Combined cycle maintenance planning

Whether it is a car or a combined cycle plant, the key to reliable operation of any power equipment is a successful maintenance plan. This helps maintenance managers plan ahead, minimize unexpected failures, and accommodate limitations imposed by financial and outage resources. Cost, outage time and contractor manpower are some of the factors to be considered.

(Figure 1. Reheater piping cracks downstream of the superheater)

Each plant may choose a different maintenance plan. Some may want to maximize the synchronization of Heat Recovery Steam Turbine (HRSG) maintenance with the overhaul of the gas turbine (GT) or steam turbine (ST) to take advantage of outage time. Others may strive to level their costs by shifting as much HRSG maintenance to the years before or after the GT and ST outage window to avoid large peaks in annual maintenance costs.

This is important when you take into account the fact that hundreds of F-class or larger HRSGs were commissioned in the 2000-to-2005 time period. This means many are about half-way through their original specified design life.

Component redesigns

Factor in the sharp increase in plants conducting HRSG cycling, and many of these HRSGs can expect to need major component redesigns and replacements in the next ten years. That means combined cycle power plants will need more focus and coordination over the next few years than they did in the previous ten to identify and plan for the likelihood of major HRSG capital projects.

There are several large HRSG projects that managers of combined cycle plants should assess thoroughly and include in any forecasts being done as part of GT and ST maintenance planning.

Attemperator piping failure and replacement is a vital area to take into account. Problems in this part of the HRSG are usually caused by a desuperheater overspray and short distance from a feedwater inlet to downstream elbow where liquid water is not able to completely vaporize before it comes into contact with a pipe and quenches it.

Desuperheater overspray is an operational issue and can occur when steam is sprayed to saturation or within 50°F of saturation temperature. But even well designed and properly operated desuperheater arrangements can experience a pipe failure.

All it takes is one leaky feedwater valve or faulty desuperheater probe. Some gas turbine upgrades from OEMs that allow lower loads and changes to the turbine exhaust conditions can aggravate this problem.

Issues with attemperator piping are no small matter as redesigned piping systems can take 40 or more weeks to design and supply.

Their assessment involves visual inspection of superheaters and reheaters for bowing, check of performance data to look for indicators of overspray, and quality non-destructive evaluation (NDE) of piping locations at risk to stress and cracking (Figure 1).

Another common area of combined cycle maintenance is duct burner replacement. This occurs due to burner failure from cracking initiated at the nozzles or from overheated, warped and burnt away components. Detection requires visual inspection, both off-line and on-line, and review of maintenance history of comparable designs (Figure 2).

(Figure 2.  A badly bowed duct burner inside HRSG)

Poor burner flame patterns or burner element failures may force a derate of a steam turbine generator from its maximum full-load, full-duct firing power output. A burner derate on one HRSG can affect not only that HRSG, it can also derate adjacent HRSGs if they are part of a 2x1, 3x1 or 4x1 steam turbine configuration within a combined cycle plant.

With full-duct firing at full load, it becomes critical to have relatively balanced steam flows across each HRSG to avoid tube over-heat problems. The challenging aspect of a thorough duct burner inspection, however, is checking elements at higher elevations.

It would therefore be wise for maintenance managers to plan on building a scaffold, mobilizing a Sky Climber or even a drone inspection as a part of a regular maintenance. This is especially important if a history of duct burner problems already exists.

(Read more in the November/December 2015 issue of Turbomachinery International)

(Bryan Grant is a Steam Turbines Reliability Engineer and the Maintenance & Repair Planning Manager For HRST. Since 1998, HRST has specialized in technical services and product designs for HRSGs, waste heat boilers, and smaller gas or oil fired power boilers.

Souren Chakirov is a Systems Engineer for HRST. With experience on over 200 boilers annually, HRST is able to provide inspections, analysis work, design upgrades and professional training. For more information, visit www.hrstinc.com)