CTOTF
The total plant concept
The CTOTF Spring Turbine Forum at The Williamsburg Lodge in Virginia’s Colonial Williamsburg, April 15-19, is coming up quickly. It is the meeting of choice for gas-turbine owner/operators who need information on more than just one engine model, so it’s ideal for generation executives, asset managers, and plant managers.
Access the agenda at www.ctotf.org and you’ll see roundtables for GE legacy, aero, and F-class engines; the gamut of Siemens engines; Pratt & Whitney FT4s and FT8s; and Mitsubishi and Alstom gas turbines as well. Digging into the agenda it’s easy to understand why CTOTF Chair Bob Kirn of TVA says the meeting is in lockstep with the group’s tagline, “The Total Plant Concept.” The meeting does not just address gas turbines. Note the sessions dedicated to industry issues, O&M and business practices, combined-cycle plants, generators, controls, electrical systems, emissions control, etc.
The CTOTF organization chart on p 96 offers a capsule summary of the subject matter addressed as well as the industry experts responsible for coverage of those areas. Want to know more about the subject-matter experts? Just access the org chart at ctotf.org and click on the person’s name for an abbreviated resume. Have a question regarding upcoming content, an idea for a presentation, etc? Use the handy e-mail feature. Want to evaluate the content in formal presentations? Just hit the IBBCS button on the home page (left-hand toolbar).
Keep in mind that formal presentations are only the tip of the iceberg, so to speak, in terms of value-added content available at a CTOTF conference. Informal presentations and discussion forums dominate the program. You can only benefit from those information resources by attending.
Not done promoting yet: Two more outstanding features of the Spring Turbine Forum are the presentation of the CCJ’s 2012 Best Practices Awards and the training benefits of CTOTF’s three-hour CT-Tech sessions on Tuesday and Wednesday evenings. You’ll get the details on the Best Practices projects that received the highest recognition from the judges; one or more ideas might benefit your plant. Regarding CT-Tech, see details on p 97.
HRSGs at CTOTF
A good example of both The Total Plant Concept™ and the value of informal presentations was the recent effort by a G-class plant manager to highlight real-world issues his plant was experiencing with a triple-pressure heat-recovery steam generator (HRSG). His main discussion points:
- Problems identified with poor design of the restraint system for the perforated plate at the entrance to the HRSG.
- Floor-plate failure and the catalyst and tube fouling caused by the release of insulation.
- Wall damage in the duct-burner bay.
- Forced outage caused by an LP steam-drum leak.
- Salt fouling of finned-tube surfaces and corrosion damage to piping from salt.
The takeaway from this presentation was that owners driven by low first cost might want to reconsider their goals and switch instead to a low-lifecycle-cost strategy. Operational penalties and the cost of design corrections can be substantial.
The first item on the speaker’s agenda was the perforated plate. The top guides for the perf plate didn’t slide to accommodate growth as intended and the top structural support for the perf plate tore loose. In addition, tee-shaped side restraints at six elevations apparently were impact points for the plate during cyclic movement. Audible banging and material distress were in evidence. The installation of so-called plate boxes distributed the impact and reduced the clearances between the perf plate and restraints.
The bottom center is the only place the perf plate is welded to the HRSG (Fig 1). The weld was found compromised at every annual inspection, leading the speaker to believe that a pinned connection may be a better alternative. What the plant did, with a measure of success, was install reinforcement plates inside the perf plate to build up the bottom center anchor (Fig 2).
The plant manager said it was common to find ligament cracks in areas of the perf plate with high porosity. Missing sections are not unusual (Fig 3). Best practice: Maintenance personnel added a man door in the perf plate to facilitate access to the back side for repairs (Fig 4).
First floor-plate damage experienced was attributed to inadequate lap seam between duct sections (Fig 5). Later, plates were damaged by impact of the expansion-joint baffle which broke loose and sheared off pins. Corrective action: Expansion-joint baffles were reinforced with ribs.
Interestingly, floor plates near the perf plate were installed with a lower pin density than plates closer to the gas turbine. Former were were more susceptible to damage by vibration and turbulence. Insulation released from areas where the plates failed fouled the catalyst face and finned heat-transfer surfaces (Fig 7). Impact of fouled catalyst was a forced reduction in engine output to meet air-quality limits, as well as a unit trip on high backpressure. Such restrictions cost the plant $1 million in lost production and efficiency one winter alone.
Duct-burner wall failure was caused by high heat flux (Fig 8). First heat deflector was removed from the burner to correct the issue.
A leak in the LP drum was identified by “whispy” steam coming up from under the insulation. A crack was found on the bottom of the drum, running longitudinally near the 6 o’clock position at mid vessel (Fig 9). Inside the drum, an angle support leg for the feedwater distribution header broke loose from where it was welded directly to the drum (Fig 10). Correction was to install a baseplate and weld the support leg to it (Fig 11).
Fouling of finned tubes from the HP economizer section to the stack was a thorn in the side of this plant manager. Ammonium bisulfate is the problem, he said. It combines with iron from the finned tubes to produce a rock-like deposit. Rust by itself is not an issue. The buildup is so acute the plant has never operated at maximum output. The as-new backpressure is about 22 in. H2O; alarm is set for about 27 in.
Annual attempts at conventional cleaning with dry ice and compressed air have been only partially effective, he noted. Backpressure is only lowered by 1.5 to 2 in., which equates to 7-8 MW. One section of the HRSG is particularly difficult to clean. It encompasses nine rows of evaporator tubes (three headers) sandwiched between two rows of superheater tubes and six rows of economizer tubes—without a passageway.
Plant staff removed the baffles from the LP economizer section to reduce the backpressure and picked up about 0.5 in. However, that raised the stack temperature and steam turbine output dropped by 5 MW.
The plant is challenged by having to burn Canadian gas with 20 ppm sulfur. Excess ammonia runs about 5 ppm (NOx emissions are limited to 2.5 ppm). Salt from nearby water is not an issue; the gas-turbine inlet is equipped with HEPA filters, which work “pretty well,” the PM says.
Attempts to solve the tube fouling problem include liquid nitrogen (no good), and a sonic horn. Latter works “to some extent,” but only near the device. Sound is attenuated quickly. The plant maintains the compressor inlet at between 35F and 40F to maintain the backpressure as low as practicable in cold weather.
Next trial will be to see if the deep cleaning procedure outlined in the article on p 82 is beneficial. If that is conducted as planned, you’ll likely find out by the fall meeting if not at the upcoming Spring Turbine Forum. Other possible solutions include the following:
- Reduce the sulfur content of fuel gas; however, this likely would not be economically viable. Removing odorants might have a positive impact because mercaptans are high in sulfur.
- Reduce ammonia use by retrofitting an ultra-low-NOx combustion system.
Another problem associated with fouled tube surfaces is corrosion of the drain system.
