| Applies To | |
| Product(s): | HAMMER |
| Version(s): | 08.11.xx.xx and higher |
| Area: | Modeling |
| Original Author: | Mark Pachlhofer, Bentley Technical Support Group |
After adding a surge tank to my model, there is an unexpected initial surge (starting right after the initial transient time step).
This can happen if the "Elevation (Initial)" and "Elevation (Maximum)" properties of your surge tank are set to the same elevation, while also having the "Has Check Valve?" property of the tank set to 'True'. If that is the case then, set the "Elevation (Initial)" to a slightly smaller value than the maximum elevation, or set the maximum higher than the initial. For example, if the maximum elevation is 347.21 ft, then set the initial elevation to 347.20 ft. This will still allow you to simulate the tank being full without causing the initial transient down surge at the beginning of the simulation.
The problem with having "Elevation (Initial)" and "Elevation (Maximum)" set to the same value is that the software is expecting the tank to start overflowing. However, with "Has Check Valve?" set to True, this implies that the surge tank is a one-way tank (flow can only go out to the system). If such a case, HAMMER assumes that the tank elevation will never get to the maximum elevation for overflow to occur.
Note that if you set "Has Check Valve?" to False, you will be able to enter a weir coefficient and weir length. If there is no check valve, the surge tank would be allow to overflow and the initial surge will not occur.
Note also that there is a known issue in version 24.00.00.24 and 24.00.00.26 in which the initial surge can occur in this condition if the initial HGL for a one-way surge tank (has check valve = true) is above the tank's maximum elevation. A sudden initial drop in transient HGL can be observed, with positive overflow "spill rate". This has been reported (reference #1449093) and is planned to be fixed in future versions. In the meantime you can set the tank maximum elevation value to be higher than the tank initial HGL, then re-compute the model.