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Determining of soluble gases in water

Determining of soluble gases in water

Determining of soluble gases in water

(OP)
Dear all.
Hello.
Due to tube failure of heat exchanger, methane gas enters into water cooling. We are going to measure the gas component in water to ensure tube failure.
Unfortunately LAB section is not able to determine gas content in water in this way.
We are considering to flash water stream in a sample cooler which is shown on attached picture. Water enters via nozzle A and exit via nozzle B. Due to flash vaporization gases accumulate in top section of sample cooler. When water level reach about 50%, two nozzle closes and LAB team would get an analyzing sample.

What is your idea? I'm concern about low pressure of accumulated gas, in a way that LAB team cannot get their samples. I would be pleased if I receive your idea.

RE: Determining of soluble gases in water

Hi,
Why don't you fix the problem and plug the broken tube(s)? That is happening everyday .
my view
Pierre

RE: Determining of soluble gases in water

(OP)
We are not sure about that...

RE: Determining of soluble gases in water

Why couldn't they take a sample of cooled water, then in an enclosure in the lab heat it while drawing a slight vacuum and analyze the off gas.
This is the way that nearly all dissolved gas analysis is done.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Determining of soluble gases in water

Yes, take a full sealed glass container to the lab and have it analyzed. That is how it is measured in well samples.

RE: Determining of soluble gases in water

Jack,

I think the problem is this phrase is confusing people "We are going to measure the gas component in water to ensure tube failure."

To "ensure" means that you you intend to get tube failure.

Did you mean "to measure tube failure" or "to monitor for..." instead?

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

RE: Determining of soluble gases in water

You have a flammable gas entering your cooling water system, where it will then travel to your cooling tower or chiller, which is likely not explosion proof. Methane is not very soluble in water - something like 10-15 ppm(w). Unless the leak is very small, your cooling water will be conveying this methane to an area that is not likely electrically rated.

Why not plug the tubes that are leaking? Are the tubesheets themselves leaking, or are there so many tubes leaking that you couldn't run the process with all the leaks plugged? The normal steps are to either plug the leaks or fix the heat exchanger.

Or are you trying to set a system up to determine if your tubes are leaking in the first place? Does your analytical lab has a gas chromatograph? Headspace chromotagraphy would be a standard way of measuring this.

If not, why not set up a methane detector in your cooling loop? If not fully enclosed (i.e. cooling towers), you could set up a methane detector above the reservoir. If you have a leak, it'll show up there.

RE: Determining of soluble gases in water

(OP)
Dear Ticl4 and all other dears...

Goal: setting a system up to determine if our tubes are leaking in the first place, using determination of presence (or ot presenece) of hydrocarbon gases in water.


Unfortunately gas detectors above cooling towers stack is out of service, and have operarional problem.

After isnurance of presence of gas in cooling water, we are going to dismantle heat exchabger and plug the fialed tubes.

RE: Determining of soluble gases in water

(OP)
Dear Ed stainless
According to our lab team statements, due to very high equilibrium vapor pressure of methane in water, they are not able to do any test...

RE: Determining of soluble gases in water

Jack,

Quote (Jack Nicholson)

Unfortunately gas detectors above cooling towers stack is out of service, and have operarional problem.

Not to be pedantic here, but you already have a safety system in place to detect methane. Just fix it.

Also, I do not think your lab people are correct. If you have a GC, you can test for methane. The high vapor pressure of methane means that headspace sampling is the typical means of sampling to the GC. The ASTM method linked by BIMR essentially formalizes headspace GC testing, although it notes

Quote:

The main caveat in applying ASTM D8028-17 and many headspace analysis
methods used to date is their applicability to samples collected at a nominal
ambient pressure, but not closed-system samples obtained (in situ, at depth) at
pressures greater than 1 atmosphere (atm) or potentially containing free
methane gas.

If you have multiple heat exchangers and need to identify the heat exchanger that is leaking, you should install cooling water sample points at each exchanger's water return line and sample them whenever the cooling tower gas detectors alarm for methane leakage. If the leak is significant enough to set off the cooling tower detector, you could bring a portable detector to each sample line and directly "sniff" each sample as you pull it.

RE: Determining of soluble gases in water

Of all the components in natural gas, only CO2 has some significant solubility. Methane, ethane solubility is very low at low pressure, ambient temps. At higher pressure, lower temps, hydrates may form.
See Perry Chem Engg Handbook for gas solubility in water at various P/T for several components, including CO2.
Tubes may have ruptured due to corrosion from (a) chlorides in cooling water (b) chlorides in formation water condensate in natural gas. Choose CRA materials to get longer life of tubes. I've not heard of any company trying to detect flammable gas on process gas cooler exit cooling water line, or if there is such an instrument deployed in reliable use.

In modern practice, direct process cooling with open loop cooling water or open loop sea water is not done for precisely this process safety risk of process fluid breakthrough into open loop cooling medium via tube rupture. Use a closed loop for direct process fluids cooling, then degas this stream to atmospheric pressure (or as close as you can get it to atm) and cool it against an open loop cooling circuit. Flammable gas breakthrough from tube ruptures is easily detected on the gas exit line of the closed loop degasser vessel. If there is only one closed loop degasser in a network of many process coolers, finding the source of rupture is still almost impossible.

Get your Company corporate management to invest in a total revamp of this unsafe open loop cooling water system. In the short term, either (a) replace the tube bundle on all process coolers with CRA tubes/tubesheet or (b) install an intermediate degasser, with gas exit floating on LP flare or atm, then pump this degassed CW to the cooling tower.

RE: Determining of soluble gases in water

You don't need to detect soluble methane, you need to detect there's a methane in your cooling water (as gas phase).
For this you either make your cooling tower gas detection system functional asap or you go hunting for the methane with portable gas detector - start with cooling water sampling/draining points as close to the suspected heat exchanger as possible.

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