Why air valves on water lines but not on hydrocarbon lines?

[stanier]

This is an old chestnut. i pose the question why the water industry insists on fitting air valves on pipelines every 1km or so. Similar sized lines carrying crude or gasoline do not have air valves.

Are there any water pipelines of sgnificant length that do not have air valves?

Are there any website links with technical papers of note on this topic.

 

[BigInch]

Air is not nearly as soluable in hydrocarbons (5% maximum or so) as it is in water, thus never available in sufficient quantities to cause an air lock. Condensed water is much more of a problem in terms of quality and inducing corrosion. Its not soluable either, but gets inside the system when humid air in contact with the HCs condenses and falls in. Water is usually removed from storage tank bottom drains.

There are one or two websites that I've seen that explain how and where to locate air valves for water lines. I would think that, if a water line profile is continuously sloping uphill, or downhill with a velocity sufficient to sweep free air bubbles along, you would not need an air valve in that segment. Do you need this air valve location info?

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[stanier]

Hi BigInch,

I have plenty of reference material that shows where to place air valves and how to size them. HR Wallingford did some great research into the phenomenon back in 2005.

The question put to me is that hydrocarbons have vapours that could come out of solution similarly to air in water. Why isnt the oil & gas industry concerned with their removal. Is it because vapour goes back into solution far easier than does air into water? Thus sufficient quantities do not exist to impair hydraulic performance?

 

[BigInch]

I was pretty sure you wouldn't be short on that material.

Keep the HC pressures above the product vapour pressures and you've got no worries about that. They have no choice but to remain in the liquid phase. Vapours are only present in low pressure tanks and those vented to atmosphere, w/o vapour recovery systems. Minimum pipeline pressures at the line highpoint must be above VP with some safety factor to avoid boiling and downstream cascade flow. Not in itself necessarily a bad thing, but expanding and collapsing vapour expansion pockets can cause vapour locks, water hammer and lot of false alarms in a leak detection system. Hence the preference to hold liquids to liquid phases by maintaining the pressures higher than VP.

Air valves in HC lines would simply vent any vapours to atmosphere and create many potentially dangerous locations across the landscape.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[stanier]

Hi BigInch,

That is what I thought.

If a water pipeline can be held at a pressure where the air remains in solution then air valves are not required either. This may be a problem with downward sloping lines. Also the water industry tends to want to scour and drain pipelines ratherthan use pigs to cleanse the line.

Air valves are also used to prevent vacuum collapsing the pipeline.

Do you see anything wrong with designing a water pipeline as an oil line? ie No vents or drains? Clean with pigs. May have to blanket the feed tank with n2 to prevent air being taken up.

The biggest challenge is convincing the civil engineers to do it differently to the last century and what all the text books say.

 

[BigInch]

I haven't looked at the air soluability vs pressure and temperature tables but theoretically I'd suppose its possible, although the pressures that might be required to do so could be quite a bit higher than what are common to the water industry. Given that air valves are used to prevent collapse during vacuums, it would indicate that the wall thickness to diameter ratios and material strengths are lower than what I typically use for oil lines.

Don't forget that air is 70% N2 and 30% O2, so I am not very hopefull that blanketing with N2 is going to make that little problem go away. I don't think the soluability will change significantly. The real problem is that water is one of the best solvents of all the chemicals known to man.

Hummm.... can pigs swim? I can't see why that couldn't work ... in theory.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[eadwine]

Anyone thought about the toxic & hazardous releases to the environment via air vents in HC lines? Water doesn't have this problem. Besides, there is a major campaign on by the Government and Industry to eliminate or minimize any and all hazardous fugitive emissions or releases. And that is tied up in the Company's Environmental Permits which regulates discharges to the Environment.

Air Vents? Quite simply, they leak. Besides, I'd rather get a bonus this year than have to pay fines to the Government and pay for the clean up of spills. Not too mention, the bad press involved.

 

[BigInch]

Oh yes. The natural gas industry has been replacing gas operated valves with with solar-electric, or gas fuel cell-electric power to eliminate CH4 emissions for over 20 years now. In another thread yesterday I mentioned that stopple fittings are favored instead of venting to atmosphere, or a vent to a flare burn. Oil production used to vent stranded gas (when it can't reach a market) to atmosphere. Since many years now in most places in the world it must be reinjected back into the well. But there are still some countries flaring it off. Vapor recovery on oil tanks is also required in many jurisdictions too. Even vapor recovery systems for transferring gasoline from gas station tank to automobiles are found in some areas of the US. Since many years in California. But I've yet to see that anywhere outside the US ... that I can remember right now.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[stanier]

Vapour recovery systems are amndated in truck loading facilities in Australia. Thy are also in refineries. Gas is still flared at gas gathering systems.

 

[rgvrider]

I think there may be two issues:

1. In water industry air valves are used liberally because they can be. No one cares (too much) if water leaks and it's a nice safe lazy option to put many in. For HC, the fluid is worth allot more and generally it is not acceptable for any of it (including vapours) to be released. Therefore they don't have the same flexibility.
2. In process plants, the pipelines tend to be in operation nearly continuously, as processing as much fluid as possible usually equals more profit. Water pipes can sometimes sit around unused, making it more likely air will settle in high points. I believe that due to the large size of many water pipe, vacuum valves are used to reduce drain and scour times.

In small membrane water treatment plants we have installed the pipe with only manual bleed valves to aid in filling. Generally the higher velocities will drag the air through the system. What sorts of velocities do you see in HC pipe? Do they have the same gradients as water pipes?

 

[stanier]

Air valves represent a high maintenance item. With chloramines in the water the EPA is becoming more concerned about leakages.

Many air valves serve no useful purpose except for filling and draining of water pipelines.

Suppliers software always over specifies the number of valves. And why shouldn't it for they are in the business of selling valves.

 

[rgvrider]

Laziness?

There is no reason why air valves need to be placed anywhere but local high points. Even then I can't really see the need for them, apart from to aid filling as I find it unbelievable that so much air could enter the pipe.

The other reason may be the water industry trying to minimise the pipe. Most water pipes can't handle any vacuum, so they'll introduce vacuum valves, which introduces air. In HC I believe most of the pipe is real schedule pipe, hence more suitable for vacuum.

Sorry I'm not answering the question on where to find technical papers.

 

[BigInch]

This might be of some use,

http://www.eng-tips.com/viewthread.cfm?qid=166360&page=14

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[cvg]

myth 1 - many waterlines sit around unused some of the time

Water transmission lines rarely sit around un-used. They generally run 24-7 365 days per year. Distribution and service lines generally do not have any sort of air valve installed regardless of whether they are sitting around or not...

myth 2 - vacuum valves introduce the air into the lines

not really, vacuum valves are installed only for emergency service if there is a line break and the line drains suddenly. Yes air comes in and when the line is repaired it is re-filled and the air is purged.

myth 3 - waterlines are not "real schedule pipe" so they can't handle vacuum

waterlines are designed for pressure flow. air valves are designed to release air during filling and to release accumulated air during operation if velocities or pressures are not high enough to prevent the accumulation. Vacuum release valves are strictly a safety feature built into combination air/vac valves to prevent collapse. For most waterlines, they are not really needed.

myth 4 - No one cares (too much) if water leaks

leakage is a concern, excess leakage is a liability. However, the sheer number of (hundreds of) miles of waterlines in the average city make the water distribution system very expensive. Sure, leakage standards could be improved at significantly greater cost to the water district. It is a matter of economics, especially since your tax dollars are what is paying for the water system, zero leakage is just not affordable or obtainable with limited funds. We all wish we could be working for a big oil company selling crude at $70 per bbl. Water sells for a few hundred dollars per acre-foot. Bonus question, how many bbls in an acre-foot?

are any waterlines installed without air valves? - yes, air and vacuum release for filling and draining can be done without a valve. Sometimes a fire hydrant can be opened or a blowoff / fill pipe can be installed to facilitate the process. Surge protection can be provided at the pumps with surge tanks. waterline profile can sometimes be designed without local high points. then no valves would be required or necessary.

 

[rconner]

The 1962 AWWA Journal article of R. Richards re "air binding" et al I refer to near the end of past thread

http://www.eng-tips.com/viewthread.cfm?qid=191919&page=97

is a good reference in this regard.

 

[dcasto]

Actually, in hydrocarbon plants we do worry about places where gases form and would "vapor lock" the lines. You will see on drawing phrases "do no pocket" on reboiler return lines or on vapor recover/flare systems. We will place high point vents in lines.

Thermosyphon reboilers require great detailed looks at what happens if there is not sufficient liquid head to move the fluids.

My favorite is injection wells. Gas and air can be entrained in the water or fluid that is being injected underground. Those gases break out and float to the top of the casing. when this happens, the level of liquid drops and therfore the bottom hole pressure isn't high enough so the pump pressure climbs until it reaches its mechanical limits and the whole process stops. This requires burping the air out. I've seen automated systems that do this.

 

[BigInch]

drifting, "lines carrying crude or gasoline".

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/