1 December, 2001

Technical Bulletin

Vogt Power International Inc. 1 of 2 4000 Dupont Circle, Suite 400 • Louisville, Kentucky 40207 • 502/899-4500 • Fax: 502/899-4690 Sapulpa Manufacturing Operations: 2110 N. Industrial Drive • Sapulpa, Oklahoma 74066 • 918/227-4202 • Fax: 918/227-4207

Sparging Systems & Stack Dampers Attn: HRSG End Users Re: Sparging Systems & Stack Dampers Observation: Many plants are currently considering the addition of a Sparging System and or a Stack Damper in belief that they will decrease

the Start-Up Time and increase the longevity of their cycling HRSG(s). Many HRSG(s) were not designed for cyclic conditions though market conditions are compelling End Users to operate them as such. As a result, HRSG(s) are being asked to perform in ways they were not designed for. End Users are asking, “What can I do with my existing equipment to limit the effects of Cycling and increase the Reliability and Longevity of my HRSG?” Well, it depends. Not only is there limited reliability data on operating plants operating in cyclic mode but, each HRSG is typically unique. If we consider that each OEM has their own design standards and approach to accommodate the effects of cycling, then there are very few rules of thumb that all parties would agree with let alone divulge. Here are some of the variables: • Definition of a Hot, Warm, and Cold Cycle • Ambient Conditions • Number of Cycles • OEM Design Criteria & Considerations • Gas Turbine (Soak Temperature & Time) • Design Considerations of the Specific HRSG • Start Up & Cool Down Rate • Delta Temperatures across the inlet/outlet of the Modules, Drains, etc. Definitions: Sparging System- System to admit and circulate heat (usually in the form of steam) into the HRSG. Stack Damper- Damper mounted in the stack that can be shut remotely typically used to reduce total heat loss from the HRSG or prevent the ingress of

precipitation. Start Up Rate- Rate at which the HRSG can be heated (ºF per minute) abbreviated SUR, this should be defined by the Designer. Cool Down Rate- Rate at which the HRSG can be cooled (ºF per minute) abbreviated CDR, generally more restrictive than SUR due the HP Drum shell

thickness. Start Up Time- The total time required to bring a HRSG in its present condition to full operating capacity. Cold Iron- HRSG secured and all components at ambient conditions. Cycle- The definition(s) vary. OEM- Original Equipment Manufacturer. General Comments: HRSG’s are susceptible to premature failure as a result of cycling and thus are limited by the SUR and CDR as defined by the OEM. The CDR is usually more restrictive as a result of the stresses generated in the high-pressure drum as it cools. Gas turbines reach 1000F in a matter of minutes, not hours. Since the HRSG must react to this input, the designer accommodates this by ensuring that sufficient steam is flowing through the modules to ensure proper cooling. It is for this reason that the SUR should be defined by the HRSG designer and not the gas turbine manufacture or power island designer/provider. The HRSG selection and design is just as important as that of the gas turbine/power island. How then can an older HRSG accommodate the anticipated cycling for which it was never designed to operate? A Cycling Study is the best way to determine the worst case scenarios and how aggressive the SUR and CDR can be. In this type of study, Operations should also be reviewed. Please keep in mind that this topic generates a lot of opinions. Again, experimental data is limited. Other means End Users are considering are Sparging Systems and Stack Dampers.

1 December, 2001

Technical Bulletin

Vogt Power International Inc. 2 of 2 4000 Dupont Circle, Suite 400 • Louisville, Kentucky 40207 • 502/899-4500 • Fax: 502/899-4690 Sapulpa Manufacturing Operations: 2110 N. Industrial Drive • Sapulpa, Oklahoma 74066 • 918/227-4202 • Fax: 918/227-4207

Sparging Systems: Sparging Systems are usually used to keep boilers warm. The source of steam is typically from an alternative source such as another boiler. Sparging Systems can decrease the total start up time when the HRSG is being started from Cold Iron. They can decrease the chance of freezing and may increase the longevity of the equipment as well. Stack Dampers: Stack Dampers are best used to decrease the total Hot and Warm Start Up time but only when the shut down is no more than 12 hours (depends on ambient conditions as well). They only decrease SUR’s for the Hot and Warm Start Up cases. When open, they offer little exhaust gas pressure drop and when closed add the benefit of limiting water ingress. Be aware that heat will migrate from the HP system to the IP and LP system causing steaming in these sections, contact a reputable firm when you are ready to consider this option. Also consider the control logic, a damper should fail open, not closed else one will risk the unintentional shutdown of the gas turbine on a backpressure trip. Suggestions: 1. Ask the OEM to what extent your HRSG was designed for cycling. 2. Ask the OEM what components on your HRSG(s) are susceptible to failure as a result of fatigue/cycling (a finite element analysis may need to

be performed on those components). These are typically the Superheaters, Reheaters, and isolable economizers/pre-heaters. 3. Ask them what in their opinion can be done to limit the effects of cycling on your existing unit(s). If Vogt Power International Inc. can assist you with your concerns, do not hesitate to telephone us. Respectfully, Jeff Daiber Director for AftermarketProducts & Services Vogt Power International Inc. Phone: 502.899.4500 Facsimile: 502.891.1960 Email: Aftermarket@vogtpower.com Vogt Power Internet: http://www.vogtpower.com