| Applies To | |||
| Product(s): | Bentley HAMMER | ||
| Version(s): | 08.11.XX.XX | ||
| Area: | Modeling | ||
| Original Author: | Jesse Dringoli, Bentley Technical Support Group |
How do I model a valve that starts in the fully closed position, then opens up during the transient simulation?
For this modeling case, the TCV (throttle control valve) element is suggested. First, set the initial status of the TCV to "Closed", then make sure the starting relative closure in your transient operating rule is 100% (fully closed). Select "DischargeCcoefficient" or "Valve characteristics curve" as the "Coefficient Type", then enter the "Discharge Coefficient (Fully open)", which represents the headloss through the valve in the fully open position.
When starting with a valve in the closed position, you may run into some challenges when computing the initial conditions.
Disconnected Demands
When demands are entered on a junction, they are assumed to be satisfied, so if there is no other water source downstream of the valve in question to supply those demands, they will need to be simulated as zero outflow in the initial conditions. However, if you set the demand to zero, you won't get a positive demand during the transient simulation when the valve reopens. If attempting to enter a regular demand on a junction with no other boundary condition to supply it, the model will still attempt to satisfy the demand, forcing flow through the closed valve, causing very large headloss and invalid results. You'll see a red user notification in this case about disconnected demands.
It is not typically to have normal demands be cut off like this though, but if you need to model this situation (zero demand in initial conditions with valve closed, positive demand after valve opens in transient simulation), you may need to use the Discharge To Atmosphere (D2A) node element at the demand locations. Configure the "Typical Flow" and "Typical pressure drop" based on a steady state run with the valve open: observe the demand and corresponding pressure or pressure head and use those. When the valve is closed, the pressure at the D2A will be zero and the outflow will be zero.
If you have a large amount of demands, the D2A approach may not be feasible and it can have trouble solving due to the differences in elevation (it can't have zero pressure at all D2As at different elevations - in reality some of the pipe would drain out but HAMMER assumes pipes are flowing full). In that case, you may want to start the initial conditions with the valve open, then configure the valve operating rule to slowly close, then wait for any transient to settle down (then you've established the "real" initial conditions), then have the pattern open the valve as you want. Since demands are always treated as pressure dependent during a transient simulation, they should properly drop to zero when the valve closes, but then increase back toward the expected normal demands when the valve opens.
If you have downstream storage in the system (tank or reservoir), then starting the valve in the fully closed position should not be a problem since the downstream system will still be connected to a boundary and water source.
PSV / PRV
If you have a PSV or PRV downstream, they may not behave as expected when the valve starts in the closed position. For example, consider a PSV downstream of a closed valve, connected to a tank at a lower elevation. In this case with the valve closed and no flow to generate headloss across the valve, you may end up with a low HGL based on the low tank elevation, and potentially negative pressures. In a case like that, one option may be to assume a reservoir at the PSV setpoint elevation (in place of the PSV+Tank).
Modeling Reference - Valves of Various Type
Disconnected Demand Nodes User Notification