17.1.1 Forced circulation

A typical vertical gas flow, horizontal tube, forced circulation HRSG with two levels of steam pressure is shown schematically in Fig. 17.1. Gas that exits the gas turbine horizontally from the left turns upward in the turning duct, flows across the horizontal tubes and exits to the atmosphere from the stack at the top of the unit. Water for the high-pressure portion of the system enters the first high-pressure economizer at the top of the unit and flows horizontally through the tubes row by row gradually progressing downward. This water flows from the exit of the first high-pressure economizer to the entrance of the second high-pressure economizer and flows through this economizer in the same way it progressed through the first economizer. From the outlet of the second economizer the water flows to the high-pressure steam drum. Circulating pumps deliver water from the steam drum to the inlet at the bottom of the high-pressure evaporator. Water enters the bottom of the high-pressure evaporator and flows through the horizontal tubes in much the way it did in the economizers, only now it flows from bottom to top. The water evaporates as it moves upward, creating a steam/water mixture of increasing quality as it progresses to the outlet. Note that the water/steam mixture flows through several rows of tubes in parallel in the evaporator to minimize flow velocities, erosion, and pressure drop. From the outlet of the high-pressure evaporator the water/steam mixture is piped to the steam drum where the water and steam are separated. The separated water mixes with water from the economizer and returns to the evaporator inlet. The separated dry steam flows to the high-pressure superheater, where it flows through the superheater in much the same way that water flowed through the economizers.

The low-pressure economizer, evaporator, and superheater function in much the same way as their high-pressure counterparts, only as a separate system. Each low-pressure component is located at the proper place in the HRSG to optimize steam production for both systems.

The horizTontal tubes are supported by vertical tubesheets that are in turn supported by beams at the top of the HRSG. The tubesheets grow downward as they heat up during startup of the HRSG. The tubes must slide within the tubesheets to accommodate longitudinal growth of the tubes as they also expand during startup.

A feedwater preheater, third pressure level, and reheater could be included but have been omitted to simplify the explanation above.

17.1.2 Natural and assisted circulation

A vertical gas flow, horizontal tube, natural or assisted circulation HRSG is shown schematically in Fig. 17.2. It looks very much like the forced circulation HRSG mentioned previously, with the primary difference being the elevation of the steam drums. Gas flows through the HRSG in the same way as above. Water flows through the economizers the same way and steam flows through the superheaters the same way. The water/steam mixture flows through the evaporators in much the same way as previously mentioned only it now relies on the buoyant forces present due to the difference in elevation between the steam drum and the evaporator to generate flow in the evaporators. Since the static liquid head is small, there are usually more parallel circuits in these evaporators than in a forced circulation unit and the piping to and from the steam drums is usually larger to minimize pressure drop. An assisted circulation unit would have pumps to help overcome the pressure drop on the steam/water side of the evaporator, especially during startup. A true natural circulation unit would not have these pumps.

Support of the tubes and tubesheets would be the same as for the forced circulation unit.

A feedwater preheater, third pressure level, and reheater could again be included but have been omitted to simplify the explanation.

17.1.3 Comparison to a horizontal HRSG

17.2.1 Serpentine coil OTSG

A serpentine coil OTSG is typically a vertical gas flow design where water/steam flow is countercurrent to the gas turbine exhaust flow as shown schematically in Fig. 17.3 and in more detail in Fig. 17.4. This design can be a one- or two-pressure design and has typically been applied to smaller gas turbines (<100 MW). The design utilizes 800 or 825 series Incoloy tubes such that it can be started up without flow through the tubes. Incoloy is good for high temperatures and for resistance to flow-accelerated corrosion. Specific material type varies due to concerns in different areas for stress corrosion cracking and other types of corrosion. Run dry operation will impact the casing design and fin material choice through the entire OTSG and may not even be possible if NOx catalyst systems are required.

Terminal headers are low-alloy steel and therefore there are dissimilar metal welds between the tubes and headers. Tubes are supported by support plates. Tube-to-tube flexibility is good as tubes can move within the support plates. Sagging of tubes between supports can lead to pooling of water or condensate. As this water evaporates, dissolved solids can be left behind, creating tube deposits.

Control of a serpentine coil design OTSG is simple once operating in that the water flow is adjusted to achieve a desired outlet superheat temperature. Control logic is a feedforward system that must predict the intended feedwater flow. Water flow distribution is also important so that there is uniformity in temperature of the flow from each tube flow circuit. To achieve uniform distribution and for flow stability, each tube circuit may have an inlet orifice. During shutdown, if steam in the coil can condense, care must be taken to ensure that the condensate is not allowed to flow into hot steam headers. Chang (Ref. [2]) describes a typical single-pressure OTSG startup that takes approximately 27 minutes. Gradual introduction of water is important to prevent hot tubing from thermal quenching, which can result in warped and bowed tubes. LP system starts would lag the HP system starts. Chang mentions various failures associated with corrosion, thermal quenching, and plugging of tube inlet orifices.

17.2.2 Benson HRSG

The Siemens Benson OTSG technology is the most common technology used for larger gas turbines (>100 MW). Most Benson OTSGs are horizontal gas flow design although more recently, this has been applied to vertical gas flow configurations. Horizontal coils in vertical gas flow configurations could have greater difficulty achieving flow distribution and stability.

The horizontal gas flow Benson technology was first used in the Cottam combined cycle power plant (United Kingdom) in 1999 (Ref. [3]). The horizontal gas flow Benson concept is shown in cross section in Fig. 17.5 and utilizes a two-pass evaporator. Water from the economizers enters the bottom of the evaporator first pass. This water entering the evaporator must be subcooled to avoid any flow issues related to a steaming economizer. Water in the first pass distributes to all tubes in the pass and flows upward. Tubes with higher heat input will naturally get more water flow similar to natural circulation designed evaporators. The quality (mass flow of steam per unit of total mass flow) of the flow leaving this first pass is approximately 50%. This two-phase flow is collected by headers and manifolds at the top of the evaporator section and led by downcomers to the entrance to distributors located at the bottom of the evaporator. The distributors are designed to discharge a uniform constant quality flow to pipes that lead from the distributor to the inlet of the second evaporator tube pass. The flow through the second pass flows upward and ends up exiting somewhat superheated. There will be a row-to-row variation in the superheat temperature due to the decreasing gas temperature as it flows over each row of tubes. The target combined outlet superheat level must be high enough to assure superheat conditions leaving each tube row.

At startup, excess water flows through the evaporator until boiling is established. A two-phase flow separator and surge vessel is located at the outlet of the evaporator. The unit is initially operated by controlling the feedwater flow (flow control mode) but once the heat input reaches an adequate level, the control system switches to superheat temperature control. The horizontal gas flow Benson evaporator configuration is illustrated in Fig. 17.6. A picture of the evaporator lower header and piping configuration is shown in Fig. 17.7. Tube bends are included at the inlet of the second evaporator pass to accommodate tube-to-tube differences in expansion.

The Benson design does not have a thick wall drum but does have separator and surge vessels. These vessels are smaller in diameter than conventional steam drums and thus somewhat thinner but still of substantial thickness.

The Benson OTSG control logic is a complex feedforward control system with various provisions and limitations for the different coil sections and surge vessel.

A completed and operational horizontal gas flow Benson OTSG is shown in Fig. 17.8.

17.2.3 Supercritical

The critical pressure of water is 3206 psia. Boilers in conventional power plants have utilized once-through supercritical steam cycles since the mid-1950s. Once-through designs are more appropriate for supercritical operation because there is no phase change from water to steam and natural circulation will not occur. The compressed liquid or dense fluid is sensibly heated from the boiler inlet to outlet. Once-through systems therefore are more conducive to supercritical operation.

Today, large modern gas turbines have enough flow at high temperature to make a supercritical HRSG design practical. Supercritical steam cycles have a higher efficiency than subcritical cycles. Supercritical steam turbines are very large so multiple large HRSGs would be required to produce enough steam for the smallest available supercritical steam turbine. A supercritical design must start up and operate under subcritical conditions. Flow distribution and flow instabilities must also be considered under all operating conditions. Flow distribution and/or flow instability must be analyzed in detail to avoid tube-to-tube temperature differences that would affect the mechanical integrity of the coils and the design temperature limits of the evaporator. With the exception of the HP evaporator, the balance of coils in an HRSG would be essentially the same. Lower pressure level evaporator sections could still be natural circulation design. Since a supercritical design is an OTSG, there is no need for a steam drum. Some startup separator vessel may be required if a minimum startup water flow is necessary.

Siemens Benson HRSG Reference List (Ref. [5]) indicates that the highest steam pressure installation is 175 bara or 2538 psia. Supercritical Benson technology has been used in numerous conventional boilers. The advantage of using Siemens Benson technology for supercritical OTSG design would be that the basic Benson configuration is known to function properly at supercritical conditions.

17.3.1 Process design

EOR HRSGs usually contain a cocurrent flow evaporator followed by a counterflow economizer as shown schematically in Fig. 17.9. Use of cocurrent flow in the evaporator serves two main purposes. First, the liquid loading in the tubes is highest where the gas temperature is highest. Second, since the saturation temperature of the steam/water mixture will drop a few degrees from inlet to outlet, the gas and steam temperatures at the evaporator outlet will be lower and the steam production will be higher for the same pinch temperature difference than if counterflow was used. The first couple of rows of tubes in the evaporator often serve as economizer surface as the water entering them has not reached saturation. Subcooled boiling may even take place in them.

Proven HRSG designs ensure that the liquid and vapor are maintained in intimate contact throughout the HRSG evaporator coil, and that phase separation is avoided. The HRSG evaporator coils must also be designed to maintain the steam/water mixture in the proper flow regime as described in a later section of this paper. By proper pretreatment of the feedwater and correct HRSG coil design, HRSGs have operated for many years without experiencing significant coil fouling or corrosion.

Since the consequences (tube overheating and failure) of a buildup of scale on the inner surface of a tube are disastrous, good fluid side flow distribution is a must. If the flow distribution is poor, some tubes will have quality higher than 85% and the buildup of scale will occur. Several complementary techniques can be used to ensure that the fluid side flow distribution is good.

First, the fluid flow is split into a number of parallel circuits that are not interrupted throughout the entire HRSG (both economizer and evaporator). The outlet of each circuit contains the same fluid and mass flow that entered the circuit at the inlet. The only difference is the quality of the mixture: water at the inlet of the circuit and a steam/water mixture of the desired quality at the outlet. There are no headers or other devices between the inlet and outlet where the flows from adjacent circuits could intermingle and then not separate uniformly.

Second, a very high fluid side pressure drop is utilized to assist the control valves in maintaining uniform flow to each circuit and to promote high, uniform heat transfer from the tubes to the two phase mixture in the evaporator. Total pressure drop across the HRSG is often 200 psi or greater. A substantial portion of this pressure drop (~50 psi) should take place in the economizer.

Several other factors influence the tube wall temperature (and thus the potential for overheating and failure):

From the standpoint of heat transfer and pressure drop selection, there are two major flow regimes: gravity-controlled flow and shear-controlled flow. Various flow patterns can be grouped into one of these two major regimes as shown in Fig. 17.10.

Charts such as the Taitel & Dunkler chart, the Baker diagram (Refs. [6,7]) or the Heat Transfer Research, Inc. (HTRI) generalized flow regime map (Ref. [8]), for those who have access to HTRI documents, can be used to determine the flow regime and flow pattern at any point in a tube.

It is necessary to maintain the steam/water flow in the shear-controlled flow regime in as much of the HRSG as possible, especially in areas where strong cooling of the tubes is required.

Special care must be taken in the design of the evaporator at the gas inlet, especially if a duct burner is used. If the water temperature has not reached saturation yet, fluid velocities are low, subcooled boiling is probably occurring and even when saturation is reached, the fluid flow will be in the gravity-controlled regime. Reduction of the heat flux to levels appropriate for the flow regime in this area will minimize the chances of tube burnout. It is also necessary to account for radiation in this area as radiation can increase the heat flux substantially.

17.3.2 Mechanical design

The mechanical design of an EOR HRSG is not much different from that of a horizontal tube superheater or economizer used in a power boiler, HRSG, or waste heat boiler.

Both conventional fired EOR units and EOR HRSGs have traditionally used schedule pipe rather than boiler tubing. This is due to several factors, mostly related to the availability of replacement pipe and return bends in remote locations. The use of standard return bends is a substantial benefit to the end user.

The horizontal tubes are supported by tubesheets. Tubesheet spacing is determined to maintain reasonable tube deflection and prevent tube vibration, much as for a conventional HRSG. Tubesheet material is selected based on gas temperature.

At high gas temperatures, water-cooled tube supports are used. These tubesheets maintain the structural integrity required and minimize differential thermal expansion between the tube coils and tubesheets.

Since gas temperatures are similar to those used in a conventional HRSG, a cold casing design is used.

17.3.3 Controls

Each circuit of the EOR HRSG should have a control valve at the economizer inlet. Quality of the steam/water mixture can then be measured at the outlet of each circuit and the control valve can be modulated to maintain the desired steam quality.