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Air Valves û Water supply Trunk Main
2

Air Valves û Water supply Trunk Main

Air Valves û Water supply Trunk Main

(OP)

I am reviewing a contractual argument between the design and build contractor and the review consultant on major water pipeline. 1.6m dia by 800 km.

The review consultant requires twin air/vacuum relief valves to be provided at “critical” locations. Critical locations are high points where a sub atmospheric pressure could result if the pipe is drained and the air valve fails to open. The pipe is not designed to withstand vacuum pressure. Surge suppression equipment is provided separately to prevent negative pressures.

The review consultant refers to duty stand by air valves !

In my experience I have never come across duty stand by air valves or doubling air valves to ensure reliability. I have also not come across any failure resulting from an air valve failing to open under vacuum conditions. Personally I keep air valves to aa absolute minimum.

I would be interested to know of any industry standard or manufacturers recommendation etc. for the duplication of air valves to ensure pipe safety. (not for surge protection).

RE: Air Valves û Water supply Trunk Main


I have limited experience, but supplying to the Scandinavian market have not yet met this demand.

If your contract just describes 'sufficient air inlet' or is unprecise, the consultant would in my mind have to come up with examples of 'common practise' or direct rules.

On the other hand: what about dimensions and air inlet amount requirements / dimensions? How is this incorporated in your contract ? (Pipe bursting air inlet incorporated?)

If this is not properly described, you would probably have to argue the side of 'common practice'.

If there is a fairly large demand for air inlet you could look into the cost of mounting a T with two smaller air inlet devices instead of one larger. (Two air inlet devices but with one service shutoff valve for T stem only) If circumstances are right this might be comparatively equal in price, or even cheaper than a larger air inlet device.

Very often air and vacuum valves are described with a diameter (connection diameter) in projects. Technically correct would be to describe air inlet and outlet capacity demands at given over/underpressure and then to check the capacities of the given type/fabricate supplied against demand.

As many dependable vacuum relief valves are constructed and calculated  'conservatively' (eg. giving fairly large reserve capacity), a technical detailed survey based on calculated demands, including air inlet at bursting conditions, could give as a result more or larger valves than 'common practice'.

RE: Air Valves û Water supply Trunk Main

A risk analysis including HAZOPS should be carried out with all the interested parties present. The level of maintenance will be a key issue. Many authorities are below critical mass when it comes to maintenance and this changes the design dynamics.

From previous posts BRIS you are Australian based? If so I would be very concerned about maintenance levels as the industry has been run down for a while. On top of that there is an abundance of maintenance "baby boomers" who will be gone in ten years. The bean counters wont allow their replacement.

An air valve is as critical as a relief valve. in my opinion should be registered and routinely tested for operation. After all it protects the pipeline from collapse due to negative pressure. You may need to check AS 4343 as to the obligations.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au

RE: Air Valves û Water supply Trunk Main

2
I don't know specifically about all involved with "duty stand" but the 1992 American Society of Civil Engineers (ASCE)publication "Pressure Pipeline Design for Water and Wastewater", a committee report of , contains specifically in section 5.5.4 Relief Valves (after talking about surge anticipating/relief valves) the following guidance,

"Air and vacuum relief combination valves are used in
transmission pipelines at critical pipeline profile high
points to release air and gas that may accumulated at
those locations. They are designed to also admit air
when a system failure occurs to prevent the development
of a vacuum in the pipeline which may collapse a portion
of the pipeline structure. Since valve operation may be
infrequent two valves are sometimes placed are a single
location to provide operation redundancy. Pipeline size
and profile, along with flow, temperature and time
considerations establish the capacity of the valve at a
particular location."

I believe in spite of this or similar guidance I have seen third-hand over the years pictures indicating that some  extents of at least a few, predominantly low ring stiffness larger diameter operating pipelines have in effect (and I suspect rather surprisingly to the Owners) been squashed flat as a flitter obviously by external pressures ( including vacuum etc.), perhaps in some incidents in combination with other complicating factors.  I think it is possible knowledge of such past incidents from some more experienced or responsible engineers may have at least in part given rise to this publication's suggestion for some "redundancy" in this area?
    
While perhaps some might argue air valve or local maintenance practices in this or even other applications may have been improved over the last few years since this last ASCE publication revision, it would appear that it is ultimately a system design decision as to exactly how potential external forces are to be dealt with.  Of course in some cases some folks have apparently employed stiffer pipes (or pipes with stiffening ring girders affixed), multiple air/vacuum valves, or other means to minimize the likelihood of failure particularly in applications when/where the consequences of failure are quite high.  

RE: Air Valves û Water supply Trunk Main

Others have mentioned "redundancy" as a possible reason for requiring two or more air/vacuum valves.  It is also true that air can accumulate at, OR NEAR, the high point of a water line.  Because of this, some designers use 2, or more valves, to try to ensure that they are able to vent such air pockets quickly.  As you may imagine, on a long, flat pipeline an air pocket can form as a very long "bubble" near the inside top of the pipe and would extend over quite a horizontal distance on either side of the "true" high point.

I've read about such things happening but haven't actually experienced it in 47 years on the water business.  Still, it seems believable.

good luck

RE: Air Valves û Water supply Trunk Main

(OP)
Thanks for the responses.  I am not in Australia. I work on international funded development projects across North Africa, Middle East and West Asia (Libya to Baku, Pakistan to Nigeria, Lebanon to Kenya). Competitive bidding on international funded projects is resulting in poorer quality engineering, lack of head office back up and consequent erosion of engineering standards. But it suits the bean counters in Washington. It is common to have one consultant producing the feasibility study and RFP documents, one producing the design and one doing the design review and none having adequate breadth of expertise. More and more I am getting stuck in resolving disputes of preference engineering with neither side having the engineering capacity to support their preferential solutions. I could solve the problem analytically with cost and risk analysis etc. but what most “engineers” want is to see it in print in a code or standard. This attitude is prevalent in Eastern Europe and former Russian states:  It doesn’t matter whether it will work or not providing in complies with the “Norms and Standards”.

Anyway that is my gripe and I am way of track. Rconner’s reference to 1992 ASCE guidelines may be what I require.

In a constantly flowing trunk line air does not accumulate at the peak but slightly downstream and there is a good argument for placing a small capacity air relief valve downstream of the peak but not for duplicating large diameter air exhaust/vacuum relief valves.

Preventative maintenance has been carefully planned and budgeted for but in reality it is non-existent. Air valves will not be maintained. I am not convinced that doubling up air valves is a solution: both are operating in the same environment and both will fail.

1)    Does anyone have any reference to a Vacuum relief valve failing to open resulting in a pipe failure? I have seen the pictures of collapsed pipe but the diagnosis has been the lack of provision of air/vacuum relief valves rather than failure of the valve to open.
2)    The T arrangement described by Garhardl is the solution proposed – I was considering recommending that one valve is set higher than the other so that the higher valve works as duty and the lower as stand-by. Has anyone attempted to get air valves to work in a duty stand-by arrangement?
3)    The critical concern is vacuum relief should the air/vacuum relief valve fail. Rather than duplicating on expensive air valves it should be possible to provide a simple air inlet valve or bursting disk as a fail safe device. Does anyone have any knowledge of such a device?

I appreciate the comments.

RE: Air Valves û Water supply Trunk Main

BRIS, with regard to your specific question "1", I suspect the most well-known example of large collapsed pipelines may still be the exposed and normally rather high-pressure section of the "Bouquet Canyon" 80"-94" (3/8-1-1/16" plate) pipeline, I think pictures of which were esconced in the 1989 (and maybe also earlier) versions of AWWA Manual M11.  According to this former reference, this pipeline "collapsed as a result of failure of automatic vacuum-relief valves".  You can e.g. also see this incident referenced similarly at one or more current sites e.g. under the "Flexibility/Ductility..." paragraph at    http://findarticles.com/p/articles/mi_m3MKT/is_22_108/ai_59284083, with these references also indicating that this line that collapsed was actually subsequently and to some extent successfully sort of re-inflated!  However, note it would appear this incident must have occurred many decades ago, of course it would follow with only the air/vacuum devices and practices etc. of the time available.  
I have heard also quite third-hand of at least a couple other vacuum-related pipeline incidents, I think including a case involving a 108” steel pipe that a picture revealed collapsed apparently after a downstream rupture in sort of an “S” form, that was referenced in a 1962 AWWA Journal article by Mr. Richard T. Richards entitled, “Air Binding in Pipelines” (though in that case there is in fact, as per the experiences you relate, an indication vacuum valves were not provided at all for that pipeline).
I do not know enough about any of these specific problems (it would appear most quite long ago), nor for that matter reliability of long past nor present air valve sizing, constructions or maintenance etc., to know if they hold any relevance to your contemporary issue.  I do know AWWA Manual M11 of course does include means for analyzing collapsing/buckling effects on buried and exposed steel pipelines, and DIPRA also has available a Technical Report, “Critical Buckling Pressure for Ductile Iron Pipe”, that can be used also to illustrate the normally greater security vs vacuum of usually at least a little stiffer, large diameter ductile iron pipes.        

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