A new solution to detect wet steam and measure the dryness fraction

Transcription

1 A new solution to detect wet steam and measure Oliver Seifert, Head of Department Product Management (Vortex and thermal flowmeters) Endress+Hauser Flowtec AG, Reinach (Switzerland) Endress+Hauser has recently introduced its new Proline vortex flowmet er family. These highly robust flowmet ers have been developed mainly for st eam applications, and t hey offer a broad scope of multivariable solutions for st eam mass and energy measurement. Two innovations help to increase safet y and to improve t he efficiency of st eam syst ems a continuous wet st eam det ection and a dryness fraction measurement. Fig.1: The new Prowirl 200 is a vortex flowmeter able to detect wet steam and even measure the dryness fraction of steam. This information can be used to compensate both mass flow and energy flow thus reducing any measurement uncertainty dramatically. Steam is commonly used for process heating. Typically, saturated steam is produced in shell-and-tube (fire-tube) boilers. Its advantages are obvious: The heat content is high and temperature can be regulated by controlling 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). When dry saturated steam passes on its energy to a process, its latent energy is released. This energy can be found in steam tables as enthalpy (h fg ). Whilst this energy is released, the steam becomes wetter, i.e. its dryness fraction (x) is reduced from 1 down to 0. What doesn t change, however, is the combination of pressure and temperature during this process. This means that ideally steam enters, for 1/ 6

2 example, a heat exchanger at 3 bar g and +144 C and condensate can be found at the exit at exactly the same pressure and temperature but the steam has lost 2138 kj/ kg of latent heat (h fg ). The problem now is: If both liquid and steam, and any state in-between, can exist at the same pressure and temperature, it is impossible to determine by just measuring these two parameters? Therefore, today there is no solution available to easily determine on a continuous basis if water is present in the steam line or not. Fig. 2: Mollier diagram for water. Example: Heating liquid water from 20 C (A) to 100 C (B) requires about 4.2 kj/ kg K of energy (h f ). In order to convert the water (B) to steam (C) at 100 C and bar abs., 2255 kj/ kg are required (h fg ). During this process, the dryness (x-factor) is increased from 0 to 1. Fig. 3: The steam quality is defined by its dryness fraction x. If x = 0: water is fully saturated. If x = 1, there is dry saturated steam. If x = 0.8, 80% of the mass of water is in a gaseous state and 20% in a liquid state. 2/ 6

3 Where is wet st eam generat ed in a st eam syst em? Wet steam can be found anywhere in a saturated steam system: At the outlet of boilers because of undersized boilers or poor boiler water quality In the distribution network because of heat losses At the point of end-use because of malfunctioning equipment (e.g. steam traps) Wet steam, however, is both a safety and an 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. Therefore, it is highly beneficial to receive a warning in case wet steam is present or, even better, to measure and compensate accordingly. Such helpful functions are optionally available with Prowirl 200. Fig.4 : As soon as wet steam is in the pipe, Prowirl 200 issues an alarm message. Why compensat e for wet st eam? We have seen above that the energy content of steam strongly depends on. Let s take our example of 3 bar g steam again (Fig. 2). 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 h f (i.e kj/ kg) 90% of the latent heat h fg of perfectly dry steam ( i.e 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 60 Euros to make a ton of steam, this steam will only have a value of 54 Euros left. 3/ 6

4 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 is beneficial for: Better internal costing and external billing, i.e. you only pay for the energy you receive. Better process control, i.e. how much energy does the process really receive? Improved efficiency. Fig. 5: Error in total mass flow measurement versus dryness fraction (x). With measurement and associated mass flow correction, the total mass flow rate measurement uncertainty can be reduced to ±3%. Output Options Through the measurement of, a broad variety of useful parameters can be generated: The dryness fraction itself Mass flow of gaseous steam Mass flow of condensate Total mass flow Heat flow (i.e. enthalpy relative to the triple point of water) compensated by 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. 4/ 6

5 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 200. The Kurtosis is related to and is used for measuring and detecting wet steam. Fig. 6: Signal response to wet steam in a Prowirl 200. The primary vortex signal is clearly affected by the droplets. For which applications is t he wet st eam measurement available? The wet steam measurement and the wet steam detection is available for: line sizes DN 25 (1") to 100 (4"), flow velocities > 5 m/ s, steam qualities from 80 to 100%, steam pressures from 0.5 to 10.0 bar g. Another important point is that enough inlet runs have to be respected to make sure that the vortex signal is not disturbed by obstructions up- and downstream of the flowmeter. 5/ 6

6 Sum m ary The presented innovations of detecting wet steam and measuring of steam offer many benefits. The most important are: Increased safety: Detecting wet steam can help avoiding water hammer and corrosion in the steam system. Increased efficiency: Detecting wet steam helps to avoid inefficiencies in the steam system, and measuring helps to measure the accurate amount of energy provided to a process. Better internal and external costing: The amount of energy purchased or sold can be determined more accurately. Endress+Hauser Endress+Hauser is a global leader in measurement instrumentation, services and solutions for industrial process engineering. The Group employs more than 12,000 personnel across the globe, generating net sales of 1.8 billion euros in With dedicated sales centers and a strong network of partners, Endress+Hauser guarantees competent worldwide support. Production centers in 11 countries meet customers needs and requirements quickly and effectively. Endress+Hauser provides sensors, instruments, systems and services for level, flow, pressure and temperature measurement as well as analytics and data acquisition. The company supports customers with automation engineering, logistics and IT services and solutions. Endress+Hauser works closely with the chemical, petrochemical, food & beverage, oil & gas, water & wastewater, power & energy, life science, primaries & metal, renewable energies, pulp & paper and shipbuilding industries. Founded in 1953 by Georg H Endress and Ludwig Hauser, Endress+Hauser has been solely owned by the Endress family since The Group has developed from a specialist in level measurement to a provider of complete solutions for industrial measuring technology and automation, with constant expansion into new territories and markets. 6/ 6