Today, I thought I would take a break from my boiler discussions to share an interesting issue that came up on a project recently. Here is a picture of it.
A lot of you may be thinking “just looks like a couple of big pipes to me, so what’s the problem?”
Let me give you a bit more information. What you are looking at is a picture of the 14″ condenser water supply and return lines at roof level on a 6 story building near the cooling towers that serve them. The towers were behind me when I took the picture. Are any of you starting to get an idea of what the potential problem might be? Here is another clue.
So, now you are probably wondering “what does a glass full of water have to do with all of this?” Notice the air bubbles on the sides of the water glass. You’ve probably seen this
before, but did you ever wonder why it happens? Here is a clue about that.
This is a graph of the solubility of oxygen in water taken from Control of Scale and Corrosion in Building Water Systems by Russell W. Lane. As you can see, the solubility of oxygen in water varies with both temperature and pressure. I’ve highlighted the curve for atmospheric pressure for your reference.
The reason air bubbles form on the sides of a glass of cold water after you draw it from the tap is that oxygen tends to become dissolved in our tap water both at the source and via the treatment process. If you draw a glass of tap water (especially in the winter when the supply supply temperatures are colder) and let it sit undisturbed, as the water warms up, the oxygen that was dissolved in it can no longer be held in solution. The result is air bubbles which adhere to the side of the glass due to surface tension.
If you study the graph above, you will notice that at the boiling point, water at atmospheric pressure holds no oxygen in solution. This is one of the fundamental operating principles behind the dearetors used in boiler plants as a part of their water treatment process.
Here is the final clue I am going to give you before explaining howall of this ties together. The first illustration below is the system diagram for the condenser water system associated with the piping we are talking about. The second illustration is an
enlargement of the portion of the diagram inside the red rectangle. (I’ve
published a copy of the diagram as a .jpg file on my Google photo page for those of you who want to look at it in more detail.)
As you can see from the overview, the system serves three different chillers, any or all of which can be in operation depending on the load. What you probably can not read unless you go download the diagram is that with the smallest unit in operation, the system flow can be as low as 200 gpm. With everything running, the flow will easily exceed 3,000 gpm in the 14″ mains.
The pipes in the opening photo are the ones that the red arrow is pointing to on the system diagram detail.
So there you have the clues, the very same ones that caused the commissioning team I am working with to wonder if we might have some operational issues we needed to address.
I’ll give you a couple of days to think all of this over and come to your own conclusions. Then I’ll post and connect the dots, including what actually happened in the field with the system as it was brought on line.
Senior Engineer – Facility Dynamics Engineering