Expansion Tank 2

[afa13]

Hey guys,
Can someone please explain to me mathematically why should the expansion tank be connected to the suction side of the pump and not at the discharge.
I can't seem to understand the point of no pressure change. Doesn't the tank itself exert pressure on the water network?
Expansion tank doesn't have a water supply nor return it is connected to the network via one pipe.
What i know is this:
Entire system is filled thorough a fill valve and then this valve is shut. When the temperature changes in the chilled water network the air trapped in the tank is compressed. But if the tank is not connected how does it detect temperature change?
Can someone please clarify once and for all?

 

[afa13]

"Example: If the Expansion tank is charged at 40psig and the pump head is 30psi.

1. If the expansion tank is installed at the suction side of the pump. The suction pressure is 40psig and the discharge pressure is 40+30 = 70psig.

2. If the expansion tank is installed at the discharge side of the pump. The discharge pressure is 40psig and the suction pressure is 40-30 = 10psig. Therefore, you need to check if this 10psig is enough for pump NPSH."
Nicole could you please explain why u added the pressures in the first part and subtracted them in the second?

Thanks!