Calculation of air side pressure in an chilled water expansion tank and setting of safety relief valve?

Question

Could anyone provide with a real time example?

Answer 1

First of all be carefull, a safety relief valve is designed to protect the expansion vessel and the system it is connected to, if this is an existing system the safety v/v should have its set pressure stamped on it or stamped on the lead seal on it to prevent alteration. Contact the manufacturer of the system.
If this is a system you are building yourself, you first need to establish the maximum safe working pressure of your system. This is governed by the component in the system that has the lowest 'maximum working pressure'. You should apply a factor of safety, i.e. set your safety valve at a value lower (50% lower)than the maximum allowed for your system.
Next you need to establish the volume of fluid in your system and the temperature change you expect during normal operations.
Then apply the coeficient for expansion of water, i.e. how much does 1 cubic meter of water expand per degree of temp increase,(sorry cant remember it).
Once you have established the volume of water that will be displaced to the expansion vessel and you know the maximum system pressure you have set your relief valve for the important factor now is the size of the vessel. For example if the vessel is 1 cubic meter and 0.5 cubic meters of water will be displaced into it the ratio of compression of the air inside will be 2. So if the initial pressure was atmospheric the pressure after expansion will be 2 bar (this assumes no change in temperature of the water/air in the expansion vessel). If you already have the expansion vessel you will have to check, using the above method, that it will be big enough to allow the required expansion without exceeding the setting of the safety valve. If not you need a bigger vessel. If the relief valve pressure is exceeded and it lifts then you risk the system going into vacuum or losing any precharge pressure when the system is at its lowest operating temperature.

Answer 2

Expansion Tank Calculation

Answer 3

I would think the relief setting would be a function of your system's pressure capability (for the maximum) and what your circulating pressure is (at the minimum). Somewhere in between these two should work.