| Applies To | |||
| Product(s): | WaterGEMS, WaterCAD | ||
| Version(s): | CONNECT Edition, V8i | ||
| Original Author: | Mark Pachlhofer, Bentley Technical Support Group |
Problem
Fire flow nodes are failing the zone or system pressure constraint due to low pressure on the suction side of a pump. What can I do to prevent this from happening?
Solution
In many cases, nodes on the suction side of a pump are connection points between a source, such as a reservoir or tank, and the pump. This is common if you have multiple pumps in parallel. As a result, the pressure at these nodes may not have a major impact on the overall fire flow results, and these nodes can thus be placed in their own Zone or excluded from the fire flow run.
Note: You may want to make sure the reservoir elevation upstream of the pump is correct and that the pump intake pressure meets the minimum NPSH requirements.
Create a selection set for nodes excluding the low pressure nodes
Updates have been added to the Automated Fire Flow routine to make it easier to get viable results and easier to understand the results that you are seeing.
A new "Nodes To Exclude" field has been added to the Fire Flow Alternative. This is an optional set of nodes that would be excluded when checking the zone and system pressure constraints. This field would be used in a case where the calculation of the active fire flow scenario already violates at least one the zone or system pressure constraints for some nodes before any fire flow is even applied.
If this occurs in a model a new user notification will be generated that says: "Pressure constraints violated before fire flow added. Use 'Node to Exclude' if needed."
This will often be nodes that are on the suction side of the pump, which are more likely to have low pressures during a typical steady state or EPS run if the elevations of the pump and the upstream reservoir or tank have similar elevations. If this occurs, open the Fire Flow alternative and find the field "Node to Exclude." This is located near the pressure constraints in the upper right of the Fire Flow Alternative.
The nodes in the selection set selected will not be considered for the zone or system pressure constraint checks.
Note: if the low pressure is occurring at a node that is also part of the Fire Flow Nodes selection set, it can still produce a User Notification about the residual pressure constraint failing, even if you add it to the "Nodes to Exclude" selection set. In this situation it should instead be excluded from the fire flow nodes selection set as it would not make sense for it to be part of the set of elements to flow. In other words, "nodes to exclude" does not mean it is excluding them from the fireflow analysis itself, but rather excluding them from the zone and system pressure checks.
Alternative Solution: Create a Zone for low pressure nodes
If you have an older version of WaterGEMS or WaterCAD, you can create a new Zone for the suction side or low pressure nodes.
Note: Unless you need to apply fire flows to nodes on the suction side of the pump, you may want to create a selection set that only includes the fire flow nodes you need to consider and select this for the Fire Flow Nodes option in the Fire Flow alternative.
To create a new zone for the suction side nodes, go to Components > Zones. In the Zones manager, create a new zone for the low pressure nodes.
If you only have one or a couple of nodes where this applies, you can select the node in the drawing and open the Properties grid. Find the Zone field and set this to the new zone. If you have multiple elements, you may want to create a selection set, highlight this in the drawing, right-click in the drawing and choose Edit Group. You can then choose the Junction FlexTable and use the global edit feature to edit the Zone column.
Once the suction side nodes have the new zone selected, they will not be considered for the zone pressure constraints.
Note: If you are using the system pressure constraint, the suction side nodes will still be considered for this constraint. In such a case, the Nodes to Exclude option described above may be useful. If you do not have WaterGEMS or WaterCAD CONNECT Edition Update 3 version 10.03.03.72, you will need to upgrade to that version to use this feature.