Do I need pressure break tanks or surge tanks on my water pipeline? 2

[Guru2]

Hi all,

I am designing a new water pipeline from one processing plant to another. The water will be pumped through a pipe of length approx 30km and diameter approx 20 inch. The elevation profile varies between 335m and 385m above sea level (see attached).

Our intention is to provide a back-pressure control valve at the end of the pipeline to increase the pressure throughout the whole pipeline to avoid it dropping below atmospheric pressure at the high points in the line. Is this the best method for avoiding cavitation in the line? It seems inefficient (in terms of "wasting" pump power), but I'm not sure what else to do.

Secondly, do we need to install break tanks or surge tanks in the pipeline? I have typically seen a break tank installed at the highest point in the line, but I'm not 100% sure why? Can anyone explain the difference between break tanks and surge tanks and where they should be installed? Feel free to indicate the recommended positions of break and surge tanks on the attached elevation profile.

Thanks for your help!

 

[stanier]

I suggest that you need to do a surge analysis before answering your own question. What you are asing for is some free engineering. You may get yourself in real trouble if you think these matters can be guessed.

Terminology in the water industry is loose at the best of times. A break tank may be a surge tank that allows flow in and out when there is a transient. Whereas a surge tank may be described as a one way surge tank where flow out is allowed but flow in is restricted to allow the surge tank to fill again slowly. Then there will be tanks with a dipping tube tha allows a cettain flow in but then the dipping tube rpevents this being exceeded. the trapped ai is then acting as a gas accumulator.

Surge tanks may be fitted with bypass check valves on their outlets.

Check out

http://www.pipingdesign.com

for some papers on surge mitigation and surge analysis. Also look at the following presentation I have delivered to IMechE/ASME/IEAust and others.

http://www.engineersaustralia.org.a...7-0528-B401-36CA-F0E6D07C1841&siteName=ieaust

Surge analysis is not simple and you need an engineer who has done it before to do the analysis and answer your questions.

 

[BigInch]

Break tanks are open to atmosphere.
Surge tanks generally operate at line pressure.
There can be some overlap in function of both. I will exploain.

--------------------- Break Tanks -----------------------

Break tanks generally control steady state line pressures, whereas surge tanks control transient waterhammer pressures.

A break tank open to atmosphere at the high point of a line, where the pressure is the lowest,

1) will always make sure that the lowest pressure anywhere in the line is never less than atmospheric pressure,

2) thereby also prevents creation of a vacuum in the pipeline and possible collapse of the pipe wall due to external pressure,

3) prevents formation of a vapor space due to pressures dropping below vapor pressure and the resulting possible pressure shocks.

4) prevents transients from vapor space creation when the pressure goes below vapor pressure and the transient created when the pressure returns to above vapor pressure levels

5) flow isolation forward and/or reverse siphoning prevented (up to the volume contained in break tank)

6) controls pressure at both the outlet of the upstream pipeline segment and the inlet of the downstream pipeline segment to equal the static pressure of the fluid level,

7) prevents transmission of waterhammer pressure and flow waves from one segment to the other (pressure changes across the segments are slowed to that of the rate of fluid level change of the tank, flow changes across the segments are slowed to the rate of change of volume in the tank)


------------------- Surge Tanks ------------------------

Surge tanks contain liquid and fluid volumes, usually under some pressure, but they are not always pressurized. Their function is pretty much limited to this effect alone.

Surge tanks limit the maximum transient pressure and flow by compression of (typ) air contained within the tank. A large change in fluid volume inside the tank produces only a relatively small change of the pressure in the tank, since the air is easily compressed. A small change in fluid volume without any air in the tank would cause a very great change in pressure, since fluids are much more difficult to compress with a given change in pressure.

Thus a surge tank accepts high flows of liquid with minimal change in pressure, thereby limiting the maximum pressure in the line at the tank location to the normal pressure at that point plus or minus the tank pressure change.



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

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

 

[msquared48]

In my experience, an open ended (atmospheric)surge tank is really nothing more than a venturi meter, with the water level reflecting the static energy head at that point in any point in time.

It is usually used in large hydroelectric projects to control surge, or water hammer, protecting the turbines and intake structure from damage in the event of an emergency shutdown. The height of the surge tank, plus a margin of safety, reflects the maximum increase in total head due to the surge.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[stanier]

A break tank may simply be a tank at the highest elevation. The rising main terminates at the break tank. The pipeline from here is a gravity flow line.

 

[BigInch]

I don't see a likeness to a venturi? A static pressure gage, OK.

A break tank can't control surge pressures. The fluid inside would simply adjust its level, free to reflect the instantaneous pressure of any pressure wave passing by. A vertical open-ended tube in the ocean wouldn't stop the fluid inside it from rising and falling with any passing wave. If the top was closed and the bottom was partially closed, then you might have the beginning of a surge tank design.


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

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