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I have a fixed head variable speed pump or variable speed pump battery (VSPB) that is not able to achieve the target hydraulic grade at the target junction, or it stays on at full speed despite the HGL being above the target (it is not ramping down to meet the target) What might be happening and how can I fix it?
Multiple VSPs Serving the Same Pressure Zone
One possible cause is when there are two or more VSP stations or VSPBs (with different target nodes and target HGL) serving the same presssure zone. Both pumps may be set to an inital status of 'On'. As our Help documentation mentions, there should only be a single VSP serving a given pressure zone. What happens is the VSPs essentially "fight" with each other.
In many cases this causes an error to result, but in other cases the model converges, but has to settle on a solution where there is an energy balance for the network and the demands are fully satisfied. Basically, behind the scenes what is happening is one pump is trying to meet the fixed head you assigned to it. Then as that happens the other pump is also trying to meet the fixed head you assigned to it. At this time the calculation engine is trying to balance the energy in the model and meet all the demands as well, so it goes through a few iterations until it stops at some number of trials, where it has converged on a solution that may requires the VSP's to run in "fixed speed override" (full speed, with a relative speed factor of 1.00) in order to have an energy balance. This fighting of the two VSP's in the model to try to maintain their fixed head setting creates hydraulic conditions that are impossible to maintain.
The issue can be resolved by avoiding multiple variable speed pumps or variable speed pump batteries discharging to the same pressure zone. You may need to set all but one of the pumps' initial status to 'Off', and check to make sure that all related data input is correct.
If you really do have multiple variable speed pump stations discharging to the same pressure zone, with different target nodes and target HGL, you may need to set them all to regular pump elements with the variable speed control variable set to "pattern", then manually define the pattern of relative speed factor over time.
Target Node HGL Influence by Other Elements
It is also possible that a VSP cannot ramp down its speed to meet the target if other elements in the pressure zone already maintain the HGL above the target. For example there may be another pump (even a non-VSP), tank or reservoir that can keep the HGL above the target if the pump in question tries to ramp down. One thing you can do to test this is to run a steady state simulation with the variable speed option turned off on the pump in question, then set the initial speed to something lower and see the effect on the target node. You may find that the pump is not able to pass any flow at lower relative speed factors and the HGL at the target node is still above the setting.
Similarly, you could encounter a situation where a boundary condition (tank or reservoir) has a large enough influence on the target node to be able to prevent the pump from meeting the target hydraulic grade. For example if the target node is right next to a reservoir, it may be very difficult for the pump to meet the target, since the hydraulic grade will be heavily influenced by the reservoir hydraulic grade. For example if the target hydraulic grade is above the reservoir hydraulic grade, the pump will need to pass a flow amount that causes a headloss in the pipe (between the target node and reservoir) equal to the difference between the reservoir elevation and the target hydraulic grade, which may not be possible. In an unlikely case like this, the VSPs are not necessary since the boundary condition is essentially already providing that fixed hydraulic grade.