6.2.1 Process steam

In process applications, the HRSG generally produces steam at the fixed pressures of the process steam system headers. Many refineries, for example, utilize steam systems where several steam producers maintain the header at a constant pressure. Here the HRSG(s) might produce steam at one or several of these header pressures depending on the need. The superheaters often have to handle some level of supplemental firing to generate additional steam flow but typically the outlet pressure is fixed over the entire range. These units also tend to function for long periods of time at relatively stable loads and pressures so that on/off cycling can be a minimal concern.

6.2.2 Power plant steam turbine

In contrast, HRSGs designed specifically for power generation typically are designed for maximum efficiency. There are generally multiple pressure levels of superheated steam generated for use in a steam turbine and/or in cooling streams for various combustion gas turbine components, and can also include steam generation for extraction to another process.

6.2.3 Steam purity vs various applications

Required steam purity is generally a function of the consumer of the steam. Steam turbine suppliers typically require the superheated steam to be of very high purity to avoid a loss of efficiency due to fouling, erosion, and/or corrosion of the steam turbine internals. Processes can generally accept steam of lower purity.

6.3.1 Tube External/Outside Heating Surface

As discussed in Chapter 3, Fundamentals extended heating surface in the form of finned tubes is used in HRSGs. In high-pressure superheaters and reheaters the addition of external finning greatly increases the tube metal temperature. Higher fin density and/or thicker fins lead to higher tube metal temperatures. Varying the amount of finning added therefore has a significant impact on the tube material selections. This allows the tailoring of different alloy materials up to each material’s maximum temperature for continuous use before stepping to the next higher grade material. In cases of supplemental firing, the radiant heat flux to the first longitudinal rows of tubes downstream of the burner can cause a significant increase in tube metal temperatures. In these cases, it is typical to utilize one or more rows of bare tubing immediately downstream of the burner to maximize radiant absorption while minimizing the resulting increase in tube metal temperature. Thereafter it is typical to find external finning but perhaps at a reduced fin density and fin height, again to limit tube and fin metal temperatures and required metallurgy.

6.3.2 Staggered/inline

Depending on gas side pressure loss, layout, etc., there can be a benefit with respect to enhancing turbine exhaust gas flow distribution when using the staggered layout. At the hot end, where the high-pressure superheaters and reheaters are usually located, this flow distribution effect can be used to improve turbine exhaust gas velocity distributions to supplemental firing equipment, catalyst beds, and other components downstream of the superheater and reheater when required.

6.3.3 Countercurrent/cocurrent/crossflow

The use of the proper flow arrangement is critical to achieve required performance at minimum cost and maximum reliability. Superheaters and reheaters are no different in this respect. For optimum heat transfer, the countercurrent arrangement is preferred. This typically maximizes the effective temperature difference and therefore minimizes heating surface. Crossflow is generally used for single-row and single-pass (can be multiple rows in parallel; see Section 6.3.5) coils. Here, single-pass refers to the working fluid making one pass across the turbine exhaust gas flow before exiting the gas path for the terminal point or reentering the gas path at some other location (Fig. 6.2).

Cocurrent can be a useful arrangement to minimize temperature excursions and provide some temperature control by using the natural pinching effect (gas temperature leaving versus steam temperature leaving) at the coil steam outlet. In some instances, cocurrent arrangement can also be used to optimize metallurgy by placing the coolest steam with the hottest turbine exhaust gas and the hottest steam in a cooler turbine exhaust gas zone. Note that a cocurrent arrangement typically maximizes heating surface and therefore first cost. However, the increase in heating surface may not be significant if temperature difference between turbine exhaust gas and steam is substantial (note that in this case the desired “pinching” effect will be reduced).

6.3.4 Headers/jumpers vs upper returns

Several different typical coil configurations are common in the HRSG industry. The simplest is a single row of tubes installed into an upper header and a lower header. Connecting piping then attaches to the nozzles on each header. It is thus possible to assemble individual single-row panels into a coil with the use of jumper pipes between the headers. The coil can be drained from the lower headers. For multiple row coils it is also possible to use return bends between individual tubes in neighboring rows for the intermediate upper row-to-row crossovers between the inlet and outlet header connections. It is imperative with any construction to properly manage internal stresses due to differential thermal growth from row to row, as well as temperature differences between adjacent tubes within a row in the coil assembly. This is especially true in high-pressure superheaters and reheaters located at the hot end of the HRSG. The use of return bends at the top of intermediate row-to-row connections provides additional flexibility between adjacent tubes within a row. In turndown cases, for instance, where greatly reduced steam flows can result in poor distribution through these hot end tubes, a “starved” hairpin that heats and grows more than its neighbors will simply lift off its upper support basically stress free. When steam flow increases and the tube receives better-distributed flow it cools and settles back onto the upper support with its neighbors.

6.3.5 Circuitry

The steam flow through a coil is directed to follow a predetermined path in each pass across the turbine exhaust gas that is set by the tube and header arrangements. Each flow path is referred to as a circuit or parallel path within the tubes in each pass. Several sample circuitries are shown in Fig. 6.3.

The designer uses a combination of flow circuitry and tube diameter to optimize performance of the superheater and reheater as well as the other heating surface. By manipulating these parameters, it is possible to find workable combinations of tube flow area that satisfy pressure loss requirements and still provide effective cooling of the tube metal. For example, in the case of the high-pressure system, pressure loss is important for minimizing pressure part thickness but has a much smaller impact on steam generation than in lower-pressure systems. Pressure part thickness is also affected by tube diameter; the smaller the diameter, the thinner the tube. Thus high-pressure superheaters tend to utilize smaller tube diameters, which can be further reduced by utilizing multiple-row circuitry. In contrast, reheaters operate at much lower pressures (pressure of the high-pressure steam turbine exhaust) but typically at temperatures and mass flows similar to the high-pressure superheaters. Reheater tubes therefore carry steam of a much lower density (or higher specific volume) than the high-pressure system. This is compounded by the relatively high superheat remaining in the cold reheat return from the high-pressure steam turbine exhaust. Reheater pressure loss should be minimized as the steam turbine efficiency is sensitive to reheat loop pressure loss. Flowing reheat steam with little pressure loss requires a significant steam flow area relative to the high-pressure superheater, for example. This generally forces reheaters to utilize multiple-row circuitry and larger tube diameters. The intermediate-pressure and low-pressure system steam outputs are very sensitive to their respective superheater pressure drops. In combined cycle systems, the intermediate-pressure steam generated is typically combined with the cold reheat return and sent to the reheater sections of the HRSG rather than going directly to the steam turbine at the appropriate stage/admission port. The intermediate-pressure superheaters and low-pressure superheaters, which are located downstream of the high-pressure evaporator in most cases, tend to use larger tube diameters to minimize pressure loss. The intermediate-pressure superheaters add superheat to the intermediate-pressure steam prior to combining with the cold reheat steam to enter the reheater. This is to maximize the high-pressure steam generation and overall cycle efficiency. In Fig. 6.4 this can be seen as the split sections of the intermediate-pressure superheater. The intermediate-pressure system of Fig. 6.4 fits into the high-pressure system of Fig. 6.4 with the hot stage of the intermediate-pressure superheater located downstream of the high-pressure evaporator.

Location of intermediate-pressure superheaters or low-pressure superheaters upstream of the high-pressure evaporator is discouraged since steam generation from these systems lags far behind in time during cold starts. By the time significant cooling steam arrives the tubes would be at the temperature of the hot end exhaust. Even with some prewarming in stages downstream in the turbine exhaust gas path of the high-pressure evaporator the thermal shock entering the portion in the hot end would still lead to low cycle fatigue failures.

6.3.6 Sliding/floating pressure operation

Sliding or floating pressure operation refers to operation of the steam turbine in a “valves wide open” type configuration allowing the steam turbine inlet pressure to change up or down with increasing or decreasing steam flow relative to the anchor pressure in the condenser. This operation can have a significant impact on the envelope of operating conditions that an individual superheater will experience. In the case of a 1:1 configuration, i.e., one combustion gas turbine/HRSG to one steam turbine, the maximum heat input to the system is typically at base load of the combustion gas turbine yielding the highest steam flows and therefore the highest pressures at the steam turbine. As the combustion gas turbine load is reduced, the steam mass flow and therefore pressure fall in tandem. This generally has a small impact on the design of the HRSG, typically raising the metal temperatures somewhat due to the reduction of steam flow while the turbine exhaust gas temperature remains high. Moving on to configurations with multiple combustion gas turbine/HRSGs feeding a single steam turbine (e.g., 2:1, 3:1, etc.), inflow and pressure increase. The case with all combustion gas turbine/HRSGs operating to generate the maximum inflow sets the maximum steam flow and therefore pressure to the steam turbine. As any individual unit is removed from service, the remaining combustion gas turbines can still be operated at base load. This results in maximum combustion gas turbine heat to each still-operating HRSG but at reduced overall mass flow and therefore pressure at the common steam turbine. The result is that each HRSG can generate its maximum steam flow at greatly reduced pressure such that steam velocities and pressure losses increase substantially.

6.3.7 Unfired/supplemental fired

Supplemental firing is generally located in the hot end of the system in order to minimize emissions and maximize the high pressure and reheat steam flows. This means that the high-pressure superheaters and reheaters can see greatly elevated gas temperatures when firing relative to the unfired operating cases. Since it is usually desirable to maintain the high-pressure superheater and reheater outlet temperatures (main steam and hot reheat, respectively) when the burner is not operating, the tube metal temperatures can greatly increase when firing due to the increased gas temperature.

6.3.8 Bundle support types

Superheaters and reheaters in horizontal gas flow, natural circulation HRSGs are generally top supported, allowing them to grow thermally down, freely hanging in tension. An alternative bottom-supported design with the superheater/reheater tubes growing vertically up in compression and carrying the additional load of piping, etc., at the top of the unit is possible but is uncommon due to the additional stresses imposed on the bottom-supported tubes. Even for the vertical top-supported tubes in a multirow coil configuration it is necessary to maintain good coil flexibility between the inlet and outlet headers.

6.3.9 Tube-to-header connections

The high-pressure superheater/reheater surfaces at the hot end of the HRSG are exposed to very large temperature gradients during transient operations such as startup, load changes, and shutdowns. For this reason, the tube-to-header connections in this part of the system are critical. Practical steps to minimize the introduction of additional stress include (1) eliminating bends in the tubes near the header as these increase stress due to the moment generated near the bend; (2) using tube-to-header connections, which provide the best reinforcement of the header at the connection; and (3) using the best inspection practices to minimize header thicknesses due to the connection. Hillside tube-to-header connections as shown on Fig. 6.5 can be used to minimize the impact of tube bends. The tube-to-header joint requires a high-quality weld.

6.4.1 Spraywater desuperheater

The basic function of a spraywater desuperheater is to atomize liquid water into a superheated steam line such that the heat required to evaporate and superheat the water is taken from the incoming superheated steam. The result is a blended steam temperature at the outlet equal to the desuperheater’s outlet steam temperature control set-point. There are many types of spraywater desuperheaters utilizing single or multiple atomizing nozzles. A few typical desuperheaters are shown in Fig. 6.6.

Desuperheaters operate under severe conditions with the spray nozzles seeing temperature differences of several hundred degrees—full local steam temperature when not spraying to perhaps a few hundred degrees of subcooling when spraying. In its simplest form the spraywater desuperheater is located on the superheater outlet as a “terminal point desuperheater” and controls the final steam temperature to the desired level. There is the remote possibility of water induction into the steam turbine or process due to unevaporated spraywater. Many codes and standards contain requirements intended to prevent the induction of liquid water into a steam turbine so that this “terminal point spraywater desuperheater” can be an acceptable option. However, many owners and steam turbine suppliers still prefer to use an alternate configuration with the spraywater desuperheater located between two superheater coils or stages typically referred to as an “interstage spraywater desuperheater.”

6.4.2 Steam bypass attemperator

One of the highest-frequency causes of high-pressure superheater/reheater pressure part failures is improper use and/or control of spraywater desuperheaters. Spraying typically highly subcooled liquid water into high-temperature steam contained in high-temperature metal pressure parts provides multiple opportunities for high stresses, component failures, and sufficient reason to consider options. An alternative type of steam temperature control is the steam bypass attemperator. In its most common form, a portion of the incoming stream is bypassed around the heating surface in a single-valve bypass arrangement and is then blended at the outlet with the portion of the steam that was heated by flowing through the heating surface. See Fig. 6.7.

The blended steam temperature is controlled to the desired set-point. Since no additional fluid (i.e., subcooled water) is being added and evaporated there is a performance gain in operating modes requiring temperature control. No heat is lost from the high-temperature portion of the system (hot end high-pressure superheater/reheater area) to perform low-grade heating of desuperheater liquid. In fact, since some of the high-pressure superheater/reheater steam flow is bypassed, tighter pinches are created and the heating surface efficiency is decreased thus decreasing heat absorption. The result of these changes is that more heat is available to the high-pressure evaporator to raise steam thereby raising the performance of the entire process. This is a relatively small but real performance gain in the high-pressure superheater. In the reheater, however, evaporating a mass unit of spraywater results in a nearly one-to-one mass unit loss of HP steam flow since the water is evaporated in the reheater (after the HP steam is expanded in the steam turbine) upstream in the turbine exhaust gas flow of the high-pressure evaporator outlet pinch. Thus using the steam bypass attemperator in the reheater represents the majority of the performance gain in modes requiring steam temperature control. Fortunately, utilizing steam bypass attemperation in the reheater is generally practical to accomplish.

Intermediate-pressure superheaters and low-pressure superheaters do not generally require steam temperature control since they are located downstream of the high-pressure evaporator pinch and the typically tight temperature pinches on all the surfaces in the colder portions of the system keep the intermediate-pressure and low-pressure steam temperatures from increasing beyond the desired range. However, when intermediate-pressure and/or low-pressure steam outlet temperature control is necessary the preferred method is the steam bypass attemperator.

6.4.3 Mixing requirements for each

The manufacturer of a spraywater desuperheater should determine the amount of piping required for full evaporation of the atomized spraywater flow. A good rule of thumb is 10 pipe diameters. For steam bypass attemperation the mixing distance is a function of the relative heated and bypass steam flows and conditions, the piping geometry approaching and leaving the mixing point, etc. Here also a good rule of thumb is 10 diameters for good mixing.

6.7.1 Steam side

Good steam side flow distribution in the tubes of the high-pressure superheater/reheater is critical to properly cool the metal pressure parts. Flow distribution is a function of the flow area within the headers and the pressure loss in the tubes between the headers. Larger header diameters and/or higher tubeside pressure loss create better distribution. There is a balance to be considered between the minimum pressure loss to create proper flow distribution and the impact of that pressure loss on the potential steam generation as discussed earlier. Additional concerns arise in supplemental fired HRSGs with all or portions of the high-pressure superheater/reheater downstream of the burner. Here it becomes necessary to consider the impact of the flame/heat distribution in addition to the steam distribution based on header and pressure loss impacts. If the flame distribution is not adequate uneven heating will occur and portions of the high-pressure superheater/reheater face area will be heated to levels higher than accounted for in the design process. Duct burners as described in Chapter 7, Duct burners are generally one of two configurations: fuel element runners that traverse the entire gas path or cylindrical cans (somewhat similar in form to register burners) that contain fuel nozzles in their center and typically fire directly downstream. These effects can often be seen in differential steam temperatures at the downstream coil exit nozzles especially if multiple outlet nozzles exist on the same tube coil. The runner style lends itself to more even heat input across the coil face by arranging the burner element axis normal to the axis of the downstream tubes. In this way all the tubes see an even heat input if the fuel distribution is correct. Burners of the cylindrical can style can result in relatively uneven temperature distributions. Great care must be taken to have sufficient coverage of the overall duct area to avoid serious issues in the downstream high-pressure superheater/reheater heating surface. Uneven heating of the turbine exhaust gas can lead to large temperature imbalances across the high-pressure superheater/reheater coil face resulting in significant differential thermal growth between heating surface tubes connected to common upper and lower headers. This can result in low and/or high cycle fatigue issues depending on the magnitude of the differential growth. Local overheating can lead to catastrophic damage to the downstream high-pressure superheater and reheater.

6.7.2 Gas side

Turbine exhaust gas distribution coming from the combustion gas turbine is generally highly nonuniform and varies with the type of combustion gas turbine model. Peak velocities can be as high as 600 ft/s and pressure pulsations can be 60-in. W.C. or more. Axial machine swirl can make the turbine exhaust gas profile equivalent to containing a 1000–1200°F×F2–F5 tornado. Significant reinforcement in first heating surface in the gas path is often required. The turbine exhaust gas flow distribution can be improved by flow distribution devices such as a distribution grid, “egg crate” baffles, etc., as described in Chapter 12, Miscellaneous ancillary equipment. These devices first must be designed to survive the already noted severe service. They also generally contribute to the turbine exhaust gas pressure loss/combustion gas turbine backpressure. As the flow passes through the heating surface, areas of higher temperature transfer more heat due to the larger temperature difference and the fact that cooler areas transfer less heat. The temperature peaks and valleys smooth very quickly over the first row(s) of the heating surface. The mass flow deviations are more severe. As the flow approaches the face of the first heating surface (or distribution grid) it sees the backpressure of the entire remainder of the HRSG gas path. The effect is to force the turbine exhaust gas flow to distribute from high-velocity areas toward low-velocity areas. Since there is very little vertical or side/side distribution within the heating surface due to the close tube spacings, acoustic baffles, and vibration supports, the flow distribution within the coil at the outlet row is very similar to the inlet distribution. Thus there will be small penalties on both heat transfer in the low-velocity areas and pressure loss in the high-velocity areas. As mentioned previously the structural/mechanical design in the first hot end coil/bundle is a major challenge. Solutions such as additional vibration supports, installing coil bumpers upstream and downstream of the first bundle/module, or tying the first two bundles together with field installed bracing have been required at various times.