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RedR TSS

gravity flow water system

Design and construction of a gravity flow water system for a community of 3000 people in Kyrgyzstan. There is already a reservoir tank above the village so this project involves constructing a spring catchment and then pipeline to the reservoir tank.

The engineers are designing a gravity-flow water supply from a spring to an existing reservoir tank above a village of about 3000 people. Previously, a borehole and pump was used to supply the tank, but this stopped working nearly 20 years ago, so people currently drink from the river. The engineers have assessed that the distribution network from the reservoir to the tap stands in the village is still in useable condition, so in this project they plan to construct just a spring catchment and a pipeline from the spring to the tank. The pipe will discharge freely into the tank. They have estimated the length of pipeline required as 6km, but until I arrived they had not surveyed the height differences of the land or measured the flow from the spring. Without calculation, they decided to use 90mm diameter HDP pipe (which they are now in process of procuring through a tender). I have now helped them measure the flow of the spring as 6 litre/s and have started to survey the pipeline route with them. So far we have surveyed 1.6km starting at the spring, and there is a drop of 150m in height from the spring so far. Based on this topography and pipe diameter, I have calculated the ‘natural flow’ of the pipeline as about 13 litre/s (using pp47 of ‘A Handbook of Gravity Flow Water Systems’ by Thomas D. Jordan). This is more than the flow from the spring, so it implies that the pipeline will not flow full and therefore will not be pressurised. Jordan states that this is not a problem for pipeline sections without any tap stands situated on the section. However, Engineering in Emergencies pp365 states that a pipe not flowing full creates problems of air-locks, water hammer and variable flow. As I understand it, there are likely to be 2 options (assuming the rest of the topography survey shows a similar gradient the rest of the way to the reservoir - this has to be checked): (a) Continue with the design of 90mm diameter pipes, knowing that the pipe will not flow full. (b) Re-design the pipeline with a smaller diameter pipe that allows the pipeline to flow full. This may need the addition of break pressure tanks to the design. If anyone with experience of gravity flow systems is able to comment on whether or not a pipe not flowing full is likely to cause problems, and therefore which option (a) or (b) is more sensible, I would really appreciate the advice.

Here is additional information since my first email:

PROFILE - Kyzyltoo We have now surveyed the whole pipeline and overall there is about a 240m drop over 5.9km from spring to tank. I am attaching the survey data and what I have plotted so that I can refer to points of interest and you have the full information.

PIPES AVAILABLE The NGO has tendered for 4000m of 90mm polythene pipe rated as 6 bar NP. It has been agreed with the community that they will purchase the remaining 2000m of pipe needed, once we specify what they should buy. 90mm pipe is the maximum diameter available; diameters below this are available too.

DESIGN PROPOSED I have proposed the following design to the engineers here, which I know is not perfect but I hope is a reasonable compromise. 0m to 300m: 300m of 90mm polythene pipe (NP6) [head loss 2.4m/100m at 6 l/s so 7.2m/300m] 300m to 1150m: 850m of 76mm polythene pipe (NP10 if possible) [head loss 6.1m/100m at 6l/s so 52m/850m] 1150m: break pressure tank (note - if NP10 of 76mm is not available, then I think we cannot put a valve on the inlet of the break pressure tank because the static head would be too high when it was closed - therefore we would plan to use a valve at the spring catchment outlet instead) 1150m to 5900m: 4750m of 90mm polythene pipe (NP6) [head loss 2.4m/100m at 6 l/s so 114m/4750m]. However note the gullies at approx 2600m, 3700m and 4300m. I am suggesting we use 90mm polythene pipe (NP10) at these points (part of what the community buys) because the residual head is > 60m at these points. 5900m: existing reservoir tank, 22m residual head in theory but no inlet valve so water flows freely into the tank.

DESIGN RATIONALE The key issue I found during design was ensuring there is sufficient residual head at 5400m so that the HGL does not drop below the profile at this point. I realise that even with the break pressure tank, the static head on the second half of the system would be too high, therefore as the 1st respondent suggests, we are planning to ensure there is sufficient overflow capacity at the reservoir tank because we will not use a stop valve. Is this a valid method of avoiding static head? 90mm pipe is the maximum diameter available, so I do not think we can include another break pressure tank in this section because of the need for enough residual head at 5400m. (My rationale for including the break pressure tank, even if we are trying to use the overflow to make a design that never experiences static head, is to at least reduce what static head there would be if there ever was a problem or blockage with the reservoir tank overflow. If this idea of the break pressure tank is not needed, then the 76mm pipe could continue to 2200m and then the 90mm pipe continues until the reservoir?).

OTHER ISSUES FROM 1ST RESPONSES We are planning a gate valve on the catchment outlet to regulate the flow to what we have designed for. Overflow is also planned for the spring catchment. We should be burying the pipe below the frost line - I will check this because it is an issue in Kyrgyzstan. Particular thanks for the comments on how to deal with possible air locks etc. Thank you very much to both the respondents, and I hope that if this is used in the workshop it makes an interesting case study!

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RedR CCDRR

gravity flow water system

Design and construction of a gravity flow water system for a community of 3000 people in Kyrgyzstan. There is already a reservoir tank above the village so this project involves constructing a spring catchment and then pipeline to the reservoir tank.

The engineers are designing a gravity-flow water supply from a spring to an existing reservoir tank above a village of about 3000 people. Previously, a borehole and pump was used to supply the tank, but this stopped working nearly 20 years ago, so people currently drink from the river. The engineers have assessed that the distribution network from the reservoir to the tap stands in the village is still in useable condition, so in this project they plan to construct just a spring catchment and a pipeline from the spring to the tank. The pipe will discharge freely into the tank. They have estimated the length of pipeline required as 6km, but until I arrived they had not surveyed the height differences of the land or measured the flow from the spring. Without calculation, they decided to use 90mm diameter HDP pipe (which they are now in process of procuring through a tender). I have now helped them measure the flow of the spring as 6 litre/s and have started to survey the pipeline route with them. So far we have surveyed 1.6km starting at the spring, and there is a drop of 150m in height from the spring so far. Based on this topography and pipe diameter, I have calculated the ‘natural flow’ of the pipeline as about 13 litre/s (using pp47 of ‘A Handbook of Gravity Flow Water Systems’ by Thomas D. Jordan). This is more than the flow from the spring, so it implies that the pipeline will not flow full and therefore will not be pressurised. Jordan states that this is not a problem for pipeline sections without any tap stands situated on the section. However, Engineering in Emergencies pp365 states that a pipe not flowing full creates problems of air-locks, water hammer and variable flow. As I understand it, there are likely to be 2 options (assuming the rest of the topography survey shows a similar gradient the rest of the way to the reservoir - this has to be checked): (a) Continue with the design of 90mm diameter pipes, knowing that the pipe will not flow full. (b) Re-design the pipeline with a smaller diameter pipe that allows the pipeline to flow full. This may need the addition of break pressure tanks to the design. If anyone with experience of gravity flow systems is able to comment on whether or not a pipe not flowing full is likely to cause problems, and therefore which option (a) or (b) is more sensible, I would really appreciate the advice.

Here is additional information since my first email:

PROFILE - Kyzyltoo We have now surveyed the whole pipeline and overall there is about a 240m drop over 5.9km from spring to tank. I am attaching the survey data and what I have plotted so that I can refer to points of interest and you have the full information.

PIPES AVAILABLE The NGO has tendered for 4000m of 90mm polythene pipe rated as 6 bar NP. It has been agreed with the community that they will purchase the remaining 2000m of pipe needed, once we specify what they should buy. 90mm pipe is the maximum diameter available; diameters below this are available too.

DESIGN PROPOSED I have proposed the following design to the engineers here, which I know is not perfect but I hope is a reasonable compromise. 0m to 300m: 300m of 90mm polythene pipe (NP6) [head loss 2.4m/100m at 6 l/s so 7.2m/300m] 300m to 1150m: 850m of 76mm polythene pipe (NP10 if possible) [head loss 6.1m/100m at 6l/s so 52m/850m] 1150m: break pressure tank (note - if NP10 of 76mm is not available, then I think we cannot put a valve on the inlet of the break pressure tank because the static head would be too high when it was closed - therefore we would plan to use a valve at the spring catchment outlet instead) 1150m to 5900m: 4750m of 90mm polythene pipe (NP6) [head loss 2.4m/100m at 6 l/s so 114m/4750m]. However note the gullies at approx 2600m, 3700m and 4300m. I am suggesting we use 90mm polythene pipe (NP10) at these points (part of what the community buys) because the residual head is > 60m at these points. 5900m: existing reservoir tank, 22m residual head in theory but no inlet valve so water flows freely into the tank.

DESIGN RATIONALE The key issue I found during design was ensuring there is sufficient residual head at 5400m so that the HGL does not drop below the profile at this point. I realise that even with the break pressure tank, the static head on the second half of the system would be too high, therefore as the 1st respondent suggests, we are planning to ensure there is sufficient overflow capacity at the reservoir tank because we will not use a stop valve. Is this a valid method of avoiding static head? 90mm pipe is the maximum diameter available, so I do not think we can include another break pressure tank in this section because of the need for enough residual head at 5400m. (My rationale for including the break pressure tank, even if we are trying to use the overflow to make a design that never experiences static head, is to at least reduce what static head there would be if there ever was a problem or blockage with the reservoir tank overflow. If this idea of the break pressure tank is not needed, then the 76mm pipe could continue to 2200m and then the 90mm pipe continues until the reservoir?).

OTHER ISSUES FROM 1ST RESPONSES We are planning a gate valve on the catchment outlet to regulate the flow to what we have designed for. Overflow is also planned for the spring catchment. We should be burying the pipe below the frost line - I will check this because it is an issue in Kyrgyzstan. Particular thanks for the comments on how to deal with possible air locks etc. Thank you very much to both the respondents, and I hope that if this is used in the workshop it makes an interesting case study!