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Static Head and Hydraulic Grade - pumping over high point.

Hi,

I created small and simple system with pump at elevation 0.0' to push water up to outfall at elevation 15.0'. Highest pipe located in the middle between pump and the outfall is at elevation 35.0' (water from pump goes from el. 0' to el. 35' and after that down to 15'). Pump shutoff is 50.0' but profile of the entire system is showing water level (Hydr. grade line) below pipe at high elevation. Should I be concern about it? End pipe is showing expected flow in results but per profile, water won't be able to reach highest pipe and go down after it. I presume static head in this case is the difference between pump and outfall (15.0' - 0.0' = 15.0') and the high point will be reached because pump shutoff head is 50.0'? Is that correct?

Thanks!

  • There are several conditions that can exist when you have such a high point along a pipeline.

    1. WaterGEMS assumes that the downward leg is not so steep that is will have partially full flow once the pipe fills and therefore the water can siphon over the top of the hill once the pipe fills. This assumes that there is no air leaking in along the pipe and the negative head does not exceed 32 ft (10 m). Depending on the downward slope and the length of a pump cycle, you may never reach this condition.

    2. More commonly, an engineer will place an air release valve at the high point. In this case the downward leg may or may not have full flow depending on slope and flow rate. The air valve also prevents gas pockets from forming at the high which could block the flow.

    3. Even if you don't have an air valve at the high point, if the downward slope is steep enough, you may never achieve full pipe flow in the downward leg.

    4. If you are in case 3 and want to keep the pipe full, you can throttle the downstream end of the pipe with any number of types of valves.

    Given that the downward leg will usually be empty when the pump turns on, there will be a transition time as the downward leg fills. Usually that time is short compared with the pump cycle time, but if it isn't, then you need to consider that you may not reach steady state. You may need to use one of the dynamic solvers in SewerGEMS if you want to study filling.

    A lot depends on what you are pumping and why.

    Answer Verified By: Nobravu 


  • Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.