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Hammer: Pelton free-surface tailrace surge

Hi

I am currently modelling a number of pelton hydro station layouts. One option may (to be confirmed) include a tailrace tunnel with a free surface. Because the tail race tunnel is so long, surges upon start-up are a concern as could flood the pelton wheels. One scenario is to include a free-surface tailrace tunnel, allowing free-board for surges. Is it possible to model this situation in hammer, where under surge the tunnel may become pressurised? If so, how do I go about this?

Thank you for any assistance.

  • Hello,

    Could you help us understand this setup a bit better - perhaps a sketch? The "tailrace" normally refers to the piping downstream of a turbine. From my understanding, a Pelton turbine is basically a nozzle discharging to the atmosphere, which then turns a wheel. For transient analysis purposes, the system ends at the nozzle (typically modeled by a discharge to atmosphere node - see our documentation or Turbine technote). Typically you would just look at the pressurized piping upstream of the nozzle for any surge problems due to closure of the valve.

    It sounds like you're saying that you're looking at the pipe downstream of the turbine which carries the water away. Could you explain a bit more about what would cause a surge in that pipe?

    Generally speaking, HAMMER's assumptions rely on pipes being fully pressurized.


    Regards,

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

  •  Hi, thanks for your reply. You are correct; I am looking at the pipe carrying the water away.

    I have attached a basic sketch. I am modelling the pelton as a TCV followed by a D2A so I can have multi-stage valve control. Typically I would only be interested in the upstream sections; however this scheme has a long tailrace tunnel. In this case I am more concerned about mass surge rather than hammer. I expect this can occur upon unit start-up (4 units) if they all start at once, there will be a surge in the tailrace as momentum builds to establish full flow. The risk is this surge will cause the peltons to choke and churn water, which is unacceptable. Conversely if all units trip, the sudden halt of water will probably cause a down surge, though this is less of a concern.

    There are two options proposed to deal with the upsurge on start-up. One is to build a surge chamber, similar to my sketch (but vented) but probably more like a cavern. The other is to make the tailrace tunnel over size so under normal operation the tunnel runs a free surface, allowing space for an upsurge - the risk here is to develop instability from transition between partial and full flow (change in wetted perimeter, and thus friction).

    Another option would be to simply place the pelton units higher, however this loses head to deal with a choking problem which only occurs on start-up.

    I hope that clarifies?

  • Thanks mattyjb, this is interesting.

    In your sketch, it appears as if the entire horizontal part of the tailrace tunnel will be submerged due to the water level in the outlet reservoir. You expect a surge may occur from the water initially filling up the vertical part of the tailrace (right under the wheels) as the nozzles first open? I'm still having trouble picturing how that could occur; maybe another contributor can interpret better and offer some advice.

    Bentley HAMMER is meant to simulate transients in fully pressurized pipes (with the exception of possible air or vapor pockets introduced at local spots) so you may have to use some other means to look into your open/partial flow surge. You may want to search around to see if there have been any papers written on the subject.


    Regards,

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

  • Hi Jesse, yes that is the situation I am concerned about. I will attempt to model the situation once I have all the info (waiting on the civil guys to provide some details) and will post it to clarify.

    As you say, if we end up looking at partial flow might have to use another tool, however I think a surge chamber will be in preference.

    Cheers