Cooling Water Dissolved Gasses
Cooling Water Dissolved Gasses
(OP)
We have a demineralized, low-oxygen cooling water system that has H2 as the cover gas in the surge tank. We have some air or O2 inleakage somewhere in the system that is raising the dissolved O2 in our cooling water (normally less than 3 ppb dissolved O2). In order to reduce the dissolved O2 levels, we began to sweep the tank with a slow ("feed and bleed") injection of N2 gas. The nitrogen is injected into the tank at less then 10 scfh and vents through the normal pressure relief valve at 5psig. By design, the relief valve can easily relieve 10 scfh at 5psig. After a day of injecting N2, we experienced an odd high-pressure event in the tank (up to at least 7 psig) along with an apparent reduction of flow in the cooling water system. I think we encountered a sudden restriction, such as relief valve plugging, in the relief line that led to the high pressure.
However, I am wondering about the dynamics of changing the cover gas from H2 to N2 and whether or not that could have played a role in this event. H2 is more soluble by volume and therefore more of that comes out than N2 goes in. Using rough numbers, the system water volume is about 2000 gallons, the tank total volume is about 100 ft3 with 25 ft3 of gas space, water temperature in the tank is around 140F. What would you expect happen as you slowly change the composition of the tank cover gas?
However, I am wondering about the dynamics of changing the cover gas from H2 to N2 and whether or not that could have played a role in this event. H2 is more soluble by volume and therefore more of that comes out than N2 goes in. Using rough numbers, the system water volume is about 2000 gallons, the tank total volume is about 100 ft3 with 25 ft3 of gas space, water temperature in the tank is around 140F. What would you expect happen as you slowly change the composition of the tank cover gas?





RE: Cooling Water Dissolved Gasses
nitrogen blanket is typically employed to prevent atmospheric oxygen ingress into the tank. The nitrogen blanket does not to remove the oxygen from the water.
Perhaps some dissolved gas came out of solution and caused the piping to become air-bound for a short time.
Would not think that changing the gas would affect the pressure.
RE: Cooling Water Dissolved Gasses
Possible?
Good luck,
Latexman
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RE: Cooling Water Dissolved Gasses
Relief valves are not precision back-pressure regulators. Most are called pop-off valves because they are designed to be fully opened or closed.
RE: Cooling Water Dissolved Gasses
- the nitrogen is used to "sweep" out O2 as well as provide a blanket. This is to keep O2 levels from building up too much in the tank and therefore keeps the levels in the water low.
- I don't think it was ignition. The normal arrangement is 100% H2 tank gas, the high pressure event did not occur until we had been introducing N2 for a while.
- Compositepro, we have been thinking along the same lines as you. However, we rely on this relief valve all the time to relieve the tank of H2 gas that is expected to leak into the water (the application is electric generator cooling). We haven't had this high pressure event occur under those normal H2 in leakage rates (say .25 ft3/hr).
RE: Cooling Water Dissolved Gasses
Also remember that as you are 'relieving' H2, N2 or whatever from the tank, that air is diffusing back into the tank. People smarter than me will have to predict at what rate, but it will happen. Laws of physics, and all....
Can the O2 that you are finding be being pulled out of any of the containment metallurgy. Water that pure is 'hungry' and will pull ions out of whatever contains it (except glass I think) in order to equalize with its surroundings.
If this is a generator H2 system, you aren't leaking air into the H2 gas past the oil seals are you?
rmw
RE: Cooling Water Dissolved Gasses
RE: Cooling Water Dissolved Gasses
rmw
RE: Cooling Water Dissolved Gasses
You wrote "air is diffusing". I'm not sure what you mean by this...back through the relief valve??? Fwiw, the vent/relief line has a loop seal in it.
RE: Cooling Water Dissolved Gasses
Diffusion is a very good thing when it comes to the CO2 we breath out not surrounding us and suffocating us, but it is a troublesome thing when you are trying to keep air out of a vessel with flow to atmosphere.
I am a ME and diffusion is "black science" to me, but I learned about it dealing with Zirconium O2 analyzers many years ago and have found the effects in lots of places I would have never suspected since that time.
rmw
RE: Cooling Water Dissolved Gasses
Is it possible that switching cover gas from H2 to N2 causes a weird transient/event that would lead to tank over pressure and/or flow reduction. Such as H2 "fizzing" out of the fluid and causing a lot of bubbles.
RE: Cooling Water Dissolved Gasses
The nitrogen blanket is not causing any removal of oxygen from the water. Nitrogen is used to prevent the water from contacting and absorbing oxygen from the atmosphere.
At the low pressure that you are operating at as well as with the high temperature, there should not be significant concentrations of gases in the demineralized water. And you are not releasing enough pressure to cause the gases to fizz. You have stated that you have only 3 ppb of oxygen.
The blanketing gas that should be used is nitrogen, not hydrogen for obvious reasons. You should upgrade the gas bleed off device as a pressure relief valve is not appropriate for this application.
Evaluate the use of a membrane device on the fresh makeup water to remove the oxygen.
I agree with you that the relief valve probably hung up causing the observed pressure transient. The pressure transient had nothing to do with the nitrogen gas.
http://www.powermag.com/water/Forgotten-water-Stat...
RE: Cooling Water Dissolved Gasses
Purity of N2 gas?
RE: Cooling Water Dissolved Gasses
It is my understanding that as we change the composition of gas in the tank (isobaric), some dissolved H2 will come out of solution. Likewise, some N2 will go into solution at a different rate. Based on the responses I've seen, any system effect from this should be minimal. Sound about right?
RE: Cooling Water Dissolved Gasses
Perhaps the set pressure on the nitrogen regulating valve is too high.
RE: Cooling Water Dissolved Gasses
We orignally started of with the N2 injection regulator set at 6 psig providing 2.5 SCFH, which was able to improve O2 levels to about 6 ppb (tank pressure is 5 psig). We did this for over a day. Then, in order to drive the dissolved O2 levels even lower, we increased N2 pressure regulator setting to 7 psig and throttled our injection rate to 7.25 SCFH. Seven hours later, we noticed high tank pressure (7 psig) and low system flow rates (based on water temperatures). After isolating the N2 supply, the tank pressure returned to normal and system temperatures returned to normal. We have since replaced the relief valve and proved that it is capable of exhausting 7.25 SCFH at its cracking pressure (5 psig).