How to measure water level in a vacuum...

[Peter Nachtwey]

@Peter Nachtwey why are you being such a jerk? The man says the tank is at 20 torra and he is trying to measure level of water.

Mas01 has not been clear about that the absolute pressure is in the vessel. He has said 20 torr and too is absolute. I have quoted him above.

If the pressure in the is 20 torr then it is near a perfect vacuum and water will boil. If the pressure is at 740 torr then it won't boil.
NOW READ THIS TO SEE THE DEFINITION OF TORR
https://en.wikipedia.org/wiki/Torr
torr is an absolute scale, not a relative one

If they're designing a test system, do you really think the mechanical/chemical engineer involved doesn't know his/her steam tables?

I have seen all sorts of screw ups.

It's worth it to bring it up, but harping on it non-stop, obviously not reading the rest of the thread, and then condescending not only OP but everyone trying to help him isn't a good look.

I have not been harping on it non-stop. Stop lying. I made a post #5 and have waited 5 pages watching you guys flail and fail because the uncertainty still persist. There is a huge difference between 20 torr and 740 torr.
Being precise is important.

@Peter Nachtwey: if you are mocking anyone about flailing, then why are you bringing steam into this? When 20Torr was first mentioned, it was mentioned as a vacuum

Torr is not relative. 20 torr is a vacuum too. See the wikipedia post.
Where was -20 torr relative to atmospheric mentioned?

What I saw is

No, no...the absolute value is 20 Torr inside the vessel.

 

[drbitboy]

torr is an absolute scale, not a relative one

d'Oh! I don't know why I forgot this. I think I immediately remembered the magnitude of 1torr used to be equivalent to 1inHg, and inHg was often used to express vacuum relative to atmospheric.

 

[g.mccormick]

The boiling point of water is relative to the pressure.
If the OP meant that the vessel pressure is at:
A. 20Torr (~ 26.6 mbara) then the boiling point will be around 72°F
B. "20Torr Vacuum" which would imply 740Torr [1ATM = 760Torr = 1013.25mbar] then the boiling poing would be around 210°F.

 

[thehoneybadger]

Nice animation!

I second that. that is a great animation. You have too much time on your hands bitboy.

 

[thehoneybadger]

Since we cannot penetrate the pressure vessel, can we instead penetrate the piping near the pump suction, to add a pressure tap (red in the image below)?

View attachment 62538​

The red vertical line representing the pressure sensor line conneting to the pipe is out of place. It is straight

 

[ndzied1]

Tables are nice but a phase diagram really shows off what is going on. At least at a static temp and pressure.

It took a while to find one that had a good resolution and log vertical scale. Many phase diagrams I found were not to scale or if they were, they were linear so the area around the triple point is so small to be useless.

My weight suggestion was based on me thinking of mass flow, not vapor change so I think it would work (if steam/water are entering and laving the vessel ... seems like a reasonable assumption) but with such a heavy tank the differences of some H2O leaving or entering would probably be hard to measure.

 

[Mas01]

Just to confirm, it is 20 Torr absolute inside the vessel.
We have a digital readout of vessel torr on the PLC. It starts off at around 760 (atmospheric), then drops slowly to 20 Torr. Then water is pumped into the vessel as previously described. Some will turn to steam, some will accumulate in the vessel. This water is pumped out once it reaches the high setpoint. Float switches are being added now, since the analogue depth sensor is not working. I'll give an update when it's ready. Thanks for the help.

 

[drbitboy]

The red vertical line representing the pressure sensor line conneting to the pipe is out of place. It is straight

Good point, the transducer should should be level with or below the pump suction, with a way for air and/or vapor in the tap to rise or purge to ensure the tap is full of liquid only.

But OP is going with float switches, so this analogue approach, which would bypass and solve the problem with the WIKA breather, is moot.

 

[Regular_Jim]

Lol at Peter still thinking water is GUARANTEED to boil at 20 torr. The only one here who's actually got his head screwed on tight when it comes to the physical phenomena inside the tank is Bit. The problem could be thought of as a problem of state, but it's much more appropriate to use pump equations here, as even if the process fluid is liquid water (which, I should say AGAIN, is more than possible, Peter) you may run into loss of hydraulic performance or even cavitation.

The whole damn debate can be solved if OP calculated NPSH and checked out the pump curve for NPSHr. Which, I'm going out on a limb because it sounds like OP is a controls guy in an organization with a process/mechanical engineer, was already considered. In other words, OP probably is looking for what he asked for: how to measure the level of a specific tank, not for recommendations on process design.

 

[drbitboy]

It took a while to find one that had a good resolution and log vertical scale.

Su-weet!

 

[Mas01]

I'm going out on a limb because it sounds like OP is a controls guy in an organization with a process/mechanical engineer, was already considered. In other words, OP probably is looking for what he asked for: how to measure the level of a specific tank, not for recommendations on process design.

Correct.

 

[g.mccormick]

So it seems like it is solved now.....

What is the next worldly problem that we can work on? I hear it is getting hotter.

 

[drbitboy]

Not that the answer has not already been provided, but here is what ASME 1967 Steam Tables say, linear interpolation yields ~21.7°C for [p CALC]. LSTG is Large Steam Turbine Group, where my dad used to work.

​

 

[NetNathan]

20 Torr is no where near a "perfect" vacuum.
The equipment I work on regularly operates in the 10-5 Torr (0.00001 Torr) range (at 2400 F even).
So to me 20 Torr is a gross vacuum level.

Pure space is around 10-3 Torr. It is 10-5Torr where the space station is orbiting

 

[NetNathan]

Engineers guide to tank gauging...
https://go.emersonautomation.com/rmt-en-l-resources-tank-gauging-guide?utm_content=22E_NA-en_NWS_RMTB_Measure_Instrumentation_JUL_EM01_0S&utm_medium=emle&utm_source=mso_nam-elqemnws-mei-zs&utm_medium=emle&utm_campaign=22grmtl-lvel_resources_eg15&elqTrackId=8bf1f8bc969243cda8afcd64bf0b6c4b&elq=77676850399a4dbf9c00684239fed887&elqaid=32370&elqat=1&elqCampaignId=21895