4.2.1 Heat transfer/heat flux

Calculation of the heat transfer coefficient between a two-phase flow and the inside wall of a tube is necessary for an accurate evaporator design. It is a complicated process, but there are numerous correlations, varying in simplicity and accuracy, available in the literature. The simpler correlations often sacrifice some accuracy as they generally assume a homogeneous two-phase flow model. The homogeneous model assumes the steam and water are flowing inside of the tubes at the same velocity. In reality, the steam flows at a higher velocity than the liquid water; this is known as separated (or slip) flow. While better correlations exist for two-phase flow heat transfer; they are generally much more complex in nature. Fortunately for the designer of an evaporator, the heat transfer coefficient inside of the tube does not have a large impact on the overall thermal performance of the evaporator. The dominant resistance to heat flow across the tube is on the outside of the tube where the heat transfer coefficient is determined by forced convection between the exhaust gas flow and the tube. That does not mean that heat transfer on the inside of the tube is not important. Heat flowing through the tube wall must be removed effectively to prevent overheating of the tube. Thus the two-phase flow pattern inside of the tube and heat flux at the tube wall are of utmost importance.

The maximum design heat flux, an important factor to consider when designing a natural circulation evaporator, should be calculated during design and maintained within appropriate limits to ensure reliable long-term operation. A detailed discussion of it is beyond the scope of this chapter; however, some basics will be reviewed below.

At higher pressures, the maximum heat flux limit is set to avoid film boiling (Ref. [1]). Film boiling occurs when the inside surface temperature of the tube is high enough such that it is not possible for liquid to remain in contact with the metal surface. A layer of vapor will exist at the inner wall surface and any liquid will be flowing in the center of the tube. As discussed previously, a vapor layer at the tube will result in a significant increase in the tube metal temperature local to the vapor blanket, which can be dangerous if it was not considered in the design.

Actual maximum heat flux for HRSG is typically well below any film boiling criteria for a clean tube wall. The problem arises due to deposits on the wall. Any kind of deposit, such as preoperational oxidation or iron transport into the evaporator, will elevate the tube wall temperature. Dissolved solids in the water will concentrate under the deposit because of water flowing through the deposit and evaporating. This concentration of dissolved solids can be corrosive if the water is not treated properly. This is especially true for low pH (<8) excursions. Preoperational acid cleaning of an HRSG is recommended to place a new unit in as clean of a condition as possible to avoid under-deposit corrosion. See Chapter 15, Developing the optimum cycle chemistry provides the key to reliability for combined cycle/HRSG plants for more information on under-deposit corrosion.

At lower pressure, the maximum heat flux limit is set to avoid choke flow instability (Ref. [2]). The instability occurs when the pressure loss associated with generating additional vapor exceeds the natural circulation driving force for flow, causing temporary oscillations where vapor can actually reverse flow direction.

In cases where the pressure is high enough to avoid choke flow instability and low enough to avoid film boiling, the heat flux can be limited by the mist flow regime (Ref. [2]). As discussed previously, the mist flow regime occurs when a high enough vapor fraction exists to tear the liquid from the walls. The wall is blanketed with a layer of vapor with water droplets dispersed through the vapor space. If this occurs, local heat transfer coefficients will be greatly reduced and local metal temperatures greatly increased.

Although “rules of thumb” have existed in the heat recovery boiler industry for many years (e.g., limit maximum heat flux to 100,000 BTU/ft²-h), the problem is far more complex than can be represented by a single number and it is possible to determine a much more applicable limit. The maximum allowable heat flux is a function of the steam conditions (pressure, temperature, and quality), flow conditions (primarily mass velocity), and geometry (tube diameter, length, and orientation). Most HRSG suppliers maintain proprietary databases and correlations to determine the appropriate maximum design heat flux under various conditions. Correlations also exist in the literature for calculating the maximum heat flux. Refs. [3–5] in addition to many others cover the subject in more detail.

4.2.2 Natural circulation and circulation ratio

Natural circulation utilizes buoyancy due to density differences within the system to circulate the fluid in the evaporator. The density of the liquid and the height difference from the steam drum water level to the evaporator inlet provide the driving force for natural circulation. Since the density of the two-phase fluid flowing upwards inside the tubes is lower (due to the boiling of water inside the tubes) than the density of the liquid water in the downcomer, the gravitational force in the downcomer is greater than the gravitational force inside the tubes. This ensures continuous circulation from the drum through the tube field without the need for circulating pumps. For today’s high-pressure large HRSGs, this driving force is generally between 22 and 28 psi (Figure 4.1).

The maximum practical drum pressure for natural circulation is approximately 2750 psig. At higher pressures, the difference in density between the water in the downcomers and the two-phase mixture in the tubes becomes small enough that it is difficult to provide the driving force needed for natural circulation.

Circulation ratio is defined as the ratio of the mass of the steam/water mixture to the mass of steam at the exit of the evaporator tube field. A circulation ratio of 5:1 means there is five times as much water flowing through the downcomer and into the tubes than steam being generated in the tubes. If 100,000 lb/h of steam is being generated in the tubes at a circulation ratio of 5:1; 500,000 lb/h of water is flowing in the downcomer (100,000 lb/h of which is boiled while the remaining 400,000 lb/h is separated in the steam drum and will reenter the drum water storage volume).

Maintaining the circulation ratio within proper design values promotes strong cooling of the tubes; operation in areas of good flow regime and assists in maintaining stable circulating flow. As with many other parameters discussed in this chapter, recommended values for minimum circulation ratio will vary with operating pressure as shown in Fig. 4.2.

4.2.3 Flow accelerated corrosion

During normal HRSG operation, a thin layer of the inside metal surface of a tube will corrode and form a protective oxide layer. This oxide layer passivates the inside surface of the tube, eliminating the risk of further corrosion.

Flow accelerated corrosion (FAC) of an evaporator is a phenomenon that occurs when the protective oxide layer is dissolved or “stripped” from the inside surface into the flowing stream of flowing water or the two-phase steam/water mixture. Since the base metal surface is exposed, another layer of the metal will corrode to form the protective oxide layer described previously. If the oxide layer continues to be removed and reformed, eventually the base metal will become thin enough to rupture, causing failure of the tube and a reduction in performance.

FAC is influenced by four main factors: water chemistry, fluid temperature, flow velocity (turbulence), and metal composition.

The influencing factors for FAC can be mitigated by:

Additional information related to FAC is included in Chapter 15, Developing the optimum cycle chemistry provides the key to reliability for combined cycle/HRSG plants.

4.3.1 Drum water levels and volumes

Typically, the water level in the steam drum is controlled by introducing an amount of fresh feedwater into the drum approximately equal to the amount of steam being generated in the evaporator and exported to the superheater. During normal operation, the water level is kept at a defined normal water level (NWL). Water levels in the steam drum are defined as:

4.3.2 Drum internals

As discussed previously, one of the main functions of the steam drum is to separate the steam/water mixture exiting the evaporator tubes, sending the steam out of the steam drum while the water returns to the drum water storage volume. There are typically two stages of separation.

4.3.2.1 Primary separator

Typically a centrifugal type separator, the primary separator is designed to separate the largest portion of water from steam. The primary separators will generally fall into two categories:

1. Baffle type separator. The baffle type separator utilizes the difference in density between the steam and water to separate them. The steam/water mixture flows around the ID of the steam drum to a baffle that turns it in a downward flow direction. The heavy water droplets continue on into the water level while the lighter steam will turn upwards towards the secondary separators.

2. Cyclone type separator. The cyclone separator utilizes centrifugal force in a different device than the baffle above. The steam water/mixture enters the cyclone and flows tangentially around the cyclone. The water will remain at the outside surface and then fall to the water level. The steam will flow towards the inside area of the cyclone and out of the top of the cyclone towards the secondary separators (Figs. 4.4 and 4.5).

4.3.2.2 Secondary separator

The secondary separator is typically a chevron style separator with a mesh pad agglomerator attached to the front of the separator. The steam flow is largely dry exiting the primary separator. The remaining small water droplets are coalesced in the stainless steel mesh pad into larger droplets. The large droplets are easily separated in the chevron style separator. Today’s modern separators will typically reduce the exiting steam moisture content to 0.2% or less (by weight). See Fig. 4.6 for a chart of typical separator efficiencies as a function of drum pressure.

4.4.1 Continuous blowdown and intermittent blowoff systems

As water is continuously circulated through the evaporator system and pure steam departs, impurities in the steam drum water volume will increase. Since most of the water is separated from the steam and reintroduced into the drum, the impurities never leave the system. As additional feedwater is introduced into the drum (with its own concentration of impurities) to replace the steam generated, impurity levels would continue to rise unless they are removed via the blowdown lines.

Continuous blowdown is a small stream of water continuously taken from the drum to a blowdown tank. The amount of water taken depends on the impurities in the drum water and the required purity in the exit steam, but is typically between 1% and 3% of the incoming feedwater flow. Continuous blowdown thus helps provide ongoing control of the water impurity levels.

Even with the use of continuous blowdown, some impurities will settle near the bottom of the drum. It is necessary to occasionally take a larger amount of flow, blowoff, from the drum to provide additional control of the water impurity level. The intermittent blowoff connection on the drum is usually located to remove flow from an area where solid particles tend to settle. Intermittent blowoff connections are occasionally located in lower evaporator drum, header, or feeder lines where solid particles may settle. The intermittent blowoff will be a much larger flow rate than the continuous blowdown flow rate.

4.4.2 Drum level control

During normal operation and startup, it is important to control the drum water level within the HHWL and LLWL defined previously. In fact, it is preferable to maintain it between the HWL and LWL. If a control system fails to maintain the water between these levels, a costly HRSG trip could occur or excessive carryover of water droplets could occur, harming steam purity and possibly causing downstream coil damage.

There are two types of drum level control typically used. Single-element control is used during startup when the steam flow is less than 30% of maximum flow. Once the steam flow is high enough, the system will switch to three-element control.

4.4.3 Startup drum level

During startup, the drum water level is susceptible to swell due to changes in drum pressure and steam generation. To accommodate this phenomenon and prevent a CT trip due to HWL, the following philosophy is used.

Before the CT is fired, the startup level is set below the NWL to accommodate the drum swell that is expected (typically the startup level is approximately 8″ (203 mm) below NWL).

Once the CT is fired, the process adds a preceding step to the algorithm. Instead of simply comparing the startup level (–8″ (−203 mm)) with the operator input, the drum level plus a predefined tracking variable, –3″ (−75 mm), is also compared with the present set-point.

This effectively holds the set-point at –8″ (−203 mm) until the drum level swells up to approximately –5″ (–125 mm). After this threshold the set-point begins tracking the current drum level with a 3″ (75 mm) offset until the set-point reaches zero (NWL), where the set-point is finally held at zero. At time T1, when the process variable settles back to zero, the level control valve is permitted to open and begin controlling to the desired set-point.

The startup set-point of –8″ (–203 mm) is based on the expected amount of drum swell and may be altered from the initial value to meet site-specific startup conditions. The purpose of the –3″ (–75 mm) tracking variable is to restrict noise in the process variable signal from prematurely switching the setpoint to zero. If the noise in the signal does not come close to 3″, this variable may be changed to an absolute value less than 3″. If, however, the noise is greater than 3″, changing the variable to an absolute value greater than 3″ must be done with caution; a value greater than an absolute 3″ may force the drum to swell too high, resulting in carryover.

4.5.1 Multiple drum designs for fast start cycles

As discussed in the introduction, the HRSGs of 2016 are seeing an increased demand for cycling during operation. In addition to cycling, many combined cycle power plants are also seeing a requirement to be “fast start” designs. While the definition of fast start can vary from site to site, fast start designs are typically required to allow the connected gas turbine to start without the use of any hold points for the HRSG components to stabilize in temperature.

This is often not possible with a standard single-drum setup. In the high-pressure system, due to the large diameter of the drum and high pressure within it, a single drum may be sufficiently thick that hold points on the gas turbine startup would be required in order to limit the heat input to the HRSG to avoid overstressing the high-pressure steam drum and other thick HRSG components.

This need can be met by replacing a single steam drum with multiple drums for the applicable pressure levels. By splitting the volume of one drum between two drums, each of the multiple steam drums can be significantly reduced in diameter. The smaller drum diameters, for the same temperature and pressure, can be significantly thinner than a single drum. This reduced thickness will allow a faster heat input ramp, and often can eliminate any need for gas turbine hold points. A single drum can be split into two, or even more, vessels to reduce the diameter and thickness as much as possible.

If using multiple drums is not in itself sufficient to reduce the thickness below the value needed to eliminate gas turbine hold points, the secondary steam separator assembly described previously can be located outside of the steam drum. Moving the secondary separator external to the steam drum reduces the volume required for steam/water separation and further reduces the diameter and thickness of the steam drum.

Higher-strength materials are an additional option that can be used to reduce the drum shell thickness. Carbon steel grade SA-516 70 has been a standard drum shell material for many years due to ready availability and reasonable cost. However, there are other higher-strength carbon steel materials that can also be used. These higher-strength materials allow a thinner shell to be used for the same set of design conditions. Depending on the design specifics and the material chosen, shell thickness can be reduced by as much as 30%.

4.5.2 Deaerators

Deaerators, when needed, are used to physically remove dissolved oxygen and carbon dioxide from the condensate/make-up water stream feeding an HRSG. High levels of oxygen in the HRSG feedwater can cause corrosion and premature failure of HRSG tubes and other components. Deaerators reduce the oxygen content to levels low enough to avoid premature corrosion failures.

Deaerators operate on the principle of Henry’s law of partial pressures (the solubility of any gas dissolved in liquid is directly proportional to the partial pressure of that gas above the liquid). Thus, the dissolved gases in the feedwater can be removed by spraying the water into a steam environment in which the partial pressure of the gas is reduced. The deaerated feedwater eventually flows out of the deaerator into a storage tank while the oxygen and carbon dioxide are vented to the atmosphere, carried by a small amount of steam. As a byproduct of this deaeration, the incoming water is heated to the saturation temperature of the steam.

There are multiple styles of deaerator design but two are predominant within HRSG systems: integral deaerators and remote deaerators.