Minimum Allowable Velocity in Pipe

[IGSAM]

Hi,
I am working on installation of water wells. The design is for 15 wells in total, each wells should give me a maximum of 50GPM. In the design we received each group made of 5 wells is connecting to a 100mm HDPE line and the 3 groups branch together in a 200mm HDPE. At the moment we can only start up one group, which means I would only have 250GPM in the 200mm.
My research on the internet mostly ended up about Maximum velocity in lines and I found no serious documents on how to calculate the minimum velocity. I read somewhere that the 'rule of thumb' for the minimum was between 0.8-1m/s. From my calculation I would only have 0.14 m/s in the pipe (but than again, even with the 15 wells I wouldn't reach 0.8m/s ... ).
So my question is, is it dangerous when I get in very low density that the water will 'stall' ? If so how do I calculate it ?

Thank you,

 

[bimr]

The minimum fluid velocity is generally set to maintain solids in suspension. That would be approximately 2.5 ft/sec for pipe installed on level grade. If you do not have solids or gas, you do not have to worry about the velocity. As long as you have some flow, the water will not go stale.

 

[EmmanuelTop]

With 250 GPM in 8" line the velocity would be close to 1.6 fps (~0.5 m/sec). Like bimr says, unless you have issues with solids which you want to keep suspended in the flow, the 1.6 fps velocity should not create any problems.

Dejan IVANOVIC
Process Engineer, MSChE

 

[IGSAM]

Thank you guys,
It answers my question.
And yeah you are right Emmanuel, my mistake I meant 50LPM.

 

[LittleInch]

You need to work out the Reynolds number for your low flow and see if you're fully in turbulent flow or not. Turbulent flow is recommended for most flows to keep the contents fully mixed.

This flow seems quite slow (note 0.14 x 3 = 0.42 m/sec for all 15 wells) so unless you have some very serious head loss issues, you seem to be using a bigger pipe than you need to...

Otherwise I agree with the others - there is no recognised minimum flow for lines with no solids.

The only other issue with low velocities is that they will not sweep out any bubbles or areas where air can gather e.g. at the top of a hill and this can lead to sluggish flow or surging if it gets too bad and air is not bled out. Also pigging becomes quite difficult below 0.5m/sec and the pig tends to "hop" along the line.

This all applies if the pressure is maintained in the line to prevent slack flow, but you haven't given any details.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IGSAM]

I do find that the 200mm is bigger than necessary. I am not too worried about the head loss, the pumps curve are design for a efficiency range of between 480ft-920ft head. Which to me I also find is an over design as the total head loss will be at around 300ft. But I am not in the design team, I receive the drawings and material to supervise proper installation.
From my calculation I will be in the fully turbulent flow.
Good point for the air accumulation, I will make sure to install a saddle + valve at the high points.

Pumps will be running 24/7 and a expending pressure vessel is connected to the system, so pressure should be maintain.

 

[LittleInch]

If total head loss includes the static lift and the minimum maintained pressure then it looks like the pumps will go off end of curve, but difficult to tell from a distance.

If so you might find the system very difficult to start or commission and hence it will become your problem....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

Quite a lot of solids can often be generated internally from a number of sources, such as weld beads, collecting water / bacteria slime, tank junk, black powder and corrosion.

I haven't heard of a well that wouldn't be susceptible to getting some sand out of it at one time or another. A reasonable minimum line sweep velocity is a good idea to keep in mind.

 

[bimr]

The system may be designed for the installation of lager pumps at a later date.

 

[DubMac]

Water wells almost always have sand, and sometimes in slugs. 5 fps is the very minimum velocity inside the pump column; assuming it is a vertical turbine.