Pumps & Systems, September 2007
With the old mechanical displacers, level fluctuations ran 4-in to 5-in with a lot of hysteresis and frequent failures, causing operators to control at much higher levels than were optimal. New magnetic technology is changing all that. Here's how.
Feedwater heaters are used to "preheat" feedwater before it is fed to the steam drum or boiler to be converted to steam.
The feedwater heater is a larger shell and tube heat exchanger through which condensate returns and any makeup water is preheated prior to being sent to the boiler for conversion to steam. This "preheating" means that less energy (fuel) is required to flash the water into steam.
The feedwater heater, simply described, is a series of tubes carrying feedwater through a tank shell. Steam is injected into the shell and heat is transferred to the feedwater inside the tubes. The steam condenses in the shell, creating a liquid level of condensate in the heater.
The level is controlled to create a seal and prevent blow-through of steam. The maximum thermal transfer between steam and the feedwater takes place when the largest tube area is exposed to the steam without allowing steam blow-through.
Condensate is allowed to drain from the shell through the normal drain. When the tubes become submerged in condensate, heat is transferred to the condensate rather than the tubes with the feedwater inside, resulting in poor heater efficiency. A relatively small increase in condensate level (2-in to 4-in) over the minimum greatly reduces the heat rate (fuel efficiency) of the boiler. Thus, tight level control is required.
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 to 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, numerous major 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).
New magnetostrictive level transmitters are loop powered and 4-mA to 20 mA, with versions in HART®, Honeywell DE, and Foundation Fieldbus. By mounting a magnetostrictive level transmitter on the outside of the heater, maintenance is very low because it is not exposed to the internal temperatures and extremes of the heaters.
The accuracy and repeatability of a magnetostrictive level transmitter is key in this situation. The performance and reliability of the transmitter allow the level of the condensate in the heater to be controlled precisely (usually at a point 2-in to 3-in above the drain).
When the level is controlled as low as possible, more tube area within the feedwater heater is exposed and additional feedwater can be heated for the same energy input to the heater. This increases heater throughput and efficiency. The accuracy and reliability of the system prevent steam blow-through. Typical system accuracy is 1-in.
To further enhance performance of these new level systems, a thermocouple and thermowell can be installed in the magnetic level gauge to measure condensate temperature. The density of condensate will change with temperature, causing a small change in the buoyancy of the float that affects the accuracy of the reading.