in specific for nuclear seismic requirements and for fairly significant water hammer loads. Putting an anchor at the very end of the line increases the stresses substantially. I have it layed out so that it is well supported but I do not have an anchor at the end of the line since it just terminates in a hose connection. Adding an anchor to the end would necessitate a more complicated layout and higher stresses.

Some clarification to address previous comments;
- the line in question is not buried.
- There would only be several feet of pipe at the end of this run that would be unsupported.
- Anchors are normally used at class boundaries for nuclear piping.
- My existing model does have an anchor that separates this line from the rest of the system. It's the adddition of an anchor at the termination of the line that i'm concerned with.

Thanks again for your learned comments.

 

[vpl]

If adding the anchor increases the stresses, I wouldn't think you need it.

However, I'm not an expert on B31.1. I'll see if I can convince John Breen to pop in.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[BigInch]

I would reason that anchor to anchor would only apply to continuous piping, not to a free end, in an effort to ensure that the analysis of a piping segment under consideration is continuous from "anchor to anchor" and any effects from piping on the other side can be assumed to be isolated to that piping and v/v. Especially as vpl mentions, when it increases stress, as the addition of any proper anchor or guide will when it does its job and stops displacement. Increasing stress is, after all, certainly not the objective. If displacement is manageable under all load scenarios, let it displace in lieu of installing an anchor where one is not needed for any purpose at all, real or imaginary.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[racookpe1978]

I have it layed out so that it is well supported but I do not have an anchor at the end of the line since it just terminates in a hose connection.

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Two "corrections" - comments really.

You implied above that you consider "tanks, pumps, and equipment" (nozzles) as "anchor points" -

They most definitely are NOT anchor points, but have to evaluated - each nozzle indepenently based on your actual nozzle conditions and plant layout and temperature changes and equipment changes - to be "mobile, agile, and hostile" to the pipe.

For example, an "anchor" point is a point that doesn't move, that resists the tendency of a pipe to expand, contract, twist, push, or pull. A nozzle, on the other hand, "might" - or might not! - itself move a long ways as the eqpt expands or contracts. The nozzle's rotation or linear movement might be the primary source of your pipe's stress.

A pump suction nozzle, on the other hand, needs to be an analysis point so the suction PIPE doesn't create ANY force into the pump nozzle and rotating seal. So, you'll have two (or more) rigid supports on the short straight pipe going into the pump nozzle. Those supports will be anchors, but your suction line analysis is going to be "through" those anchor points and will end up some place upstream where other more flexible supports are holding the pipe.

The first "anchor point" of your little 3/4 inch will "move" depending on where that 16 inch line goes (in x,y,z global directions). If, somewhere in the middle of the short 3/4 pipe run, there is a pipe support that "sways or swings back and forth, then you've restricted movement in "z" (up and down) and "y" (north-south maybe) but not in "x" (east-west).

The "hose connection" is NOT an anchor actually, but might be a load point: I've seen lots of force being put on hoses during outages themselves that in turn pull the pipe. A "free" end of a pipe cannot be an anchor, nor can it be a load point (other than gravity or as pointed out mechanical overloads as somebody jerks the hoses sieways), but you have to analyze those loads.

You can also properly "analyze" pipe stresses by stating that "Based on evidence of available loads and the actual as-built pipe geometry, there is no substantial force on this section of 3/4" pipe under all known operating and shutdown condtions and therefore no need to perform a detailed pipe stress analysis."