Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations IDS on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

high pressure saturated steam

Status
Not open for further replies.

Sebra

Mechanical
Jan 29, 2015
6
we are taking samples from a HP saturated steam line at ~7000kPa pressure and 500degC temp. as this is a sampling line it is only a 1/2" SS tube. the sample will be taken to the analyzer unit close by. Do we need to do stress analysis for the line or . we can use rule of thumb for supporting tube as a 1/2" pipe is very flexible.
 
Replies continue below

Recommended for you

Similar small dia tubing is often used when conducting a drum downcomer circulation test, and it is neccesary to provide generous flexibility loops to account for teh thermal growth of the tubing as it heats from ambient 20 C to the saturated temp of as high as 370 C. The tubing usually uses swagelock type fittings, and these can fail if thermal expansion is not accomodated by flexibility loops. These are temporary test tubing, and the root connection to the boiler must have a minimum 3/4" pipe connection with block valve before swaging down to tubing. I think the boiler code has requirements for these connections.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick
 
I would be more concerned about the stress on the connection for the 1/2" line into the bigger line. that's where you can generate high stresses / failures quite easily.

The tubing is only very flexible if you allow it to be. We have no sight of your design so very difficult to say if an experienced stress engineer can "eye it" and pronounce sufficient flexibility or not. 500C is quite a lot of expansion.

What is the sample pot or unit? Are people nearby / operating valves? It's very easy to apply more force than you think to a small line trying to open or close valves.

Failure of any line at that pressure and temp is dangerous so if in doubt, do the analysis.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
22, which code excludes 2” and under from stress analysis?

Huub
- You never get what you expect, you only get what you inspect.
 
Im going to delete my post. I let my thinking affect my reality.
Good thing you checked your code.
.
 
Not sure what to think of that. But I presume there isn’t any such code, correct?

Huub
- You never get what you expect, you only get what you inspect.
 
It was a whole lot easier to delete it, than try to prove it. [blush] This is a bit embarrassing.
Thank you XL83NL. After that, I'm going to get my meds changed.


Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.
 
Nothing to be ashamed of. We’re all here to help and learn. Hence my question 😊

Huub
- You never get what you expect, you only get what you inspect.
 
Could it have been an old provision of the flexibility clause that was dropped. I have a fuzzy image of such a phrase.
Now I'm making it worse.

Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.
 
The idea that 1/2" pipe or tubing is "flexible" does not negate the need to provide flexibility loops to the pipe run; the stress or strain that the pipe experiences is related to the ratio of the force divided by the available area of the pipe section, and although the force is low, the section area is also very low. The expansion from 20 C to 370 C will impose tremendous strain if no flex loops ( or sliding supports for the samplng pot) are provided, and stainless tubing is also susceptible to ratcheting at high strains. Also, pipes under 3/4" NPS need to be protected from inadvertent workers stepping on them. If you ever watched insulators at work, it is surprising that more pipe failures don't occur.
"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick
 
22, what's up with all the username changes? IIRC, you've been 35, 28, and now 22.

It’s been a while (1980-90’s) , but I do recall projects in my business unit (BU) where stress analysis for 2” and under (or under 2”?) pipe was declared not needed by my BU. Our process temperatures are low, ambient to 85 C, and our piping is rarely > 4”. We use packaged boilers. I really don’t know how this aligns with Code, or if Code changed over the years, but I do recall the BU practice.

Good Luck,
Latexman
 
Most of the piping systems we have in our installations are tubing, and piping usually NPS2” and under. In general we perform a detailed pipe stress analysis (using autopipe) for design temperatures roughly 200 deg C and higher. Unless the lines were initially routed with proper thought of supporting and inherent flexibility, most lines require some adjustment of supporting and/or routing. Once that’s good, usually the nozzle loads are the real problem. Especially on pumps and other rotating equipment. That’s the real caveat.

Huub
- You never get what you expect, you only get what you inspect.
 
Yeah. Maybe I can see that from a company practice note from somewhere. I think I have only ever used 2" on a drain, bypass/pressure balance around a valve, or short blow down stack from time to time. Never for any straight length over 8ft that attached to same temperature pipe. Thinking about it, I can say the same for 3" and 4" as well. I haven't done any pipe over 212°F since leaving refinery work a long, long time ago. That shows.

Sorry. I'll try to be more careful.

Name changes; I can't decide what it should be. I'm sure something will occur to me eventually.



Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.
 
We also need to consider how much the main pipe moves. This could be 100s of mm and the sample tapping piping needs to allow for this. Can be better to mount the sampling device on a bracket on the main pipe.

There are test calculations in the codes to see when thermal stress is not needed.
 
Thanks everyone for sharing your experience and thoughts. The reason that I raised this was because of an argue that I had with our design contractor. As I believe some level of formal stress analysis is required but they want to leave it to the installation contractor to do that. good points about the expansion loop, the movement of the main pipe, connection of the sampling line to the main line as potential failure point.
@davefitz , Can you please let me know which code are you referring to B31.1?
 
The issue is often that routing of small bore stuff, especially tubing, is left to site run, so it's difficult and often not worth it to design the precise routing at the design stage.

As said by a few others, 1/2" is both flexible and fragile. A lot depends on the details of the end attachments and how it is supported.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
sebra,

ASME section I ( power boilers) has paragraphs that discuss instrument connections to the boiler, and B31.1 does as well. As a practical matter, the minimum size connection to boiler piping needs to be at least 3/4" sched 40 ( or greater) to support a person standing on it, as often happens in the field.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick
 
Exactly how are you going to collect and analyze the sample?

7000 KPa at 500 deg C is hot and highly pressurized (obviously) - until it runs into a deadleg sample line for a few minutes. Then it becomes "cooler" but still dangerously hot slug of highly pressurized water that will blow out of the line. Then,once the one, two of more slugs of hot water are all blown out, it becomes live stream again.

Your analysis will probably show no significant stress IF the original movement of the sample port into the large line is taken into account. (Larger line moves, sample tap moves.) The rest of the sample line like all 1/2 and 1/4 inch (12 and 6 mm) tubing needs to be protected from access, wear, and physical damage as others have noted. The sample line should be routed with continuous drain angle (1/4 inch per foot or greater) to minimize water slugs into the sampler or its drainpoint.
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor