Pipe support questions 3

[NewMechanicalEngr]

Hi Folks,

I'd like to know if you can share your opinions with me on some basic pipe support questions?

Summary:

Will be running a few 2" and 3" CS pipes with max 450 deg F (hot insulated - 2" calcium silicate). Spec calls for 2" to be socketweld and 3" to be buttweld. Route of each is relatively short (approx 25 feet) and each has a couple elevation changes and horizontal 90's. The max straight run length is 7 feet. I will be adding one steel support to rest on and a new concrete support also. The ends will be bolted to existing flanges. I plan to use pipe shoes at the supports due to insulation. I figured thermal expansion is about 1/32" per foot, so my max expansion would be just over 3/16".

I would like to get your opinions on the following questions:

1) Does this sound like something that needs stress analysis? Is there a criteria or rule of thumb to help determine if stress analysis is required?

2) Is 3/16" thermal expansion great enough that it would warrant using guides instead of fixing the pipe shoes?

3) Should I install slide plates or can I rest the pipe shoe directly on steel (i could embed a piece on the concrete support if this would be required).

Thanks in advance for your opinions!!!

 

[pennpiper]

You wrote: "The ends will be bolted to existing flanges."
Flanges of what?

 

[NewMechanicalEngr]

One end will be flanges on a skid that is being prefabricated and delivered. The skid is basically two 2" control valves in parallel in a symmetrical horizontal configuration. The outlet of each control station comes together and leaves the skid in a single 3". The piping on the skid will be attached to the skid steel using 2" S/80 pipe supports welded directly to the pipe. There will be one support on each pipe at each end of the skid (total 6 supports - 2 on each pipe). FYI: Total length of skid piping between fixed supports is 5'6".

The other end of each 2" going to the skid will be connected to flanged valves that are directly bolted to nozzles on shell side of an two exchangers.
The other end of the 3" is going to be connected to a flange on a vertical pipe.

 

[pennpiper]

A1) This does not sound like something that needs stress analysis. Yes there is a rule of thumb to help determine if stress analysis is required. If NPS x Temp (F) > 1500 formal Stress analysis is not required

A2) 3/16" thermal expansion is not so great that it would warrant using guides or fixed the pipe shoes?

3) You should not require Teflon slide plates. You can rest the pipe shoe directly on steel pipe supports. Embed a steel piece in the concrete support.

 

[pennpiper]

OOPS,
If NPS x Temp (F) > 1500 formal Stress analysis is not required.

This should read:
If NPS x Temp (F) less than(<) 1500 formal Stress analysis is not required.

Sorry!

 

[NewMechanicalEngr]

Thanks pennpiper. I was on the fence with this and your guidance helped tremendously. I will go with the pipe shoes on steel as discussed.

Sincerely!!!!

 

[BigInch]

Or U bolts.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[XL83NL]

A good practice for helping you determining when to do stress analysis might be PIP (Piping Industry Practice) PNC 0004, "Piping Flexibility Analysis Criteria for ASME B31.3 Metallic Piping". It gives 2 (one for SS and one for CS) indices on when to do stress analysis, when formal, etc.
Very useful and small practice.

 

[BigInch]

I say U bolts, because I don't think I've ever seen shoes on such small line diameters.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[europipe]

Big, I think he should use shoes.
(insulated lines)

Greetings

 

[DSB123]

pennpiper,
Using your criteria if I have a 2"nb line and it is at 749 Deg F (398 Deg C) so NPS x Temp <1500 then no stress analysis required. What if the total length is say 500m and the pipe support guy is in-experienced and sticks guides at every support and the line is routed relatively straight??

 

[europipe]

DSB, When inexperienced guys are designing, anything can go wrong,
every designer knows there is only one stop or anchor in a staight piece of line. and guides where they belong.

 

[StevenHPerry]

The Size x Temperature criteria, as I would apply it, is a screening guideline for pipe stress. It does not relieve anyone of the requirement to use common sense (e.g. rotating equipment, or excessive movements at a tie in point).

Something else to look at is the "inherent flexibility" equation that appears in B31 codes. (B31.3 319.4.1(c)). If you can satisfy that requirement, you have some basis in code to not do stress analysis without relying on judgment based on previous experience (... or free advice from the internet peanut gallery).

- Steve Perry

http://www.linkedin.com/in/stevenhperry

This post is designed to provide accurate and authoritative information in regard to the subject matter covered. It is offered with the understanding that the author is not engaged in rendering engineering or other professional service. If you need help, get help, and PAY FOR IT.

 

[BigInch]

OK, I'm seeing the 450[&deg;] just now.

But .. little shoes.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[pennpiper]

DSB123,
The answer I gave about Pipe Stress Criteria is just a short form to answer the specific question.
When I teach Piping Designers and Stress Engineers (to work together) I have a longer answer. I also have an article published on pipingdesigners.con (

http://www.pipingdesigners.com/Training - Section 4.htm

) that deals with "What a Piping Designer needs to know about Pipe Stress."

Here is my longer Pipe Stress Criteria answer:
Pipe Stress Criteria #1

The question:
What are the criteria for which lines need formal stress calculation?”

My answer:
You will get lots of opinions on this one. Here is what I use and teach.

1. “The 1500 rule”
(from David Diehl, COADE)
a. If the line size (nominal pipe size) times (x) the temperature (degrees F) are below 1500 then the line "may" not need formal stress analysis.
Example 3”(x) 400 degrees (F) = 1200

b. If the line size (nominal pipe size) times (x) the temperature (degrees F) are above 1500 then the line "may" need formal stress analysis.
Example 4" (x) 400 degrees (F) = 1600

2. If the line (regardless of temperature) connects to a pump, a compressor or other alignment sensitive equipment then it should be submitted for formal stress analysis.

3. The "May" noted above relates to the application of good judgment when considering other factors such as metallurgy and the wall thicknesses above schedule 80.


Pipe Stress Criteria #2

The question:
What is the minimum recommended temperature or temperature range for which flexibility analysis should be done per ASME B31.3?

My answer:
I do not have a copy ASME B31.3 (Old or current 2009) so I can't cite any specifics therein. However I would be surprised if B31.3 would give a minimum temperature for the requirement of flexibility analysis. There are just too many variables.

Flexibility Analysis might come in a number of forms and should consider many situations.
Forms:
- Visual Review
- Informal Calculations
- Formal Analysis for excessive stress caused by common factors (Temperature induced expansion, Dead/Live Loads)
- Special Analysis (i.e.: Analog studies for reciprocating compressors, seismic studies, Sea-Force studies for Offshore Platforms)

Many engineering companies may set some minimum analysis criteria on a project by project basis but there is always the exception for even "That" project.
Exceptions may include:
- Very large diameter pipe
- Very heavy wall pipe
- Very thin wall pipe
- Exotic Alloy Pipe
- Attachment to strain sensitive equipment (Pumps, Glass lined vessels, etc.)
- Extremely low temperature (Cryogenic) systems
- Any pipe installed in Arctic locations
- All other lines below the stated minimum level on a job installed in a place like Oman or Kuwait

Even if the Code makes a recommendation on this subject each Piping Lead and the assigned Pipe Stress Engineer are responsible for the proper engineering and design of the project.

 

[DSB123]

pennpiper,
That's more of the answer I would have expected. The first "simple" screening criteria was much too broad and could have led someone up the garden path/in the wrong direction.

europipe,
I was trying to guard against these in-experienced people using a simple criteria. There are many in the engineering game now who have had the PDMS training, for example, and route/support lines with no consideration of temperature. At this present moment we have a line which has been "supported" by a pipe support "designer" who has used "U" bolts all along it's length with the line design temperature of 135 Deg C. There are sections of line "locked in" by the U-bolts.

 

[BigInch]

Thank God that it wasn't me.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[simplemath2]

You may use ASME B31.3 319.4.1 to rule out the stress analysis requirement. It may end up same as equation provided by pennpiper.
But when someone has to approve your design, it may helpful behind a code.