negative pressure

[Tadas Jancis]

Hi,

Can anybody advise me how i should evaluate negative pressure at a node? I started using boreholes with pumps and pipe system include. At the end of system epanet shows negative pressure at a junction, for example: node elevation - 100, total head - 98.65, but outlet - 98 and result is -1.35. Is there any risk of vacuum of  something?

[Daniel Antonio Colocho Arévalo]

It means that EPANET is demanding that continuity equation be fullfill. This is because this is an obligatory point in hydraulic science.

 

The reason that this happen is because you have some pipe diameter so small that the headloss is so big. Chek your diameter and you´ll see that giving a mayor diameter your headache go so far.

Bye.

 

Daniel Colocho

Civil Egineere

Guatemala

2012/10/9 Tadas Jancis <tada...@gmail.com>

Hi,

Can anybody advise me how i should evaluate negative pressure at a node? I started using boreholes with pumps and pipe system include. At the end of system epanet shows negative pressure at a junction, for example: node elevation - 100, total head - 98.65, but outlet - 98 and result is -1.35. Is there any risk of vacuum of  something?

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Atte.

    Ing. Daniel Colocho
danco...@gmail.com

Paz y bendiciones a usted y su familia.

 

La forma en que jamás DIOS rechazará nuestra presencia es presentándonos humillados a lo sumo y humildes, reconociendo que sin DIOS valemos menos que la nada y que lo que nos da valor para ser merecedores de su amor es que fuimos redimidos por el Redentor Amado. (Sal 51:17 y  62:9)

 

La única vida que vale la pena vivir es aquella en la que Dios es primero, luego y por último. Todo lo demás será añadido con abundancia en misericordia, amor, prosperidad y benignidad.

[Tadas Janciauskas]

Headloss is not so big, it does not exceeds more than 10.

If i try to to simulate big headloss (reduce pipe diameter) then the pressure becomes positive, but i guess it is not correct way?

[Daniel Antonio Colocho Arévalo]

Can you share your design file to give a best advice? Is the *.net one.

2012/10/10 Tadas Janciauskas <tada...@gmail.com>

[Tadas Janciauskas]

attached for assessment..

[Stephen LaFrance]

Your valve 103 is set to "open" and it free flows.  The pipes are large enough that they can gravity flow to the reservoir without building up positive pressures.  Set the valve to "none" to control the flow and you´ll get positive node pressures.

[Tadas Janciauskas]

So the main and last condition is to use PBV and set setting for example 2?, i was specially left PBV in open position, because i thought it is rather normal to have negative pressure at the end of system. Us i understand, system simulates  what head they need corresponding to outlet elevation..

[Lucas Vasconcelos]

Negative pressures occurrence in EPANET means that in a given point the pressure within your piping is below the atmospheric pressure, assumed around 1 bar. This is, however, an unlikely situation in an existing system, as it is highly likely that an air vent or even an open faucet is allowing air to enter, causing an abnormal and hard to model situation that reduces the flow in ways that are hard to evaluate.

Also, EPANET has no limit for negative pressures. In flawed models it even generates flow with some absurd negative pressures. There is a real limit, though, below of which the water boils in the piping. http://www.engineeringtoolbox.com/boiling-point-water-d_926.html

A system's curve represents the amount of head you need to cause a certain flow through the piping system in permanent regime. There are many workarounds to obtain this curve from Epanet, but its simulation results do not give this curve. What it really does for you is calculating the balance point to a given set of conditions (demand, diameters, rules, headloss coefficients, pump curves and etc...) using a model. 

This way, you can say (roughly) that the result shows you the obtained flow to a given pressure, and not the pressure needed to obtain a given flow.

[Tadas Janciauskas]

Hi,

sorry for delay..

This drainage model of excavation is going to be operate more than three years (i showed half of system), so it has work stability all that time. In my case there are several type of boreholes with known fluid capacity (all of them) and the problem is how to keep system stability when pumps are going to work separate depending on water level in borehole. Thats why tried to use PBV to control head when some pumps are stand by. Other wise i will avoid negative pressure at the end. Operational costs is not primary here, the main is system stability hole year.

At the end, is the any possibility to attain water hammer when junction is at negative pressure?   

[Lucas Vasconcelos]

Water hammer might cause water column separation during transient events if pressure is too low in a region. If it happens, water hammer models are likely to become untrustworthy. I've been testing WANDA for this month, though, and it shows credible results.

[Tadas Janciauskas]

What it is WANDA?

[Lucas Vasconcelos]

WANDA is a commercial software for transient analysis. As you asked about water hammer, I remembered it. It shows some credible results when the water column separates (as you have high regions and high points in your model, this possibility might bother you), and imports data from EPANET.

Unfortunately, as an extensive and precise water hammer analysis does not add value to my company's projects, we won't be able to buy the software (it is quite expensive for Brazilian standards), so I can not help you with it unless you finish your model before my trial license expires (wednesday). You can ask for one yourself, though. 

[Lucas Vasconcelos]

Oh, this is their website:

http://www.deltaressystems.com/hydro/contact

if you want to ask for a trial license! :-)

[Mohsen Amiri]

Hello everyone
Who can answer my questions about watergems?
Do you know another forum which relates to watergems?
Please help me.
Mohsen

[Paulo Guzman]

Sir can you help me solve my problem in epanet. I have a model but it says negative pressures in the report. should i send it to you so you can check? 

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[Faizan Ahmad]

can anybidy solve this issue of epanet

[Faizan Ahmad]

How to calculate diameter of pipe so for feed this diameter into epanet?

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