venting of pipelines 1

[alizafari1]

Hello to all;
We have a GRP underground pipeline for sea water transportation. Because of clash with another pipeline, a portion of pipe moves up and after a couple of meters moves down in the previous elevation, and therefore makes an air pocket in the line. Naturally we need a vent hole on the top of the elevated pipe for hydrotest. The question is: do we need this vent hole for venting of air before transportation of sea water? I mean can accumulation of air obstruct the pipe?

 

[zdas04]

A lot of people talk about gas as "obstructing the pipe", but that is not really accurate. I think of it as disrupting the pressure rebound on the downstream side of the hill.

When a line is liquid-full (disregarding friction) the pressure at a given upstream elevation will be the same as the pressure at the downstream of the hill at the same elevation. If the line is not liquid full, then the pressure at the upstream bottom of the hill will include the hydrostatic head of however much liquid is in the upstream leg (usually pretty close to the height of the hill). The pressure at the bottom of the downstream leg will be less because the downstream leg is farther from full. Sometimes the pressure at the bottom of the downstream leg will be very low for a significant hill.

Bottom line of all this is that if you are just jogging a few feet to get over the foreign line in pretty flat country then you are not going to see a huge problem. It can be big if there are significant elevation changes.

To evaluate the net effect of multi-phase flow, I generally get very close to actual conditions by doing a 4 step process: (1) calculate the single-phase pressure drop in the line (for gas I use the Isothermal Gas Equation, for liquid I use Darch-Weisbach); (2) calculate the 2-phase friction using Duckler (see GPSA Engineering Field Data Book); (3) I use the Flannigan Correlation to get the dP due to elevation changes; and finally (4) I add the three dP's together and am always within 20% of observed conditions (better than the high-dollar multi-phase flow models generally do). The second step for you probably isn't applicable since you don't seem to have much gas. The third step is important for your process. It is also described in GPSA. In this step you add together all the up-hill portions of your line traverse (disregard downhills). It can result in a big dP. In your case if the dP is more than you can tolerate put in the vent. If not, ignore it.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[rconner]

I think if you were to do a complete review of the literature, I believe it will review the answer to your literal question as worded is "yes". Will the degree of obstruction that at least initially results cause a problem with your specific filling and service conditions, I think that is another question. e.g. if flow velocity is high enough, it is possible the air might be in essence "scavenged" (removed). If you are hinting/leaning you might not remove the air at all, it probably should be mentioned however that along with the above mention most standards and literature say that air should be removed (as unnecessary complications in hydrotest/interpretation can result).

 

[bimr]

Agree with zdas04 that a couple feet in elevation change would not make much of a difference.

Manual air release valves are sometimes required during the initial startup of a water pipeline to allow the air to be released. After the initial air release, the valves are generally not required. Automatic valves should be used only as a last resort. Automatic air release and vacuum valves have been used on steep grades to prevent the collapse of thin walled pipe during a fast drainage during an emergency.

The correct term for the flow obstruction is air binding. Air binding may occur is the design of the pipeline system is inadequate. Designing the pipeline velocity to more air down the pipeline is usually the most attractive design approach. The velocities required are within the normal design range for transmission pipelines.

You have not presented actual the actual design condition so I have included some references to allow you to under the issue.

 

[BigInch]

"After the initial air release, the valves are generally not required."

I don't do water pipelines, but that certainly is not to my understanding, and most certainly not what the air valve manufacturers say. Their theory is that water can always contain a high content of air in solution that becomes free air due to temperature and pressure changes, making the air valves a continuous necessity for the lifetime of the pipeline.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[bimr]

"I don't do water pipelines" enough said. Do you really expect a supplier to tell you that his device is not needed? At best, he will tell you to put the device in "just in case".

The use of air valves and relying on air valves is risky because of uncertain maintenance. Designing the pipeline velocity to more air down the pipeline is usually the most attractive design approach. The velocities required are within the normal design range for transmission pipelines. The velocities are identified in the linked document.

 

[BigInch]

Interesting. So, totally unnecesary. There sure are a lot of air valves installed in these water pipelines. I see um all over the world. I was counting all the air valves on a pipeline I was driving by Saturday afternoon and they must have had two every couple of km, if that far.

You'd think that water pipeline engineers would catch on, but looks like they have not.

Thanks.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[BigInch]

They seem to have some convincing arguments. What happens when you have to stop the pipeline? Won't the air come out of solution when the temperature, or pressure changes? If it forms an air lock across a couple of real high points, how do you restart?

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[zdas04]

I do a lot of produced water pipelines and in those the free gas never stops being a problem. It has been over 30 years since I last messed with piping sea water, but as I recall, we had manual air vents on several of the cooling systems on the ship I was on. I seem to recall a PM that required periodic venting.

The trouble I have with steam-trap style air eliminators is that they always spit some water with the air, sometimes a lot of water. The pipe around them is always nasty and covered in rust. I've never seen a pipe failure from this surface corrosion, but I've had several clients that were really worried about it. I've got a provisional patent on a device that captures the gas for use, but it really only makes sense then the gas is a saleable product, it is too expensive for low-pressure air.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[bimr]

On the typical water transmission main, there is no problem starting back up.

I am not saying that air release or vacuum valves are never required, just that you should try not to minimize the use of them. If you have a real steep high point, an air release may be required.

 

[BigInch]

Now we're getting somewhere.

If it ain't broke, don't fix it. If it's not safe ... make it that way.