Condition/Stutural analysis of Cast Iron Pipe 1

[rnjroles]

Can anyone point me in the right direction for a standard practice for evaluating the stability of an existing cast iron water main.

I am to provide analysis for a 40 year old main under a section of highway that will be widened. Need to know that construction activities as well as proposed conditions will not shorten the service live of the existing pipe to any great extent.

I have found a good deal of information on Ductile iron but not much on cast iron. The pipe was manufactured and laid in 1968.

Thanks

 

[PEDARRIN2]

Go to

http://www.cispi.org/

This will hopefully help.

 

[bimr]

"The most significant event in recent pipe history was the development of ductile iron in 1948. In a relatively short time, ductile iron pipe gained widespread acceptance. The specifying of ductile iron increased to where all iron piping installed for the past several years has been ductile, and gray iron pressure pipe is no longer manufactured. Improvements in casting methods, processing, and metallurgical technology are continually producing superior-quality pipe, capable of satisfying the specific piping needs of the 20th century and beyond."

http://www.acipco.com/international/pipeandfittings/ductileiron/history.cfm

"Since cast iron pipe often lasts for centuries, changing requirements and improved manufacturing techniques have resulted in some piping materials becoming obsolete. Although no longer manufactured, many piping materials continue in service. In addition, AMERICAN has continually made joint and configuration improvements over the years. In doing this, some joints and materials have been eliminated."

http://www.acipco.com/adip/engineeringlibrary/historicaldata.cfm

"This data provides limited information pertaining to some of our obsolete materials."

http://www.acipco.com/adip/products/Sect12.pdf

 

[rconner]

You don't mention details of what size etc. of line is involved, and whether or not the existing pipe is installed in some sort of casing under this highway, and these tidbits along with local codes etc. may be perhaps be helpful in this particular analysis. In any case, you may be interested also in the suggestions in

http://www.dipra.org/faq/corrosion.cfm#FAQ9

(and perhaps also the standards and references related to such rigid gray cast iron pipe bedding and design, manufacture, testing/quality control, and installation etc. mentioned in past threads at e.g.

http://eng-tips.com/viewthread.cfm?qid=167007

&

http://eng-tips.com/viewthread.cfm?qid=161752

. Beyond this, I think many case studies by many entities can be found with a good search engine. Gray cast iron pipe could have been specified by Owners and supplied by manufacturers in the year 1968.

 

[rnjroles]

Thanks for thr responses - the pipe is 12" C.I. class 26 which has an o.d. of 13.2 and wall thickness of 0.65 inches.

I have a reference that is based on ASA Specification A21.1 (AWWA H1) which provides tables for selecting thickness class given a known depth of cover and laying condition. If I use this (after reducing the wall thickness by a corrosion factor times the number of years in service) I find that the pipe (at least the part that is left) is not suitable. This differes from the results I get when I apply the equations provided in AWWA Spec C150/A21.50-02 Thickness Design of Ductile Iron Pipe (Using a modulus of elasticity of 14.5 x 10^6 for C.I.). Using this later approach I find that the pipe is good with a safety factor of ten.

Which do I use?

 

[rconner]

You are in my opinion using a reasonable design reference standard in A21.1 (or for that matter tables I think based on same in A21.6 or A21.8 etc.), and I would NOT use the A21.50 ductile iron pipe standard.
However, I would not necessarily trust a conclusion of "unsuitable" based on "reducing the wall thickness by a corrosion factor times the number of years in service" for cast iron pipes. e.g. I personally picked up a freshly broken shard/piece (where I could see the fresh metal fracture all around) of an old buried 6" gray cast iron pipeline that was being burst in an upsizing operation to 8" ductile iron several years ago, and while I could see some small casting inclusions from very old melting processes, I could observe no depth of corrosion penetration whatsoever on the entire othberwise quite clean fracture faces. I was told by the Engineer the 6" pipeline was at that time 110 years old! (and I still have that coupon in my desk as a souvenir).

 

[greggy]

We perform structural evaluation of buried cast iron lines here at my water utility by performing a tap and examining the "cookie" that comes out. Preferrably we tap for a 6 inch branch and install a run and a hydrant while we are at it. Otherwise, tap at least a 4 inch and examine the metal coupon that comes out for thickness, and condition and solidity of the metal. If the tap is not needed, just cover with a repair sleeve.

 

[bnd1]

Dependng on the size of the main I'd say this migh be a good time to replace it! Have not heard of any records or data being kept on 40yr old CI pipes. DI much the same as CI bt of course much more ductile so the same parameters will not apply.

 

[rconner]

A common effectiveness of iron pipes has been proven far beyond the 40 years time frame mentioned in this thread. [E.g. you may be interested in the information contained in

http://www.dipra.org/pdf/CIPCC-CIPSC.pdf

and

http://www.dipra.org/pdf/ddm.pdf.

]

 

[kwdwaterengineer]

It is the age old question that arises when a major road project is planned;keep the main in service or replace it.

I happen to have a 1967 edition of A21.1 (AWWA H1) The corrosion allowance is 0.08 in. I assume that is what you were using. Table 1-1, 12 inch pipe at 150 psi, at an 8 foot cover on the laying condition F (the most severe) requires a thickness of 0.58 inches. Will you be exceeding thes conditions after the road is built? Are the soils corrosive in the area?

I am not too familar with A21.1 since it is an old method, but reading into it as I make this post, one also has to subtract another 0.08 inches due to casting tolerances. That puts you at the depth and condition for a 0.49 inch main. I could not find a safety factor from built into A21.1., but would assume that it is at least 2:1 if not 3:1 by looking at the ultimate burst values for each of the corresponding pressure ratings.

It is easy to simply replace the pipe if you do not have to pay for it. Depending on the situation, the main may not be fully depricated financially, this may be a factor.

The advantages with the main as is if you are comfortable with the installation is: it is most likely cement lined, it is the same outside diameter as new ductile iron main allowing for off the shelf fittings, it used either tyton on SBT rubber gaskets when installed, not leadite. Leadite, the lead substitute is causing numerous problems years later.

The disadvantages are: it is cast iron, it could have been droped when installed causing a microcrack, and not burst for another 10+ years when it is more than 50 years old, you may not know how it was layed and backfilled, most mains I have seen break around this vintage are circumfrential splits, likely caused by poor installation methods. Also you may not know how others will treat the pipe, I have seen large compation equipment at full amplitude pound on subgrade, then the construction crew wonders why the cast iron main only a few feet below the subgrade breaks.

Good Luck making your evaluation.

 

[rnjroles]

kwdwaterengineer

"I happen to have a 1967 edition of A21.1 (AWWA H1) The corrosion allowance is 0.08 in. I assume that is what you were using. Table 1-1, 12 inch pipe at 150 psi, at an 8 foot cover on the laying condition F (the most severe) requires a thickness of 0.58 inches. Will you be exceeding these conditions after the road is built? Are the soils corrosive in the area?"

Do you happen to have a thickness listed for greater cover? Say 10 or 12 feet? I'm looking at a table I found in a text book which is listed as table 6.4 of ASA 21.2-1952 (C.I. pipe Centrifugally cast in metal molds for water and other liquids) and for laying condition A (flat bottom trench w/ loose backfill) 8 ft cover I get 0.52. I don't have a reference for laying condition F - you said worse case so I'm thinking it could be a flat trench with the pipe on blocks.

To clarify - I know the laying conditions are "A", I know that the pipe has a wall thickness of 0.65" and I know that the pipe is centrifugal cast in metal molds and is identified as class 26.

What I don't know is some of the logic/mathematics behind the tables listed in ASA 21.

Thanks for the help.

 

[kwdwaterengineer]

I have up loaded table 1-1 for 12 inch pipe for 18/40 iron strenth and table 1-2 for 21/45 strenth. all being said, unless you had other info, I would assume the 18/40.

I have to admit, I am educating myself on a standard that is old as I am. The more I have read I have realized that the corrosion allowance is included with the thickness design in Table 1-1, so one does not need to add in the additonal 0.08 for casting tolerance and 0.08 for the corrosion tollerance to the allowable thickness.

I haven't taken the time to scan all 45 pages of the standard. I've attached the narrative to the tables. I hope this will help. Let me know if you need the remaining portions of the standard.

Take Care

 

[kwdwaterengineer]

Apparntly I can only load one file at a time.

Here is the Table 1-1 for 12 inch 18/40

 

[kwdwaterengineer]

Here is Table 1-2 for the 12 inch 21/45. There is a table 1-3, but it is for cast iron gas piping.