Sch 80 Vs Sch 40 with small diameters

[cgmgsa]

Dear all,

All pipes with small diameter (<2") that I´ve used were always Sch80 despite that their process conditions allowed in some cases Sch 40 (cheaper option).

Reasons that I´ve always heard to justify this are on one hand that welded union is less problematic with Sch 80, and on the other hand that availability of the stock in case of Sch 40 may be also a problem in case of a big stock.

The fact is that I´ve followed this common practices all times assuming that they were right, but I wonder if the use of Sch 40 (cheaper option) may be interesting in some cases, asumming of course that it is technically possible to use a Sch 40.

Any clarification about this topic?

Thank you in advance for your help.

Kind regards.

 

[SNORGY]

You *can* use SCH 40 if you specify that in-line threads are not allowed. Otherwise, by the time you add up the potential effects of 12.5% mill tolerance, 1.6 mm corrosion allowance and 1.753 mm thread depth, you are left with a pipe that's good to about 300 kPag (50 psig) according to Code (assuming B31.3 and A-106-B).

Based on my experience, the overwhelmingly common industry practice is to specify minimum SCH 80 for 2" NPS and smaller.

 

[SNORGY]

Make "300 kPag LESS THAN 50 psig". Terribly soft conversion on my part. My apologies.

 

[moltenmetal]

You can weld and thread sch40 perfectly well, as long as you take the depth of threading into account in your wall thickness calculation. You can weld sch10S perfectly well also. The only place a competent welder will have trouble (of a sort that heavier pipe might reduce) is if you try to fill the entire weld bevel of a 3000# o-let fitting for a branch- then you'll end up with a banana! And yes, in stainless steels, using sch80S is going to cost more- significantly more- than using sch10S or sch40S, even at sizes of 2" and smaller.

Sch80 is no more available than sch40 in carbon steel, and no more available than sch40S or sch10S in common stainless grades. And of course, the normal form for sch10S is welded rather than seamless.

Some firms like to simplify their internal maintenance inventory by standardizing on the heaviest schedule. They value the additional mechanical robustness against corrosion and abuse offered by the heavier pipe as well. They of course don't use 150# stainless fittings either, favouring 3000# or in some cases, even butt-welding everything. Is that good value for money? Doing so definitely costs more, and whether or not that additional cost offers useful, meaningful value is a matter of debate to say the least. For a substantial new build with large amounts of pipe 2" and below, applying your maintenance stockroom's standardization to the specs used for that big new project is definitely going to represent a significant amount of additional cost with a benefit which is questionable at best.

I would agree with you that for welded carbon steel services, and for threaded carbon steel without internal coating, sch80 makes an insignificant cost difference relative to using sch40 and provides a much longer service life by providing more corrosion allowance. But in galvanized 150# threaded services, using sch80 pipe is a waste in my opinion.

 

[zdas04]

I can live with the Sched 80 in 2-inch and smaller (I think the derates above are pretty generous, but I've given up fighting it). I was recently in a HazOp where an Engineer demanded Sched 160 in 1-inch threaded. I asked "why?" and she got up and returned with the company standards book. The standard called for Sched 80 in threaded 1-inch, but had a footnote that the values in the table were minimum and heavier wall pipe also satisfied the requirements. She took the footnote as requiring a schedule increase for each pipe size reduction. Everyone else in the HazOp was fine with the Sched 80. You really have to watch out for folks adding steel for the sake of adding steel.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[moltenmetal]

David, it doesn't matter what schedule you pick- some weak-kneed idiot is going to try to go one heavier out of a misguided notion of "safety". Overdesign of that sort is rife in our industry in my experience- every pair of hands the design passes through wants to chain on an additional prudence factor. I've seen many instances where those chained safety factors actually made the result unsafe.

 

[zdas04]

LOL. Me too. Too often.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat