Minimum slope, probably needs to be clarified a bit (positive/negative slope) and I want to make sure we're discussing potable pressurized water right? I've had discussions with water municipalities concerning steep slopes for hillsides where I am using restraint joint pipe, but that would be more of a maximum pipe slope. Maybe a few more details would allow us to help you better.
Yes, BigDog23, it is about a pressurized, potable water pipe. The pertaining municipality would like to see a certain slope since the water consumption can approach near zero for some periods. They are concerned about incrustations and settling of suspended particles if there is no slope over a long distance. Now I need some good arguments to dispel their concerns. Thanx!
Is the main chlorinated? Is there is not adequate flow, then the main will lose its chlorine residual. Locally, we use either hydrants or 2" blow-off valves to flush lines that lose chlorine residuals. Clifford H Laubstein
FL Registered PE 58662
To me, the Munipality's argument has no merit. If they have suspended solids and low flow, solids will settle out regardless of the slope, and it would have to be a steep slope for settled solids to drift towards a low point. And what value would this be? If they have a true SS concern, they should better filter the water. Higher velocities may help, but only way to do that would be to circulate the water, or use a smaller pipe size.
My 2 cents,
Rising Mains generally follow the contour of the land. Slopes are inevitable but are not purposesly added to the reticulation system for when one gets into flat areas any solids are going to settle anyhow. Eventually there will be a flat spot even right up to the users tap.
If you have slopes in a potable water system you will end up looking like a sewage system with pump stations every so often else your carriers will strike magma.
A water filtration plant is whats required here or the authority will have to embark on routine maintanence and flush their system even using pigs if required.
For pharmaceutical (water) application we maintain 1 inch in 10 feet slope in the direction of water flow. This is as per the recommendation of International Society of Pharmaceutical Engineering (ISPE). Perhaps this gives good drainability.
If the water is potable there shouldn't be any SS problems. There may be encrustation or whatever if the water is hard, but a minimum slope won't prevent that.
If it's surface water pre-treatment then there may be some solids etc, but if your collection point is OK then this will be minor and provision of scour points and relief valves is enough.
There are minimum velocity rules to avoid deposition of solids in sewers - 0.7m/sec is a number used in pressure mains. Note that most sewers and water pipes have periods of low-flow and static periods.
Gibfrog has a good point re. preserving a disinfectant residual if it's a treated water main.
And for $64K, the leading question is 'What sort of settlement and encrustation problems have you had before?'
AndyLe
i believe what hansid said is correct you have to maintain a min slope to enable entrapped air to go out or you will end up with air pockets and a large reduction in your pipe area, you will need high velocities to get rid of them and your existing pump may not be able to achieve the required velocity, also for maintenance issues , if your pipe burst it will be difficult to drain it if you have a flat section on either side of a washout,
I just want to add one clarification to a pretty good discussion. Slopes should be considered for transmission mains and are not that important for the average distribution main in the residential street. Trying to maintain slopes in distribution mains will be more trouble and cost that its worth since there are other means to maintain the main like hydrant flushing and corporation stops for removal of air....
BobPE is right, I like to design systems where the hydrants are placed at low and high points of the pipe for easy evacuation of air, complete draining (in seasonal systems)and complete flushing. Slopes will facilitate in air evacuation, but I think the individual from the municipality is perhaps a bit mis-informed regarding the buildup of material. In a pressure system, assuming the water meets guidelines for potability (turbidity) if there is no flow the accumulated solids on the bottom of the pipe are not going to "flow" or travel anywhere, regardless of slope. A good flushing program will evacuate the accumlation on the bottom of the pipe. If they want to see slopes, then do like I do and show it to them with an exaggerated vertical scale on the profile. That should do the trick. KRS Services
Watermains are pressurized, so slopes are not important. The only thing you have to worry about are MAX slopes, as in, the most a joint can deflect. But minimum slopes? Nope, that is utter nonsense.
I agree with you that the municipality's request or suggestion is utter nonsense, but alas, they are the client. For those whom are not too familiar with the municipal structure, the Directors, Superintendents and other managers are in a position of supreme power and have the legislative ability to request just about anything they wish. It boils down to politics and ideals.
The municipal official believes that a minimum slope for watermains are required, no big deal. Have the municipality provide examples of where he/she has seen slopes work and proceed accordingly. Like I mentioned previously, I provide some slopes too, but I was in a northern environment and utilized them to evacuate air and to drain some mains in the winter (park closures). Never for the buildup of fines settlement.
Hi Chris - For raw water transfer mains I adopt a minimum velocity of 0.45 m/sec. Slope is irrelevant (Does your client expecting the sediment to role down the slope? - In which case does the pipe slope only in the direction of flow. I can't believe that even the least informed client would believe that there is any connection between slope and sediment movement in a pressurised water pipe).
For treated water you should be removing the suspended sediment before it goes into distribution and not carry it through to the consumer. If the main operates at variable flow then the solution is to flush it periodically.
For trunk mains I use a minimum slope of 1 in 500 towards an air valve and 1 in 300 away from an air valve.
For distribution pipes adequate air relief is provided through hydrants and consumer connections and, as stated by BobPE, it is not worth trying to maintain a minimum slope.
For steep slopes a concern can be high velocities during initial filling.
Did an engineer ask this or some political hack? If an engineer, ask him where and when he studied this and where in the AWWA this requirement is written.. If political hack tell him 1 inch per foot. Then show him where the main comes out of the ground.
Please understand the design, construction and maintenance.
In design, the engineer must consider the construction and operation side also.
My experience in Pakistan and UAE suggest that by providing a minimum slopes against and with flow, the operation and maintence becomes much safer. Otherwise, the designer must preapre an operation manual with details of valve operation schedules/nr of turns to be open/close in specfic duration. The designer must produce calcuation for airvalve sizing,filling and emptying process. Check negative pressure.
By having defined low point (washouts), it will be much safer to darin the pipeline to carry out the emergency work.
Similarly, having defined high points (airvalves), it will be quick and safe to bring the pipeline in operation. Otherwise, it will take hours and might force section by section filling.
The minimum slopes are very useful when the water transmission systen is having working pressure 24 bars and having high velocity.
The design is always done with assuptions and the assuptions are modifed based on the degree of relibility.
The minimum slopes must also take care of other utilities.
