Rate of flow from a pipe drain

[Zalamay]

I am trying to calculate rate of flow from drain of pipe. pipe size is 6" about 100 feet long, and drain size is 3/4". It is a gravity flow and medium is water. there is a large piping system which we have to drain for shut down, we have to calculate the time it will take to drain that.

Thanx, Zalamay

 

[cvg]

so, how long do you think it will take?

 

[chicopee]

Bernouilli's equation is a start. Assume pipe is flowing full and make adjustements for entry & exit losses, friction factor and density of medium, elevation changes. Perhaps velocity changes and back pressure building have to be accounted if large piping system plays a factor while the 6" pipe is draining.

 

[ione]

This has been discussed many times here (just to quote one thread

http://www.eng-tips.com/viewthread.cfm?qid=282722&page=11

or look for gravity fed pipe).

In gravity fed pipe all the potential energy (available head) is dissipated as pressure drop.
You can use D’Arcy Weisbach equation for friction losses, running some iterations with different flow rates, until results converge. You have also to account for entrance and exit losses as pointed out above.

 

[Zalamay]

Is it correct to assume 6" dia pipe, 100 feet long as a vessel and calculate time for draining it through 3/4" drain.

 

[bcd]

Yes, but you need to account for the reduction in pressure as the fluid level in the pipe decreases. Initially, you will have a differential pressure across the 3/4 inch drain that equates to the full height of the fluid. To get time, you will need to break this up into segments as the flow rate out of the bottom will decrease as the fluid level (and pressure) decreases. One way to get an estimate is to break the problem down into say...10 sections. Calcuate the average DP for the 1st 10 feet and the flow rate out the bottom for that DP. Convert the flow rate to time using the volume for that segment. Then do it for the next segment and continue. Add up all the times and you will get a close estiamte of the time to drain that takes account of the lower flow rates as the vessel drains.

 

[btrueblood]

You do have a way to vent air to the uphill section of the pipe?

 

[ione]

Wow, what a slender tank!

Have a look at this thread

http://www.eng-tips.com/viewthread.cfm?qid=289494

 

[Zalamay]

Thanks every body for the response, What is the best/easy way to find out/calculate Pressure drop (DP).

Thanks once again

 

[waterpipe]

Zalamay,

Do not forget two issues:
- The role of the pipeline slope (steeper--> more discharge)
- To control the maximum flow velocity during draining

Depending on your pipe slope and drain velocity, you might need to use an orifice at the discharge to limit / control the drain velocity.

Then I would like to point out that the correct direction during the design phase is to size the drain to reach a certain acceptable draining time. you can not wait 3 days to drain 5 km of you pipeline. So normally you size the drain valve to achieve a certain draining time. However you are doing it vise versa here, looking for the drain time based on the size of the drain valve. I understand this could be a case dealing with an constructed pipeline.

Once, I had written an Excel worksheet to calculate the draining time based on the pipeline diameter, length and slope. Unfortunately I cannot find it in my archive at the moment. While I keep searching for it, you might find this article of particular interest:

Procedures for Calculating Drainage from Pipeline

Hope this would help.

 

[Zalamay]

Waterpipe, Thanks a lot for the valluable advice I will greatly appreciate if you can show your EXCELL sheet also.

Thanks