Expansion Tank - Pressure (p2) at high Temperature in chilled water system 1

[orator]

I have gone through many threads regarding expansion tank location, pressure requirements etc. One thing which is still not clear to me is the pressure P2 at highest temperature. I understand that this pressure is slightly less than the relief valve setting. I have two questions in this regard.
1) why pressure p2 is related to relief valve setting. I see that the system pressure cannot be more than the discharge pressure of pump (which is when pump is ON in chilled water system). So why my pressure P2 cannot be pump discharge pressure.
2) Please let me know where in a chilled water system this pressure relief valve shall be located.

 

[EnergyProfessional]

forget about the pump, it just adds pressure to whatever your system pressure is. your system pressure will increase with temperature. the expansion tank will buffer that a bit. but pressure will increase. the larger your expansion tank, the less increase. Hence the sizing requirement.
At maximum temp the expansion tank still needs to be large enough to limit pressure to be below relief valve.

your relief valve value depends on location, component heights and ratings. for example some radiators are only rated to 55 psi, most valves are rated to 125 psi. so you need to see what the pressure at relief valve will have at location while pressure at the lowest radiator does not exceed 55 psi.

Spend the money to buy the book referenced on this site. It will help you understand a lot. there is a really good section about expansion tanks, point of no-pressure-change etc.

 

[Compositepro]

EP, what is your definition of system pressure?

Orator, P2 has no relationship to relief valve pressure. It can be positive gage pressure you want that is lees than relief pressure. In most systems I've seen the expansion tank is located at the highest point in the loop and the pressure is often set to 15-20 psi. The expansion tank may also sometimes be used as the air separator/vent to vent excess air from the loop. The only reasons to not place the tank at the highest point are accessibility issues or to keep the tank in a controlled environment.

 

[LittleInch]

orator, EP,

I think you're missing the point here.

Each system is different in that height and gravity play a big part in any system, as well as the location of any of the components, none of which are fixed.

The relief valve is there to ensure that the pressure at any point within the system does not exceed the design pressure of the lowest rated component, whether that is the pump, a valve, an item or the pipe.

Your expansion tank should be designed such that it can accommodate all the known expansion in the system without the relief valve lifting (I assume this is what you mean by P2?).

In a closed loop system or indeed any system, the discharge pressure of the pump is the inlet pressure plus the pressure generated by the pump. As your inlet pressure (the system pressure) can vary depending on the temperature of the water, the discharge pressure of your pump can also vary.

As said above, there are so many variables associated with any system that you can't have fixed ideas about this, you need to look at each system and work out the pressures involved. E.g. if a pump is located at the top of a building, it's discharge pressure could easily be lower than the pressure on the ground floor.

In such a way there is no set location for a relief valve. It can be anywhere on the system and set at a pressure to protect the lowest rated item in the highest pressure location.

Again if it is at the top of the building then its set pressure could be much lower than, say, the pressure rating of a radiator or AHU on the ground floor.

Does this make it clearer?

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

 

[orator]

Thanks littleInch. Its more clear now. I have expansion tank and pump at ground level. Maximum pressure the mechanical equipment can withstand in my system is 80 psi and my pump head is 50 psi. I will now design a tank considering maximum pressure of the tank as 80-50 =30 psi. That means that when the pump is on, the system pressure is already 50 psi and i dont want it to cross 80 psi. Therefore tank must be able to handle 30 psi excess pressure if there is expannion in system for any reason. I hope am on right track.

 

[LittleInch]

I think you're getting there, but it all depends on the highest point of your system.

Normally you want the highest point in a static situation to be more than atmospheric pressure, usually by 1 bar / 10m head.

I don't know what your height is from highest point to lowest point, but this will get you a head / pressure at your pump / tank point.

Then you need to work out in a static situation what your expansion tank needs to do to maintain a positive head at minimum temperature and how much expansion you need to allow for when the water system heats up to ambient temperature. All depends on the temperature range and the volume of your system.

Then you can find highest static pressure at max temp. in your case this shouldn't be more than 30 psig

Then add your pump differential head and this tells you whether your system pressure is exceeded or not.

A schematic sketch would be useful showing heights and location of elements



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

 

[Drazen]

once you separate thinking on pump differential pressure from system static pressure, you will reach much better understanding.

in closed circuit expansion tank is the only device that controls pressure, so static pressure at expansion tank connection depends only on temperature. variations in total pressure along the system which relate to pump differential pressure are relevant for flow balancing, not for static pressure elaborations.