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How are the pump flows used in the transient analysis derived in HAMMER?
There are some additional points of interest with pump flow in the transient analysis compared to the pump operation in the initial conditions or in WaterGEMS/WaterCAD.
In WaterGEMS and WaterCAD, reverse flow through the pump is not allowed, and the pump will only operate in a single quadrant. In HAMMER, you can have cases where reverse flow through a pump is possible. Because of this, a four-quadrant curve is used to define the flow in the transient analysis.
HAMMER comes with a number of pre-defined four-quadrant curves. These are selected by going to the transient tab of the pump definition dialog, found by going to Components > Pump Definition.
The specific speed provides four-quadrant characteristic curves to represent typical pumps for each of the most common types. There are a number of values to choose from in the pulldown menu. The specific speed you select should be as close as possible to the manufacturer's catalog to assure the correct values.
While the specific speed defines the four-quadrant curve that is used, the curve itself is dimensionless. Because of this, the initial pump operating point (the head(nominal) and flow(nominal) from the initial conditions) is still important. All of the points on the four-quadrant curve are relative to initial operating point. The quadrant curve data is dimensionless, so a change to the initial conditions operating point of the pump can have an effect on the quadrant curve that is used. Additionally, HAMMER assumes that the pump or turbine is operating at the best efficiency point.
Because HAMMER can use four-quadrant curves, you will sometimes see a case where the pumps seem to flow at values that do not match up with the pump definition. When this occurs, you will likely need to look into the pump definition and specific speed used to assure that the setup of the pump is correct and matches well with the expected pump flow.
In addition to the specific speed, the initial head, and the initial flow, nominal speed and inertia (also assigned in the Transient tab of the pump definition) can impact the results. Changes to speed and inertia will affect hydraulics. This then could cause a change in where the pump operates. For example, the inertia affects how the pump changes speed, and the speed is part of the four-quadrant curve. Taking into account that head and flow are used as well, you can see that everything is interrelated, and the proper input is needed to assure accurate results.
Note that you can also construct your own four-quadrant curve, which could give you more control over the results. This is often more trouble that it is worth, as there are a lot of curves available for use. But those that are used are dependent on a lot of factors, including the initial flow and head. You can find more information on making your own four-quadrant curve at the Help topic "Pump and Turbine Characteristics in Bentley HAMMER."