You need to get the thread depth tables, so you know how much to subtract from the wall thickness of the pipe. Then there's the tolerance on the dies, the wall thickness and grade of pipe, what kind of fitting, corrosion/erosion allowance, etc. Check the "Piping Handbook" by Nayyar - it's got a section on this topic.

clp-

I'm not sure there are rating classes for threads similar to flange classes. Someone else on the forum will probably clear that up. I checked B.1.20.1 and found no ratings. I guess what I'm wondering is how threaded connections could be economical at even moderate pressures with even small diameter pipe. You kill so much of your pipe wall thickness by cutting the thread that you're putting in a lot thicker pipe than you need. Socket welded connections might be a much better choice for small pipe.

jt

For a guide see ASME Sec VIII Div1 UG-43(e),Sec I PG-39.5

As noted previously, the design pressure formula in B31.3, B31.8, etc. essentially reduces the WT for the thread depth.  B1.21.1 or just about any piping handbook will give you the thread dimensions.  (Note: pipe threads are tapered.)

Most engineers use Sch 80 pipe for 2" and under pipe so you will have plenty of WT normally for pressure.  The reason for using Sch 80 is that small diameter pipe is so flexible the heavier WT gives you some structural rigidity so the pipe will not sag excessively.  So the selection of Sch 80 is related to deflection and not pressure stress.

If your next question is distance between support points this issue has been discussed many times on this forum.  I always use the MSS standard.

Team Member: clp (Mechanical),TBP (Mechanical),deanc (Specifier/Regul),jte (Mechanical), 1969grad (Mechanical).


Piping Classification Specification engineer shall verified the calculation of rating.

ASME B16.11-1996
 
FORGED FITTINGS, SOCKET-WELDING AND THREADED


TABLE 2 CORRELATION OF FllTlNGS
CLASS WITH SCHEDULE NUMBER OR WALL
DESIGNATION OF PIPE FOR CALCULATION
OF RATINGS

Class
of Fitting Designation No    Type of Fitting
    Schedule
    Wall
Designation
2000    Threaded    80    XS
3000    Threaded    160    
6000    Threaded        xxs
2000    Socket-Welding    80    XS
3000    Socket-Welding    160    
6000    Socket-Welding        xxs
            

NOTE:
(1) This table is not intended to restrict the use of pipe of thinner or thicker wall with fittings. Pipe actually used may be thin-ner or thicker in nominal wall than that shown in Table 2.
When thinner pipe is used, its strength may govern the
rating. When thicker pipe is used (e.g., for mechanical
strength), the strength of the fitting governs the rating.


Leonard Stephen Thill
www.thill.biz

jte - There a numerous applications in which threaded pipe is preferable to welded. In many, if not most plants, hot work permits & fire watches are required. In locations requiring a fire watch, there is instantly a requirement for two people. Even a small job can easily turn into an all day adventure for a welder and fitter, by the time the paperwork, set-up and fire watch is finished. The same small scale job can often be accomplished in a couple of hours by one fitter with a couple of pipe wrenches.

1969grad - I have any number of piping books, but the great majority do not contain pipe thread depth tables. They routinely have all kinds of other info on threads, but curiously, only a couple of them include the information on how deep they're cut.

TBP (Mechanical)

I agree There a numerous applications in which threaded pipe is preferable to welded only if an apporved Piping Classification Specification is varified and calculation support the Piping Classification for specific Piping Class.


clp (Mechanical)
Please see additional detail in the Standard ASME B16.11-1996.

lst

Much of this is stated above, but to summarize.

Threaded joints have the same pressure rating as the threaded pipe.

For most applications, the pressure rating of the pipe is reduced because the thread depth is considered a mechanical allowance, which reduces the wall thickness available for pressure.  An exception is the high pressure piping rules in Chapter IX of ASME B31.3, in which under certain circumstances, thread depth does not need to be subtracted.

Fittings, such as ASME B16.11 threaded fittings, are given the pressure rating of matching pipe.  B16.11 states what schedule pipe is matching for which class of fitting.

If you have the book, Process Piping, The Complete Guide to ASME B31.3, thread depth (per ANSI/ASME B1.20.1) and B16.11 matching pipe can be found in Appendix I.

cb4 (Mechanical)

Dr. Charles Becht IV, Yes, I do have your book: Process Piping, The Complete Guide to ASME B31.3.

Also, Standard ASME B16.11-1996

Lst

Isthill - it needs to be remembered, though, that most general industrial plants (which constitute the vast majority of plants) operate at relatively low pressures and temperatures. It's unusual to see plants in this catagory with boilers rated for over 300 PSIG, and most are for 150 PSIG. Most of these operations are filled with 150 or 300# malleable fittings. Depending upon the grade and wall thickness of the pipe used, the fitting can often be the limiting factor.

TBP (Mechanical)

NOTE

Class 300 with CA=.125
Nominal Pipe Threaded Pipe Inches
1/2"  =  XXS

TBP (Mechanical)

NOTE

Class 300 with CA=.125
Nominal Pipe Threaded Pipe Inches
1/2"     =  XXS   Class 6000
3/4"- 2  =  160   Class 3000

varified and calculation support the Piping Classification for specific Piping Class.

Isthill - I have to ask - what kind of piping in general industry is it necessary to take a 1/8" corrosion/erosion allowance?

Hi TBP (Mechanical)

TBP (Mechanical)what kind of piping in general industry is it necessary to take a 1/8" corrosion/erosion allowance?

Refinery & offshore



lst

Refinery & offshore isn't what I'd really term "general industry". I would use that term to describe food processing, auto assembly/parts, general manufacturing like corrugated box plants, etc. For plants like that, 1/8" corrosion allowance is very large - double the maximum I've ever seen taken. If you have to take a corrosion allowance in plants that fall under my definition of general industry that takes you into pipe heavier than sch 80 for 2" and under screwed, the wrong material has been selected for the service, and/or there are serious water treatment/chemical problems.

To all,

"cbt" has answered the question with both precision and authority. There is no "pressure rating" for threaded fittings, and the ASME B31.1/B31.3 calulated minimum piping wall thickness must account for threading...

I am conviced that the purchase of the book he suggested would eliminate at least half of all the questions asked on this and other forums....

Based on some of the comments noted above and in other forums, I also suspect that "lsthill" is a robot........

MJC

  

MJCronin (Mechanical) and All Team Members


Mr. MJCronin (Mechanical) I total agree with your in regards to (cb4) Dr. Charles Becht IV, book: Process Piping. Also, Having the ASME CODE on CD is a grate Engineer Tool's

Note: Process Piping is use at the University Of Houston for the new Piping Engineer in the Houston work force.


Mr. MJCronin, NO! Leonard Stephen Thill "lsthill" is not a robot........Just an Old Piping Engineer.

www.thill.biz

A general observation.

Many people who very helpfully respond to posts regarding piping have backgrounds that are typically central generating stations, petro-chem, refinery, offshore platforms, etc. Plants falling into this catagory are the "big leagues" of piping. Many, if not most of the people posing the questions and/or reading the responses, are from general industrial backgrounds, and have never, and will likely never, see a forged steel screwed pipe fitting. These plants are full of 150 & 300 malleable, 125 & 250 cast iron fittings, and sch 40 or 80 pipe thats A53B ERW at best, and very commonly A53F (which I understand has largely gone out of production - much rejoicing...). These plants don't have specialized piping engineering departments, or even AN engineering department. More and more, they don't even have an engineer dedicated full time to their plant. Lots of companies have an engineer covering projects spanning all disciplines looking after 2 or 3 plants. Most of these plants no longer have "pipe shops", either. Just understaffed maintenance shops with electricians and millwrights, and the line between them gets more and more blurred as well. For most applications in general industry, the fittings listed in B16.11 are serious overkill, as is sch 160 pipe.

I've worked in plants ranging from a little low-pressure steam heating plant to a 2000 MW central station, and lots in between. What makes perfect sense in one particular type of plant, is often just plain nuts for monetary or code/technical reasons, in another type of plant.

Whether you're somebody who does piping as your full-time gig in the big plants, or someone who's range of duties include pipe in general industry, and hasn't got the budget to buy or time to plow through a series of ASME code books, the "Piping Handbook" by Nayyar is probably the best single engineering book on piping I know of. Put a copy on your shelf. The answer to most of your piping questions are in there somewhere.

TBP-

Agreed. It is always useful to understand the perspective of the people you are conversing with. With that in mind I'd encourage everyone to go to the top of this page and click on "Personal Profile" and fill out the "Tell Other Members About Yourself" section.

jt

MJ Cronin:

I suspected the same thing about "Isthill"  I still think he's a robot or an IBM HAL 9000!

MJ Cronin AND jay165,
                      Think you are both spot on. Perhaps he's an "old" abacus.