Vapor pressure, negative gauge pressure in a pipe, air bubbles - question about good design practice 2

[TomaszKruk]

Hello,

I feel really dumb to be confused by this, but I started to work with liquids other than water and this whole vapor pressure thing seems to go against my previous practice.

First of all - I work mostly with pumping either industrial sewage or water from surface intakes. What it meant for me was that I should design an air valve at any major high point in the system, cause the air will be released from the liquid, and might become trapped in the system, compromising it. I never cared for vapor pressure - I assumed that water I deal with is nowhere near a perfect liquid, so "gas emission" (don't know how to put it) is expected regardless of the temperature and pressure.

Now I'm to design a relatively simple system, but for a toxic liquid that will be pumped to a tank pressurized to near atmospheric pressure with an inert gas. Any part of my pipe that is above tank's inlet nozzle elevation will have slight negative gauge pressure. The liquid is "clean" - only possible contamination it could come to contact with is the inert gas in the tanks. Since the absolute pressure in the pipe would be way above vapor pressure for the liquid - I should be ok, right? I could increase the pressure in piping by moving control valves, but I'd avoid it if unnecessary.

The line will be equipped with valves and flanges for flushing with inert gas, but these will be manually operated.

My understanding is that air bubbles will form if the pressure in my pipe falls below vapor pressure of the liquid for given temperature. Thus trapped air in hot water systems is a much bigger problem than for cold water applications. I assumed stuff like sewage and untreated water doesn't follow the same principle because of organic and inorganic matter carried by the liquid. It will release gas bubbles due to processes happening in time, as the liquid flows.

Please help

Regards,

 

[LittleInch]

Tomasz,

I've read this a couple of times now and couldn't really find a question or issue to answer.

Don't confuse vapour pressure with entrained air or gas. The former is a number specific to the liquid at a certain pressure and temperature.

The latter a bit less exact, but solubility of the gas in the liquid in question can be determined at different pressures and temperatures

Also I've neve quite worked out why water and sewage people like all these high point vents, but then I tend to work in higher pressure / higher velocity systems. But as long as your flow velocity in a full pipe is >1m/sec, the liquid flow will simply blow out all air bubbles etc to the next tank or atmospheric chamber.

If your new system is pumped then you need to look at pressure drop in the pipe to get pressure at any point. Also it would be height above the fluid level in the tank, not the inlet nozzle where when the flow ceases the pressure could be below atmospheric.

But I still don't really understand what your question is....

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

 

[TomaszKruk]

Hello,

Honestly the question comes from me not having the leg of design guideliness to stand on.

I wanted to confirm that negative gauge pressure is not bad, as long as it doesn't reach vapor pressure of the liquid. I come from water systems, where high points are bad for warm water, and pretty much ignored when the water is cold, and treated. Thus I started to wonder about entrained gas, vapor pressure and other things I usually am free to ignore (sadly, I admitt).

When it comes to entrained gas releasing - I checked different lines the customer has against pipe diameters and the velocity they seem to maintain is around 0,5 m/s. This is what I was going for as well. Could you provide some tips about verifying if entrained gas might be a problem? I'm more than willing to do the reading (you've already helped a lot with providing the proper english term) but maybe you could point me somewhere. I already know they don't use any release valves. Could it be that it's just not a problem for some process liquids? I'll talk with the customer, but want to educate myself beforehand.

Regards,

 

[1503-44]

High pressure squeezes most bubbles down to nothing, so we don't have to worry about entranced air, or vapor pressure for that matter very much, unless it's at pump sections or at very high points. Their low pressure systems can air lock at almost any moment.

So, Thomas, be careful if you go into pump sections, as cavitation is easy with low pressures and air bubbles will reduce pump performance. Some thin pipe walls will also have collapse problems if vacuum becomes too great. Keep air entry and vortexing at tank suctions to a minimum to avoid air locks. Vapour pressure severely affects NPSH requirements at pumps.

1m/s is a better minimum flow velocity. And generally keep it below 3m/s.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Correct, negative guage pressure is generally not "bad", but the entrained gas issue is harder to determine.

0.5 m/sec seems very low for most piping system which normally operate in the 2 to 4 m/sec range. Inside plants the pipe runs are relatively short and hence pressure drop isn't the issue that it is on longer pipelines, so pipe sizes are usually smaller to save CAPEX.

For the entrained gas, you need to look closely at where this mysterious liquid comes into contact with a gas, be it air, Nitrogen or any other gas, especially under pressure. Then find some data on the solubility of the the gas in your liquid, or maybe need to do some tests. You usually find less gas is entrained at higher temperatures for the same pressure.

But if the system is pumped and not flowing under gravity or syphon, then just size the lines properly and you simply don't have an issue.

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

 

[TomaszKruk]

Thank you both. Gonna read up on the matter, check the lines on the site, and consult with the guys actually using the systems.

 

[TomaszKruk]

Hello,

I've received incomplete demand data, and the velocities need not to be that low. Problem solved. The nagging feeling of impending doom approaching - receds. Thank you both

 

[GBTorpenhow]

Negative gauge pressure can also possibly lead to air ingress at mechanical connections, you reference an inert purge, might want to consider if air/oxygen intrusion into your toxic liquid is a potential problem if there are flanged joints, valve packing, etc. present.

 

[bimr]

Tomasz,

You should also consider the flow velocity. If you are working in water and wastewater, you can eliminate the effects of water vapor and bubbling through an appropriate fluid velocity. See pumping station design:

PSD