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Customers from all kinds of industries need valves of all varieties for their high-demand processes. Valves are available in a huge number of materials for any application imaginable. A few of these materials are PVC, carbon steel, stainless steel, cast iron, and ductile iron. In this post we will focus on two of those materials: ductile iron and cast iron. The argument of "ductile vs. cast iron valves" has been going on for decades, as not everyone knows the differences.

Luckily, comparing valves made of ductile iron and cast iron is simple, as these materials have just a few major differences. We will outline properties of both materials and then go through the pros and cons of each. By the end of this blog post, you will be an iron expert!

Cast Iron Valves Specifications

Cast iron (also known as gray iron) is a type of metal that has been in use for hundreds of years. It is an alloy made from 96%-98% iron, 2%-4% carbon, and small amounts of silicone. It has impressive temperature tolerance, with some cast iron capable of handling temperatures over 2100F (1150C). When it comes to pressure, cast iron's strength depends on its pressure class. The two most common are class 125 and class 250. At less extreme temperatures, class 125 cast iron flange is rated for pressures between 150 and 200 psi. Class 250 cast iron is a bit tougher, with pressure ratings from 300 to 500 psi. These pressures can vary by end type.

Cast iron is strong and will usually go undamaged even after going through intense vibrations. The main drawback of cast iron valves is that they are not very ductile at all. Virtually any bending will cause cast iron to crack and become useless. One great quality of cast iron is that it is not very expensive. As far as metals go, cast iron is usually the most economical option, which makes it a solid alternative for people on a budget.

Ductile Valves Specifications

The second material we're looking at is ductile iron. Ductile iron is a more modern iron alloy that is made with nodule-shaped graphite. This gives the material excellent ductility, so it will not necessarily break when bent. The temperature limit is a bit lower than cast iron, but it is still quite high at 1350F (730C). When it comes to pressure, ductile iron valves also use pressure classes: 150 and 300. At standard outdoor temperatures, class 150 ductile iron keeps a seal up to 250 psi. Class 300 can stand up to pressures as high as 640 psi.

Ductile iron has excellent corrosion resistance, tensile strength, and yield strength. Unlike cast iron, ductile iron does not break when it is bent, so it is more suited for high demand applications. Ductile iron is a strong and reliable material for pipes, fittings, and valves. One drawback is price. Ductile iron is typically more expensive than cast iron due to its more complicated chemical makeup. If your application requires a tougher material, however, you may need to spend a little more.

Ductile vs. Cast Iron Valves

Now that we've looked at both options individually, we will compare and contrast ductile and cast iron valves. Hopefully, this will help you decide which is best for you! We will compare these materials on durability, ease of installation, and price, so you can learn the most important details. For some more technical differences, check out this helpful article.

Durability

• Cast Iron: Cast iron is a strong material that can handle higher pressures than just about any plastic. When rated pressure of a cast iron valve or pipe is exceeded however, it can bend and crack. Cast iron has virtually zero ductility, meaning it is rigid and will crack if bent. Cast iron will also rust over time, so regular maintenance must be done to keep your valve clear of corrosion.
• Ductile Iron: Ductile iron also has impressive structural integrity. It can handle high pressures and temperatures without flinching. Thanks to its ductile nature,
  this material is also less likely to crack when bent. Ductile iron will also rust over time, but not as easily as cast iron.

Ease of Installation

• Cast Iron: This material gives the user many options when it comes to installation. Cast iron valves are typically bolted using flanges. Welding is difficult, but not impossible. Our cast iron valves are available with flanged ends, as this is a great method for high or low pressure applications.
• Ductile Iron: Ductile iron, because of its structural similarities to cast iron, gives the user many of the same installation options. Welding is not recommended,
  but bolting with the use of flanges is still the best option for most applications.

Price

• Cast Iron: Cast iron valves, due to their simpler manufacturing process, will save you a significant amount money compared to ductile iron valves. If you do not need the added benefits that ductile iron provides, cast iron is the smart move.
• Ductile Iron: Ductile iron is the more expensive option, but for good reason. It has properties that give it clear advantages over cast iron in many situations.

There you have it! Now you know that this is not an "ductile vs. cast iron valves" kind of issue. One is not always better than the other. It all depends on what you are using it for!

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PVC vs Ductile Iron for water mains?

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PVC vs Ductile Iron for water mains?

PVC vs Ductile Iron for water mains?

Pump2005

(Civil/Environmental)

(OP)

28 May 14 21:29

I work for a water company and the town engineers have decided to replace a percentage of DI mains with PVC through a grant program. Since we don't currently have any PVC mains, what are the pros and cons of going with PVC. I just don't see it as strong enough and easily damaged during excavation and almost impossible to trace and can hydrants be connected? Thanks in advance. Rob

RE: PVC vs Ductile Iron for water mains?

bimr

(Civil/Environmental)

29 May 14 15:20

Other than what you have stated, it may be less expensive.

RE: PVC vs Ductile Iron for water mains?

cvg

(Civil/Environmental)

29 May 14 16:12

long term O&M costs may not make it less expensive.

however,
hydrants can be connected to pvc lines
locating pvc lines is relatively simple, but you should be installing tracer wires and warning tape in your trench. with the traceable locator tape, not a big deal
it is certainly strong enough to be used for municipal water distribution service but maybe not on major transmission lines

RE: PVC vs Ductile Iron for water mains?

rconner

(Civil/Environmental)

3 Jun 14 00:04

fyi http://www.nbcmatoday.org/PDFs/pvcdoc.pdf

RE: PVC vs Ductile Iron for water mains?

jgailla

(Geotechnical)

3 Jun 14 15:35

I'd like to know who commissioned that report, rconner.

RE: PVC vs Ductile Iron for water mains?

bimr

(Civil/Environmental)

3 Jun 14 17:07

The report just identifies the reason that some municipalities use PVC and others use ductile iron. It is a personal preference, the same as rconner's opinion.

No one should expect the report to be definitive and pass a peer review as there are too many odd assumptions.

There are also errors in the report. Plastic pipe was actually invented during WWII by the Germans because it enabled the piping system to be repaired quickly after it was destroyed by the strategic bombing.

Oddly enough despite all of the assumptions, the report actually supports the use of PVC pipe because it states that PVC pipe is 4.7% less expensive than ductile iron pipe. That may not seem like much money for a small project, but 4.7% adds up quickly.

Some have estimated that the cost to repair and replace U.S. water and sewer pipes is as high as $660 billion. 4.7% of that is $31 Billion which is a substantial cost difference.

It reminded me of the story about automobile daylight running lights. Each automobile light may be only 55 watts or so and DRL's sound like a good idea. But when you add up the cost for 55 watts times 254 million cars in the U.S., it adds up to 27,000 megawatts, which is a tremendous waste of fuel.

RE: PVC vs Ductile Iron for water mains?

IronDogg

(Civil/Environmental)

5 Jun 14 19:26

Cool thread! I find it interesting as our town is on a semi aggressive replacement program to get rid of all our cast iron and ductile iron mains. We use only C900/905 PVC DR-18 for all repairs and capital replacements and new subdivision installs. The only exception are crosses. Apparently PVC crosses are not yet adequate to withstand all the forces over time, so we still use cast iron crosses with sacrificial anodes attached whenever a cross is needed. We have been using PVC water mains since about 1981 and have not had any leaks on PVC that were not due to original installation error of some kind (eg. over insertion, dirty joint, etc). Advantages of PVC would include less expensive to purchase, less expensive to install; it's much lighter and easier to install, does not corrode, is flexible, can come with internal restraints (i.e Terra Brute, Cobra-lok, bulldog, et al) and is convenient for directional drilling, easy to tap, blah, blah... The disadvantage to PVC would be that you will no longer be invited to the iron pipe parties anymore (probably not the PCCP parties either)...
It might make a difference, so I will mention that we have a minimum bury of 9 feet.

RE: PVC vs Ductile Iron for water mains?

bimr

(Civil/Environmental)

5 Jun 14 23:05

PVC Pipe Association (PVCPA) staff will present information on a 20-year-old AWWARF research study titled “Evaluation of Polyvinyl Chloride (PVC) Pipe Performance.” The study includes the following:
• Survey of utilities, consulting firms, and government agencies
• Results of testing of excavated pipe that had been in service

According to the “Discussion” chapter of the report, the objectives of the study were:
• To perform a comprehensive evaluation of the use and performance characteristics (including performance limits) of PVC water pipe
• To conduct the necessary research and analysis to resolve problems and concerns raised by the first objective
• To report results of analyses along with conclusions and recommendations

This half-hour webinar will address:
•Survey planning
•Survey data for various pipe products
•Survey data for PVC pipe
•Results of laboratory tests on excavated PVC pipe
•What 1994 results mean for 2014


Date: Wednesday, June 25, 2014
Time: 10:00 a.m. – 10:30 a.m. Central Time
PDHs: 0.5
Price: $49.00

RE: PVC vs Ductile Iron for water mains?

rconner

(Civil/Environmental)

6 Jun 14 17:29
In this regard, and while I am otherwise rather thick-skinned, I furthermore do feel that saying selection of pipe material is simply a matter of "personal preference" is at best a gross over-simplification and at worst an affront or insult to those utility folks and consulting Engineers in many countries (and in ours in the USA who frankly have done the best job of supplying the best quality water to the most folks and at the lowest real cost of anywhere in the world, and for a very long time.) While no doubt many have their favorites for piping based on their experience/level and other factors as for any capital etc. asset, a whole lot goes into any arguably rigorous selection process beyond this. While I guess it is true all pipes do indeed have at least a similarly sized hole in both ends, beyond that similarity the most common pipe materials that have been around for a long time are really apples and oranges. A whole lot of factors are involved in achievement of a successful pipe system and life cycle (the cheapest is certainly not always the best), and the linked evaluation touches on at least many of them.
As to historical "error", by multiple accounts I've heard ballyhooed from the pvc pipe industry pvc pipe was actually first produced in Germany in the 1930's, and before the (bombing etc.) of WWII. While I have seen in the ditty at
As to the post from I see a newcomer to these forums suggesting a utility apparently wants to "semi-aggressively" pursue replacement of all iron piping in their system. I hope for the benefit of tax/rate payers they proceed cautiously, While there have been some problems with some of huge quantities of particularly relatively brittle and often also unlined gray cast iron piping out there, iron piping has overall been providing (even with the technology of the day) overall pretty good service for hundreds of years, and in some cases continuously! On the other hand, I am aware of a good many relatively young pvc and other plastic pipelines that have been replaced in their veritable infancy over the last 40 years. I suspect those folks that originally bought those pvc lines also thought highly of the promise of the new material. It seems funny to me however that when folks are promoting plastic pipes, they want to exclude from consideration multiple categories of breaks or leaks from considerations of pipe materials (e.g. "those due to original installation error of some kind ("eg. over insertion, dirty joint, etc", tapping explosions blamed on procedures or tools, third-party damages etc. ) In my opinion, and while I agree with as throughrecordsas possible, all these things and others should be "on the table" as differing pipe materials are considered, as when such problems occur it makes little or no difference to the tax/rate payer and/or other inconvenienced public who/what caused same.
I would like to welcome "IronDogg" to these forums. I believe the experiences you presented are indeed atypical (by any chance you are located e.g. in Canada/Alberta? or the very far Northern United States, as I think I have heard some equally strange report from that direction for many years?)

[and p.s. to bimr I have a family member who has a 2000 model Toyota Corolla with daylight running lights/DRLs. I guess I have to admit I kinda liked this really safety feature myself in at least some driving conditions, as it is hard to put a price on safety and security. While I have thought this could well wear out bulbs quicker, I never thought about the aggregate power consumption; however, while I guess a cheaper-type car could have been purchased many years ago it still gets more than 30 mpg and has been quite reliable for now approaching 300,000 miles. Everyone have a good weekend ;>)]

I discovered the linked file I provided in a Google search some time ago when I was looking for comparisons of pipe materials, and I supplied same as I felt it was responding to what the OP requested (and it appeared to enforce to enforce prior good response of others). This is clearly a report from a consulting engineer (whose firm/name is there) to a municipal authority (per URL also there) for a pipeline project. While I frankly do not even know what pipe was selected for this project, but I guess I wouldn't be a bit surprised if responsible selectors chose the one that had the higher installed cost.In this regard, and while I am otherwise rather thick-skinned, I furthermore do feel that saying selection of pipe material is simply a matter of "personal preference" is at best a gross over-simplification and at worst an affront or insult to those utility folks and consulting Engineers in many countries (and in ours in the USA who frankly have done the best job of supplying the best quality water to the most folks and at the lowest real cost of anywhere in the world, and for a very long time.) While no doubt many have their favorites for piping based on their experience/level and other factors as for any capital etc. asset, a whole lot goes into any arguably rigorous selection process beyond this. While I guess it is true all pipes do indeed have at least a similarly sized hole in both ends, beyond that similarity the most common pipe materials that have been around for a long time are really apples and oranges. A whole lot of factors are involved in achievement of a successful pipe system and life cycle (the cheapest is certainly not always the best), and the linked evaluation touches on at least many of them.As to historical "error", by multiple accounts I've heard ballyhooed from the pvc pipe industry pvc pipe was actually first produced in Germany in the 1930's, and before the (bombing etc.) of WWII. While I have seen in the ditty at http://www.sewerhistory.org/articles/compon/pdfs/p... that "damage" to some of the 1930's lines (I thought when I read this to explain why many old pipe/line testimonials were hard to come by?) was in fact blamed on the subsequent WWII, while I guess I'm not surprised this thread is the first I have heard of the claim that it was invented because of that bombing! What might be most interesting/revealing is how much pvc pipe (e.g. percentage relative to other pipe materials) was selected by the inventing country with most long-term experience in the decades that followed the 1930's and 1940's??As to the post from I see a newcomer to these forums suggesting a utility apparently wants to "semi-aggressively" pursue replacement of all iron piping in their system. I hope for the benefit of tax/rate payers they proceed cautiously, While there have been some problems with some of huge quantities of particularly relatively brittle and often also unlined gray cast iron piping out there, iron piping has overall been providing (even with the technology of the day) overall pretty good service for hundreds of years, and in some cases continuously! On the other hand, I am aware of a good many relatively young pvc and other plastic pipelines that have been replaced in their veritable infancy over the last 40 years. I suspect those folks that originally bought those pvc lines also thought highly of the promise of the new material. It seems funny to me however that when folks are promoting plastic pipes, they want to exclude from consideration multiple categories of breaks or leaks from considerations of pipe materials (e.g. "those due to original installation error of some kind ("eg. over insertion, dirty joint, etc", tapping explosions blamed on procedures or tools, third-party damages etc. ) In my opinion, and while I agree with as throughrecordsas possible, all these things and others should be "on the table" as differing pipe materials are considered, as when such problems occur it makes little or no difference to the tax/rate payer and/or other inconvenienced public who/what caused same.I would like to welcome "IronDogg" to these forums. I believe the experiences you presented are indeed atypical (by any chance you are located e.g. in Canada/Alberta? or the very far Northern United States, as I think I have heard some equally strange report from that direction for many years?)[and p.s. to bimr I have a family member who has a 2000 model Toyota Corolla with daylight running lights/DRLs. I guess I have to admit I kinda liked this really safety feature myself in at least some driving conditions, as it is hard to put a price on safety and security. While I have thought this could well wear out bulbs quicker, I never thought about the aggregate power consumption; however, while I guess a cheaper-type car could have been purchased many years ago it still gets more than 30 mpg and has been quite reliable for now approaching 300,000 miles. Everyone have a good weekend ;>)]

RE: PVC vs Ductile Iron for water mains?

IronDogg

(Civil/Environmental)

11 Jun 14 19:35

Thanks for the welcome! :) Yeah, I am in the great white north. By semi-aggressive, I just mean that the water main replacement program focuses on those two pipe types. It’s mainly based on things like leak frequency on areas or sections of main, and of course over 90% of our main leaks are on iron. We do have sections of cast iron that is about 100 years old with no recorded leak frequency and we also have had ductile with corrosion leaks after only 20 years. Soil conditions make a big difference.

RE: PVC vs Ductile Iron for water mains?

bimr

(Civil/Environmental)

11 Jun 14 21:14

http://www.sewerhistory.org/articles/compon/pdfs/p...

The AWWARF research study noted above should be the best reference source to answer the poster's query.

Thanks for setting us straight on the bombing story, rconner.The AWWARF research study noted above should be the best reference source to answer the poster's query.

RE: PVC vs Ductile Iron for water mains?

excavator1

(Civil/Environmental)

20 Jun 14 10:38

rconner

I just joined, we are a water main contractor and we are installing HDPE DR11 8" Water Main with Mechanical Joints at the Gate Wells and Hydrants, along with ductile iron sleeves, tees etc.....
From what I have researched, can you tell me the correct or acceptable hydrostatic test method to be used on this pipeline, or pressure testing procedures. From what I have researched and found out that there is no pressure testing other than the normal test procedure for make up water used after an hour for pvc, or ductile systems. I have seen that a hydrostatic test or pressure test in NOT a leak test, and that Totally fused systems with electrofusion couplers or mj adaptors you can use ASTM F2164, in which I dont think is applicable to our case. We have successfully tested HDPE systems built as I described above with many engineering firms using the same procedures as ductile. But I have One firm that in my view wants to make me use something that is not necessary.

Thanks in advance

RE: PVC vs Ductile Iron for water mains?

rconner

(Civil/Environmental)

20 Jun 14 14:09

Just a couple questions that might help you get advice (and from perhaps more knowledgeable folks than I).
In what country are you located? What exactly is this firm asking you to do that you feel is unnecessary?

RE: PVC vs Ductile Iron for water mains?

excavator1

(Civil/Environmental)

21 Jun 14 11:29

USA

I believe that a leak test procedure with 5% pressure loss is not the proper test. ??? (astm f2164) I have been testing similar situations with the PVC or Ductile Specificaiton in a pipe bursting situation with HDPE and Ductile Mega lugs, inserts, tees, etc... gal/hr, per inch per mile etc.. all this with two other engineering firms, I am having a situation with this one engineering firm, although we had passed the leak test i am reluctant to use this as a standard on this particular project.

RE: PVC vs Ductile Iron for water mains?

bimr

(Civil/Environmental)

22 Jun 14 21:18

http://www.performancepipe.com/en-us/Documents/PP8...

Leak testing is described in ASTM F2164, “Standard Practice for Field Leak Testing of Polyethylene (PE)
Pressure Piping Systems Using Hydrostatic Pressure.”

Here is the link for HDPE pressure and leaking testing:Leak testing is described in ASTM F2164, “Standard Practice for Field Leak Testing of Polyethylene (PE)Pressure Piping Systems Using Hydrostatic Pressure.”

RE: PVC vs Ductile Iron for water mains?

rconner

(Civil/Environmental)

24 Jun 14 23:50
1. Note first from the aforementioned first portal that this web page for ASTM F2164 leads off with the statement, "5.1 If required by the authority having jurisdiction, hydrostatic pressure leak testing may be conducted to discover and correct leaks or faults in a newly constructed or modified polyethylene or crosslinked polyethylene pressure piping system before placing the system in service.") While the the very first prepositional phrase of this ad (I believe verbatim from a "Significance and Use" section of the original F2164 standard circa 2002) appears to indicate that field hydrostatic testing of pipelines is optional, in my opinion a well-run field hydrostatic test on a new pipeline is instead absolutely critical to provide some degree of protection for all parties to any pipe material piping design and installation, and ultimately the tax/rate payers. This language may be contrasted e.g. with that e.g. of ANSI/AWWA C600 for ductile iron piping systems that reads more unequivocally in Sec. 5.1.2 Workmanship, "All pipe and appurtenances shall be installed and joined in conformance with this standard and tested under pressure for defects and leaks in accordance with Sec. 5.2 of this standard..." then per Sec. 5.2.1.2, "...Following the installation of any new pipe or any valved section thereof shall be subjected to a hydrostatic pressure test." [I would particularly note e.g. that per AWWA C906 governing requirements for USA polyethylene pipe manufacture, individual polyethylene pipes are not required to be pressure tested by the manufacturer/at the factory, not even considering all that conceivably can happen to the pipe later/before final installation and burial!]
2. This as well as the criteria from the linked document bimr provides seems quite liberal in the overall scheme of things. Lets say we are going to test a normal new 2,000 feet long 8" diameter underground water distribution pipe line that has been fully installed in the buried position it is intended to serve, and in our first case say it is polyethylene pipe to be field tested say to 160 psi.When I pull up the "Technical Note 802" on the website at

It appears at least some USA manufacturers of polyethylene pipe are indeed promoting specifications requiring field hydrotesting per ASTM F2164. You can read at least the overview/entre' to the latest version of this standard,, and certainly purchase same if you wish and are responsible for knowing the details therein, at the portal http://www.astm.org/DownloadStandardA.html?ASTM%20... . It appears a historical 2002 version of this standard, not controlled as I believe there is a 2013 version available, can even be read for at least the time being at http://nutmeg.easternct.edu/sustainenergy/EnergySe... . A few points to consider in looking at what is meant by field testing of pipelines. The most common basic types of piping used for water service, including polyethylene, pvc and ductile iron all go back to near WWII or thereabouts. However, it is some interesting that while I'm sure various manufacturers and AHJ's have had their own and likely differing opinions concerning same, no sort of widespread utility consensus USA field testing protocols for plastic water pipes e.g. ANSI/AWWA standards have really been available until quite recent years.1. Note first from the aforementioned first portal that this web page for ASTM F2164 leads off with the statement, "5.1 If required by the authority having jurisdiction, hydrostatic pressure leak testing may be conducted to discover and correct leaks or faults in a newly constructed or modified polyethylene or crosslinked polyethylene pressure piping system before placing the system in service.") While the the very first prepositional phrase of this ad (I believe verbatim from a "Significance and Use" section of the original F2164 standard circa 2002) appears to indicate that field hydrostatic testing of pipelines is optional, in my opinion a well-run field hydrostatic test on a new pipeline is instead absolutely critical to provide some degree of protection for all parties to any pipe material piping design and installation, and ultimately the tax/rate payers. This language may be contrasted e.g. with that e.g. of ANSI/AWWA C600 for ductile iron piping systems that reads more unequivocally in Sec. 5.1.2 Workmanship, "All pipe and appurtenances shall be installed and joined in conformance with this standard and tested under pressure for defects and leaks in accordance with Sec. 5.2 of this standard..." then per Sec. 5.2.1.2, "...Following the installation of any new pipe or any valved section thereof shall be subjected to a hydrostatic pressure test." [I would particularly note e.g. that per AWWA C906 governing requirements for USA polyethylene pipe manufacture, individual polyethylene pipes are not required to be pressure tested by the manufacturer/at the factory, not even considering all that conceivably can happen to the pipe later/before final installation and burial!]2. This as well as the criteria from the linked document bimr provides seems quite liberal in the overall scheme of things. Lets say we are going to test a normal new 2,000 feet long 8" diameter underground water distribution pipe line that has been fully installed in the buried position it is intended to serve, and in our first case say it is polyethylene pipe to be field tested say to 160 psi.When I pull up the "Technical Note 802" on the website at http://www.performancepipe.com/en-us/Documents/PP8... , the first thing I see are all kinds of caveats with regard to tests and exactly what tests do and do not mean (though I wonder a little bit why, that is another story and I'll not worry about that here). As I know my required test pressure is 1-1/2 times my maximum service pressure, I will choose "Alternate 2", that I see is lead off with, "Immediately following the initial expansion phase, monitor the amount of make-up water required to maintain test pressure for one (1), or two (2), or three (3) hours. If the amount of make-up water needed to maintain test pressure does not exceed the amount in Table 2, no leakage is indicated." I choose 2 hours duration, as I know from my experience that that has long been the MINIMUM required/vetted for many years duration of AWWA C600 (for other material), so I look at their "Table 2 Test Phase – Alternate 2 – Make-Up Water Allowance". I see that for my two hour test the allowable make-up water is 1.0 U.S. gallon for every 100 feet of 8" pipe in my testing extant. For my 2,000 feet long project, I thus run the not too hard calculation (2,000 ft/100 ft)(1.0 Gal.) and come up a full 20 gallons (most of the bottom half of a 55 gal drum) of water they say I should allow the Contractor to pump back into this small hdpe pipeline if necessary to maintain pressure etc. Let’s now look at case two, that I had chosen instead ductile iron pipe for this project. Looking at Table 11-6 of AWWA M41, AWWA C600 (or tables in any number of major utility testing specifications re-created there from on the web) you will find that at 160 psi the testing allowance to maintain pressure is no more than 0.77 U.S. gallons per hour testing duration per 1,000 feet of pipeline involved (meaning I must multiply this amount by (2,000 ft/1,000)(2 hr/1 hr) ft or 4 to get the total testing allowance for such modern ductile iron piping installation that is in fact ~3.1 gallons, or milk jugs, of water). Now before we proceed further, let me add some more information, also from AWWA standards. Unlike ductile iron pipe, some folks may not know plastic pipe manufacturers are not now necessarily required by minimum AWWA standards e.g. AWWA C906 to hydrostatically leak or pressure strength test each piece of pipe at the factory, and I daresay little or no for hdpe water pipe is likely so tested. Also and on the other hand, anyone who has been around field installations of pipes very long understands that reasonable field testing allowances from the practical standpoint are necessary for the real world. It is virtually impossible to make sure all pipelines are laid precisely like profile lines on a CAD drawing, or that air release devices end up located precisely at every high point as shown on drawings (so that no air is trapped in pipelines), said air goes into and out of solution as it will, temperatures heating or cooling over the test duration have an effect on pressure (particularly on trapped air in accordance with Boyles, Charles, and Gay Lussacs etc.), pipes can settle or move at least slightly as they are pressurized (changing test volume some), water can soak at least a little into e.g. cement mortar linings, and indeed particularly for hdpe pipes there is also some rather complex change in test volume due to Poisson as well as Bourdon behaviors of the viscoelastic material (that are frankly more dependent on the pipe embedment and layout, as well as variations in piping, than some numbers in a table!) etc. In other words, pumping 3 gallons of water back into a ductile iron pipeline or seven times that allowable amount back into a polyethylene pipeline (the latter claimed outrageously by the vendors to be "jointless" and "leak-proof")"after the same two hour test doesn't mean either one of them has leaked that amount of water.

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