Why is the HGL above a Pump higher than what I would expect?
Advanced feature on modeling overflows
In the data grid for a pump is an importing field called SY_IDOVER. This is the G_ID of the pipe receiving the overflow at a pump station if the pump and its wet well does not have the capacity to take care of all the incoming flow. If there is no physical pipe that receives the overflow then SY_IDOVER should be set to zero and the entire overflow will be lost to the collection system.
If you have drawn an overflow pipe that is attached to the pump, then the G_ID of that pipe should always be defined in the SY_IDOVER until the collection system has been “walked” (topology), otherwise you will get a walking error. Once the collection system has been successfully walked and the HYDRA_SEQ (Sequence of analysis) field is set, then it is possible to direct the overflow to another entity (link or node) downstream. It is very important that the overflow be directed to an entity downstream (one with a HYDRA_SEQ greater than that of the pump), as the recalling of the overflow into that entity must take place after the pump has calculated the quantity of overflow.
While most users will have no reason to set the SY_IDOVER to an entity other than the one that is actually attached to the pump station, some advanced modelers have good reasons to inject the overflow to an entity downstream. Typically this is for two reasons:
||To re-inject the overflow into the downstream system so the overflow will not be lost and distort the quantity of flow sent to the treatment plant, or|
||To direct the overflow to another lateral without drawing the entities that represent the by-pass.|
Whatever the reason, there are some cautions in attempting this use of the SY_IDOVER field.
When setting the HGL up at the pump, the HGL up of the discharge (pressure line) is ignored as it is assumed that the downstream HGL cannot move upstream through the pump itself. But the Hydra® Engine does look at the overflow and will allow an elevated HGL in the overflow to move upstream through the pump. The problem of selecting some downstream entity other than the one attached to the pump is that it is very possible that its elevation is above the pump and may have significant flow, even if there is not overflow from the pump itself. The Hydra Engine then uses this downstream entity to set the HGL at the pump overflow and then moves that HGL upstream of the pump station.
This may not be a good thing and the cause for your concern. There is a way to prevent this from happening. Simply enter a “SET IgnoreOverflow” command just before the pump station, then when calculating the HGL for the pump station, the Hydra Engine will ignore the overflow and you will probably get much more predictable results.
As a summary then, if you elect to set the SY_IDOVER to an entity other than the pipe physically attached to the pump, consider placing a “SET IgnoreOverflow” command just upstream of the pump station command. This SET command is only active for the downstream pump, so it must be placed in front of every pump station where you wish this logic to take place. One easy way to place the SET command just before the pump is to click on the pump in the Graphical User Interface (Hydra) and then double click on the SY_CMD field. When the dialog box pops up, select “NEW” and name the paragraph “OverOff” and then enter “SET IgnoreOverflow”. You can then use this design paragraph for all pump stations where you wish this logic to be used.
NOTE: You must be using HYDRAENG.DLL version 220.127.116.11 or later to use SET IgnoreOverflow feature.