10 replies to this topic

[56200358]

Hello,

we want to transfer water by gravity(length of pipe is more than 1000m). In design criteria max velocity for gravity line is 0.6 m/s.

why shall be velocity low in gravity line?If the height difference between the source and destination is such that the fluid is transported to the destination, but the velocity in the 18-inch line is 2 m/s, is there a problem?

[katmar]

I will give a short answer here, but you really should not ask a question in an existing thread if your question is not relevant to the thread. Maybe one of the admins will be kind and move your question to a new thread.

In my understanding, the low velocity recommendation for drains applies to drains where the the inlet is open to the atmosphere and it is possible to draw air into the pipe. High velocities will entrain air which can cause noise, vibration and other problems along the line and also at the discharge point.

On the other hand, if the inlet is always flooded and no air can be entrained then there is no reason to avoid high velocities. One exception would be if there are entrained solids and you need to avoid erosion. In pumped water schemes where water is pumped to a higher elevation when excess electricity is available, and then the water is run down again to generate electricity when demand is high, the velocities often exceed 10 m/s.

[Pilesar]

Your design criteria may be based on keeping the Froude number low enough so that the line is self venting for gravity flow. Will the line have pockets where vapor could be trapped and impede flow? In your pipe, bubbles in the line will be carried along with the water unless vapor pockets can form. If bubbles are no issue, then I see no problem with water in gravity flow at 2 m/s in an 18-inch line. Larger lines have lower relative friction due to wall effects and can handle higher velocities. If I were building this pipe, I would consider a smaller line size. Calculate the erosional velocity and calculate pressure drop. 

[_1angelia23]

Hello,
we want to transfer water by gravity(length of pipe is more than 1000m). In design criteria max velocity for gravity line is 0.6 m/s.
why shall be velocity low in gravity line?If the height difference between the source and destination is such that the fluid is transported to the destination, but the velocity in the 18-inch line is 2 m/s, is there a problem?

According to Norsok-p-001,
the velocities shall in general be kept low enough to prevent problems with erosion, water-hammer pressure surges, noise, vibration and reaction forces. In some cases a minimum velocity is required.

Edited by _1angelia23, 05 March 2025 - 02:27 PM.

[_1angelia23]

 

Hello,
we want to transfer water by gravity(length of pipe is more than 1000m). In design criteria max velocity for gravity line is 0.6 m/s.
why shall be velocity low in gravity line?If the height difference between the source and destination is such that the fluid is transported to the destination, but the velocity in the 18-inch line is 2 m/s, is there a problem?

According to Norsok-p-001,
the velocities shall in general be kept low enough to prevent problems with erosion, water-hammer pressure surges, noise, vibration and reaction forces. In some cases a minimum velocity is required.

 

In addition see attached typical velocities as mentioned in" Rules of thumb for chemical engineers''

Attached Files

•  drain.jpg   35.63KB   1 downloads

[56200358]

 

 

Hello,
we want to transfer water by gravity(length of pipe is more than 1000m). In design criteria max velocity for gravity line is 0.6 m/s.
why shall be velocity low in gravity line?If the height difference between the source and destination is such that the fluid is transported to the destination, but the velocity in the 18-inch line is 2 m/s, is there a problem?

According to Norsok-p-001,
the velocities shall in general be kept low enough to prevent problems with erosion, water-hammer pressure surges, noise, vibration and reaction forces. In some cases a minimum velocity is required.

 

In addition see attached typical velocities as mentioned in" Rules of thumb for chemical engineers''

 

Thanks Veteran. In attached table has written for "gravity separaator flow". My project is transfer water from 50000m3 reservoir at top of site to 10000m3 reservoir in bottom of site by gravity (more than 1000 m).in fact this is transfer by gravity in pipe line not transfer by gravity in line between two equipment.

Edited by 56200358, 06 March 2025 - 12:31 AM.

[_1angelia23]

Hello,
we want to transfer water by gravity(length of pipe is more than 1000m). In design criteria max velocity for gravity line is 0.6 m/s.
why shall be velocity low in gravity line?If the height difference between the source and destination is such that the fluid is transported to the destination, but the velocity in the 18-inch line is 2 m/s, is there a problem?

According to Norsok-p-001,
the velocities shall in general be kept low enough to prevent problems with erosion, water-hammer pressure surges, noise, vibration and reaction forces. In some cases a minimum velocity is required.

In addition see attached typical velocities as mentioned in" Rules of thumb for chemical engineers''

Thanks Veteran. In attached table has written for "gravity separaator flow". My project is transfer water from 50000m3 reservoir at top of site to 10000m3 reservoir in bottom of site by gravity (more than 1000 m).in fact this is transfer by gravity in pipe line not transfer by gravity in line between two equipment.

the velocity for gravity liquid drain line, is also mentioned in the table (1.5 - 4)ft/sec

[breizh]

Hi,

Consider reading this document to support your work.

Katmar and Pilesar have given you good pointers.

Good luck

Breizh 

Attached Files

•  gravity_fed_system_2_sizing_en.pdf   4.57MB   10 downloads

[Pilesar]

Breizh, you find such interesting papers! That one makes me want to find a poor village to get water to.

[56200358]

Thank you all for the answers.

[breizh]

Hi,

Thanks, Pilesar.

many places around the world are waiting for you!

Breizh