For years, original equipment manufacturers (OEMs) of heat-recovery steam generators (HRSGs) have supplied distribution grids (DGs) as a means for controlling the undesirable velocities, temperatures, spin angles, and pulsations generated by certain gas turbines. In the past, the combustion turbine OEMs readily provided information about the velocity profiles of these units, but in recent years that information has been increasingly more difficult to obtain, let alone substantiate.
The fact that some turbine installations produced large disturbances led HRSG OEMs to take the strong-arm approach of adding a distribution grid. The presence of a DG can produce a pressure drop of up to 2 inches (water column), adversely affecting the turbine's performance and/or necessitating an increase in the size of the HRSG. Although the use of guide vanes has proved successful, the technique also proved to be more black art than science. The number of failures that have resulted has made many HRSG users averse to using guide vanes.
The challenge for designers of HRSGs is to develop analysis tools that allow them to thoroughly understand the fluid dynamic storm raging at the entrance to the HRSG and use those tools to develop designs that reduce the flow disturbances and induced vibrations. As a result, many manufacturers of burners and HRSGs either have purchased software programs or contracted for computational fluid dynamics (CFD) analysis or cold-flow modeling of the problems (see box).
This replacement of intuition by science is a positive development; eliminating flow and vibration problems requires a comprehensive understanding of the interaction between fluid dynamics and installed hardware. Although the level of that understanding has increased significantly over the past decade, some key design information still remains a closely guarded secret today, and some OEMs are even attempting to patent certain devices for changing flow distribution.
Optimizing design velocity
The amplitude and uniformity of gas velocity through sections of the HRSG affect not only the unit's heat-transfer and burner operation, but the performance of downstream emissions control systems as well. The desirability of reducing an HRSG's footprint and materials costs has led all OEMs to continue pushing the design envelope. But some may have gone too far; there are at least two OEMs whose success with aggressive ductwork configurations have raised questions about what is proper gas flow performance.
Together, proper design and careful analysis can overcome most duct design challenges by evening out the distribution of gas flow across the heating surface of a burner. Every burner has an ideal approach velocity and flow distribution. Louisville-based Vogt Power International, Inc. prefers not to use increased baffling to provide them because doing so can cause reliability problems. For example, some HRSG OEMs use long runners to guide flow to burners located in large cross-sectional areas of the inlet duct. But such runners are susceptible to failure due to gaps in support along their length, turbulence, uneven thermal growth, etc.
Another design challenge that CFD analysis is particularly good at meeting is a common one: minimizing the footprint and interconnecting ductwork of a gas turbine–HRSG combination. For one particular installation of a General Electric 7FA turbine, Vogt used CFD to minimize locally high near-wall velocities and backflow, reducing ductwork by 30% (Figure 1). On another project with the same goal, another OEM reportedly designed a duct that left the top 20% of heating surface open, reducing the effectiveness of the design and failing to meet performance goals.
1. Go with the flow. Flow velocity distribution in the entry duct can vary significantly and can seriously impact HRSG performance. Computational fluid dynamics (CFD) analysis can determine the most effective design and placement and design of a distribution grid.Courtesy: Vogt Power International
A final bit of practical advice in the field of HRSG burner design: Don't be tempted to increase the burner firing rate to increase steam production. Doing so either will worsen the distribution of gas flow or cause premature failure of pressure parts or liners by overheating them. In some jurisdictions, increasing the firing rate also will increase the ammonia slip of the selective catalytic reduction (SCR) system downstream of the HRSG above permissible limits.
Following are some of the parameters that Vogt takes into account when configuring HRSG inlet duct geometry:
- The gas turbine's exhaust geometry and direction.
- The dimensions of the heating surface module.
- The location of duct burners.
- The mass flow rates and average velocities of the gas turbine's exhaust.
- The mass flow distribution at the entrance to the heating surface.
- Local velocities within the inlet duct: Near duct walls, and at the entrance to the first heating surface.
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