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?

 

[LittleInch]

An expansion tank can be anywhere in a closed circuit. They tend to be on the inlet to the pump a this runs at a lower pressure than the discharge and the tank could be at a lower pressure. In high rise buildings you often find the tank at the highest point.

To work the expansion tank needs to be connected to the circuit. Water will flow in and out of the tank a it needs to so that the air pressure in the tank equals the pressure at the point where the tank is connected. The tank acts on pressure change caused by expansion or contraction of the water in a closed circuit.

There are many posts here with diagrams which might help.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[willard3]

Start here:

http://bellgossett.com/literature-library/#category/631

 

[cdxx139]

To piggy back on willards suggestions, start here:

http://documentlibrary.xylemappliedwater.com/wp-content/blogs.dir/22/files/2013/02/TEH-1196B.pdf

knowledge is power

 

[afa13]

thank you guys! enlighting posts however i need the mathematical explanation for not installing the expansion tank at the discharge side of the pump.

 

[LittleInch]

There isn't one.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EnergyProfessional]

expansion tank needs to be at the point-of-no-pressure change, which is the suction side of the pump. there the pressure always is the system pressure ( based on system temp and expansion of fluid). If you connect oit to discharge side of the pump, the pressure fluctuates with pump operation (that is what a pump does, create pressure)

 

[LittleInch]

Energy, I agree the common place is the inlet to the but this will also change from the static pressure when you have no flow.

The preassure on the inlet side will probably vary less than on the discharge side, so if you want to use numbers the I suppose you could look at static pressure at no flow compared to operating pressure which should be a smaller number than on the discharge side. Other than that I don't believe there is a "mathematical" solution.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EnergyProfessional]

the static pressure should not change a lot since any expansion due to added heat is buffered by the expansion tank. the fill system also is set to a specific pressure. Assuming your system pressure is 12 psi, there won't be much fluctuation at the suction side unless something fails (i.e. pressure reducer). On the discharge on the other hand you may have 12 psi whne pump is off, or 12 psi plus the head the pump generates. If that is a 30 psi pump, your discharge location experiences 12-42 psi changes. you probably could throw away the expansion tank membrane after some weeks...

 

[afa13]

When the pump operates, the pressure differential of the pump will higher than zero. The pressure at the discharge will be equal to the pump head whereas the pressure on the suction side will be equal to the pump pressure minus the pump head. is this why the expansion tank should be installed on the suction side? to compensate for that pressure and thus decreasing the effect of cavitation?
If so then i'm starting to get a better grisp on this matter if not then i have a lot more to read lol.
One more thing, only one pipe is connected to the expansion tank and this one pipe serves water going in (expansion) and water going out (pressure exerted by the tank), right?

 

[EnergyProfessional]

pressure at discharge will be system static pressure (measured at suction) plus your pump head.

only one pipe to expansion pump since it either goes in or out, not both at the same time.

Get this book

 

[afa13]

Thanks for the confirmation

 

[nicole10g]

I was told by my senior that in a closed loop system, pressure never change at the point the expansion tank is installed. This is because the function of the expansion tank is to regulate the pressure in the system. The expansion tank can be installed at the suction or the discharge side of the pump.

For a low rise building, the expansion tank is always installed at the suction side of the pump because of NPSH.

If the expansion is installed at the discharge side, the pump NPSH need to be checked. This installation is normally for high rise building to reduce the system pressure.

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.

 

[SAK9]

Nicole why does NPSH come into play for a closed loop pumping system such as a chilled water loop?

 

[nicole10g]

This is because the expansion tank is regulating the pressure of the system. Based on my previous example if the pump head still remain as 30psi and the expansion tank charging pressure is changed to 20psig and is installed at the discharge side of the pump. The discharge pressure is now at 20psig and the suction pressure is 20-30 = -10psig. In this case this pressure will be below the NPSH of the pump.

 

[willard3]

In a closed system, NPSH doesn't make any difference.

 

[nicole10g]

I could have been wrong on the NPSH. But still we have to prevent the suction side to go into negative pressure.

 

[Compositepro]

I think Nicole is correct. If an expansion tank is on the discharge side of the pump in a closed loop, there are a number of scenarios where the pump could cavitate due to low suction pressure.

 

[EnergyProfessional]

nicole10g: the pressure at discharge side of pump for sure changes. if you don't believe us, just take a pump that is off and put your finger on discharge side. then turn it on... and, pressure? Of course. that is what a pump does (or a fan).

 

[Compositepro]

The purpose of an expansion tank is to buffer pressure changes. If the tank is large enough there will be no pressure changes at the tank. So if you place the tank at the the discharge of a pump in a loop (which is a bad practice), the pump discharge pressure will not change. What will happen is that the suction pressure on the pump will drop. The head developed by the pump in either case (tank on discharge versus suction) will be the same, as long as the pump does not cavitate.