Freeze protection, Minnesota style
Best Practices Award
Challenge. At a plant located in one of the harshest winter environments in the continental US, personnel are faced with multiple challenges to ensure immediate dispatch availability, especially after being idle during prolonged periods of sustained subfreezing temperatures.
Originally, the facility included a lot of outdoor equipment—including the gas turbine (GT), the heat recovery steam generator (HRSG), and several associated subsystems. To further complicate matters, the facility had no auxiliary boiler.
Solution. After commissioning the plant in fall 2007, and with winter fast approaching, personnel took a broad-brush approach to address several immediate issues at hand. First, the team recognized the need for a detailed checklist that included sign-off accountability for specific instrument verifications.
The team implemented an “Offline 32 Degree Action Log” to guide an operator through every portion of the system to verify non-freezing conditions, such as draining each HRSG drum by a small amount while observing the response of the drum level transmitter. To ensure that non-responsive transmitters were consistently identified, ambiguous wording such as “periodically start pump and monitor for freezing” was removed from the steps.
Next, temporary heaters and tarps were procured for each location that was identified as being more susceptible to freezing.
The team also established a heat-trace testing regime to periodically record individual circuit amperages. A significant drop in periodically monitored supply breaker amperage could be used as an indication of an “open” in one of the heat-trace branch circuits.
Additional heat-trace LED indicating lights were procured and installed, providing an “easy to notice” indication of voltage. These visual indicators are used in conjunction with the amperage monitoring plan as a comprehensive set of troubleshooting data points, allowing for easier and more accurate problem resolution.
Operating under a requirement to maintain HRSG drum levels in a ready-to-start status, and with no auxiliary boiler to provide sparging steam, the team established detailed criteria to determine when the gas turbine was required to be started to prevent the HRSG and associated steam system from freezing. The time between GT starts had to be optimized to limit fuel and starting costs.
To accomplish this, specific HRSG temperatures are trended in conjunction with weather forecasting to determine the anticipated time the plant can be idle before requiring a “HRSG freeze protection” start-up. Interestingly, a detailed return on investment (ROI) analysis did not justify the capital and operational expenses associated with the installation and operation of an auxiliary boiler, because of the uncommon occurrences of extended plant idle periods.
Over time, plant personnel established a list of action items from lessons learned during that first winter to reduce freeze-related events in subsequent winters. In one example, heat-trace testing was scheduled earlier to allow sufficient time for potential repairs before sub-freezing temperatures arrived. Long-term freeze protection solutions
A “pre-winter” freeze prevention plan was drafted with each specific task entered into the computerized maintenance management system. Due dates were set to allow ample time for completing each task before the cold weather arrived. Examples of the plant pre-winter plan include high level task descriptions, such as:
- Verify all heat-trace circuits and record amperage readings.
- Lay up the evaporative cooler system.
- Begin daily use of the “Offline 32 degree action log.”
In all locations where temporary heaters were required, the facility owner and plant personnel designed professionally engineered enclosures to complement the plant architectural scheme. Several items were considered during construction such as:
- Is additional heating and ventilation required?
- How much and what type of lighting will be required?
- How can the enclosure be accessed when maintenance is required?
Results. “The devil is in the details,” explains the plant manager. “Like our plant, most facilities have a freeze protection checklist. If your facility has struggled with freeze-related outages, review your checklist and ask if it requires the CRO and outside operators to sign off on each check they perform.” If your checklist doesn’t have that level of detail, he suggests trying the plan implemented above.
In one of the nation’s coldest locations, the facility has experienced only one hour of startup delay caused by freeze-related issues in the past four years, primarily because of the detail-oriented checklists, creative engineering system implementation, and the dedication of plant personnel to proactively identify upcoming conditions.
Bob Burchfield, plant manager
Doug Klar, operations manager
Tim Mallinger, lead O&M technician
The entire O&M staff
HRSG chemical feed system improvement
Challenge. To maintain a condensate pH of 9.2 to 9.6, plant operations personnel routinely transfer an amine/ammonia blend containing 40% ammonium hydroxide from a 55-gal drum to a mixing tank where water is added, and then pump that solution into the condensate system. During this process, personnel are exposed to a significant risk of chemical burn or spill because of the portable chemical barrel pump and hose used to transfer the amine/ammonia blend.
Plant personnel conduct approximately 80 of these chemical transfers per year, and the operator must wear a full chemical suit with boots and a face shield when transferring the amine/ammonia blend. Further, because the mixing tank is not fully enclosed, ammonia vapors in excess of 100 ppm create a potentially hazardous breathing atmosphere in both the HRSG chemical feed room and the BFW pump room. Plant personnel must open doors to increase ventilation.
Solution. Plant staff eliminated the process of manually mixing water and chemicals by replacing the existing day tank with a portable 330-gal tote of concentrated product. First, plant staff contracted with an engineering firm to provide an engineered drawing for the containment calculation, and for tank and pump positioning.
Next, plant staff contacted the chemical provider to supply a 330-gal self-contained tote, and to provide specifications for the new pumps required for the higher mix ratio, as the amine/ammonia blend would no longer need to be mixed with water.
Finally, the team sent out a request for quote to several contractors for the purchase and installation of the stainless steel containment, a tote stand, and pump stands. The team successfully implemented a fully contained amine/ammonia porta-feed system.
Completely eliminated the hazardous atmosphere created by mixing the chemical in an exposed tank. Results. After performing this modification, the plant noticed several advantages to the new amine/ammonia porta-feed system. Some of the more important advantages include:
• Improved the accuracy of chemical dosing by eliminating the step of blending the chemical with water.
• Significantly reduced the likelihood of a chemical burn or spill by eliminating the need to transfer the product with a portable barrel, pump, and hose.
• Significantly reduced the likelihood of a chemical burn or spill by changing the number of chemical transfers from approximately 80 per year to a single, annual change out of the tote, which is performed by the chemical provider.
Doug Klar, operations manager
Tim Mallinger, lead O&M technician
Shawn Flake, I&C technician
Matt Murray, I&C technician
Bob Flicek, mechanic