Correct density to use in a steam generator level guage

[mrtangent]

Dear sirs,

I am struggling to understand how to calculate this. If you can recommend a reference please let me know.

We have Steam generate generated at 100psig. The density of steam at 0.27 m3/kg(approx). but water at 100psig proir to boiling is 909 kg/m3 (approx). Does the rate of steam impact the reading. i think it must do but i cannot figure out how to adjust with rate. or ensure the right factor is used? or does it not make a difference.

We have a kettle boiler, so i assume there is a transition region where the system changes from 100% water to 0% water. Average density is the mixture average density of steam/water.

i Pressure on Level transmitter =
+ Water phase only phase pressure (density * gh)
+ transition phase (densityaverage * gh)

I dont seen how to convert transition phase density / height or calculated from flow. I've read some text books but it does not seem to cover it.

 

[KiwiMace]

The density of the water will make a difference to the level measurement, but the pressure of the steam will not - provided you are using a DP transmitter with upper tapping on the vessel; as required on a closed vessel. The pressure acts on both sides of the txr and nulls.

Transmitters can't usuually sustain the steam on the element, so you fill wet legs with condensed steam. This is hot, but way less than the SH steam temp of the boiler. This is often done by filling manually and then hosing the tubing to speed up condensation and a complete fill.

Your level will be determined from h=P/(rho.g) calulated for both sides of the txr, with proper compensation for the wet leg height and fluid density (rho). With the upper tapping being constant high side, the txr ends up naturally reverse acting - high level is low reading, often this can be reversed in a smart txr setup.

 

[katmar]

James,

You are correct that the steam bubbles in the water will lower its density, making it difficult to convert a pressure difference signal into a true height. However, you can calculate the mass of water present if the horizontal cross sectional area is fairly constant between the two pressure tapping points.

The mass of water is the product of the cross sectional area, the height and the density. But, as CinciMace has pointed out, the expression for the height is inversely proportional to the density. When you substitute it all into one formula the density cancels out and you get the mass directly.

But why do you need to know the height or the mass? The steam generator manufacturer should specify minimum and maximum pressure differential readings for safe operation.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[mrtangent]

I believe as rates increase the density will drop of the liquid. Therefore unless i change the calibration density will be running the unit at higher rates (as we are now). I want to calculate the sg to input so we ensure the vessel is not overfilled.

it is an existing unit so the manufacture will not be interested (there not in that business now anyhow).