Pressure build up in a pipe system containing solvent

[EnginE3R]

Ok, having a bit of trouble here. there seems to be a gremlin in a system at work that is causing a pressure build up but we cannot find the cause.

in essence, the system is run off a huge vertical storage vessel, the solvent is pumped out via a pump at approximately 40psig. This then feeds two dispensing guns, 4 large mixers, 5 washers and a dispensing room with three outlets. The pump is only activated when a dispensing point is operational, and stops as soon as operations cease. The issue at hand is that there is a large pressure build up, intermittently, of up 150psig. Enough so that it can blow through a dispensing guns trigger operation.

Im looking for any help or ideas at what could cause such a build up within this kind of system and to this magnitude. Any ideas are welcome and happy to explain further details if you need more information.

thanks

Sean

 

[LittleInch]

Is there any potential for a bit of backflow from the dispenser and you're getting some sort of reaction occurring inside the pipe which is then limited by the small amount of material either forced back into the mixer pipe or somehow "sucked" into it?

Have a close look at the end of the dispenser - is it below liquid level? Can it get reacting fluid back flowing into the pipe??

Still better to solve this at source, but a pressure relief back to the tank would be a good idea in any case.

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[EnginE3R]

Zeus, its a in in house blend of several solvents. While it is rather potent, if it was a solvent reaction or thermal one then the build up would be a constant effect. But it only happened after this specific machine was activated.

LittleInch, the dispensing pipe into the mixer is at the top of the vessel, and the feeding pipe even higher. There isn't any way that some other material could work its way back into the pipe by that route. Also it wouldn't explain why the pressure build up suddenly stops when the hand gun is used briefly.

 

[LittleInch]

Well it suggest there is something different with this machine or location.

If you only suck in a small amount of material then it will only generate a certain amount of gas which is then relieved as soon as you operate the gun and then there is no more material to create any gas.

It's not easy to guess remotely, but it's all I can think off.

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[Jboggs]

I once encountered a steam system that kept losing steam and condensing even though the pressure and temperature both remained in the range in which we should have had steam. We discovered that one of the tire presses fed by this system had an internal leak (faulty check valve) that was allowing high pressure inert gas (nitrogen) to leak into the steam lines. So the steam wasn't at the gage temperature and pressure. It was at its own partial pressure, and accordingly condensed. Do you have some other higher pressure source of another gas connected to that one machine? If so, you might have an internal leak from one source to another.

 

[EnginE3R]

Okay, so I got tired of all the second hand data and set up a test myself. Two of the 4T mixers would be filled with 1.8T of solvent and the results and data recorded. What I found was that the normally operating machine stopped quietly, as designed. When the other machine stopped, it sounded like a truck had been driven into it. This would suggest that a rather severe Water Hammer would be the issue.

The real question on my mind now is; Why does the pressure build up so slowly after such an immediate event? After the water hammer, I recorded the pressure of the system every 5 minutes for 20 minutes:

0 minutes - 3 Bar
5 minutes - 8 Bar (+5)
10 minutes - 11.5 Bar (+3.5)
15 minutes - 14 Bar (+2.5)
20 minutes - 16.5 Bar (+2.5)

So what do you guys think could cause the pressure to build up so slowly over 20 minutes, rather that instantaneously?

 

[BrianPetersen]

Water hammer isn't going to do that.

There's something else going on that you haven't figured out yet.

What happens if you leave a valve open somewhere? Obviously the pressure won't go up (which leads into the prior suggestions that you've got thermal build-up in a closed system), but how much comes out of that valve and over what time period?

 

[chicopee]

Is the temperature of the solvent increasing during flow?
What happens to the pressure when the operations cease during plant closure?

 

[LittleInch]

you've got me stumped at the moment.

It still looks thermal related to me, but maybe there is somewhere a device which is taking out the high pressure then slowly pumping it back in?

I still think you're going to need to physically check the entire pipe run versus your P & ID / schematic to see if anything else is connected to this system that isn't on your drawing.

There is something weird going on here.

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[btrueblood]

Is this solvent reactive with air/oxygen? Is it possible that air is gurgling up the line as the solvent dumps into the mixer, and the air bubble then slowly reacts with the solvent (releasing a gas phase) to build pressure after the valve closes?

 

[3DDave]

Look at the system with a thermal camera or at least a non-contact thermometer. I will bet that somewhere along the way that a line runs next to a heat source. Maybe it's a radiator or just under a sunny window. But the slow climb seems heat soak related.

 

[EnginE3R]

BTrueBlood, the solvent is Butyl Cellosolve Acetate, which I'm pretty sure isn't particularly reactive with air, but its definitely an idea to see what it does react with.

3DDave, Its worth a try, but again the issue is that as soon as the pressure build up is relieved once through the dispenser, the pressure stops rising. A pressure increase from the sun, or some other constant thermal input, would resume after the pressure is relieved.

Furthermore we had another build up last week, completely different machine was used. Over the course of 100 minutes, it went up to 40 bar at a constant gain. When the valve was cracked open the pressure relief, from 40 bar down to 6, dispensed just over 7kg of solvent. Now (if we have a high estimate of 100m pipe with an average of 2 inch dia) that means there was a low approximation of 4% more fluid than there should be (closer to 5% with a slightly more accurate approximation). Which for a line that has an in-compressible fluid in it, is quite high. Furthermore the 40bar reading is simply the limit of the gauge that's on the line at the moment. When we relieved the pressure, in order to prevent damage to the gauge, it was still climbing at about 1 bar every 5 minutes.

I have a few more tests planned out for now, but this issue is just getting more and more peculiar by each passing week.

 

[LittleInch]

At that sort of volume it implies some sort of gas production.

The reason it might stop once you relieve it is that once the gas expands and forces some fluid out of the pipe then more gas production has a gas volume to expand into where the pressure rises much slower. Aslo if it is a small section where you're getting heat then maybe it's all evaporated.

I still think you need to run / view the entire line to see if you're getting hot spots (anything over 75C will make it boil) or some strange connection not on the piping diagram.

When you get to this condition, is it possible to start the pump and flush the line through a dispenser to see if any gas appears?

Anyway thanks for the update - too many threads just stop without any further information

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Also: If you get a response it's polite to respond to it.

 

[EnginE3R]

Well, we are getting some thermal cameras in for the electrical engineers so could use that when they arrive to look at the line. Although according to the data sheet on BCA, the boiling point 1atm is 184-195 Celsius. So it is unlikely to boil, and 99% of the pipework is visible from the ground so it would be obvious if it went past a heat source that could put that much energy into the line.

Whenever the pressure builds up, we relieve it through the dispenser. As far as we can tell there is not gas getting out.

 

[3DDave]

It doesn't take much. A bud I worked with was with a big aircraft company and he said a bunch of them had a great idea - use self-sealing disconnects to the vertical stab so that the tail could fold (not sure which plane; may have been a military bomber??) without draining, refilling, and bleeding the air out.

So they did all the design work and went through all the budgeting and so forth and got a test item set up. All worked great with none of the draining, refilling, and bleeding they had to do before the self-sealers were used.

Then came the great news. Their test plane had ruptured a hydraulic line in the tail. Without a place to vent expanded fluid, the warmth of the sunshine was enough to exceed the ability of the lines to contain the fluid.

A short time after, when the hurt had worn off enough to talk about it, an old guy sitting nearby overheard the conversation. He said, "Yup, same thing happened 40 years ago when we tried it."

Which gives me an idea. Is the supply tank typically cooler than the rest of the factory? It may just be the cold fluid from the tank warming on the way into the distribution lines. I can imagine a bunch of interactions based on how much insulation there is, how much thermal mass there is, thermal conductivity, and time dependency. It may be that the solution is to warm the tank to close or slightly above the room temperature.

 

[LittleInch]

Good thinking. I like that idea. It is difficult to get to the volume being seen but I don't know what the expansion coefficient is of this stuff?

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[EnginE3R]

It is a good idea, And I shall endeavor to have a look and see what kind of effect it may have.

The solvent is kept in external tanks, so if we say for argument that the temperature outside is 4C, the solvent line is about 60% outside with the tank, the other 40% is inside the factory which is kept at a cool 19C. I personally cant see 15C temperature difference making a 40 Bar increase. None the less, I will attempt to run some calculations to see what I can find.

 

[btrueblood]

"the solvent is Butyl Cellosolve Acetate, which I'm pretty sure isn't particularly reactive with air"

Depends on how you define reactive, but it does react:

see

https://en.wikipedia.org/wiki/2-Butoxyethanol_acetate

and similar entry for 2-butoxyethanol.

Both will react with oxidizers, and air (containing oxygen) is one such; in the atmosphere, the reaction is catalyzed by pollution (sources of OH and similar radical species). Granted, it is likely to be a slow reaction if at room temperature, since the stuff will not burst into flame with contact with air. Might be worth a small scale lab test: half fill a tube with solvent (the other half being air), then cap it and measure any pressure rise.

Slow reaction in air (vapor phase presumably) yields formates of various types, and some aldehydes (propanal being one, which also has a fairly high vapor pressure), which should be identifiable by a decent lab chemist; if you have such on staff, give them a catch sample from the pressure relief and let them analyze it for those contaminants. From a quick look on google using "oxidation of butoxyethanol" as search term.

I do agree that the bp of this solvent is quite high, and it's unlikely that heat alone is creating a vapor bubble...but if some reaction creates lighter species, and more of them, from single molecules of solvent...well, you get my drift.

 

[LittleInch]

Coefficient of thermal expansion of this stuff is 0.0009 per C according to Dow. That's about 4 times water at 20C. Easily enough to get the pressure but not the volume you found.

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Also: If you get a response it's polite to respond to it.