CCJ Archives: 1Q 2005

Tight specs, good engineering, quality manufacture ensure reliable control of steam temperature

By Dr Sanjay V Sherikar, PE, Control Components Inc, and Peter Borzsony, CCI International Ltd

Attemperators—desuperheaters if you prefer— have been around for a long time. Like many other power-system components, they have performed well on large base-load steam units owned by regulated utilities. However, the boom in building combined-cycle plants for merchant service in the early 2000s created new hurdles for components that hardly got a yawn when specifications for these plants were written. Recall that most of the combined-cycle plants were designed for base-load operation. So, when it came to specifying a system for steam temperature control it was business as usual. They worked reasonably well in large base-load steam plants, so why not combined-cycle facilities? Owners assumed that off-the-shelf solutions were fine. more

The COMBINED CYCLE Journal’s First Annual

Best Practices Awards

For Management
For Safety
For Operation and Maintenance

Gas-turbine-based powerplants compete against nuclear, coal-fired, and hydro generating facilities for a share of the electric power market. Product price is of primary importance and competition is keen. Large GT-based plants-most installed within the last five years-are challenged today by the high cost of natural gas and the need to pay down construction debt. By contrast, nuclear, coal, and hydro facilities generally are 25 or more years old and virtually debt-free. Their fuel resources also are much less expensive compared to natural gas. Hydro plants, for example, do not even pay for “fuel.” more

Growing experience with P91/T91 forcing essential code changes

By Jeffrey F Henry, Alstom Power Inc and ASME Task Group

With today’s combined-cycle powerplants operating at more demanding steam conditions and in more rigorous cycling duty than facilities built only a few years ago, designers increasingly are specifying creep-strengthenhanced ferritic steels-such as the modified 9Cr- 1Mo alloy-for use in critical sections of the steam plant and heat-recovery steam generator (HRSG). Use of the modified 9Cr-1Mo alloy, commonly referred to as Grade 91 (“P91” for piping and “T91” for tubing), for high-temperature applications can result in substantial reductions in component thickness compared to weaker alloys, such as Grade 22. The thinner wall produces substantially reduced thermal stresses and thereby improves service life. more

HRSGs for small combined-cycle and cogen plants

Size matters, or so it seems. Take  heat–recovery steam generators , for example. Mention “HRSG” and most people in the power business think of the triple pressure units in service at large combined-cycle plants. Articles in the trade press focus on these units, as do papers at major industry conferences. That’s interesting, because during the recent building boom associated with gas-fired generating facilities, so many HRSGs were built by two or three manufacturers for service behind only two or three gas-turbine (GT) models one could almost say they were “mass produced.” Not much to talk about under that scenario. more

Why generators fail

By William G Moore, PE, National Electric Coil

Planned outages for gas-turbine-based peaking, cogeneration, and combined-cycle plants are scheduled based on the inspection and maintenance needs of the gas turbine/ generator as prescribed by the OEM (original equipment manufacturer). Owners of machines in regular use often schedule two outages yearly— spring and fall, when the market for power is soft. Others opt for one major outage annually. For peaking turbines in limited use—say a couple of hundred hours per year of operation—the time between overhauls may stretch from two to four years. more