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Improving Safety in Your Steam System


April 17, 2016  


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The Prowirl 200 F Vortex flow meter accurately detects and measures wet steam and the dryness fractions in a steam system. It offers a multivariable solution for steam mass and energy measurement that will increase plant safety and improve the efficiency of steam systems.

Steam is commonly used for process heating. Typically, saturated steam is produced in shell-and-tube (fire-tube) boilers. The advantage of saturated steam is that the heat content is high and temperature can be regulated by controlling pressure. Wet steam is the region on the Enthalpy line where both water and vapor exist, whereas superheated steam is steam that has been heated past its evaporation pressure. When looking at the properties of dry saturated steam it can be seen that it is exactly at the border line between wet steam and gaseous superheated steam (vapor). Superheated steam is typically used for transportation of steam through a plant and/or for turning turbines for power generation.

Wet steam is not desired as it has a safety and efficiency concern:
• Water hammer may result.
• Priming of boilers results in carry-over of salts into the steam system and thus leads to fouling and corrosion.
• Wet steam contains much less energy than dry steam.

Example: Heating liquid water from 20 °C (A) to 100 °C (B) requires about 4.2 kJ/kg• K of energy (hf). In order to convert the water (B) to steam (C) at 100 °C and 14.7 psi abs., 2255 kJ/kg are required (hfg). During this process, the dryness (x-factor) is increased from 0 to 1.

Wet steam can be found anywhere in a saturated steam system:
• At the outlet of undersized boilers or poor boiler water quality
• In the distribution network due to heat losses
• At the point of use because of malfunctioning equipment

Why compensate for wet steam?
We have seen that the energy content of steam strongly depends on the dryness fraction. Let’s take our example of 43.5 psig steam again (Fig. 1). Now let’s assume that this steam has a dryness fraction of 90%. In reality this means that in the pipeline we find:
• 100% of the sensible heat hf (i.e. 604.7 kJ/kg)
• 90% of the latent heat hfg of perfectly dry steam (i.e. 0.9 x 2138 kJ/kg = 1924 kJ/kg)

Most relevant for heat transfer is the latent heat (condensate will be returned to the boiler), i.e. steam with a quality of 90% will only have 90% of the energy of dry, saturated steam available. If we assume that it takes about $15 Dollars (CAD) to produce 1,000 pounds of steam, this steam will only have a value of $13 Dollars (CAD) left. Therefore, the innovation of dryness fraction measurement and mass/energy flows compensation (using this information) results in a strongly reduced measurement uncertainty and error in compensated steam flow output.

This real time information will allow for:
• More accurate internal costing and external billing as you only pay for the energy you receive
• Improved boiler and steam system efficiency as process adjustments can be made proactively to maintain the steam quality

What output information can be provided by the Prowirl F 200 Vortex meter?
Through the measurement of the dryness fraction, a broad variety of useful parameters can be generated:
• The dryness fraction itself
• Mass flow of steam
• Mass flow of condensate
• Total mass flow
• Heat flow (i.e. enthalpy relative to the triple point of water) compensated by the dryness fraction
• Delta heat (difference between enthalpy contained in the steam and the enthalpy contained in the condensate, compensated through external temperature values read in)

These parameters can either be assigned to the display or to the analog (max. 2) or digital outputs, e.g. HART, Profibus and FOUNDATION Fieldbus.

How is it done?
At a steady flow rate and stable process conditions, the volume flow in a dry steam application will result in a stable vortex signal over time. If liquid water droplets are present, however, this will result in variations of the vortex signal, i.e. the droplets superimpose a second signal which has an effect on the signal’s Kurtosis (“peakedness” of the probability distribution) which is analyzed by the Prowirl F 200. The Kurtosis is related to and what is used for measuring the dryness fraction and thus detecting wet steam.

Summary
The Prowirl F 200 has the unique ability of detecting wet steam and measuring the dryness fraction of steam. This single flow device will increase safety and efficiency in your steam system thereby:
• Increasing plant availability by reducing the number of steam traps and strainers needed
• Increasing efficiency and lifetime of your boiler and steam systems by reducing water hammer events
• Increasing plant safety by limiting exposure to potential hazardous process conditions
• Optimizing your steam quality with real time process information that provides immediate process disturbance data that can be corrected and maintained

by Chris Cousineau, Flow, Pressure and Temperature Product Manager

www.ca.endress.com