Feedwater heater control: Level


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Feedwater Heater Control: Bring Your Efficiency to a New Level -- Kevin Hambrice, Global Factory Sales and Marketing Manager-Level, K-TEK, A Member of the ABB Group

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  • Historically, mechanical displacement type level systems with mechanically driven pneumatic controllers have been used. These types of controllers have two basic issues. First, the mechanical nature of these level controllers requires high maintenance due to seal and bearing failures and air supply issues. In fact, one plant used maintenance costs and an average 5-year life of the equipment as cost justification for an upgrade of the level systems. Second, the varying liquid density of both the water and the steam in a feedwater heater creates significant errors in the measurement with a displacer. A change of only 0.1-sgu in the liquid will result in a 12.5 percent error and, as more commonly seen, a change of 0.2-sgu will create a 25 percent inaccuracy with a displacer. With the ranges for these heaters typically being 48-in to 120-in, it is common to see the heater level change 6-in to 8-in without a change of feed or extraction rate. Most power plants compensate for this by simply controlling the level at a point above the maximum inaccuracy of the displacer. Typically, this point is 8-in to 10-in above the drain – which greatly impacts the heater efficiency. Another point is that “displacers” are typically pneumatic and cannot be easily tied into a distributed control system (DCS). This means the advantages of varying heater level to control temperature with load cannot be realized. Early upgrades used “hydrostatic” level transmitters, which proved unsatisfactory due to their inaccuracies caused by changes in the condensate density due to temperature, thermal expansion and contraction of the shell.
  • To solve these problems, power producers in the U.S. and international locations have applied the newest magnetic level gauges and magnetostrictive level transmitter technology to their feedwater heaters. The design of the newest magnetic level gauges is rugged, yet simple, and can be certified to ANSI / ASME B31.1 “Power Plant Piping Requirements.” These gauges use a magnetically coupled float level indicator for local visual indication. The float design and magnet technology provide very high magnetic flux density at the measurement point to prevent “decoupling” of the indicator on high-pressure heaters. To eliminate the maintenance problems with mechanical transmitters and controllers, new magnetostrictive level transmitters can be utilized. A very accurate (1/32-in) level transmitter is actuated by the magnetic field in a level gauge. It is simply strapped to the outside of the level gauge housing. Magnetic level switches can also be mounted in this fashion with certain magnetic level gauges.
  • The real money is in the improved control. With the old mechanical displacer, level fluctuations ran 4-in to 5-in with a lot of hysteresis. This poor performance, coupled with frequent failures, caused operators to control at much higher levels than were optimal. With the new technology, users can control the level systems with their DCS to a level set point of 9-in. Through this precision level control, they are able to maintain a 5-deg differential on their heaters and get 6 percent more efficiency, which equates to an overall cost savings.
  • Several power facilities around the world have found magnetic level gauges and magnetostrictive level transmitter technology so economical that redundant systems were installed on each heater. They simply do not want to run without this type of system in place. Discussions with several plant engineers indicate a payback in less than six months on efficiency gains alone. Plant operators have also reported huge maintenance savings, since sightglass repairs are eliminated.
  • ABB is recognized globally for expertise in designing, manufacturing and supplying a wide range of high quality, accurate and reliable measurement, analytical and control solutions that are certified to international standards. These are all supported by a comprehensive worldwide network of manufacturing plants, calibration laboratories and service centers where hundreds of engineers provide the application support and service you need. The result is that ABB is far more than an product supplier. ABB is a partner to help your business grow and be more competitive. ABB – your partner of choice for measurement solutions
  • Feedwater heater control: Level

    1. 1. WMP-113-1 Feedwater heater control: Bring your efficiency to a new LEVEL ABB Automation & Power World: April 18-21, 2011 © ABB Inc. May 31, 2011 | Slide
    2. 2. WMP-113-1 Feedwater heater control: Bring your efficiency to a new LEVEL <ul><li>Speaker name: Kevin Hambrice </li></ul><ul><li>Speaker title: Global Factory Sales & Marketing Manger - LEVEL </li></ul><ul><li>Company name: K-TEK, A Member of the ABB Group </li></ul><ul><li>Location: Prairieville, LA </li></ul>© ABB Inc. May 31, 2011 | Slide
    3. 3. © ABB Inc. May 31, 2011 | Slide Feedwater Heater Control Why is feedwater heater level control important? <ul><li>Heaters preheat feedwater before boilers converted it to steam </li></ul><ul><li>“ Preheating” reduces energy to flash the water into steam </li></ul><ul><li>Liquid level of condensate is formed as steam condenses </li></ul><ul><li>Level is controlled to create a seal and prevent steam blow-through </li></ul><ul><li>Accurate and repeatable level measurement keep boiler fuel consumption down </li></ul>
    4. 4. Feedwater Heater Control What are you controlling and why? © ABB Inc. May 31, 2011 | Slide Injected steam condenses and acts as a seal to prevent blow-through of steam in the heater Maximum thermal transfer between steam & feedwater takes place when largest tube area exposed to steam Small increase in condensate level (2-in to 4-in) over minimum greatly reduces the heat rate efficiency)
    5. 5. Feedwater Heater Control Traditional Devices Used for Feedwater Level Measurement <ul><li>Mechanical displacement type level systems with pneumatic controllers have been used </li></ul><ul><li>Two major issues </li></ul><ul><ul><li>Mechanical nature requires high maintenance due to seal and bearing failures and air supply issues </li></ul></ul><ul><ul><li>Varying liquid density of water and steam creates significant measurement errors in displacer. </li></ul></ul>© ABB Inc. May 31, 2011 | Slide Sight Glass Displacer Differential Pressure Conductivity Probes
    6. 6. Feedwater Heater Control Best Practice Feedwater Level Control <ul><li>Magnetic level gauges and magnetostrictive level transmitter used to improve control </li></ul><ul><li>Performance and reliability control condensate level at 2” to 3” above the drain </li></ul><ul><li>More tube area is exposed to heat and additional feedwater with the same energy input </li></ul><ul><li>Increases heater throughput and efficiency </li></ul><ul><li>Accuracy (±1”) of the system prevent steam blow-through </li></ul>© ABB Inc. May 31, 2011 | Slide Certified to ANSI/ASME B31.1 360º Magnetic Ring 75 Grams Buoyancy
    7. 7. Feedwater Heater Control Precision Measurement for Changing Process Conditions <ul><li>Thermocouple can be installed to compensate for density changes of the condensate </li></ul><ul><li>Small small change float buoyancy affects accuracy </li></ul><ul><li>Simples correction in DCS, allows lower condensate levels in the heater </li></ul><ul><li>Improves efficiency even more </li></ul>© ABB Inc. May 31, 2011 | Slide
    8. 8. Feedwater Heater Control Proven Success <ul><li>“ We saw a return on investment and efficiency gain within 6 months” </li></ul><ul><li>“ I’ve been pleased with the significant maintenance savings with the elimination of sight glass repairs.” </li></ul><ul><li>“ We were looking for a reliable solution that would be accurate and repeatable – we got this with the MLT technology.” </li></ul>© ABB Inc. May 31, 2011 | Slide
    9. 9. Improving Power Plant Efficiency ABB – Your partner for Measurement Solutions <ul><li>World class instrumentation solutions built on over 100 years of experience </li></ul><ul><li>Built upon the leading names and brands in the automation world: </li></ul><ul><li>Unrivalled in its scope and applications expertise </li></ul><ul><li>Solutions certified to international standards </li></ul><ul><li>Worldwide network of sales offices and manufacturing plants </li></ul>
    10. 10. © ABB Group May 31, 2011 | Slide