Thermal Relief - test of relief assumptions
Thermal Relief - test of relief assumptions
(OP)
I have done several a bunch of RV calcs recently where thermal relief protection was the controlling case. I would like to get critical feedbackon some assumptions. For simplicity assume a cooling water system. I have been calculating the expansion coeficient using the delta temp/sp vol between operating and max temp.
For the more narrow case of a dead headed pump (i.e. infinite energy input), I have been using the saturation temp at the Ps/P1 to perform the expansion calcs. The essentially means that the motivation results due to the vapor pressure of the water.
However, I would like to test my themodynamic assumption that this relief is at saturation temperature. The only other "process" that I can envision is that liquid expansion would cause pressure to increase, but since this is a non-compressible fluid that would not make total sense.
Please give me your ideas.
For the more narrow case of a dead headed pump (i.e. infinite energy input), I have been using the saturation temp at the Ps/P1 to perform the expansion calcs. The essentially means that the motivation results due to the vapor pressure of the water.
However, I would like to test my themodynamic assumption that this relief is at saturation temperature. The only other "process" that I can envision is that liquid expansion would cause pressure to increase, but since this is a non-compressible fluid that would not make total sense.
Please give me your ideas.





RE: Thermal Relief - test of relief assumptions
RE: Thermal Relief - test of relief assumptions
I think you are not properly identifying your situations and may be misusing the term thermal relief (= thermal expansion?) I typically use this term to refer to situations where I have a fluid "blocked in" or trapped in a fixed volume. Energy then moves into the trapped fluid, often just heat transfer from ambient or solar gain, etc. If we're talking about a vapor, pressure increase is negligable in every case I've seen. If we're starting with a liquid, my first point is throw away the thought that a liquid is non-compressible. (You use this term as if its volume can never change.) In fact, introducing energy into a fixed volume liquid filled system can generate enormous internal pressures. As the liquid's temperature increases, it tries to expand in accordance with its thermal coefficient of expansion. The liquid phase volume readily changes with temperature changes (albeit by a modest amount). It is much less responsive to volume changes caused by pressure. Think about it - it totally makes sense.
Doug
RE: Thermal Relief - test of relief assumptions
Your thoughts on this approach? What would you assume for the relief temp?
RE: Thermal Relief - test of relief assumptions
My question then becomes "starting with a subcooled liquid, and putting heat into the process, how would you calculate the relief temperature"? The obvious choice seems to be the saturation temp at the relief pressure. However, I am asking whether thermodynamically you feel I could reach the relief pressure before reaching the saturation temperature.
Thanks again for your thoughts.
RE: Thermal Relief - test of relief assumptions
RE: Thermal Relief - test of relief assumptions
Next, if it can and the piping would just be barely over the next pressure clas, I'd have a PSV on the discharge back to suction that can handle the full flow at the pump curves where the pressure exceeds MAOP.
Next, if you assume pump can deadhead or recirculate and build up heat, then you would assume that the energy put into heating the liquid and vaporizing it would be (100%-pump effiency in %) * HP at that point on the curve. That energy would go into the vaporizing of the fluid at the worse case scenerio.
RE: Thermal Relief - test of relief assumptions
Regarding Bill's response to my earlier comment, for a completely liquid filled system that you block in, you could theorize that relief could be required almost immediately. Once isolated, any heat results in increased temperature = liquid expansion. Initial relieving temperature would then be initial liquid temperature. I don't really think that's realistic, but I can't put a different lower limit on relieving temperature. Can you?
RE: Thermal Relief - test of relief assumptions
Based on your earlier comment,and comments from lizking, I believe I will take one of the cases I have worked up and estimate the line volume, then assume 1st law thermo and estimate how fast the temperature (therefore expansion rise) might be and ask the question "is it likely that plant will catch". If yes, then I will assume a subcooled liquid.
RE: Thermal Relief - test of relief assumptions
When you have heat input from a pump you'll first have thermal relief(s)(liquid expansion), followed by a vapour relief when the medium reaches saturation temperature.
RE: Thermal Relief - test of relief assumptions
RE: Thermal Relief - test of relief assumptions
Solar thermal build up can also be taken careof with a 1/2" check valve around one of the valves that blocks the system in.