Airlock or other problem with gravity feed piping

[Olmec Sinclair]

Hi all,
(apologies in advance for metric.... if this is an issue I can do conversions, hopefully you get the general idea)

I have a length of about 80meters of 60mm diameter poly pipe that collects water from a small stream and transports down a creek bed and then back up onto a terrace. The height difference from inlet to outlet is about 0.25meters. Height difference from inlet to lowest point is perhaps 15 meters.


My problem is that the pipe only ever runs at about 20% capacity. I am assuming there is some kind of airlock in the system... or could it be due to pipe friction and small elevation change?

Considerations / observations

• The first 15 - 20 meters of the pipe has been covered by debris, could be a problem under here
  
• There is a ball valve at the lowest part of the pipe so I can drain. If I open this I get a very powerful stream of water (full 15m of head).
• Opening the valve and slamming closed it makes an impressive hammer but doesn't

Any ideas on what might be preventing a full flow at the outlet? Tips on how to locate the location of the airlock if that is likely culprit. Any other wisdom or advice much appreciated.

 

[frodo]

metric messed with my head.

 

[FishScreener]

A quick guess, not having done the math, is that the friction loss in 250-ft, of 2.5-inch pipe is pretty close to the head difference of 10-inches.

At 10-gpm flow, the friction loss, (ignoring the fitting, and entrance/exit losses), would be 4-inches. Which leaves only six-inches for any fitting and entrance/exit losses.

Good news is: that if you add a pump on the outlet, it will have positive suction head, and hold a prime. I’m assuming this is an off grid location. There are some good solar powered pump systems available.

 

[Diehard]

What exactly does, "runs at about 20% capacity" mean?
That pipe is 20% full?
You show 0.25 meters of head from entrance point to exit point. What about head of water above the entrance point of the pipe? Or is that what the 0.25 meters is representing.
What's your flow in liters/min?

 

[Diehard]

Regarding that Hydraulic Ram Pump, that is fascinating, to say the least. I can't believe I worked for engineering firms for over 40 years, doing miscellaneous hydraulics for a variety of applications and never once heard of this principle. Not that we ever had an application for it but I would of thought, I would have heard or come across it somewhere. I love it!!!

 

[Olmec Sinclair]

What exactly does, "runs at about 20% capacity" mean?
That pipe is 20% full?
You show 0.25 meters of head from entrance point to exit point. What about head of water above the entrance point of the pipe? Or is that what the 0.25 meters is representing.
What's your flow in liters/min?

Yes, I mean that the pipe is only 20% full at the outlet. If I lift the outlet I can raise it about 0.25 meters before water stops coming out, this is how I am gauging the head....
At the inlet, the water in the stream just covers the pipe by a few centimetres / inches
The flow at the inlet can be quite high but if it can't go down the pipe it just continues down the stream

I have made a ram pump like this and it worked well until the stream goes dry and requires restarting....
http://www.blockhill.co.nz/pumping_with_free_energy
this stream is quite variable in flow so I am trying to keep things simple.

FishScreener might be correct about the pipe friction. Anything I can do to confirm this is the case? I supose I could pump water into the pipe to force out any air and see if this improves things (until the air accumulates again...)

 

[Diehard]

Yes, I mean that the pipe is only 20% full at the outlet. If I lift the outlet I can raise it about 0.25 meters before water stops coming out, this is how I am gauging the head....
At the inlet, the water in the stream just covers the pipe by a few centimetres / inches
The flow at the inlet can be quite high but if it can't go down the pipe it just continues down the stream

I have made a ram pump like this and it worked well until the stream goes dry and requires restarting....
http://www.blockhill.co.nz/pumping_with_free_energy
this stream is quite variable in flow so I am trying to keep things simple.

FishScreener might be correct about the pipe friction. Anything I can do to confirm this is the case? I supose I could pump water into the pipe to force out any air and see if this improves things (until the air accumulates again...)

Have you measured the flow rate so as to allow you to determine what the friction factor would be at that flow?