Overcoming Level Gauging Challenges In
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This can F drive turbines and produce power, The amount of heat energy (Btu) is divided by the drive a major improvement in overall boiler per- accurate level measurements at criti- amount of electricity generated (kW) over time. formance. cal points in the process are necessary, but they Essentially, the heat rate indicates how much fuel A typical feedwater heater uses a tube-and-shell can be particularly challenging. A high degree of must be burned to generate a specific amount of design in which it pumps water through the tubes accuracy can provide greater operational efficien- electricity, and the lower this number is the better. surrounded by steam that is diverted away from cy and avoid conditions that might cause degrada- Obviously, the heat rate has a substantial effect the turbine (see Figure 2). While this reduces the tion or failure of equipment such as the turbine. on the efficiency of a plant. For a medium-sized amount of steam available for the turbine, the gains This article focuses on level measurements for utility operation of 500 megawatts (MW), a 1 per- realized more than make up for the loss because two specific applications: feedwater heaters and cent improvement in heat rate can save $500,000 the overall amount of fuel consumed goes down. boiler drums. The basic methods used in these in fuel. Consequently, feedwater heaters are used Typically, the feedwater temperature increases applications are examined, and case studies illustrate each approach. Depending on a given Figure 1. Reheating feedwater plant’s configuration, either — or possibly both before it is pumped back into the boiler improves overall efficiency. — application could reside at a single location. All graphics courtesy of Emerson Automation Solutions Application 1: Feedwater heaters When steam comes out of the exhaust vent of a turbine, it goes into a condenser where it is suf- ficiently cooled to condense back into water (see Figure 1). This creates a negative pressure on the back side of the turbine as well as a larger pres- sure differential from the inlet, which helps pre- pare the water to be sent back to the boiler and used again. Water coming out of the condensers is hot, but if further heated, it can increase boiler efficiency. The colder the feedwater is the harder the boiler has to work to return it to steam at the desired pressure. This effort is quantified as a boiler’s “heat rate,” which is the measure of energy put Copyright © 2017 by Grand View Media. All rights reserved. No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the publisher, except in the case of brief quotations embodied in critical reviews and certain other noncommercial uses permitted by copyright law. The material in all Grand View Media publications is copyrighted and may be reprinted by permission only. Reprinted with the permission of Grand View Media and the author[s]. Figure 2. Steam intro- duced into the shell heats condensate flowing through the tubes. “A typical feedwater heater uses a tube-and-shell design in which it pumps water through the tubes surrounded by steam that is diverted away from the turbine.” from 110ºF to 570ºF as it passes through this heat º exchanger. This is significant since even a 5 F Pressure (psig) Temperature (deg F) Water Specific Gravity Percent Error from SG of 1 increase reduces the heat rate by 0.1 percent, so small improvements add up over time. 100 338 0.897 10.3 200 388 0.866 13.4 Maintaining correct level While this concept is very straightforward, complica- 300 421 0.844 15.6 tions emerge when trying to control steam flow to the 400 448 0.825 17.5 feedwater heater. The steam is in the shell and heats the liquid through the tubes. To get maximum heat 500 470 0.816 18.4 transfer and efficiency, the steam should condense 600 489 0.808 19.2 fully within the heater, so there should be some water 700 505 0.778 22.2 accumulation inside. Any steam blown out represents unrecovered heat, and the water accumulation helps 800 520 0.764 23.6 protect against tube leaks, so some condensate within 900 534 0.752 24.8 a specific level range is desirable. At the same time, too much condensate accumulating in the heater reduces 1000 546 0.739 26.1 efficiency. The tubes should be mostly exposed to the Table 1. The degree of error characteristic of DP level instruments and float level sensors increases as pressure and temperature levels increase. steam for maximum heat transfer. The worst situation arises when the steam sec- tion becomes fully flooded with condensate. This Real-world level solution in the heater — affect density and, therefore, the not only drastically reduces efficiency, it also can A three-unit coal-fired facility that burns low- accuracy of the level reading, the system did not cause condensate to back up all the way to the tur- sulfur coal in the western United States wanted make an adequate correction. So, as the tempera- bine, blasting water into the blades along with steam. to improve its level measurements. Units 1 and 2 ture changed due to variable unit loading, so did This can damage the blades and reduce turbine life, of the facility are conventional sub-critical designs the level, even though the indicator said it was potentially causing catastrophic failure of the tur- from the 1970s. Unit 3, constructed in 2010, is consistent (see Table 1). It was not uncommon bine. Effective level control to protect against turbine super-critical and has extensive emission con- to have a 20 percent error, which caused a loss in water induction is so critical that American Society of trols. All three units have similar feedwater heater efficiency. Mechanical Engineers (ASME) Code PG.60.1.1 calls trains, each with three heaters connected in series. To solve this problem, the plant installed a for feedwater heaters to be outfitted with two direct- Until recently, the feedwater heaters were outfit- Rosemount Model 5300 guided wave radar (GWR) level readings via sight glasses or one direct-level ted with pneumatic systems built around triple- level instrument (see Figure 3) on one of the feed- reading combined with two indirect readings, plus redundant, differential pressure (DP) level mea- water heaters in a three-heater train as a test. This high-level switches.