Structural consideration during pipe schedule selection - ASME B31.3 3

[Meck91]

I am trying to calculate pipe wall thickness of ASME B31.3 Carbon Steel NPS 2 & NPS 3 piping system. The design pressure of this system is very low (~ 110 Psig) & design temperature as 50 degree C. This results in very low value of tmin (i-e., pipe wall thickness required to withstand internal pressure only). For simplicity lets consider tmin as 1 mm and corrosion allowance as zero. On the basis of above calculation I can use NPS 2 Sch. 5 and same Sch. 5 for NPS 3 pipe. But i know that there will be issues with the weldability of this thickness of pipe, deflection/buckling might be other issues with the selection of this pipe schedule.

Now the question is, "How much i have to add in this tmin so that the selected pipe schedule will be structurally stable". Should i go to Sch. 10, Sch. 40? Any method to calculate the pipe wall thickness required for structural integrity ONLY, so that I can add both thickness (pressure thickness and structural thickness) and than select the next commercially available pipe schedule.

Would appreciate your feedback.

Thanks,

Meck91

 

[BigInch]

The standard guideline to avoid problems in shipping and handling metal pipe (steel) without putting on the kid gloves is a Diameter to Wall Thickness ratio of somewhere <= 100

If you decide to do a structural-type calculation, it is usually based on some realistic concentrated load hanging at the center of the pipe span, for example, or from some future maintenance load or accidential impact, perhaps 100 lbs, or 1000 lbs, or a ton, or 10 tons if it is a pipe adjacent to a boat landing on an offshore platform. Look at your client's design specs, or lacking that, anything you can reasonably justify. A maximum allowable deflection will often control, rather than an actual stress allowble.

Independent events are seldomly independent.

 

[C2it]

Big Inch is quite right. And don't forget that 300lb hairy arsed fitter climbing over your delicately designed pipes !

 

[LittleInch]

You might theoretically save a little on your pipe costs, but in reality no one has these small schedules available in C Stl so you will wait longer and it might actually cost you more to get it (get some quotes) and all the flanges etc machined to the same schedule. Also your pipe fitters will hit you with a spanner if you visit site as 1.3mm thick pipe will be a bitch to weld....

Unless you've got hundreds of metres and can protect it from excess spanning, people stepping on it / hanging off it, other accidental loads, I would go for schd 40 (STD) as a minimum as you will get lots of parts off the shelf and it has that indeterminable "bit more meat" that for a plant is what is actually required. Your no corrosion allowance sounds great from the inside perspective, but if you've ever been around a plant after 5 to 10 years, you'll see why a bit more meat is a good idea for external corrosion.

As BI says, many clients and contractors have standard pipe data sheets which limit the schedule no you can go down to precisely because there are many other issues to consider other than internal pressure containment for plant piping.

My motto: Learn something new every day

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

 

[gmoney21]

As mentioned above the limiting factor in your design will be pipe support spans with such a small wall thickness, the design may be sufficient but install of the piping very impractical.

As a standard on our site we use no less than sch 80 for all 2" and under CS piping as the cost of material, install and future maintenance for smaller wall thickness runs quite a bit more over the useful life of the piping. If using SS this no longer applies as material cost can be quite different with increasing wall thickness.

 

[MJCronin]

I agree with LittleInch....

First, I have never encountered a carbon steel piping system without a corrosion allowance. How can you ensure that there will be no interior or exterior corrosion through the design life of the system ? The typical CA for carbon steel piping is 0.0625 inches...... but could be more. What is your service ?

Second, You should consider that piping with a wall thickness thinner than Sched 40 is not suitable for threading.

Third, Schedule 5S carbon steel piping is hard to find and requires additional pipe supports due to its short span.

Fourth, Schedule 40 CS piping is available everywhere and can be threaded or socket welded

Fifth, Where is your piping line spec for this plant ?

 

[BigInch]

Actually there are many pipelines that do not use corrosion allowances. And corrosion continues to be is a great problem where corrosion allowances have been used on many other pipelines. Pinhole and under-the coating corrosion at disbonded coating locations has been known to eat straight thorough a 0.0625" corrosion allowance in a very short time. Corrosion is the largest cause of pipeline failures in the US. The obvious conclusion is that a corrosion allowance is not an effective, or economic method for corrosion control, to the point where IMO, it sounds good in practice, but isn't worth it's weight in #%&^*#%^@ when it comes to actual effectiveness.

Independent events are seldomly independent.

 

[LittleInch]

I agree that a corrosion allowance is not always useful, but my key point on the OP was that his 1.3mm of schd 5 wt would be very quickly eaten by any external corrosion. I must admit I don't like the long term internal CA used as corrosion doesn't occur in equal lots or 0.1 mm/year. An allowance for external corrosion is generally not worth it as it is not able to be reduced / controlled once it starts so in reality doesn't give you any real additional protection.

Recently I was involved with a pipeline system where some lunatic worked out the theoretical corrosion rate for CO2, divided the corrosion allowance by it and then decided to save a bit of OPEX by not injecting corrosion inhibitor for that number of years. Result - before the end of his non injection period they had a leak and a few years later had to replace the entire pipeline due to so many corrosion holes / damage.

Summary - Schd 5 is a huge risk unless it's inside a building, guarded from people standing / climbing on it, totally devoid of corrosion issues and can be laid on supports which already exist for something else. All highly unlikley and it's a bitch to buy and build so go for something thicker.

My motto: Learn something new every day

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

 

[curtis2004]

Meck91,

Use Sch40 as a minimum for 2" and 3" piping. Use Sch80 for threaded nipples.
There is no point of using thinner material for Carbon Steel piping since no real savings can be realized by doing so.

Curtis