minimum slopes for water transmission lines 4

[arj00]

What is the minimum acceptable slopes for large water transmission lines(dia. 60 inch),can we use 1 per thousand ?
Is there any standard which give the minimum slope?

 

[JStephen]

I wasn't aware that there was any minimum slope- thought lines were level where ground was level.

 

[BRIS]

Generally for water mains at normal velocities (circa 1 to 1.5 m/sec) the minimum I would specify is 1 in 500 towards an air valve and 1 in 300 away from an air valve. 1 per 1000 for a 60 inch is probably ok towards the air valve but I would suggest you go for 1 in 500 away from the air valve.

 

[BRIS]

My colleague has come up with the following : minimum gradient = 1 in 120 D/(V^2) (SI units D in m, V in M/sec) . This is the critical gradient at which air will travel uphill to the air valve against the flow.

A number of Air valve manufactures give recommendations

 

[BRIS]

http://www.hrwallingford.co.uk/projects/air_in_waterpipelines/

 

[LHA]

I don't get it. The water is under pressure, and moving. The air valve is at the high point. The pressurized fluid hits the air valve, where the instantaneous internal pressure drops, if there is compressible fluid (like air) entrained within the incompressible fluid (water). Entrained air expands and comes out of solution when it hits this air valve.

Why is a critical gradient for air travel necessary?

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[cvg]

i don't believe it is an instantaneous phenomena, therefor some of the entrained air gets released beyond the air/vaccum valve and then has to travel back upstream against the flow.

 

[BRIS]

If you lay the pipe horizontal in practice due to laying tolerance and differential settlement you will have a series of crests and hollows - it is impossible to remove the air from the crests and with air in the pipe it is impossible to pass a pressure test.

 

[rconner]

I am not aware of standards specifically requiring water mains to be laid to a specific slope or grade, and I agree with one or more posters that such mains in profile view most generally follow the "lay of the land". It is however true that some air will probably accumulate in any high points of lines as claimed by another, I would think particularly if the lines have been filled very slowly and/or there is no flow. While whatever degree of accumulated air in new lines may or may not not cause practical problems in tests or service (I have heard many over the years say that at least large quantities of air at obvious high points have caused some problems), I think many testing standards do require that as much air as practical (for many reasons, I suspect including safety, and that some large air pockets might "air lock" future flow in some circumstances?) be removed from the installed pipeline prior to test. It would appear that laying pipes very carefully on a specific grade might help to make this most efficient, though perhaps in some areas at some additional construction cost when specified?
On the other hand, I have also heard that once water pipelines are placed in service, there may also be some significant abilities of ensuing flow, at least under some conditions, to "scavenge" at least some quantities of accumulated air from some high points, so same does not cause any practical problem even if there is no air valve (or air valve "working") there.
I think there have been a few technical papers written on air effects and removal from pipelines, including some from folks who market air valves; however, I have also heard that some air valves can malfunction and/or be concerns as future contamination points.
I guess exactly where all these lines should be drawn so to speak, with all issues considered, is the province of responsible engineers.

 

[BRIS]

IHA- if the pipe slope after the air valve is less than the hydraulic gradient then the pressure is reducing even though the pipe is falling. Entrained air comes out of solution as the pressure reduces. It then has to travel back up the pipe to the air valve.

 

[rconner]

I took a quick look with the link provided by BRIS at the HR Wallingford et al website, with description of apparently ongoing research and reference list having to do with air in pipelines (with a goal it appears toward the preparation of a contemporary guidance manual on this subject). It is an impressive collection of researchers and references.
I noticed in "Section 9. Information from practical experience, Section 9.2 Pipeline Profile" on pg 67 of the literature research, that slight "minimum slopes" (to and from air valves etc.) similar to what others have provided in response to this inquiry are mentioned in this paragraph, but the paragraph also ends with the statement, "Some designers however are using flatter slopes in some cases and avoiding the requirement for installation of air valves, particularly in areas where maintenance and access are difficult."
I assume this requires some knowledge and/or experience of the designer with the specific conditions involved with that application condition that allow this to be done (in other words it appears they assume/know the air may be acceptably moved, with all parameters considered, along the pipeline in at least in these instances.)
This issue appears to be a very complicated subject with many available references, and I will add one I have seen but didn't happen to notice on the Wallingford list that also may be of interest to some -- "Air Binding in Pipelines", by Mr. Richard T. Richards published in the June 1962 AWWA Journal). Air is of course in most pipelines for many reasons and can have many effects (some of which can be deleterious in at least some circumstances when they occur, as explained in many references on the subject).

 

[rconner]

I took a quick look with the link provided by BRIS at the HR Wallingford et al website, with description of apparently ongoing research and reference list having to do with air in pipelines (with it appears a goal toward the preparation of a contemporary guidance manual on this subject). It is an impressive collection of researchers and references. I noticed on pg 67 in "Section 9. Information from practical experience, (sub-)Section 9.2 Pipeline Profile" of the Literature Review document, that slight "minimum slopes" to and from air valves etc. similar to what others have provided in response to this inquiry are mentioned in this paragraph. However, the paragraph also ends with the statement, "Some designers however are using flatter slopes in some cases and avoiding the requirement for installation of air valves, particularly in areas where maintenance and access are difficult."
I assume the latter requires some knowledge and/or experience of the designer with the specific conditions involved with those pipeline application conditions that allow this to be done (in other words it appears they assume/know the air may be acceptably moved, with all parameters considered, along the pipeline in at least in these instances?)
This appears to be a potentially very complicated subject (depending on the actual pipeline conditions) and with many available references, and I will add one more reference I am aware of but didn't happen to see on the Wallingford list that also may be of interest to some (as it discusses a great many different types of pipelines and air removal scenarios/methods, at least that were known at the time) -- "Air Binding in Pipelines" (by Mr. Richard T. Richards and I think published in the June 1962 AWWA Journal).

 

[cr1973]

I've design and overseen construction of tens of thousands of feet of water main, anywhere from 8" to 30", and never designed for a minimum grade.

Always placed air releases/hydrants at the high points. NEVER had a line fail a pressure test.

Sure, you might spend the first 30 minutes actually pushing the air out of the line, but after that, they all held the required pressure and time durations.

Minimum gradient = debunked

 

[BRIS]

cr1973 are you referring to water transmission pipelines or distribution pipelines. By your reference to hydrants I guess you are referring to water distribution mains with hydrants and connections. Whereas the question refers to long water transmission pipelines where the only means of air release is through air valves. I am not sure how you always place air valves/hydrants at high points if your main is flat??.

I have never worked in the US but I have worked in many other countries of the world and standard good practice is to lay water transmission pipelines to a minimum gradient. Yes you can place air valves/hydrants at close spacing and keep flushing through to remove the air but is that a good substitute for proper design ?.

With a 50 mile long 60 inch pipe you will spend a lot more than 30 minutes and use an awful lot of water trying to purge the air. I recently commissioned a 800 km long 4.0m diameter transmission pipeline and we would not have got very far if we had not designed to a minimum gradient.

As a designer you have to decide when it is appropriate and when it is not necessary to design to a minimum gradient.

Minimum gradient = debunked=Professional indemnity insurance =debunked

 

[cvg]

I'm not sure there was any reference in the posts to a 50 mile long pipeline or any particular length. With normal conditions, the pipeline will follow the existing ground elevations and therefore will generally have some slope. Installing with a minimum slope higher than the natural ground is generally not economical as the pipe must remain below ground. If the ground is extremely flat, low and high points can be designed into the pipeline to provide points for blowoffs and air release, but again this is not generally necessary as you will frequently be dipping under or over existing utilities etc. Air release and vacuum valves do need to be placed frequently and hydrants are almost always installed for flushing and filling service.

 

[Denob]

Most of the literature that I've looked at seems to suggest that air and vacuum release valves be placed frequently, as cvg points out. It always seemed to me that that they suggest they be placed TOO liberally. It's obvious that air valves be placed at peaks but why at transitions in slope? For example: place an air valve at the point where the slope goes from increasing to increasing LESS...it's still increasing so won't the air bubbles continue to rise, albeit at a slower speed?

 

[JStephen]

Denob, perhaps the ones who recommend where to put the air release valves are also the ones who sell the air release valves? : )

 

[BRIS]

The question was asking what is the minimum acceptable gradient for laying a water transmission line. I accept that there is no reference in the post to length but large diameter transmission pipelines are often several kms long and should always be designed to provide a minimum gradient. If the natural slope above minimum gradient or the pipe has to be dipped under services then obviously the minimum gradient is exceeded and is not a concern.

We recently designed 150 km of 1.2 m diameter transmission line in Kuwait. The maximum difference in ground level over the length of the pipeline was less than 8.0m. There were practically no service crossings along the length. We adopted a minimum slope and prepared a typical saw tooth profile. Granted that this is more expensive than designing a flat pipeline but I am sure a flat pipeline would have provided the Client with persistent problems. Air in pipelines can have serious consequences in respect to surge as well as difficulties in testing. I have never seen a hydrant on a transmission pipeline.

Air valves should be provided at changes in slope where the slope changes from less steep to steeper than the hydraulic gradient.

 

[JStephen]

While surfing the web, I ran accross an AWWA Manual, which I suppose would give typical US practice on the issue at hand:

Manual M51, "Air-Release, Air/Vacuum, and Combination Air Valves"- Selection, installation, and maintenance of air valves in potable water systems.

From

http://www.awwa.org.

 

[stanier]

Oil, gasoline, slurry and other pipelines dont have air valves . Why water lines?

Do other product pipelines have minimum slopes?