Volumetric flow

Hi Guys, I noticed an annubar being used to measure steam flow. Why are they measuring temperature and pressure as well?

Well, they are possibly interested in determining the mass flow rate as well as the volumetric flow rate. Because steam is compressible the density varies with both temperature and pressure. You need both to convert volume to mass.

Oh k. And is there an equation to convert from volume to mass?

Theboss said:

Oh k. And is there an equation to convert from volume to mass?

Click to expand...

mass = volume * density

I know that, but where does temperature and pressure fall into density as an equation?

If the steam is super-heated it follows the general gas law:

http://www.engineeringtoolbox.com/ideal-gas-law-d_157.html

If not, it gets complicated.

http://www.engineeringtoolbox.com/vapor-steam-d_609.html

When is steam regarded to be superheated according to pressure and temperature?

Theboss said:

When is steam regarded to be superheated according to pressure and temperature?

Click to expand...

If you look at a phase diagram for water there is a line between the liquid and vapor phases. This shows the boiling point for any pressure. For instance, at 1atm pressure, the boiling point temperature is 373.15K (100C).

As the pressure increases, the boiling point temperature increases as shown by that phase change line. Steam that has just formed and still has the conditions on the phase change line is said to be saturated steam. When more heat is added to the steam, it moves away from the line and it becomes super-heated steam.

Super-heated steam has more energy than saturated steam and, at least back 30 years ago when I briefly worked in this field, it was common to characterize the amount of superheating by how higher the temperature was above the saturation temperature. For instance we made a boiler / superheater package that delivered steam at 1500 psi with 600 degrees of super heat (no metric back then). At 1500 psi the boiling temperature is about 600F so the steam coming out of the superheater was 600+600=1200F (649C).

There was a big field behind the building and for testing they had a pipe that ran to the middle of the field and released its own clouds .

I currently have a vortex flowmeter measuring steam flow and a pressure transmitter after the flowmeter which sends a 4-20 signal into the vortex flowmeter.
These are the process variables from the meter
Volume flow =205.5 m3/h
Mass flow =1655.7 kg/h
Temperature =201.13 deg c
Calculated saturated steam pressure =15.96 bar
Density =8.05 kg/m3
Pressure (from pressure transmitter) = 4.9 bar.

My question is how is flowmeter computing these variables to output mass flow.
I look at a steam table and at 201.13 deg c the corresponding pressure is around 15.96 bar which gives a density of around 8.05.
I'm happy with that but the pressure transmitter has a pressure of 4.9 bar and seems on have no effect on the calculation.

The flowmeter is setup to for external pressure which is reading 4.9bar but it seems like according to the steam table 15.96 bar is the correct pressure. How and why is this?