Air Vents Again

[Vortechs]

I have a micro-hydro that's been running for 20 years. It's a 3" HDPE pipe with 130 ft of head. I have always had air issues with the system due to the nature of the environment it's in.

The system is on a rocky mountain side where the pipe runs downhill over parallel ridges of rock which means there are unavoidably some dips, though I have done my best to minimize them. The intake filter is at the bottom of a rock pool which is fed by a waterfall. The water comes off of the mountain and contains a lot of small particles of peat which build up in the pipe and coat the insides.

To cope with all this I need to regularly flush the pipe through full bore to clear air and peaty goo. Last summer I spent 2 days cleaning out the pipe with a rubber scrapper system that I came up with, this increased the flow a lot and surprisingly so did the new adjustable nozzle I installed at the same time, the increased  power output was great but also led to it sucking more air bubbles in and worsening the air problems.

Is there a self bleeding air vent system that I could install on this setup to deal with this? I've tried researching but cant seem to find much information on it.

Thanks,
Trev

[Mary B]

Not self bleeding but a vertical off the high spots where air would collect would capture it. Add a valve on the end to open and bleed air out. If you make it 2-3 feet tall it can trap a lot of air before needing to be bled off.

[Vortechs]

Unfortunately the top end of the pipe is very inaccessible and I need to vent the air regularly. Most of the air seems to accumulate just where the pipe rises up from the bottom of the pool through the front of the dam then drops away steeply down hill, at that point the top of the pipe is only just below the water level of the pool, so doubt it would be able to push the air up out of it anyway.

[MagnetJuice]

I wonder if drilling a 2mm hole at the highest part of the pipe would work.

A small amount of water could come out of the hole, but not enough to affect the output.

If that would work, it would be maintenance-free.

Ed

[joestue]

I would go with a bigger hole. 1/4"poly pipe is like 10$ for 100 feet. Can run it up the hill far enough to reduce the waste flow.

[camillitech]

I  had similar issues with my system 70M of head and 900M long 90mm MDPE penstock. Drilling a small hole at the high spots certainly cured the problem but then people kept reporting the leaks. This was fine when it was to me but I became concerned when I was approached by the local water company because some concerned tourist had spotted the 'leak' in my black pipe. This was despite it being some 7 or 8 miles from the nearest mains water pipe and in the middle of nowhere. Consequently I fitted 'automatic air bleed valves' for water irrigation systems at the high points. Sorry my Internet is pish at the moment and I can't find the link. They were plastic around £20 each ,1/2" BSP threaded and have been doing their thing for about ten years.

I also see you have bother with peat lining the penstock, me too and I recommend you 'pig' it. Works a treat, I was always frightened of doing it and the 'pig' getting stuck but I've done 3 or 4 now on penstocks of 450 and 900M with no issues, piece of cake ;-) https://lifeattheendoftheroad.wordpress.com/2022/01/22/pigging-the-penstock/ Good luck

[camillitech]

Here is the valve I used at last ;-) https://javelindirect.co.uk/product/1-kinetic-air-release-valve/

[Vortechs]

camillitech! That's really great info thank you

Is there a minimum distance or height down from the filter for the bleed hole/valve? I cant get over the feeling it would just suck more air in there at the top. Obviously it must be at a lower point than the level of the intake filter as it says: "This can also be used in pipelines to avoid siphoning. Once the pressure drops in the line, the valve opens and lets air back into the pipe, breaking the siphoning action."

Nice blog! I salvaged about half of my 250 meters of 3"  (90mm OD 75mm ID) black HDPE pipe from beaches around here, it's apparently used by the salmon farms as a float around the tops of the cages.  Had some great fun trying to get it into a roll small enough to tie on my car trailer, ended up with a 12 foot roll tied vertically on the trailer - got some strange looks on the way back home

I was terrified of the 'pig' pushing a big sausage of peat down the pipe and blocking it solid. After 18 years running continuously most of the pipe had about a quarter inch coating the insides, so I made up a scraper for drain rods that would allow the water and peat to pass the scraper, that meant I could do it while the pipe was still partially running. Here's a picture of the scraper I made. With thirty 1 meter drain rods I was able to do it with a connector at least every 60 meters, which meant I only had to put one extra one in. How did you get the rope through the pipe to pull the pig through, that was my other worry about using one of those foam ones?

[bigrockcandymountain]

Usually a pig is just pushed by the upstream pressure.  No rope or anything required.  They work pretty good.  All the gas fields use them for keeping water and sludge out of their buried pipelines. 

If you think about a 3" pig with even 30psi behind it, that is 212lbs pushing it down the tube.  Hook an air compressor up behind it and up it to 120psi and your over 800lbs of force. 

I would probably do a series of pigs starting with quite a loose fit for the first cleaning.  After that, a quick pass here and there should keep the pipe happy.

[SparWeb]

No direct experience with hydro but I am wondering if this has been tried: A settling tank?

If a tank is installed near the upper end (but not too close to the intake) then the velocity of material passing through the tank would be slow, allowing solids to drop to the bottom before the water flowed out the other end.  Air would rise into a head space.  If the tank is lower than the water intake, a riser would keep the head in balance.  I could see this looking a lot like a septic tank but adapted in ways to allow much more flow.  Even the pump-out of accumulated debris could be done with the typical suck-truck.

Just seems like it would help both of your problems at the same time, but I wouldn't know if it has a fatal flaw that hydro users would know about.

[MattM]

People put very tiny holes in pumps to allow gas leakage.  The fluid loss is neglible if the hole is kept extremely small.

[camillitech]

Sorry for the delay, Christmas got in the way  The vents I just put at my high points. I think there are 3 or four along the 900m length. The first one is around a couple of hundred meters from the inlet. Perceived wisdom is to install an 'air break' quite close to the inlet. A kind of Tee with a standpipe pointing skywards well above the height of the inlet. This allows any air in suspension to escape before it goes down the line. I have done perhaps 3 or 4 hydro schemes without this but all the ones I see 'professionally installed have it and the one I'm currently installing needs it. As for 'pigging' as has been mentioned, it is common practice in the oil and gas industry and there is an awful lot of pressure pushing your pig. I had exactly the same misgivings as yourself about my peaty pipes. I'm in NW Scotland and it's the peat that makes the whisky taste so good  Just turn your hydro off, stuff your pig down the inlet a few inches then turn it on and wait for the water and peat to come out the other end followed by the pig. A well designed system, unlike mine and probably yours will have an exit Teed into the penstock just before the turbine. If not you'll have to remove your jets or break a joint for the pig to exit. Usually one pass is enough but after 10 0r 20 years probably worth doing another. Once you are confident with the technique then a piece of sponge will suffice. Though I think I'd be buying one of the pigs with an abrasive front face to start you off. 

[camillitech]

People put very tiny holes in pumps to allow gas leakage.  The fluid loss is neglible if the hole is kept extremely small.

Aye, this does indeed work just fine until people keep banging on your door or worse still reporting the leaks to local authority's  I even placed a sign above my 'leak' saying it was intentional but that still didn't stop well intentioned citizens reporting it. I guess it depends where your scheme is but mine is seriously remote and it didn't stop it happening  :'(

 

[Vortechs]

Perceived wisdom is to install an 'air break' quite close to the inlet. A kind of Tee with a standpipe pointing skywards well above the height of the inlet.

Thanks for your reply, caillitech.

I can see that installing the Tee somewhere below the level of the inlet with a pipe up higher than the inlet would normally work, as it will allow the bubble to escape without the water leaking as it would maintain the level of the pool in the vertical pipe.

Talked to a retied plumber over xmas and he agreed with me that any hole above the level of the inlet will suck in air. The problem I have is the pipe drops steeply downhill from the inlet pool and all the air seems to collect in the hump where it rises out over the edge of the pool. When I bleed flush the pipe open full bore, I can count to 150 and then all the air comes out in one big burst, so don't think a vent lower down will solve the issue unfortunately.

I've ordered a few foam pigs and will try one out next summer when I'm cleaning the filter, if it works OK then I'll put one through every year, it would save me a lot of time as rodding the whole pipe took me 2 days work. Did you use the 3" foam pigs on a 75mm ID pipe, as the 3" says it's 100mm millimeters which is 25mm larger than the ID of the pipe, do they compress enough?

[camillitech]

Hi Trev,

would I be correct in assuming that you have in effect a siphon? That is your penstock is initially above the level of the loch or wherever you draw your water from? If so then yes your plumber friend is correct and secondly it will always cause you grief  Mine is the same, this is because as the water travels up the initial hump it's pressure decreases and any dissolved air in suspension escapes and forms bubbles in the high spots. However, so long as the high spots are below the level of the water this is not a problem. The issue is only ever at the initial 'hump' were the pressure in the pipe is below the pressure outside. As for the pig diameters I had the exact same dilemma as you and to be honest I can't remember which pig I went for. Good luck

[Vortechs]

It's a partial siphon, the pipe rises up from the bottom of the pool and goes through the concrete front of the pool, but the top of the pipe is about a foot below the top of the pool water level, so it doesn't need priming just self starts. In the past I mostly had air problems after heavy rains or in the summer droughts when the pool level drops and it sucks air, but now with the increased flow it seems to be sucking down bubbles from where the water falls into the pool all the time.

What kind of foam are the pigs made from that you used, is it like the soft yellow foam of a sofa cushion? The next size they have below 100mm is 70mm which would be smaller than the pipe ID.

[mab]

Are the bubbles a significant issue? If I'm running my system at low flow, a bit of air in the 'knees' doesn't have much effect, and at high flow, any air that's gotten in  tends to come out the bottom fairly quickly - as the bubbles go down the pipe the pressure rises and the bubbles become less buoyant.

I can see that with a waterfall dropping into your collection pool you are likely to get air bubbles continuously introduced, but then the standpipe method earlier ought to let them out.

On the subject of pigging: i too was a bit dubious about buying a 100mm pig for a 78mm i.d. pipe - might have been easier if i could hold one and gauge how 'squidgy ' it was - but I'd have had to buy one unseen,  and they described it as 'quite firm'.

In the end i made my own out of closed cell foam packaging (sort of foam used as an alternative to polystyrene in packaging (and pool noodles I'm told)) - cut disks of foam with two rigid plastic washers for the ends all held together with threaded rod. I don't have peat but have silt which can settle in a sump (dip) in the pipe. i don't know how peat would behave; but as the pressure behind the pig goes up as it gets further down the pipe, i think if the pig goes down the pipe at the top, then it should keep going as it gets harder to stop the further down it is.

If in doubt i suppose you could make a pig with a small hole to allow a flow of water to wash the scrapings out ahead of the pig? Or you could start with a pig made with a foam piston and loo brush head.

[camillitech]

Hi Trev,

just found the invoice for my pigs. They were described as 70mm 3" soft foam pigs and were perfect for my 90mm OD 75mm (I think) ID MDPE x salmon farm pipe. Got mine from http://www.pipe-equipment.co.uk/

Good luck

[Vortechs]

Thanks camillitech, that's a great help. I'll get one but might also try making a Squeegee like the 'Pull-Throughs' with the rubber rings, as if it can push/pull a foam pig through then why not one of them? It would be good to have something reusable that I didn't have to buy from abroad, and if it got stuck I could use the drain-rods with a hook on the end to pull it out.

Are the bubbles a significant issue? If I'm running my system at low flow, a bit of air in the 'knees' doesn't have much effect, and at high flow, any air that's gotten in  tends to come out the bottom fairly quickly - as the bubbles go down the pipe the pressure rises and the bubbles become less buoyant.

I can see that with a waterfall dropping into your collection pool you are likely to get air bubbles continuously introduced, but then the standpipe method earlier ought to let them out.

On the subject of pigging: i too was a bit dubious about buying a 100mm pig for a 78mm i.d. pipe - might have been easier if i could hold one and gauge how 'squidgy ' it was - but I'd have had to buy one unseen,  and they described it as 'quite firm'.

In the end i made my own out of closed cell foam packaging (sort of foam used as an alternative to polystyrene in packaging (and pool noodles I'm told)) - cut disks of foam with two rigid plastic washers for the ends all held together with threaded rod. I don't have peat but have silt which can settle in a sump (dip) in the pipe. i don't know how peat would behave; but as the pressure behind the pig goes up as it gets further down the pipe, i think if the pig goes down the pipe at the top, then it should keep going as it gets harder to stop the further down it is.

If in doubt i suppose you could make a pig with a small hole to allow a flow of water to wash the scrapings out ahead of the pig? Or you could start with a pig made with a foam piston and loo brush head.

The air has to be vented every few days at the moment. If I don't the pipe pressure drops by about 50% in a week. It doesn't come out the nozzle unless the pool level drops to where it's sucking in air, so that only ever happens in summer droughts. The drop from the pool is about 1 in 3.5 for about 40 meters, so it doesn't seem to be able to pull it down such a steep drop and stays at the top.

Best solution I can think of would be to hire a jack hammer next summer and cut a channel through the rock into the pool deep enough so the pipe is running up hill all the way and then re-concrete the front of the pool. Any air should then vent itself out the filter - just hoped there was an easier solution.

[camillitech]

Aye Trev,

Best solution I can think of would be to hire a jack hammer next summer and cut a channel through the rock into the pool deep enough so the pipe is running up hill all the way and then re-concrete the front of the pool. Any air should then vent itself out the filter - just hoped there was an easier solution.


Sadly you are probably right, it's the only way to resolve the problem entirely. However I seem to have got my system more or less OK, by covering the pipe with earth so the temperature difference is not as great. Digging it in as deep as possible and keeping my fingers crossed :-) This is the first year that I have not had to start off the siphon with a water pump. I put this down to 1, an exceptionally high water level due to the rain fall and the pipe getting covered with vegetation. I did the jack hammer thing when I installed it ;-)

[Vortechs]

The situation with the hydro pipe deteriorated until I was unable to completely bleed the air out at all and within a few days it lost all pressure. From the surface of the pool everything looked OK so I figured it had to be a partially blocked filter, unfortunately the river was in flood and it took 4 days before it went down sufficiently enough for me to be able to safely get in and pull out the filter. During that time I found out how crap my battery bank really is... 

After cleaning the filter I set the pipe and filter up higher in the pool so they're horizontal with no drop down. I can get away with this in winter as the pool is always overflowing, but it puts the filter closer to the source of the bubbles from the waterfall. Since doing it the stream has been in flood twice and I've had no issues with air at all. I think in the summer I will build up the front of the dam higher so the filter and pipe can be like that all the time. It'll mean my waders wont be high enough so I'll need a pair of chest waders to be able to get at it. 

Trev