I am working on a water distribution system with 3 booster pumps stations (each with 2 pumps) that operate based on elevated tank water level. The town also has two elevated storage tanks. I set up simple controls for each of the pumps based on the tank start and stop HGL's. I also gave each pump a pump definition based on the pump curve. I also set up VFD controls for each of the pump, selected a control node and a control pressure for each of the pump. I calculated the control node pressure by taking the (tank elevation - pump elevation)/2.31. I ran a steady state fire flow scenario setting the tank elevation to 990. That run was ok.
I want to now run another scenario assuming the tank LWL as the initial tank level, however does that mean I have to change the control node pressure based on the new tank level? I am confused as to why set up simple controls if this is the case.?
Hello,
If the required pressure for the variable speed pump doesn't change based on the water surface elevation of the tank, I am not sure that it would be required to change the pressure setting for the control node. If you consider a case where you are modeling an extended period simulation, the control node pressure setting would not change as the tank connected to the pump empties and fills.
If that doesn't help, I would need some additional information from you.
Regards,
Scott
I am a little confused, wouldn't the pressure at the control node be dependent on the initial tank elevation during a steady state run?
my understanding is if my tank water level is 990, my control node for that pump will ensure it will achieve the desired pressure to fill the tank to 990. Which is why I calculated my control node pressure base don my pump elevation and desired tank elevation? So if we lower the tank initial elevation, wolnt we need less pressure at the pump control node?
By changing the pump elevation, the head needed from the pump would change as well. With a variable speed pump, the pump will adjust the calculated relative speed factor to maintain the pressure at the control node. If you keep the control node pressure setting the same and change the level in the tank, you would likely see the pressure at the control node is the same but the calculated relative speed factor is different.
More information can be found in the variable speed pump wiki.
Are you saying that you are setting the VSP target HGL equal to the initial elevation in a tank, because you want the VSP to maintain a fixed elevation in the tank (if the tank drains, it should fill back up to that target HGL and if it fills too much, the pump should slow down so it settles back down to that target HGL?)
If so, then your statement sounds reasonable - if you want to change the target tank water level, you would change both the tank initial elevation and the VSP target HGL to be equal. If the tank has both an inlet and outlet pipe, you could also try setting the target node to the tank itself (which causes it to try to match the inflow with the outflow) which is mentioned in the article that Scott shared.
I do not recall hearing other users trying to do this, so it might help to explain what you are trying to achieve with this strategy. Typically you would want to have a tank cycle (fill and drain) so that the pump can turn on for some time and then turn off (in some cases energy cost can be reduced by filling the tank during times when the energy rate is less). Plus, cycling would help with water quality.
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Jesse Dringoli said:Are you saying that you are setting the VSP target HGL equal to the initial elevation in a tank, because you want the VSP to maintain a fixed elevation in the tank (if the tank drains, it should fill back up to that target HGL and if it fills too much, the pump should slow down so it settles back down to that target HGL?) If so, then your statement sounds reasonable - if you want to change the target tank water level, you would change both the tank initial elevation and the VSP target HGL to be equal. If the tank has both an inlet and outlet pipe, you could also try setting the target node to the tank itself (which causes it to try to match the inflow with the outflow) which is mentioned in the article that Scott shared.
this is exactly what I am trying to do.