First, it looks like you're using an old version of HAMMER - 08.11.02.31 (SELECTseries 2). The latest version is 08.11.06.113 (SELECTseries 6). I'd recommend upgrading if possible, as we've had numerous improvements over the years. You can read more about what's new in each release in the below link (scroll to the HAMMER section and look at the articles starting with "What's new in...") SELECTsupport TechNotes And FAQs Second, you could benefit from using Scenarios and Alternatives so that you have one single model file instead of one for each of the situations or configurations you are assessing. You could use the Active Topology alternative to model the different surge tank orifice configurations (orifice between pipes vs. pipe headloss) and the Transient alternative to model the different turbine operating cases. Check out this blog post from our Hydraulics and Hydrology Video series for more: Better Model Management with Active Topology With that said, the main issue in question is with the headloss in the pipes adjacent to your surge tank. From looking at the model, the issue is related to the near-zero flow in the initial conditions. For a transient simulation, HAMMER always uses a Darcy Weisbach friction factor calculated based on the pipe's initial conditions headloss. If that headloss is considered zero, it's calculated based on the friction coefficient. The problem in the case of your load rejection models is that the flow is slightly higher than the "flow tolerance" calculation option but has zero headloss, resulting in a zero friction factor for the pipes and zero discharge coefficient for the Orifice Between Pipes node. In the load acceptance, you'll see that the initial flow is slightly higher, enough to cause a small headloss in the pipes and orifice (add some decimal places to see). See below article which explains more: Transient pressure wave not dampening or unexpected lack of headloss The zero friction factor is seen under Reports > Transient Analysis Reports > Transient Analysis Output Log > search for the section called "Pipe information". The pipes adjacent to the surge tank are found at the bottom of the list. If you fix this issue, you'll see headloss in the transient simulation. Since these pipes are very large, a suitable solution may be to simply increase the flow tolerance so that it is higher than the flow in the pipes in question (the ones resulting in zero headloss). For example in the "rejection with orifice" model, try a tolerance of 1.0 l/s (the initial flows are about 0.3 l/s). From this, I was able to see headloss during the transient simulation.
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