Siphon effect

[Hawkeyes]

Hello everybody,

 

I'm looking for some informations about siphon effect. I have to design a filter with backwashing working with siphon effect. So I don't understand how starts a siphon effect ? What are hydraulics equation to study this effect ? What is the influence about bend's form ?

 

Thank you for your help.

[insane_alien]

bernoulli's equations are all you need for analysing a siphon. they're quite simple. depending on the application, you may need to concern yourself with vapour pressure as formation of vapour bubble can cause the siphon to fail.

[Bignose]

I guess you could get really fancy and work based off the Navier-Stokes equations. But, i_a's got it right in that for the vast majority of applications, Bernoulli's equation is going to be more than sufficient. Especially if you find the Bernoulli equation with friction terms in it which will help you account for the influence of any bends.

[Mr Skeptic]

You could probably get away with using just the height difference (of the water levels of the siphon source and destination) to calculate the pressure at the end of the siphon. Of course, if you are wanting to use a siphon to siphon water into the same tank, that won't work so good.

[insane_alien]

to calculate the pressure at the end of the siphon.

 

the pressure at the exit of the siphon will be atmospheric assuming it has not been submerged. the pressure head is converted to a velocity head, this is the basic principle of siphon operation.

[Mr Skeptic]

For an open siphon, yes, but if he wants to use that to force water through a filter, that would slow the water down so it would have pressure rather than velocity.

[insane_alien]

no, the pressure will still be atmospheric, you'll just have a friction term in bernoullis equation.

[katmar]

Hawkeyes, sorry to reply so late - I hope you are still following this thread.

 

Designing a siphon system is not easy, and I would recommend that you engage a professional engineering consultant before you spend any money on piping and equipment. But I can at least give you the basic principles here.

 

The strategy that you have to follow is to get the driving force causing the liquid to flow to match up with the factors that counter that driving force. The driving force is simply the static pressure difference between where the water is coming from (i.e. the surface of the water in the filter) and where it is going to (i.e the discharge point of the siphon pipe if it is not submerged). This pressure difference is calculated by Pressure = density x gravity x height. Obviously the water must flow from a high point to a low point.

 

There are two factors that "consume" this driving force (these were mentioned by insane_alien in 2 separate posts). The first is the friction of the water flowing through the pipe and fittings, and the second is the acceleration of the water. The acceleration term is usually called the velocity head or the exit loss. A free program that will enable you to calculate the friction loss is available at http://emka.xs4all.nl/dP/

The practical implementation of the Bernoulli equation which incorporates the friction effects is called the Darcy-Weisbach equation.

 

You need to determine what flowrate you require to achieve the backwashing of your filter. Then you need to work out what size of siphon pipe will give you this flow with the available driving force.

 

That is the simple version. To actually design the system you need to look at it in more detail to ensure that the siphon pipe remains full and how you will start and stop the siphon. You could make some expensive mistakes here and it would be best to get professional help.