11 replies to this topic

[steffanwaters]

Hi all,

 

I have been reading a thread that was active a while ago now on Siphon Pipeline Design but have decided to open a new forum thread, and am hoping you can help me with my siphon problem. 

 

The thread was Siphon Pipeline Design: 

http://www.cheresour...peline-design/ 

 

Breizh added a link to "Chapter 8. Hydraulic Formulas used in Designing Fish Farms" and section 3.7.2 is similar to the system I'm working on. I've worked through Example 7 with the example values, and duplicated the page to work off my values which is on another sheet named Cochwillan Calcs. 

http://www.fao.org/d...4e09.htm#3.7.1 

I have attached the excel sheet of my calculations to this thread. I understand the Heffs and HeffT values need be below their allowable value for the design of the siphon to be satisfactory. I want to understand the implications of what would happen with my current proposal. 

 

The scheme I'm working on is a 90kW hydropower scheme in the UK utilising a Crossflow turbine. The design flow rate is 1.62m3/s using 1.5m ID twinwall plastic pipes. The route for the penstock uses an existing culvert, which we will pipe through from the intake to the turbine house. The intake is lower than the culvert invert level (levels are on the excel calculation sheet). The normal intake water level will mean the 1.5m pipes will only be half full at design flow so I want to investigate if and how this would prime and utilise a siphon effect to fill the pipe. I have allowed for stubbed flanges at the pipe highpoint for a vacuum pump if required (and air valves if required). I'm not sure exactly what I need.

 

I have worked out the Froude number to be 0.2390. I understand it's below 1 therefore in the subcritical flow region. On the thread before it's mentioned a Froude number of between 0.3-0.7 is optimal for that specific example. Can you clarify how this will affect my system? The flow velocity seems to be low for my system but reading your comments on the thread, it seems the Froude number is the key to the siphon. 

 

I've attached pdf drawings also of the Cochwillan scheme to give you an idea of the layout. 

 

I look forward to hearing back to you and would appreciate any comments that you have. 

 

Regards,

 

Steffan

Attached Files

•  Siphon Calcs Example and Cochwillan.xlsx   42.62KB   28 downloads
•  COCHW_C101_R01 General Arrangement.pdf   462.79KB   18 downloads
•  COCHW_C102 Complete Section View.pdf   256.28KB   16 downloads
•  COCHW_C123_R01 Intake Elevations.pdf   165.62KB   16 downloads
•  COCHW_C142 Turbine House Elevations.pdf   91.83KB   14 downloads

[katmar]

A few questions before we get down to details. Your spreadsheet gives the required flow as 1.62 m³/s but you have calculated the flow as 11.0 m³/s. Does your pipe really need to be so oversized? How is the flow rate controlled?

 

The spreadsheet (J16 - J18) shows the elevation change between the inlet and outlet to be -7.92 m, but the drawings give it as -4.03 m. Which is correct?

 

The submergence of the inlet looks very small to me.  Have you checked for vortexing?  You do not want to draw air into a siphon. I would be uncomfortable with a siphon designed to run with the pipe half full of air.

[breizh]

Steffan ,

Harvey is a master in fluid dynamic . Please answer his questions .

 

Breizh

Edited by breizh, 09 March 2016 - 06:12 AM.

[steffanwaters]

The design flow rate of the turbine is 1.62m³/s so that is the desired flow to go down the pipes to generate 90kW. The pipes were sized initially to keep the velocity below 1m/s. There is a sluice gate an the intake to control the flow rate. Yes I've noticed what I've done by calculating the 11m³/s on the spreadsheet, that considers the velocity over the full elevation change which is why it's so high but as I said, the design flow rate is 1.62m³/s which occurs around 30% of the year for the river (Q₃₀ flow). 

 

The elevations on the spreadsheet between the inlet and the outlet water level is -7.92m. I'm not sure where you've seen on the drawings that the elevation change is -4.03m but the value on the spreadsheet are correct. The intake water level is at 19.162, the water level above the outfall channel floor is 11.240 (calculated based on the channel width and considering the water level in the river at the design rated flow). 

 

Yes, based on Gordon J.L vortices at intakes theorem with 1.5m pipes, design flow rate of 1.62m3/s (v = 0.9167m/s) the minimum submergence to prevent air entrainment is 610mm. The bellmouth entrance to the pipeline is therefore located 610mm below the minimum water level. 

[katmar]

Thanks for the quick answers. The 4.03 m elevation change I calculated was a mistake, but I would question using the full differential that you have taken. I have not used this type of turbine myself so I have assumed that the head available is only from the surface of the supply to the centreline of the turbine inlet. This would be 5.84 m, which is still much more than you need to get the 1.62 m³/s that you require so it is probably not an issue.

What is the reason for wanting the velocity below 1 m/s? The document referenced by breizh recommends a minimum velocity of 2.6 m/s for a siphon diameter of 1200 mm. Your Froude number of 0.239 means that any air that enters the pipe will be trapped. A rule of thumb is that the Froude number should be kept below 0.3 in a line that should be self venting. In your case self venting is exactly what you do not want - you want the air to be removed. A Froude number greater than 0.5, and ideally more than 0.7, would be the target.

It is good that you have addressed the question of required submergence, but the greater submergences recommended in breizh's reference would make me feel more comfortable.

A pipe diameter of 900 mm would give you a velocity of 2.5 m/s and a Froude number of 0.84 - and a much cheaper installation. Any air that enters a pipe of this size will be rapidly flushed out. But this also raises the question of whether entrained air is any problem for the turbine. As I said, I do not know these turbines and don't know whether air would be a problem. If air is a problem I would definitely go for a larger safety factor on the submergence.

[breizh]

Hi ,

 

Just a comment , how do you intend to prime the line ?

 

Breizh

Edited by breizh, 10 March 2016 - 01:51 AM.

[steffanwaters]

Hi both,

 

With the Crossflow turbine, the head available is from the intake water level to the outfall water level due to the draft tube beneath the turbine. You can see the layout on the turbine house layout I've attached where the draft tube end is submerged. For you information, the draft tube is a conduit which connects the turbine runner exit to the outfall, where water is discharged at atmospheric pressure. The purpose of it is to reduce the velocity of the discharged water to minimise the loss of energy at the outlet. If the velocity is higher, the kinetic energy would be higher meaning the losses would be more. In other words we use a draft tube to have more net available head across the turbine to produce more power.

 

Just to correct my previous answer and clarify - the sluice gate will be used to shut off the water supply to the intake area, but the flow rate will be controlled by the inlet guide vanes on the turbine. 

 

The reason for wanting to keep the velocity below 1m/s is to keep the losses down in the pipeline to maximise the head available. The total head loss as a percentage of gross head is 5.44% for a 1.5m pipe. If we used a 900mm pipe as you mentioned the total head loss as a percentage of gross head would be 21.24% which would be too much of a head loss and compromise the economic feasibility of the scheme because it would significantly affect the energy capture.

 

We can use a vacuum pump if necessary to remove the air prior to starting the siphon, though it would be nice to avoid this is possible. With the current arrangement, it sounds like it would be needed. If we used a vacuum pump to remove air from the highpoint on the pipe at turbine start up, and assuming no air would be able to enter the siphon through the intake assuming the submergence was satisfactory with a safety factor, what are the implications of having a Froude number of 0.239 (1.5m pipe) in this case? I understand that any air that would enter the system wouldn't get flushed out with this Froude number but if it was primed at start up, would this be ok?

 

Looking at the spreadsheet I put together for these calculations, the allowable suction head on the siphon H_s is higher than the H_effs value considering the upstream head losses which satisfies the siphon design given in the reference. Regarding the allowable downstream head on the siphon H_T, this is lower than the H_effT value considering the downstream head losses along the 1.5m pipe. I don't understand fully what can I conclude from this?

 

Regarding the air entrainment causing a problem for the turbine, I believe it's fine at start up but not desirable for prolonged periods but I'm not entirely sure so I'll answer that in another response once I've clarified with my colleague later on today.  

 

Thank you,

 

Steffan

[katmar]

I couldn't find a head vs flow chart for the Ossberger, but a theoretical calculation shows that you will need 7 or 8 m head at the turbine inlet, so I understand that you are basically compelled to use the large diameter pipe.  The bit above the water level is not very high, so the pressure won't be much below atmospheric and it should be easy to prevent air leaking into the pipe through flanges and fittings

 

So your only real concern is air being entrained into the inlet, and you have addressed this problem by designing for a safe submergence. You will probably need a vacuum pump for start-up so if air does turn out to be a problem you may have to run the vacuum pump manually now and then, or connect it to a pressure switch at the turbine inlet so that it can run automatically when needed.  The pipe is such a large diameter that if the air ingress is small you may find you need to run the vacuum pump only every few weeks, if at all.

 

I also do not understand the significance of the H_eff values and I don't have time to work through that reference in detail now.

[steffanwaters]

Thanks Katmar.

 

Yes that's right, with an overall efficiency (turbine 82%, gearbox 98% and generator 95%) of 76.3%, the net head required is 7.47m to be able to produce 90kW from a flow of 1.62m³/s. I don't have the head flow chart for the Crossflow turbine either that I could send you but yeah as you said, we are compelled to use the large pipes. 

 

I will re-do some calculations to check the minimum submergence at the intake to ensure what I calculated before was ok and allow a safety factor. 

 

I have allowed for a DN100 stubbed flange for the vacuum pump connector on the plastic pipe highpoint so I will spec and source a vacuum pump for this application. It is a good idea to connect it to a pressure sensor at the turbine inlet so that it can run automatically when needed so I will look into that. 

 

Also, I have allowed for extra stubbed flanges at the pipe highpoint for air/vacuum valves or combination air valves because I wasn't sure what was needed. Following this discussion, I don't think we'd need either on this system. Do you have any comments on this? 

 

Ok that's fair enough regarding the H_eff values, I appreciate the time you've spent looking into this. I will dig deeper into that to see what I can conclude. If anyone else has any comments I would be very interested to hear from you. 

 

Regards,

Steffan

[katmar]

Something that just occurred to me now is the need for the plastic pipe to withstand a bit of vacuum.  You won't need much vacuum to draw the water up into the section in the culvert, but large diameter pipes are notorious for collapsing under vacuum. You have probably checked that already, but just in case...

[breizh]

http://www.ossberger...s-water-plants/

 

Hi ,

 

May be interesting to take a look , with reference to efficiency , 80 % up.

 

The size of the pipe could be enlarged at the mouth of the turbine only .

 

Breizh

Edited by breizh, 11 March 2016 - 01:50 AM.

[steffanwaters]

Hi both,

 

Katmar - yes that's right. The 4kN twinwall plastic pipe is constructed in box sections and is tested to withstand a negative pressure of up to 0.5 bar. We would be lifting the water by a height of 850mm in the pipe (worst case), so this should be fine.

 

Breizh - The expected efficiency Ossberger have specified for this turbine is 82% and as mentioned in your link, a higher efficiency can be achieved in some cases but for my calculations I have been using 82% as a design efficiency for the turbine excluding the drive efficiency. If we enlarged the pipe at the mouth of the turbine only, we would still have significant losses along the pipeline due high velocity, elbows in the pipe route etc which is why the large pipes were chosen. However, I understand what you were saying, that if the efficiency was higher, we could afford a higher head loss by using smaller pipes.

 

Thanks,

 

Steffan