Upgrading Your Feedwater Heater Level Controls: Is It Worth It?

by Don Hite, Power Industry Business Development Manager

A global outlook for the installed base of coal-fired and, to some degree, nuclear power generation can leave decision makers staring into a crystal ball when deciding whether or not to pursue investments in technologies that improve feedwater heater level control and ultimately plant efficiency (heat rate), while significantly reducing maintenance cost.   In North America, for example, coal is the environmental equivalent of a four-letter word. On the other hand, China, India, Japan and Southeast Asia, as well as other regions, see their vast coal reserves as a valuable asset upon which their long-term energy security is hinged. Nuclear power shares a similar dilemma, with public concerns about safety and the upfront engineering cost associated with change as factors affecting industry development.

I am not going pretend my crystal ball is any better than the next guy’s, but I am willing to step out on the limb a bit to share some insight from my work with various clients in the power industries. Hopefully, the information will be helpful to those who are entertaining the “is it worth it?” aspect of feedwater heater level controls.

ROI PAYBACK IN AS FEW AS  TWO YEARS
Countries that view coal-fired and nuclear generation as part of their long-term strategic plan can realize gains in efficiency and maintenance sufficient to recover the cost of retrofitting their level controls at roughly around two to two-and-a-half years. Of course, there are a number of factors that come into play including, but not limited to, the performance of existing controls relative to feedwater optimization. Nonetheless, there is certainly value when you factor efficiency, environmental and maintenance benefits into the equation, which ultimately drop to the bottom line. In regions where the future of coal-fired generation is up in the air, i.e., retiring due to age, environmental constraints/impact, overall inefficiencies and so forth, I see plants pushing the timeframe out to five or more years, where the first two years are spent recovering the investment through operational, performance and maintenance gains and profits are realized for the duration of the plant’s lifecycle. Note the nuclear power industry may experience a slightly longer cost recovery period due to additional front-end engineering requirements.

The scope of the balance of this post is not to highlight the application and cost benefits of the Magnetrol® Eclipse® guided wave radar (GWR) as compared to traditional level technologies employed on feedwater heaters prior to…. let’s say the 1999-2004 timeframe.  I’m assuming you have already opted for the technology, so I focus the discussion on the path to retrofitting an existing installation to a state-of-the-art configuration as well as some cost considerations.   If you’re not at this point yet, you can download our literature at eclipse.magnetrol.com, which details the upside of GWR, or feel free drop me an email to discuss specifics at dhite@magnetrol.com.   Additionally, if you would like more information on how the technology can improve performance – thus, heat rate – please download the whitepaper “Heat Rate and Feedwater Heater Level Control” from Magnetrol’s website via the link below. Be advised this is one you won’t want to put down.

Feedwater Heater Level Control

COMMON RETROFIT SCENARIOS:
For those of you who are continuing on with the conversation, there are three consistent feedwater heater retrofit scenarios I encounter in the field relative to coal-fired and nuclear generation.

The first is upgrading from torque tube (displacer) technology to GWR, which can be as simple as removing the “guts” of the torque tube (displacer) and replacing with the GWR utilizing the existing chamber. Or, it can be more complex if the original installation is a top-in/bottom-out type process connection with a control scheme comprising an imbedded PID (set point) controller for local, pneumatic control to maintain “normal water level” in the feedwater heater. Yes, pneumatic controls are still out there. I used the latter scenario in my example (Retrofit Option 1) since it is the most expensive route and requires additional hardware to install the GWR and maintain the existing control scheme. It is easy enough to eliminate portions that may not apply to a particular installation or a less complicated scenario, e.g., we can delete the PID controller and I/P converter if the torque tube transmitter and pneumatic controls have been upgraded to transmit a 4-20mA signal to the DCS.

The second most common retrofit involves upgrading a differential pressure installation where no existing, external chamber is available to accommodate the GWR, but we can take advantage of available process connections. This scenario falls more in line with the example in Retrofit Option 2 relative to cost.

Last, but not least, is retrofitting an RF capacitance or magnetic level indicator coupled with a magnetostrictive-type level transmitter. This scenario is pretty straightforward since, in most cases, the existing external chamber can be used, making it a simple matter of removing/replacing the current technology with GWR. Adding an external chamber (Retrofit Option 2) covers situations where this is not the case or where an Aurora® configuration (Orion® Instruments magnetic level indicator with integral GWR) is preferred.

HP Feedwater Heater

Triple redundancy on HP feedwater heater: Two standalone ECLIPSE guided wave radar units and an AURORA configuration (ORION Instruments magnetic level indicator with integral GWR)

RETROFIT OPTION 1:
Here we pursue a “kill two birds with a single stone” approach in that this option offers a solution for retrofitting the torque tube level transmitter/controller to GWR technology, as well as upgrading the sight (glass) gage to reduce overall maintenance costs*, while at the same time providing a higher margin of personnel safety. As mentioned, this is a worst case scenario in that the process connections for the torque tube (top-in/bottom-out) are problematic when retrofitting with a top mounted technology and the pneumatic control scheme at this site was to remain intact.

Taking advantage of the side/side process mounting configuration of the existing sight (glass) gage location on each feedwater heater (Figure 1) is the most straightforward method of retrofitting the torque tube level controllers with an AURORA configuration (magnetic level indicator with integral GWR).

Magnetic Level Indicator

Although this method adds some cost to the instrument itself and may require the introduction of isolation valves, it eliminates plumbing modifications to accommodate a side/bottom process connection configuration (Retrofit Option 2) for the GWR, along with the bonus of alleviating ongoing sight (glass) gage issues. By the way, I mention the isolation valves only because they are integral to the sight (glass) gage in some instances. If the valves are separate, we can use what is in place.

This is a practical approach since sight (glass) gages are not mandated in the ASME boiler code relative to feedwater heaters; hence, they are rarely encountered in new feedwater heater installations. The AURORA (MLI with integral GWR) offers the best of both worlds: highly accurate level indication along with the host of other GWR benefits, coupled with an easily readable and redundant visual indication. The cost of replacing/repairing the sight (glass) gage in the above image would more than offset the additional cost of the instrumentation. Installation would simply be a matter of removing the sight (glass) gage and socket welding the necessary isolation valves (as required) and the AURORA configuration in its place.

To maintain a control scheme consistent with an existing pneumatic design, the new system will include a PID (set point) Controller and I/P converter as an interim set point control solution. The PID controller will accept the 4-20mA feedwater level signal from the ECLIPSE GWR, and calculate the control output where it will be converted to the corresponding pressure (3-15 psi or 3-27 psi) by the I/P converter for appropriate valve operation to maintain “normal water level” in the feedwater heater. The PID controller and I/P converter can be easily removed and the 4-20mA output signal routed directly to the I/O cabinet for the DCS in the future or if the existing control scheme can accommodate the analogy output of the GWR. (Figure 2)

PID Controller Schematic

*The primary failures of sight/tubular (glass) gages are due to leakage from a steam cut or failed seal or integral valve and etched glass. The repair process goes something like this: Remove the unit; sandblast; shave surface; install repair kit; reassemble and torque bolts; reinstall on vessel; bring vessel up to temperature and back down; verify torque values. The following details the cost associated with repair of these devices.

Retrofit 1 Budget

Budgetary Considerations (Retrofit Option 1):

  • AURORA (MLI with Integral GWR)
    • ECLIPSE Model 706 Guided Wave Radar with Coaxial Steam Probe
  • PID Process Controller
    • Outputs:  4-20mA
  • I/P transducer, non-explosion proof
    • Input – 4-20mA
    • Output: 3 – 15 psig/3 – 27 psig
  • Other Items
    • Enclosure for PID Controller and I/P Converter
    • Isolation Valves (if existing integral to sight (glass) gage)
    • Labor

RETROFIT OPTION 2:
Utilizing the existing torque tube, RF capacitance or magnetic level indicator chamber is one of the easiest and most cost effective methods of retrofitting a feedwater heater to incorporate more advanced level control technologies. The process is simply a matter of verifying the existing chamber can accommodate GWR, ensuring the GWR comes with the proper mating flange and subsequent removal and replacement.

As alluded to earlier in the conversation, there are installations, where the existing chamber will not suffice for whatever reason – usually, the top-in/bottom-out process connection configuration (Figure 3). In this situation the cleanest route to implementing the GWR solution is to replace the existing chamber with one incorporating a side/bottom or side/side process mounting configuration. This also applies to differential pressure retrofits where no external chamber exists; however, the original level transmitter process connections will be used to mount the external chamber and the GWR.

Chamber_Side_Bottom_Config

Level control scheme is consistent with the options described in Figure 2.

Budgetary Considerations (Option 2):

  • ECLIPSE Guided Wave Radar with external chamber
    • Model 706 Guided Wave Radar with Coaxial Steam Probe
    • Chamber: (side/bottom or side/side process connections (as required)
  • PID Process Controller (as required)
    • Outputs:  4-20mA
  • I/P transducer, non-explosion proof (as required)
    • Input – 4-20mA
    • Output: 3 – 15 psig/3 – 27 psig
  • Other Items
    • Enclosure for PID Controller and I/P Converter (as required)
    • Labor

 

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