I am modeling a pump trip event in HAMMER. I have a sealed hydroneumatic tank with a total volume of 5,000 gallons and have set the initial gas volume to 2,500 gallons.
My pump trip occurs after 20 seconds into the simulation. As a result, I expect my gas volume in the tank to be 2,500 gallons within first 20 seconds when nothing has happened yet. However, the gas volume shows an upward trend before pump trip.
To see if the initial gas volume would stabilize in the absence of any event (such as a pump trip), I ran the model for 300 seconds with no pump trip (or any other event). The initial gas volume in the tank rises during the first 50 seconds of simulation and stabilizes at around 2,800 gallons.
Why is this happening? I was under the impression that if I set the initial gas volume in a sealed tank, HAMMER would assume that volume is under steady state pressure (whatever it is based on initial conditions since there is no field for me to enter the initial gas pressure the way you can do for a dipping tube).
Jesse, I uploaded my model to your server.
The scenario to look at is: SAWWTP(Closed10K)-HP(DT2.5K)-EV(Bladder5K) CC-ARVs
I am running it for 100 seconds with no surge events.
The initial gas volumes in the hydroneumatic tanks start off at correct values (based on my pV^n=K calcs). However, in the absence of any transient event (no pump trip or valve closure), the volumes do not stay constant. Thanks, Parsa.
Thanks for sending the model. The initial surge is due to the "Air volume (initial)" field for air valves CAV2-CC and CAV3-CC. They were set to a non-zero value, which conflicts with the initially positive pressure at that air valve. So, during the transient simulation that small initial air is immediately released and you see the non-steady results when things should be steady. Set these both to zero to resolve this. I have added a note about it in this related wiki article: Troubleshooting an unexpected Initial Surge during a transient simulation
You may still notice some small initial movement after fixing the initial air volume issue, and this appears to be related to the selection of "unsteady" for the Transient Friction Method calculation option. It is suggested that you use the "Steady" option (which maintains the initial pipe friction factors) unless you have a need to use unsteady. If you need to use the unsteady method, the "unsteady - Vitkovsky" option may produce more stable results. See more here.
Jesse DringoliTechnical Support Manager, OpenFlows ProductsBentley Communities Site AdministratorBentley Systems, Inc.
Answer Verified By: Parsa Pezeshk
Thanks a lot! I really appreciate your help and guidance.