sizing valves

[duika]

i need to calculate the size of the draining valves for a pipeline. can anybody help with formulae?? usually we project a size of 8 inches but i want to check the criteria of this value.

 

[DLANDISSR]

Duika:
You do not give enough information.

The usual criteria is to drain the line in some time period.

To do this you would need:
The main pipe size; Length; elevation difference; fluid density and viscocity; type of valve; drain pipe size, length and geometry.

Given the above information an estimate of the time required to drain the pipe could be made.

 

[duika]

Dlandissr:

The information we have in this moment is:

The main pipe size: 40 "
Length: 580 m
Elevation difference: 82 m
Fluid density and viscocity: water 20ºC
Type of valve: globe
Drain pipe size: the same of the valve we choose

Do you think we can drain the pipeline in less of 1 hour?

 

[TD2K]

You have about 125,000 gal in that line. Over 1 hour, that is an average flow of about 2000 gpm, within the capacity of an 8" valve.

Since I don't have experience in this, I would want to do some quick hydraulics to see whether I will get that flow out through my 8" drain line (is it short or very long, does it go to an open pond, tank or just on the ground?).

Assuming during the draining the pipeline is open to atmosphere, I'd look at the initial pressure to your drain line as a result of the head in the pipeline and check the flow rate. You said you had 82 m elevation change so the inlet pressure will be about 115 psi, a fair amount of pressure. Estimate using this flow how much water is removed in 10 minutes, check where the water level should be in the pipeline, adjust the head and therefore flow and continue (shorten or lengthen the intervals as needed). Those calculations shouldn't take very long and will give you an idea if the hour is a reasonable estimate.

That's likely overkill to anyone with experience in this area but since I don't have a feel for it, it would give me, with a minimum amount of work, a good idea if the hour looks quite reasonable. If the time to drain it isn't critical, say 1 to 4 hours is fine, the 8" line should be fine.

 

[TD2K]

One other point. Make sure you have sufficient venting capacity to allow replacement air to get into that pipeline as you drain the water to prevent a vacuum forming. I don't know what the vacuum rating of a 40" pipeline typically runs but I'm guessing its quite a bit less than full vacuum.

 

[duika]

TD2K

Thank you for your comments. i think your figures are ok.

in order to prevent the vaccum forming we are to project an air valve of 12".