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.
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