varying head

[YuriB]

In a closed water circulation system with 2-way valves at the consumers and the constant head control, when the valves close, the system resistance and head increases, the system curve comes up and the control through an invertor lowers pump's rpm. Pump curve goes down and the head is kept at a set (constant) level - the horizontal line on the flow-head diagram. But what is going on when the varying head control is used - when the line on the diagram is kept inclined and there are two set points - the head and 1/2 head (Grundfos)?

 

[BigInch]

The pump is gauging speed between the speeds needed at each of the two fixed set points and the speed at the moment, then adjusts the instantaneous head set point proportionally.

Independent events are seldomly independent.

 

[YuriB]

I understand it so. To set constant head control you imitate full demand (open 100 % valves), set the pump speed and balance the flows (adjusting the speed) untill all demands are satisfied. You note the head (for instance 10 m) and make it the SP. The pump then will be keeping this head.
To set varying head control you do the same only the control (Wilo) automatically introduce another SP - 1/2 Head - that is 5 m . Between these two points the pump is supposed to proportionally run. Only I do not understand how the control knows that, for instance, at the demand 100 % (not 50 % or 75 % ) the head should be adjusted to exactly 10 m, and, at which % demand it should be 5 m ?

 

[BigInch]

It feels a pressure change and knows its speed now. It deduces what speed it should run for the new pressure and sets its new speed.

Independent events are seldomly independent.

 

[YuriB]

Thank you seem to be right.

 

[BigInch]

Seem?

Independent events are seldomly independent.

 

[YuriB]

Are. Thank you!

 

[MTES93]

Are you referring to the auto-adapt mode Grundfos uses? Essentially this thing is figuring out where it needs to run on the system curve based on power and speed (I think.. need to look at it again). From design flow-head and control point pressure minimum it plots a system curve. Using power and speed it 'auto-adapts' to the required point on the system curve...

Now you have me curious, I'm going to have to look at that again....

 

[YuriB]

I think it is only the speed the control is guided by .... the "third dimension" .