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cockswain's avatar

How is water vapor getting into this process plumbing?

Asked by cockswain (15186 points ) July 12th, 2011

Where I work we have chemicals running through pipes. One particular chemical needs to have low water content, so we analyze the source keg for ppm of water (spec is <25ppm). It passes the spec, and if we flow out of a downstream tap it passes there as well. If we let it sit in the pipe for hours, it gradually builds up ppm of water at the tap above 25ppm. Flowing about 150–200 mls through the ¼” pipe gets back below spec.

Why this is confounding is the fact that the pipes and joints have been leak tested and the chemical is under about 10 psi. So how can the water vapor enter the system at the tap?

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18 Answers

Tropical_Willie's avatar

May not be an answer, but 10 psi is less than ambient. Was the leak test for a vacuum or for positive pressure?

If it is a vacuum leak it would bring in moisture.

cockswain's avatar

I have a He tank pressurizing the fluid to 10 psi in addition to ambient. This is how it is pushed out the taps. The leak test was for positive pressure, as is the way the fluid is dispensed. Get the confusion yet?

Tropical_Willie's avatar

I’m confused too.
Is the He wet ? Or has a H2O leak or condensation in the He loop (for cooling)?

Kayak8's avatar

How do you clean your pipes?

cockswain's avatar

I don’t think the helium is wet. If it was, then I would expect the solvent to be uniformly wet and flowing 150mls or so out of any valve would not improve the problem.

There isn’t a helium loop for cooling. Just a high pressure gas tank that is pressurizing a solvent keg. The fluid is pushed by the helium pressure out of the keg and into the pipes. I’m taking samples for H20 content at the dispense valves along the pipe.

Sorry, it’s tough to describe. Thanks for asking questions, I hope it’s getting clearer.

@Kayak8 We’ve only cleaned them by pushing several liters of the solvent through when the system was plumbed.

CWOTUS's avatar

Knowing nothing else about the chemical you’re talking about or the processes you’re running or how your system is designed, I’m guessing you have one of two things:

1. Condensation in the line, where the chemical is somehow evaporating what little water it does have into a standpipe just before the tap, perhaps (or maybe even separation of the water, if the chemical is allowed to stand and if the water is less dense than the chemical, then the water is floating to the top of the standpipe),

or

2. Contamination is being introduced via the Helium, depending on the process used to bottle it, and how good the supplier’s quality control is, or the contamination is introduced from some other part of the piping, or the testing samples and equipment used to check the water concentration.

cockswain's avatar

@CWOTUS Good points. We’re pushing acetonitrile though process plumbing to feed machines. It has greater density than water, but is a very hygroscopic solvent.

Regarding your first point, see what I said above about how purging the pipe at any given valve restores the solvent back to spec. In other words, only the first 150mls gets above spec, then it’s good again.

A good question I don’t have an answer to is about the dead volume of the pipe. I’m not sure, I’ll need to measure and calculate. It’s possible that ~150mls is close.

Regarding your second point, possibly the He isn’t dry enough but it would surprise me. Over time, one would expect the whole supply keg would get contaminated and the whole system would reach a steady ppm of water equilibrium. And one wouldn’t be able to purge a bit to restore the solvent back to spec.

You guys are coming up with great questions/suggestions. Keep it up, someone might hit the right answer and solve my problem. Or at least give me a good test to try.

cockswain's avatar

I made a typo. I meant to say acetonitrile has lower density that water.

Kayak8's avatar

Have you checked your source acetonitrile for its H2O content? You haven’t mentioned the temperature of the apparatus and contents . . . are there any oscillators in the vicinity?

cockswain's avatar

Yes. We always test every new keg before placing it on the system. The system operates at room temp. What do you mean by oscillator?

Kayak8's avatar

Anything that shakes

cockswain's avatar

Yes, but really just vibrations of small motors moving carriages on tracks. That is the equipment to which the solvent is plumbed. What are you thinking?

Kayak8's avatar

I am still suspicious of your source acetonitrile. Here is what I was reading . . . may not be relevant, but I am just brainstorming to see if anything “sticks”

cockswain's avatar

But we test it before it goes on the system. We have checked the kegs after being on the system a while and found them to still be within spec. Also, note that purging the first 150mls from the pipe brings the solvent back in spec.

I’ll look at the paper.

CWOTUS's avatar

It seems to be a function of mixing vs. not mixing. So I’m still thinking that a separation is occurring somehow when the system is left standing. Either that or your test apparatus (or process) itself has contamination (or methodology) issues.

It’s possible that the new keg (or the entire process line) is not as good as you think it is (the test may be failing to register an out of spec condition that occurs with new source material), or the test of the system after it has been left at rest for awhile is bad, giving you a false positive.

cockswain's avatar

You could be onto something, but I don’t know. The system is not being actively mixed. Since we can watch the ppm of H20 increase at the valve over time and flushing fixes it, it seems to be a local effect. But I’m going to calculate the dead volume in the pipe today and see if basically we are emptying the entire contents of the standing pipe and then the fresh supply from the keg is replacing it.

Our testing apparatus and methodology are so excessively anal-retentive that I do not suspect it at all. You’ll have to trust me on that one. I definitely believe the results we are getting from our testing methods. Besides, we are seeing this phenomena consistently: ppm is low, it stands, it gets high, and goes back down with flushing.

If I have a problem with the process line, that gets back to my original confusion. I’m using 316 SS all around (pipes, fittings, valves). Acetonitrile does not react with it. We’ve been doing this for years at this facility. The only reason it caught my attention is that I personally installed this whole new run of plumbing and noticed the water content issue. After asking around, it has been observed by others that this happens and no one has really ever cared to investigate it too much.

Some further info: the same issue has been observed with a pipe containing acetone instead of acetonitrile. The spec on acetone is <5000ppm. I’m currently unaware if the acetone will exceed this spec from sitting in the pipe, only that it increases.

You’ve got me thinking about this separation angle. I don’t know enough about fluid dynamics to know how some H20 in the system will behave in acetonitrile (or acetone). If it is pressurized, does the water “slip” by the solvent in the keg and accumulate in the pipe? I don’t know. Maybe when I flow from a valve, there is some inertia that remains after closing the valve, and the water is still moving in the direction of the pipe, but faster than the solvent?

tedibear's avatar

Have you tried to blow out the tank and the line with dessicated air? You can rent an air dryer and blow out the line with very low pressure dry air. The other thing that may be happening (other than condensation) is that some of the solvent has absorbed moisture – most likely if it is exposed to ambient air. After sitting in the pipe, the solvent with the most water is heavier than the rest of the solvent and moves down. I would assume this pipe is at the bottom of the tank. Those are just some random thoughts.

Just so you know, that ^ response was from Mr. tedibear, not from tedibear. I got confused after the third sentence. :P

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