Physics/plumbing question

[Unclejake]

Dear all, I just installed several  140 lineal metre runs of 40mm Alkatene pipe (gravity fed) and am getting less than ideal water flow out of one end of the single pipe that's charged with water.

I may have a partial air lock..... but without spending ~ $400 on tee connectors, valves and freight (and a couple of weeks wait) I can't discount a partial air lock.

QUESTION: Is there a way to  test for an air 'restriction' before cutting the pipes at the obvious high point of the run and adding bleed valves? I.e. is the pressure going to be the same with or without the air restriction? I suspect a pressure test is useless but am no expert. The water flow seems lousy... but I accept there's a lot of friction on 140lm of pipe (even at 40mm i/d).

 

 

[h4nd]

Add height at the high end, and chuck a sponge in?

-and there's the pic, you've got height

[Unclejake]

^ Internet is slow here. Sorry. Can't do much about it.

What's your thinking wrt a sponge?

[azzurro]

Im no plumbologist, but

Could you coil the pipe in the low spot, run again to purge any air, close the tap then relay in the trench?

That highspot would always be where any air in the pipe would accumulate (from Say thd header tank being empty for whatever reason?)  so a bleed nipple thing there would be a good plan in the long run. Even a tee to a tap would do it

 

[Unclejake]

14 minutes ago, azzurro said:

Could you coil the pipe in the low spot, run again to purge any air, close the tap then relay in the trench?

 

That would work to expel the air,  but due to untold trees is physically  difficult and would only work once (if there's even an air restriction - and I honestly don't know if there is), but I'd have the same problem again if the tank ever ran dry. Laying the empty pipe through the trees is ideally a three man job. Managing it full of water would be next level harder. Two of us can barely lift an empty coil onto the ute. 

I'm trying to think of a way to test if there's an air restriction without drilling a hole in $4,500 of pipework, but I'll be buggered if I can think of a way!  I figure the pressure in the pipe would be the same with or without the air and I haven't thought of a better way to check it (yet). LOL

 

 

[NickJ]

Running the numbers through an online calculator gives this graph, pressure is roughly 1 bar per 10m of height.
https://www.copely.com/tools/flow-rate-calculator/

https://www.engineeringtoolbox.com/hydrostatic-pressure-water-d_1632.html

This graph will also be based on a straight bit of hose, bends etc add even more loss, anecdotally from troughs on the farm, flow rate really does become quite pitiful in 40mm even with a stonking great pump in the shed!

Looks like you'd expect to loose at least half the flow rate compared to at 28m from the tank if that fits what you're seeing?
 

 

[Unclejake]

Yikes. That's alarming reading. Thanks NickJ. I'll measure flow on Friday. If it's ~ 50lpm then I guess that's as good as I can expect.

Really appreciate it.

[ThePog]

Is the tank sealed? Ie can air get in to take up space as it is emptied or is it always fed by the high pressure side?

[Unclejake]

The tank is vented mate, but I understand why you're asking. 

For the avoidance of doubt: My first drawing wasn't that clear (sorry) The 8 metre and 18 metre numbers on the right of the drawing are vertical measurements. The tap that performs well is about 40lm of 40mm pipe away from the tank and about 18m below the top of the full tank. She's a gusher.

The tap on the left of the drawing is 140lm of 40mm pipe away from the tank and about 8m below the top water level of the tank.  That's the tap that I needed most (40mm ball valve with a 25mm PVC washdown hose and nozzle.... but I pulled the hose off the 40mm ball valve this afternoon and the flow was still lame.

Someone fax me some more gravity please. Ha!

[gibbon]

if it's a big enough slug of air you might be able to pick it up by tap-testing it? you'll feel like an idiot tap-tap-taparooing down a zillion meters of pipe though

I would probably do a proof of concept first, half fill a couple of meters of spare pipe, cap it, bend into an arch and test to see if there's a difference in sound 

[440bbm]

Whats the height difference in the 8m section between the tank outlet and the highest hump part in the middle. Whilst you are correct that the total amount ( 8m)is the overall fall  the part in middle rises and then back down you are better to remove that section from the equation for more accurate data. it does count but not accurately if the distance is actually over a bit of distance, it works agaisnt you not for.
You will definately  see a heavily reduced flow rate at only 8m fall compared to the 18m so really should have increased the pipe dia to help combat this. Sure, you wont get 'pressure' but you will have alot more volume which is good ( er ).

As nick said above, 1x atmosphere ( bar/14.5psi ) per 33ft or 10m of 'depth' ( fall )..

I highly doubt you will be experiencing an airlock, provided there is a reasonable distance between the hump in the middle and the water tank level. the amount of water flowing through SHOULD be fast enough to force that out. unless you can piss faster than the flow ( then there is another blockage somewhere )

 

Whats the frequency of using the 'wash down' hose in question?    may be cheaper to get a 12v half fucked battery and run an electric water pump and a solar panel to recharge the battery in these moments compared to changing he 140lm of pipe thats too small. 

As an idea of flow rate, if you increase to 50mm hose for that 140m section and use 0.8bar the flow rate doubles to nearly 100ltrs/min 

 

[Unclejake]

Thanks.  I should have done the maths before ordering the pipe. 50mm would have been much better

I reckon my high point in the 40mm is around 6m below the tank. 

I'll measure flow as soon as I get back there, but I'd guess it's around 30lpm. . I instlled the washdown to wash salty re-bar prior to concrete and to compact sand so it needs to be a decent flow. I'll have a look at diesel fire pumps as well as 12v. Long term this bore water will be toilet flush, washing machine and outdoor garden tap.

I ran an additional 40mm for rainwater from the same tank stand, but intended a 240v pump for the rain (which is about a year away from being use

 

Really appreciate the advice. 

 

[440bbm]

Diesel water pump also another great idea.

Or you could look at those 1000l IBC tanks or 44g drums to use as further storage where the upper tank fills these smaller tanks at the end of the 140lm 8m drop section( albeit slowly ) - so ideally you have  enough short term storage to use when you want to wash something, and have a short  but large diamter hose and there is ya volume.. it is a bit jerry rigged but works well and is cheap. 

[ThePog]

The hivemind is working hard out on this one, I am enjoying it.

[h4nd]

15 hours ago, Unclejake said:

^ Internet is slow here. Sorry. Can't do much about it.

What's your thinking wrt a sponge?

A weak moving seal to allow the water to push the air thru. I got the idea from watching the concrete pump truck boys cleaning after a splooge.

Nick J above seems to have your numbers though. If you pump from the tank up a U-bend another 10m, you'll only get ~ +30%, not worth the effort.

4 hours ago, Unclejake said:

intended a 240v pump for the rain

Just a heads-up. I worked on Purepods: The 240V pumps were very energy hungry compared to the same thing in 12 / 24V DC.  Only an issue if you're running off Photons, probably.

[morkster]

At our last place we had about 450m of 50mm from the artesian bore at the road with about 7m of fall over that distance.. 

I can tell you that if any air got into the pipe; flow stopped; the artesian wasn't enough to fill the pipe. I used to have to pump water back up the pipe to the road and reconnect to the bore while still pumping (this ensured no air).. then things would be rosy and good flow would resume

[440bbm]

22 minutes ago, morkster said:

At our last place we had about 450m of 50mm from the artesian bore at the road with about 7m of fall over that distance.. 

I can tell you that if any air got into the pipe; flow stopped; the artesian wasn't enough to fill the pipe. I used to have to pump water back up the pipe to the road and reconnect to the bore while still pumping (this ensured no air).. then things would be rosy and good flow would resume

what was the rate of fill / trickle that it came out at?  yes this would be the case provided the flow rate out of the artisian bore was lower than the flow rate of the 50mm hose then this is of course the case. 
doesnt have the pressure to push the resistance weight of the bubble out to resume flow.

[morkster]

'doesnt have the pressure to push the resistance weight of the bubble out to resume flow."

Yes i think this^ the artesian wasn't enough to fill the pipe at the high points. Once air free it flowed well with i think a siphoning effect too

[Unclejake]

Howdy gents, it's a beautiful day here so I abandoned paid work very early.

In reference to my first drawing: 

The lowest tap delivers 120 litres per minute without the washdown hose attached 

The troublesome tap delivers 67lpm without the washdown hose, but 37lpm through the 20 metre long 25mm i/d PVC washdown hose and nozzle. 

Ergo: I reckon I need a spray/stream nozzle better suited to low pressure than the Skellerup Master Blaster I was sent.

Should be an easy fix. Yay!

 

[440bbm]

Great work and even better work diagnosing the main source of restriction. 

Didn't notice you mention that you had another 20m of 25mm hose at the end of that 140lm. 

Get rid of the 25mm hose and put some 50mmm on it, and just adapt it down right on the nozzle for max cheapness. the nozzle should be OK but that 25mm hose will be the killer.