Gravity feed and Break Pressure Tank Help

[DodaPump]

Looking for some help on a gravity feed greenwater effluent line. I am new to these threads finding them while searching key words on the web. I work in the dairy effluent pumping business designing systems for farmers. Now I have been asked to design a gravity feed system which is outside what I have dealt with before. I have a HDPE lined pond with a 160mm drain pipe out the floor of the pond. There is 5 metres of head above the outlet. the outlet is 365 metres above sea level. I need to get this down to 60 metres above sea level. The pipe line will take a path down the hill that is 3.3 kilometres in distance. There is no power available along this pipeline. I would like to use 90mm alkathene with a 77.5mm ID, it is 8 bar rated. after reading some of the threads on break pressure tanks Im thinking of constructing 4 break pressure tanks, one every 80 metres of elevation drop down the line to 130 metres of elevation which should leave 70 metres of head pressure above my 70 m3 tank at the bottom. Not fully knowing about break pressure tanks, is this idea a sound way to get the effluent down to my irrigation equipment. I look forward to hearing back from big and little inch and all the others.

 

[LittleInch]

Dodapump.

It sounds like a reasonable way if you're limited on pipe you can use - Looking it up alkathene is MDPE or PE80??

Your issues as I see it are:
what are the properties of this stuff - "greenwater effluent" doesn't mean much to me - does it have solids, does it have fibrous materials?
A 90mm pipe isn't very big - what sort of flow rate are you thinking of? The drop is a 1:10 slope on average which might be enough under gravity, but have you checked.
How you control this line and how you start / stop / control is the key. Without power you are looking at fluid powered valves, but these might clog internally or in the control piping.
Iff there is unlimited storage at the end point and you can accept a whole line fill and the contents of the ponds then you might be able to just flow from one end to the other via the break ponds / tanks without risk of overflow. Please advise what the flow regime is - continuous, start stop / needing to be controlled?

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

 

[bimr]

You should consider the use of some type of screening to avoid plugging the line.

As long as the slope continues downward at a relatively constant slope, you can use a gravity line. Without a break tank of some sort, if you valved off the flow at the end of the line, you would over-pressurize the pipe with 305 meters of water head.

It will also be necessary to install air releases along the line, otherwise, the line will become air bound.

You should be able to get 12.9 l/s of water at the outlet.

 

[DodaPump]

Thank you littleInch and bimr for your response. firstly the green water is the term we use for the effluent after all the solids have been separated out. this is a 2 pond system where all effluent gravity feeds to the 1st pond then only the watery liquid is passed through to the 2nd pond. there are no fibrous materials or solids in this system. I will be guided by any suggestions made on pipe size for this project. MDPE PE100 8 bar pipe is the most common pipe used here for effluent lines. It is cheaper than the MDPE 75mm 8 bar pipe.
The drop is very close to 1 in 10 and is an almost constant slope as it goes down apart from 3 small rises before going down again. Once the entire system is connected to the pond exit pipe it will remain open to the bottom and under pressure at all times. The line that goes in will be trenched and buried. Every 100 metres of pipe length I will be connecting one of our steel 2 way steel hydrants with a gate valve on the inward flow leg. This for me is just a safety feature should any section of the pipe be damaged in future by diggers or machinery then they will have an isolating point every 100 metres. The exit pipe to the pond currently has a 150mm slide valve with a large cantilever handle on it, the pond is 2/3 full. What I would like to do is have a line to the 70 m3 tank at the bottom of the hill where we will be installing a progressive cavity pump to then irrigate out to the surrounding land.
the line going down the hill will always be open and under pressure back to the pond.
Ideally I would like to be getting about 10 L/s flowing into the 70 m3 tank. This might even be a 110 m3 tank. I have no knowledge yet about break pressure tanks but want to learn as fast as I can. I will be guided by your direction as to how many break pressure tanks we install.
I have seen some rough pictures of these tanks. Some have a ball cock on the inflow inside of the tank. Some had a pipe entering the tank and a pipe exiting the tank and no ballcock.
Excuse my ignorance on this subject but if I have a break pressure tank where the flow races in under pressure, swirls around the tank then exits out the other side and continues down the hill, is this a break pressure tank. Does the pressure coming into the tank not then pressure the tank to the same level and in turn pressure it directly out the tank. Or does this merely act to slow the speed of the flow down.
Regarding the comments from bimr, thank you for your help and anymore, I have planned again to put our steel two way hydrants on any high spot with a Europa swing check valve on the hydrant bridge. We usually always take the spring out, this is how I was planning to purge the line of any air when it was being filled the first time.

http://www.hitechenviro.co.nz

Getting to the bottom of this line, the feeder pipe will enter the 70 to 110 m3 open lid sump. This will need a ballcock arrangement in the sump to stop the flow when it is full. Will I need a break pressure tank quite close to the sump to lower the pressure on the ballcock.
On the safety side of this problem which is an environmental disaster waiting to happen is, what happens if the ballcock breaks for any reason. Do I split the incoming line into 4 and have 4 ballcocks to limit the possible flow should one cease to work. I can have the electrician wire up a high pond level alarm as well if that is a better solution.
If the break pressure tanks are the answer to this system then I would really appreciate some help on their design, how many and size. My apologies for being long winded, it is frustrating not having any knowledge on this. Thanks.

 

[BigInch]

Break tanks are open tanks, or vented tanks, open to atmosphere, so they always remain at atmospheric pressure. You cannot physically have pressure greater than atmos pressure at those points where break tanks are installed. They are a means for pressure control in your pipeline. You need to have one tank at a maximum of every 70m of elevation drop of your pipeline to ensure that pipe does not become over-pressured when flow stops, say for example if one of the valves were closed, the pipe clogged up, or the pipe collapsed flat, stopping flow but without actually breaking anywhere. Each tank should be able to hold the contents of the pipeline's volume in the distance to the next tank upstream. Minimizes chance of overtopping spills at any tank, if for any reason, you could not reach shutoff valves quickly.

 

[dhengr]

DodaPump:
You said..... “4 break pressure tanks, one every 80 metres of elevation drop” and “ MDPE PE100 8 bar pipe is the most common pipe used here for effluent lines. It is cheaper than the MDPE 75mm 8 bar pipe.” I haven’t run any numbers here, several of the other guys have those types of calcs. on the top of their heads, so I’ll leave that to them. But, a couple things come to mind here, those pressure break tanks can’t be cheap, what with construction, including foundations, the tanks and all the extra piping. Then there are also any problems and ongoing maintenance of the tanks, whatever that might be. Why not eliminate the four tanks and just use a pipe, fittings and control devices which can tolerate the pressure at the various elevations as you go down the hill. Changing the pipe size or wall thickness as you go down hill. Put an energy dissipating device at the lower outlet to control the outflow into the lower pond. Better yet, put a small turbine at the bottom of the pipe line to generate enough power to at least control operate your control systems on the line and any valves, etc. The power would be there as long as the fluid was flowing. You are proposing an expensive, more complex system just so you can use some cheaper pipe. Maybe penny wise and pound foolish.

 

[BigInch]

Because if he doesn't use break out tanks, he will need much higher pressure allowable pipe, or pressure relief valves and relief tanks. Anytime the flow is stopped, the pipe will overpressure at every 70m of elevation change.

 

[katmar]

If there are no valves in the line, and blockages can be prevented, then there is no reason for the pressure in the line to ever exceed the pipe rating. If there are local high points then those would have to be carefully calculated for pressures at the upstream low points. In a line of constant slope the flow rate will simply increase to the point where the friction losses exactly match the static head available.

A similar situation occurs in deep level mining where diesel has to be gravity fed to underground tanks to fuel the locomotives. Mines can be kilometers deep and the piping would have to be uneconomically strong. The problem is overcome by eliminating all underground valves and feeding from a batch tank on the surface.

It would be better to put your effort into extra screening at the source tank and doing a bit of extra trenching to avoid high spots and just letting the line run at its natural capacity. With the slope you have, air removal would not be a problem. You will get velocities in excess of 2 m/s and this will be more than adequate to flush the air out.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[dicksewerrat]

Use a better pipe and HDD the thing in. You will save a lot of time.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bimr]

One would think that finding a constant slope over 3.3 km would be an unusual scenario. That is why you need the break tank.

You can also use pressure reducing stations rather than break tanks.

 

[DodaPump]

Thank you to everyone that has taken their time to reply to this post. I can use different pipe sizes and bar ratings and am now looking at PN25 alkathene pipe in 90mm and 110mm OD. To bimr, is there some way I could get a design for a pressure reducing station. I have only found some sketches on the net that are not clear.

http://www.hitechenviro.co.nz

Allan Crouch. Thank you again

 

[BigInch]

And if the pressure reducing stations fail, you will still need that relief valve and relief tank.

 

[bimr]

Pressure reducing valve. These would typically be installed in a vault.

http://cdn2.hubspot.net/hub/230272/...ides/106-PR-Product-Pages.pdf?t=1436905042552

http://www.cla-val.com/documents/pdf/E-90-01.pdf

 

[cvg]

don't forget air/vacuum release valves. if you place a valve at the top and shut it quickly, you could pull enough vacuum to collapse the pipe. I am not convinced that break tanks are necessary, unless a flow shutoff valve is to be installed at the bottom.

 

[LittleInch]

dodapump,

Let me summarise where I think this thread is up to / going.

This system really is quite small which I think is being missed sometimes here. with your 90mm PE pipe, my estimation is that you have about 16m3 inventory in your pipe length of 3.5km.

Whilst your route picture is nice, what you need to develop from that is a fairly accurate profile to decide where to place your tanks if that's the way you go.

I think you have three main options
Plan A - Install your system as you described. What you need to do is try and get the first section to be the highest head loss per unit length so that this becomes the bottle neck in your system and so on down the line by a margin of 3- 5%. You can do this by judicious selection of the break tank location or perhaps adding a certain length of reduced size pipe. That way you can install a relatively small tank - say 3-5 m3, and just let the system run. As the flow capacity out of the break tank should exceed the flow in, the tank should run empty unless something happens to block flow. Then you have two options - either have someone shut down the inlet and then accept you might have a small spillage or install a simple float system to shut off flow if the level gets too high, but normally is wide open. If you don't want this option the install the float control valve but you may still find that some tanks are emetpy and some full as it is difficult to get the head loss per unit length exactly equal between sections and even a small discrepancy in capacity over time will lead to the tank filling or emptying.

Plan B - Install with the pipe you describe but in a single length. This works so long as the fluid flow is continuous. If you then have a blockage or flow ceases from the lower end then you could significantly exceed the design pressure and you have a burst pipe. Hence this system really needs no valves along the route and hence you always need to maintain an ullage or spare tank of 16m3 so that when you stop flowing in, the line can drain down. Optionally you can install fluid powered pressure regulating valves like those bimr says - just be careful to get ones which accept non potable water.

Plan C - Install higher strength pipe able to withstand full static pressure if the bottom valve is turned off.

Only you can really determine which is most suitable, what level of risk is there about potential spillages and the overall cost of installation and operation.

Your idea about valves every 100m is not required and would lead to high expense for little benefit. Your route maps shows you're not in a place where people would go and dig this up and more valves equal more chance of leaks. If it is damaged or needs repair, just shut the top valve and let it drain down.

If this is to be spread on the fields then I can't see why you would go for anything more complicated than the simple gravity system but pease let us know how you decide to go.

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

 

[DodaPump]

Thank you to everyone that posted on this thread, your thoughts have led me to a final design. 90mm PE 100 12 bar pipe will be used as the gravity line. 3 open to atmosphere tanks of 15 m3 each will be installed, one approximately every 800 metres of pipe length which is very close to every 80 metres of elevation drop. Tank inlet flows will be controlled by Big Boy ball cocks in conjunction with stainless steel gate valves in the line every 200 metres as a safety feature should there ever be a flow loss to the immediate environment. At the bottom of the line will be a 110 m3 tank also level controlled to 50% of its volume with a Big Boy ball cock. A high pond alarm system will be installed to this tank at the 80% volume level which will trigger visual and audible alarms as well as text alert to 3 cell phones. As well as this tank, the three plastic 15 m3 tanks will also have the high level alarms installed.

I look forward to any comments on this design.

 

[LittleInch]

Sounds good but I still think the in line valves are a waste of money. By the time you know of a really big leak, just turn off the supply from the upstream task and let it drain down. You certainly don't need stainless valves.

You just need to ensure you can get flow equal to 1m/sec or more to sweep out air and avoid any silting up low spots.

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

 

[bimr]

Sounds like a reasonable design concept.

 

[BigInch]

Too many valves have been known to actually increase the probability of leakage. Trick is to get as few valves as possible located in the best places to control pressure and to maximize their ability to control leaks, ie a valve near the end of the pipeline isn't very good for preventing leaks from the upstream half of the pipeline, or the downstream half for that matter.