how often do I need an air release valve

[beerginger]

I am installing a new pipe line and would like to incorporate air release valves into the system. The problem is I don't know how often I need these valves. If anyone knows of a formula or book or website or anything that has this information it would be very helpful. Thanks in advance.

 

[vpl]

beerginger,

I'm a bit confused here. You wanted to know "how often you need these valves." Are you meaning[ol][li]How often do you need to vent through these valves or [/li][li]On what spacing should I put the valves?[/li][/ol]

If your question is the latter, then it's somewhat easy: the valves go at the high points in the installed piping; one valve to each high point.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[quark]

Well said by VPL. If your piping is open ended liquid piping, you may require none. If it is a closed loop piping then you have to install them at the highest point. A thermostatic trap is a good air vent incase of steam piping.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[stanier]

Check out valmatic or ventomat websites . Thye have software for designing the system with location and type of air valves required. The Ventomat site has some other good technical information on air valves.

Be carreful as some older styles of air valves can actually cause pressure transients. The Ventomat is designed specifically to mitigate surges in a system.

 

[PUMPDESIGNER]

stanier, that Vent-O-Mat link was good, have some good looking products and technical papers.

PUMPDESIGNER

 

[BRIS]

VPL has given you the answer -you need one valve at each high point. That is if you have a main transmission pipeline. If you have a distribution pipeline with connections then generally you can allow the pipe to vent through connections and you don't need air valves. Air valves need maintenance.

For a main pipeline you also need to ensure that the air moves to the air valve. Typically pipes should be laid at a minimum gradient of 1 in 500 when rising to an air valve and 1 in 500 when falling from an air valve.

On long lengths you may also need air valves at changes in gradient - generally spacing should not exceed 1 to 1.5 km.

Having decided where to locate the vales you will need to decide what size to make them. There are three requirements

1) to vent large volumes of air during filling
2) to allow air into the pipe during emptying (some pipe materials can collapse under negative pressure i the pipe is drained faster than the valve will admit air into the pipe.
3) to vent small volumes of air that come out of solution during service.

To cater for these requirements it is common to use double orifice air valves that contain a large orifice to allow air evacuation and air entry during filling and emptying and a small orifice that allows venting of air accumulation during service.

To size the air valves you need to consider the filling rate and the rate that the air will need to be evacuated during filling. If you have a pipe material that is susceptible to collapse under vacuum you will also need to consider the maximum drain down rate and the rate you need to allow air into the pipeline to limit the sub atmospheric pressures.

The Vent-o-Mat design guides will give you the methodology but beware Vent-o-mat are in the business of selling their valves and if you follow their guides you will end up with larger valves than some of us would consider necessary.

.

 

[stanier]

Pumpdesigner has come back with some good information.

I would add another need for air valves and open a complex subject in doing so. Air valves can be used to mitigate pressure transients resulting from loss of power pump stop. Computer modelling will show that a large air valve may in fact cause higher pressure transients than a smaller orifice valve. This works against you when you select an air volve to exhaust large volumes of air. Hence the double orifice type valve. Large orifice to exhaust the air and small orifice when the approaching air water interface is close to the air valve.

It is this latter scenario that resulted in the research for the Ventomat style of Kinetic double air valve.

In respect of the size and number of air valves I agree that the Ventomat software rpedicts more and larger valves than necessary. the local Ventomat representative in Australia, AVK valves, acknowledges this and will work with you to review a design. I have also come across this problem with Valmatic's selection software.

It must be remebered that air valve suppliers are not design consultants receiving fees for the design engineering of a system. So there may well be some conservatism in their software. I agree with Pumpdesigner that you need to understand the theory , research and application of air valves. Thier misapplication is as serious as for a relief valve! Typically unforeseen pressure transients could compromise the thrust blocks in a pipeline, will result in long term fatigue damage of pump seals, bearings, valve seats, instrumentation etc.

A read of Fluid Pressure Transients in Pipelines by ARD Thorley is worthy of your time. It is out of print at the moment but any good technical library or authority should have a copy.

My conclusion is that the technology of ball type kinetic valves has gone the way of 8mm filmimg, DOS, Video 8, black & white TV and other legacy products. There are better designed valves out there. Yes they do cost more up front but they prove to save money by their reduced maintenance and reduction in risk for your pipeline.

Also sit the air valve on a short branch on a blind flange fitted to a full size tee to allow a point where the air can accumulate and egress from the system.

 

[BRIS]

Using air valves to relieve surge pressures is a contentious subject for potable water pipelines.

There are two problems:

1) on initial pipeline filing the large orifice air valve of a double orifice vlve will slam closed as water reaches the ball. This will set up a transient pressure in the pipeline. If analysis shows this to be a problem then you need to send more on a slow closing air valve. Vent-0-mat is one option.

2)During service if there is power failure causing a sudden pump trip and the resultant negative pressure wave causes pressures in the pipeline to go subatmospheric air will be drawn into the pipeline. This air will then be expelled under the subsequent positive pressure wave. Sudden closure of the air valve will cause secondary pressure waves which when combined with the initial pressure waves may lead to rupture of the pipe.

The argument for using slow closing valves such as Vent-o-mat is that allowing air into the pipeline provides a cushion which reduces the waterhammer pressure wave. Because the air is subsequently expelled slowly through the slow closing valve there are no adverse secondary transient pressures. However, for potable water few authorities will permit a design that allows pipeline pressures to fall below atmospheric. The concern is that any polluted water that is sitting in the air valve chamber may be drawn into the pipeline. There is also concern that if a pipeline is subject to subatmospheric pressure polluted water may be drawn into the pipeline through spigot and socket joints. For this reason the Vent-o-mat arguments have not found a great deal of support in the water industry.

It is unusual for excessive surge pressures to compromise thrust blocks. Thrust blocks are usually designed for test pressure plus a nominal factor of safety and as such have adequate factors of safety to resist surge pressures. Furthermore shock loading from surge pressures is usually too short lived to mobilise failure of the thrust block. The main failures I have observed are longitudinal splitting of pipes, collapse of pipes and flange failure.

However, this is straying from the argument - I would say that there are many good ball type air valves on the market - don't get too hung up on the Vent-o-mat sales talk and paying for air valves you don't need.

 

[stanier]

The message from BRIS makes some good points in relation to air valves. I would like to extend the discussion as it is fleshing out some misconceptions on these devices.

1) Filling of pipelines should always be done slowly, in a controlled manner, to ensure that htere is no damage to air valves and other devices in the system. It would be foolhardy to rely upon the Ventomat or any other air valve to prevent that.

2) The problem of air, water or pollutant ingress into potable water lines exists because there are already air valves designed into potable water lines. This has been argued as necessary to prevent air restricting the capacity of the pipeline. If one looks at a crude oil, petroleum or slurry pipeline there are no such devices? Is it a myth? I dont think so because there are many reported instances of air in the line causing hydraulic restrictions. Then why dont other industry pipelines suffer from the same problem?

Yes, one has to be wary of sales talk in engineering matters. However Ventomat are able to back up their point of view with independent research. Many of the ball type air valve manufacturers have copied a design and have no idea of the way a kinetic air valve works. The original kinetic air valve was developed by Glenfield and Kennedy in Scotland. Glenfield and Kennedy still manufacture an air valve but have moved away from the ball type in their design. They also research the product. Recently Glenfield and Kennedy were purchased by AVK valves. Who is marketing Ventomat in various parts of the world , AVK valves. There are other manufacturers who do or have done for them research into the behaviour of air valves. They however have not made their fndings as public as Ventomat.

We are now seeing products being manufactured in the far east with little or no technical back up. Copies of designs of yesteryear. No science , just economic rationalism. Let us see the ball type air valve manufacturers publish their research that counters Ventomat's critique of their design. Has there been any obvious technical discussion other than branding Ventomat as "sales talk". Where is the technical argument and facts?

Ventomat valves have been installed in many installations in Australia and have successfully mitigated the waterhammer transients previously occurring. The cost benefit analysis of reduced maintenance etc will out weigh any increase in cost of a more sophisticated valve. As more and more are sold to enlightened authorities the benefits of mass production will see the price come down. This will be the death knell of the legacy design.

 

[PUMPDESIGNER]

stanier brings up and pushes a great issue.
I read a "technical" paper by an air vent manufacturer about four years ago. Follow their advice and your system will look like it has warts growing all over it for the number of valves they recommend, and their approach is ....
Just to be sure, Just in case, something like that.

I have decided to address each system diligently, and put in the valves needed, and not to just "make sure" or "just in case".

If velocities ever exceed some value, say about 2 feet per second, the air is swept onwards, depending on slope and fluid type.

If the fluid is saturated or near saturated with dissolved gas/air, that is a problem, especially if temperatures drop.

Why not be careful in the design? It seems silly to start specifying hundreds of warts on the pipe, that gets engineers a bad name quick, especially after someone goes around plugging them up cussing the guys that specified them, because those people know why the engineer did it, "Just to be Safe".

I have to say this however, I have never seen any engineer give in on this, it is like superstitions, they have no valid reason other than it makes them feel good.


PUMPDESIGNER

 

[BRIS]

.I apologies for straying from the original question but I think a couple of points need some further explanation

1) "Filling of pipelines should always be done slowly, in a controlled manner, to ensure that there is no damage to air valves and other devices in the system" – In fact the only way a safe filling rate can be derived is from analyses of the air valve characteristic. The main parameters in determining the permissible filling rates are the allowable surge pressures, air valve evacuation rates and their closure characteristics. (We have had to develop pipeline filling procedures for some of the largest pipelines in the world - including 2000 km of 4.0 m diameter PCCP pipe. I don't think we were foolhardy in relying on the air valves performing as specified.!).

2) For pumped pipelines air valves may provide a surge alleviation solution. The surge analyses will reveal whether or not the pipeline goes sub atmospheric on a pump trip. The solutions to prevent the pipeline going sub atmospheric are a) increase pump inertia, b) add air vessels/feeder tanks c) provide by-passes around the pump or any combination of these 3.

What Vent-o-mat proposes is that pipeline is allowed to go sub atmospheric and the air valves are designed to give controlled rate of air expulsion to a) act as a cushion and b) avoid secondary water hammer effects from valve slam. This is ok for non potable water pipelines but for potable water pipeline few authorities will allow this solution because of the risk of contamination – As international consultant engineers our company policy does not allow us to propose this solution because of the safety risks.

If the pipeline is not allowed to go sub atmospheric then the vent-o-mat valve offers no advantage over a standard ball float valves.

3) The vast majority of pipelines are operating successfully with ball float type air valves.

4) Slow closure of ball float type air valves can be achieved by the addition of a globe style check valve. Most air valve manufacturers/suppliers will provide a compatible air/check valve assembly and also the characteristic curves for the assembly.

5) Petro/chemical pipelines are not my field but I believe that the initial pigging pushes the air through the pipeline.

The Vent-o-mat valve is an excellent product and I have specified it on several projects. But it may not be the answer on potable water pipelines. Ball type air valves with a globe check valve will also give slow/non-slam closing characteristics and surge suppression if require

 

[TylerJett]

I have to agree with BRIS, the filling of pipelines should always be done slowly & controlled. However, this isn't always the case for whatever reasons.

One other site to check out is Crispin Valve. They have quite a bit of technical information I find useful.