Greeting all,

A flow meter (orifice plate differential pressure) installed on vacuum steam line (-850 mbar).

Mostly the flow is zero (emergency line)

The transmitter connected to the line through impulse tubes with condensate pots.

CASE
The transmitter reads about 80 TPH while actually zero.

After inspection we found that the vacuum sucked all the filled water.

We made sure that there is no leakage at all and filled the pots again.

But after returning the transmitter to the service, again suck all water.

Please advice.
Thanks in advance.

 

I doubt the water in the condensate pots is 'flowing' backwards into the steam line, rather I think that the condensate pot water is boiling off in the presence of such a low vacuum.

According the the engineeringtoolbox.com site, water at 150mbara boils at 54.4 degC, or 129.9 degF

https://www.engineeringtoolbox.com/water-evacuation-pressure-temperature-d_1686.html

The condensate pots could be heating up from thermal conduction through the impulse lines, radiant heat from the steam line or convective heat from the environment.

Keeping the condensate pot water cooler than the boiling point of water at your system's operating vacuum pressure would prevent it from boiling off.

Can the source of the heat that is heating the condensate pots be reduced so the condensate pots remain below the boiling point of water at operating conditions?

 

I doubt the water in the condensate pots is 'flowing' backwards into the steam line, rather I think that the condensate pot water is boiling off in the presence of such a low vacuum.

According the the engineeringtoolbox.com site, water at 150mbara boils at 54.4 degC, or 129.9 degF

https://www.engineeringtoolbox.com/water-evacuation-pressure-temperature-d_1686.html

The condensate pots could be heating up from thermal conduction through the impulse lines, radiant heat from the steam line or convective heat from the environment.

Keeping the condensate pot water cooler than the boiling point of water at your system's operating vacuum pressure would prevent it from boiling off.

Can the source of the heat that is heating the condensate pots be reduced so the condensate pots remain below the boiling point of water at operating conditions?

Appreciate Dear,
So what do you suggest?
Do you prefer to replace impulse tubes with remote seal?

 

Well, if you can find a vendor whose remote seal and fill fluid can tolerate 150mbara at your operating temperature, be my guest, but be aware, there are charts or tables showing the limitations of remote seals and their fill fluids at various operating temperatures and vacuum pressures.

See one vendor's specs for vacuum vs temperature for various fill fluids in the charts below.





On the one chart, Neobee and Syltherm 800 are nowhere near the vacuum your system pulls.

On the other chart, CTFE fill fluid or a "special engineered high vacuum model" might work, depending on whatever your system' temperature/pressure operating conditions are, or maybe not. Other vendors are likely to have similar limitations because they all use the same fill fluids.

If using remote seals doesn't fly (which I suspect it won't), you can put on your hard hat and do some on-the-site analysis. Go look at the location of the condesate pot and determine what is heating the condensate pot that is causing the boil-off effect.

The 55°C boiling temperature is very close to the temperature at which you can burned by touching a surface at that temperature. If it were me, I'd grab the can of black spray paint, the Infrared non-contact pyrometer, and anything the measures air temperature (thermometer or a DVM with a temp sensor) and check out
- the temperature of the steam line (how much radiant heat) in the vicinity of the pot,
- the steam line-to-pot impulse tubing (how much conductive heat) and
- the air temperature immediately around the condensate pot

Then I'd figure out how reduce whichever heat source(s) is the culprit.

 

David_2,
Excellent advice.

I would like to tell the story of the " bubbler".

We simply routed a 1/4 inch copper line to the upper pot though an adjustable needle valve. The slow trickle of cool water stopped your well documented effect.

he air temperature immediately around the condensate pot
was the cause.

This approach did however have issues. We piped 5 grain water to the pot and the needle valve would have to be opened and flushed weekly. We just put in on the weekly transmitter schedule of blowdowns.

Although I left that job years ago, I still know the folks that stayed.

That was nearly 30 years ago, and as far a I know is still that way.

 

What a beautiful example of the KISS (Keep it Simple, Stupid) principle, "We simply routed a 1/4 inch copper line to the upper pot though an adjustable needle valve. The slow trickle of cool water stopped your well documented effect" and still working 30 years later. I love it!

 

David_2,
Just FYI
The calibration procedure for the needle valve was equally complicated.

One would open the needle valve until drops of water would form in the gauge glass and slowly descend. Anything more or less would cause the level transmitter to give false readings.

 

Curious_One,

Is the cooling water just dripping onto the surface of the condensate pot for its evaporative cooling effect or is the the cooling water is being added to the condensate pot through the fill port on the condensate pot?

 

Curious_One,

Is the cooling water just dripping onto the surface of the condensate pot for its evaporative cooling effect or is the the cooling water is being added to the condensate pot through the fill port on the condensate pot?

David_2,

We tried external cooling. Too much water needed and would have had to install some sort of drain.

Then we went internal. I was concerned that the 5 grain water would contaminate the condensate system. 5 grain water was nearby so the experiment began. We had plans to change to 0 grain water but the contamination of the condensate system was never detected during our 3X daily boilerwater/feedwater testing.

As with anything that works, just leave it alone!!!

 

The flow must be so low that it is like a differential relay (constant flow regulator) for a bubbler, like you mentioned early on, 'the bubbler'. Interesting that the flow is low enough that the chemist didn't pick up the 5 grains of hardness in the condensate. As you say, if it works, leave it alone!

 

David_2,

I have no measurements of that process that made that stupid thing work. No steam tables at that pressure or lack of pressure.

However, while in the USN we made 0 grain reserve feedwater from the ocean.

I believe that the flashing in the condensate pot was occuring even with the addition of the 5 grain cooling water. The cooling water was merely a balance.

I believe that the 5 grain also flashed to steam as well and deposited its hardness into the condensate pot that was blown weekly.

I await your thoughts.

 

My domestic water heater experience convinces me that the hardness minerals precipitate out (we have really hard water). I find brownish chunks of precipitated minerals, about size of cereal granola, collecting in the bottom of my domestic hot water heater when I have to change-out the lower heating element.

The weekly blowdown of the condensate pot might very well clean out any accumulation, whether precipitate collects in quantity depends on the hydraulics of the blowdown.

A lot of plants put a pipe Tee at the bottom of the drop leg from the condensate pot opposite the transmitter's pressure port and put a nipple and cap at the bottom of the Tee which extends below the port elevation. The extended drop leg acts as a well that collects any deposits: sediment, dirt, whatever in the impulse line.

I'll bet if the installation at the plant you're familiar with has an extension leg at the bottom of the drop leg and the cap were removed that you'd find it full of minerals that precipitated out over the years and managed to float down the drop leg.