Compressed Air Piping

[olmedo]

a)A piping design guideline that I read specifies that compressed air pipings shall be supported independent of other pipings. Can I know what is the rationale for this rule?

b) Why is it that pipe schedule 80 are specified for compressed air pipes less than 2 inch diameter while larger pipes are schedule 40 only?

 

[Montemayor]

olmedo:

Compressed air pipe (as well as all other piping) should always be supported independently of other pipes - i.e., not resting on, hanging from, welded or joined to other pipe. The reason is that you can't ensure the stability or stress integrity of that pipe if it is not independent. For example, what if an adjacent pipe is used as a support for the compressed air pipe and that adjacent pipe undergoes water hammer or vibration. The compressed air pipe would be subjected to those unexpected stresses. It's a bad situation where you can't account for everything you designed for that compressed air pipe. This is not a rule, it's just common sense.

I don't know about your pipe specs and why they call for schedule 80 for pipe < 2&quot; diameter, but normally plant pipe specs take into consideration the quality of the compressed air (wet & corrosive vs. dry), mechanical integrity, and the method of installation - especially if you are using pipe racks with 10 to 15 feet spacings between racks. You'll find that Sch 40 pipe < 2&quot; will deflect and sag considerably when supported at these distances. Sch 80 pipe is more robust due to the added metal and takes more abuse & use.

I hope this explains some of the reasons.


Art Montemayor
Spring, TX

 

[olmedo]

Thanks Montemayor, shall it mean also that compressed air pipes must not be placed in the same rack with other pipes.

 

[Montemayor]

Olmedo:

Not really.

Pipe racks are (or should be) designed to support a variety of pipes or the sake of organization, safety, maintenance, and economy of scale - i.e., you have ONE pipe rack system, not a pipe rack for every service. This is normally done in process plants; however, there are cases where special racks are used - like in the instances of storage areas, special hazardous substances (such as Fluorine, Bromine, etc.) which are considered very special cases for concern.

Some pipe racks also carry electrical conduit and trays - much to my dislike. Normally they are designed for strategic reasons like easy access for maintenance and block valve locations as well as positive slopes for easy and safe drainage.


Art Montemayor
Spring, TX

 

[bjones1]

olmedo,

In answering your second question about Sch 80 pipe being used on <2&quot; pipe is mainly due to the increased pipe wall stress on small diameter pipes (as well as corrosion, etc.) By definition &quot;stress&quot; is the ratio of Force to Area, i.e. S=F/A - therefore one can see that the stress increases as the inside area of the pipe decreases. In my experience with Power & Chemical piping design, it is not unusal to spec heavy wall pipe in small pipe diameter applications.

Of course pipe wall calculations should be performed according to current codes.

bjones1

 

[TBP]

Pipe 2&quot; and under is very often threaded. (Over 2&quot; is normally welded or Vic.) Cut a thread in sch 40 pipe, and there isn't much wall left. For plant compressed air systems in the usual 100 - 150 PSIG range, sch 40 for 2&quot; and under is usually OK. However, to simplify in-plant rules and standards, many operations just have a blanket statement that &quot;all steel pipe 2&quot; and under will be sch 80, regardless of the service&quot;. This reduces inventory costs, and makes it hard to use the wrong pipe in a given application. It also provides much more mechanical strength for typical levels of abuse plant piping suffers, such as being hit by fork lifts, chain falls being rigged off piping to work on pumps, etc.

 

[olmedo]

Thanks bjones1, But I think the formula s=pd/2t will be more appropriate than S = F/A in this situation.

 

[bjones1]

Olmedo,

I agree - I was making a general statement about how stress is defined in simple terms. However, TBP's reply is more in line with a real life reason for the Sch. 80 pipe being used in this application...

Good point TBP...