SCOPE DESIGN CONSIDERATIONS

Decisions made during the design and construction phase of the HRSG can have a tremendous impact on the chemistry control that can be used to prevent corrosion.

Auxiliary Steam Generating System - steam from auxiliary boilers may be used for brief periods at start-up, used to maintain vacuum during longer outages or used for sparging of the HRSG. Purity of steam from auxiliary boilers should be the same as that of steam produced in the HRSG. Contaminants in auxiliary steam may make cleanup of the steam/water cycle more difficult at a subsequent start-up.

Cascading Drum Blowdown - in multi-pressure steam drum HRSG units, cascading of steam drum blowdown is a common design configuration. Cascading blowdown generally refers to the process in which the continuous blowdown from a steam drum is discharged into a lower pressure steam drum to capture blowdown energy and improve cycle efficiency. In a three-pressure HRSG unit, the receiving drum for high-pressure (HP) blowdown is normally the intermediate-pressure (IP) drum. In a dual pressure HRSG unit, the receiving drum would be the low- pressure (LP) drum. In many HRSG designs, LP drum water is used for steam attemperation purposes. In this case, cascading to a LP drum is unacceptable. Although cascading blowdown improves overall plant efficiency, it can result in chemistry-related concerns and issues that must be addressed at the design phase. For example, low molecular weight organic acids produced from degradation of certain organics in the HP steam drum can cause low drum water pH in the receiving drum.

Drums receiving blowdown from a higher pressure drum must themselves have a blowdown system adequate for appropriately controlling their own drum water chemistry. For start-up and emergency operation, provision should be made for independent control and discharge to waste of normally cascaded blowdown.

Condensate/process Return polishing - condensate filters/polishers can be very effective for removing contaminants and corrosion products from the feedwater thereby minimizing deposits in the HRSG. Their inclusion is considered standard practice for once-through HRSGs, for drum HRSGs that operate on an oxygenated treatment (OT) program, and for units that collect condensate from an industrial process or from a central heating system. They should also be strongly considered for HRSGs that operate at greater than 1800 psig (12.5 MPa) or on any HRSG that will operate on an all-volatile treatment (AVT) program. These systems should be equipped with adequate monitoring of returned condensate and reliable systems for quick dumping of out-of-specification condensate upstream of the polishers to avoid contamination of not only the steam/water cycle, but the condensate polishers as well. See Table 8b.

Condensers - chemical treatment is greatly simplified if there are no copper alloys in the system including the steam condenser. Copper alloys possess excellent heat transfer characteristics but are susceptible to ammonia attack on the steam side, chemical and biological attack on the water side, and have lower allowable maximum flow velocities than ferrous alloys. If copper alloys are not present in the combined cycle plant system or in a steam host's system, where such exists, the feedwater pH can be raised above 9.3 using ammonia or amines to minimize flow accelerated corrosion (FAC).

In selecting replacements for copper alloys, stainless steel alloys are often considered. Stainless steel, however, is susceptible to attack by chlorides and microbiologically influenced corrosion (MIC) from cooling water. Additionally, some stainless steel condenser tubes have suffered pitting damage if the cooling water contains even trace amounts of manganese. Titanium alloys eliminate these issues, however, titanium tubed condensers may be more costly and tubes must be shop-welded into the tube sheets to minimize leaks. Both titanium and stainless steel alloys offer superior erosion resistance as compared to copper alloys, allowing the use of higher maximum cooling water flow velocities. Cost, size, velocity, and cooling water chemistry are all factors that must be considered in the selection of condenser materials.

Chemical Feeds - the proper feed and control of chemical treatment is critical to the successful operation of any steam generator. In HRSG systems it is important to provide the capability to feed treatment chemical to each individual steam drum. This will enable the adjustment of individual unit chemistry as required by varying operating conditions. The treatment chemicals should be fed downstream of any economizers and injected so that they are mixed thoroughly with the incoming feedwater. This can be accomplished by feeding through a chemical feed quill of suitable alloy into the center of the feedwater line prior to its entry into the steam drum, or by feeding a high volume dilute solution of chemical into a suitably designed and positioned distribution header within the steam drum.

Feedwater and condensate system pH control chemicals such as ammonia and amines should be fed where they will provide pH control for all carbon steel and copper alloy components in the makeup and feedwater systems. Feed into the condenser hot well or to the condensate pump discharge will serve this purpose in many systems. In facilities with condensate polishers, feed should be to the discharge of the polishers.

For those systems requiring an oxygen scavenger (reducing agent), the feed of chemical into the condensate pump discharge is preferred.

In facilities with condensate polishers, feed should be to the discharge of the polishers.

Low-pressure HRSG systems operating with lower quality feedwater (e.g. softened makeup or low-quality process condensate return) typically use different treatment injection points. Oxygen scavenger (reducing agent) and amine treatments are usually injected into the deaerator storage section while the boiler water treatment chemicals are injected into the feedwater.

Cycling Operation Considerations - since many HRSGs and combined cycle plants are developed to operate as cycling units, several additional points need to be addressed in the design phase to plan properly for this mode of operation. The HRSG blowdown and blowoff treatment and disposal system should be sized to accommodate and safely dispose of blowdown at a rate of at least 5% of design steam flow. The sizing criteria for cycle chemical feed pumps should be based not only on the normal operating feed requirements, but also on start-up feed requirements which can occur on a daily basis in a cycling unit. The pumps should be sized for the start-up drum blowdown rate of 5% of design steam flow in addition to the normal operating steam drum blowdown conditions.

Deaerator - combined cycle units are normally supplied with feedwater deaeration systems in one of three basic configurations. Pressure deaeration may be accomplished with a stand-alone deaerator or a deaerator integral to the HRSG. The integral pressure deaerator design utilizes the LP steam drum as the feedwater storage tank. Vacuum deaeration is achieved through the use of a special deaeration section and spargers located internally in the condenser.

It is recommended that the HRSG be furnished with a separate standalone deaerator that is designed to remove dissolved oxygen in the feedwater under all conditions, particularly during start-up. The stand-alone deaerator provides benefits that are not available when an "integral" type deaerator, integral either to the HRSG or the condenser, is utilized.

Steam produced in an auxiliary boiler or steam from an alternate source can be beneficial for short-term lay-up and start-up of an HRSG.

A stand-alone deaerator can operate with auxiliary steam to produce deaerated feedwater prior to the firing of the combustion turbine. Auxiliary steam also provides other advantages such as maintaining heat in the HRSG and vacuum in the steam turbine condenser in a cycling plant.

A stand-alone deaerator can also provide steam attemperation water that allows for the use of solid-alkali treatment to mitigate two-phase FAC in the LP evaporator.

Documentation Requirements - as a minimum, all critical parameters including all on-line analyzer data (see Table 2) should be recorded every two hours, including during the commissioning period prior to commercial operation. The preferable method is to retain chemistry data electronically. All treatments, blowdown settings, and pump settings should also be regularly recorded.

Duct Burners - HRSG operation with and without the need for duct burners in service should be carefully reviewed and evaluated at the design stage. In some cases, HRSG chemistry requirements and system operating conditions will be different when the duct burners are in service as compared to operation with no duct burners. Different rates of firing may produce alternate scenarios that need to be considered; especially since duct burners generally do not distribute heat evenly across the HRSGs' heat transfer surfaces. The chemical treatment program should be developed considering the worst-case scenario of the various anticipated operating scenarios for each particular unit.

Depending on the HRSG design, high rates of firing with duct burners have been known to create chemical hideout, particularly in HP evaporator tubes. A low solids (or no solids) treatment program may be required to prevent deposit accumulations when the HRSG is being fired with duct burners. Since the utilization of the duct burners is not predictable, prudence requires that the treatment program be designed for the most severe (i.e., highest heat flux/pressure) conditions. This optimum chemistry must be determined by performance testing.

Plant Materials - corrosion generally is affected by conductivity, pH, and oxygen concentration. In systems with mixed materials, high pH and oxygen may lead to corrosion of the copper and copper alloys, while low pH leads to corrosion of carbon and low alloy steels. If the pH and oxygen are low, the steels are subject to flow accelerated corrosion (FAC).

For all- ferrous systems, the level of dissolved oxygen typically present in the effluent of a properly functioning deaerator (< 7pg/L) may be acceptable without the addition of chemical oxygen scavengers (reducing agents). High pH values in the feedwater that protect ferrous alloys create amounts of ammonia that may be corrosive to copper alloys.

Flow accelerated corrosion (FAC) has been found in many HRSGs. Testing and experience has shown that mild steel alloys that contain chromium are far less likely to develop FAC.

Owners/Engineers should specify use of alloys containing a minimum of 1.25% chromium in the areas susceptible to FAC. The primary areas for FAC in HRSGs are all economizers and the LP evaporator. Chromium-containing alloys will significantly improve FAC resistance in systems.

The HP and IP evaporator feedwater lines and attemperator piping may also be susceptible to FAC depending on design, materials, and operation.

Copper-alloyed materials in the condenser, or in a host's steam system, limit the feedwater pH that can be used. This lower pH makes the HRSG more susceptible to flow accelerated corrosion (FAC). It is strongly recommended that copper materials not be used in combined cycle plants. Aluminum alloys are unacceptable in the HRSG water and steam cycles.

Makeup Water Treatment System - combined cycle plants are subject to frequent start-up/shutdown cycles and the makeup water treatment system and related storage facilities must be designed and sized accordingly. Ideally, the makeup treatment systems for a cycling unit should be sized for the highest demand during start-up and maximum steaming rate. The start-up case should include an initial blowdown rate of 5% of design steam flow as well as any other potential water consumption that may be required during start-up operations. The maximum operating case should include blowdown, steam to hosts, sampling, and miscellaneous cycle losses, as well as any additional water uses such as combustion turbine inlet fogging, combustion turbine steam power augmentation, and steam injection for NOx control. In situations where other considerations dictate that a smaller makeup water treatment system be installed, storage facilities must be sized to provide sufficient water at the rate and volume required during peak demand periods.

The design must include adequate makeup treatment capacity to allow continuous operation during normal and abnormal circumstances, such as the loss of returned condensate from the steam host or mechanical problems with the water treatment system. Adequate treated (softened or demineralized) water storage should provide the plant with sufficient response time for corrective action.

In design and specification of the equipment required for processing plant makeup water, care must be taken to ensure that representative source water analyses are obtained and provided to the manufacturers prior to procurement. These water analyses should include seasonal variations in water quality parameters including historical data for at least two years. The more water data that are provided the more satisfactorily the equipment can be designed to meet the specific needs of the plant. Crucial parameters that should be included in all water testing for design of these systems include pH, seasonal temperature variations, organic concentrations, alkalinity, hardness, full ionic analysis, total dissolved solids, reactive and total silica, total suspended solids, oil and grease contaminants, and heavy metals.

Purity Requirements for Chemicals - the chemicals for the steam/wa- ter cycle have to fulfill the following purity requirements to avoid impairment of the recommended quality of steam and water as shown in Table 1. The purity may be proven either with certification by the chemical supplier or with laboratory analyses. Dilution water should be high- purity demineralized water. Trisodium phosphate is the only phosphate recommended. Trisodium phosphate could be supplied as a dry powder or as solution. If supplied as a solution, the user should request a Certificate of Analysis to ensure that it does not contain contaminants.

Typical Purity of Aqueous Ammonia (10-29%)

Table 1.

Chloride as Cl

<2.0 ppm (mg/l)

Sulfate as SO4

<1.5 ppm (mg/l)

Sodium as Na

<1.0 ppm (mg/l)

Iron as Fe

<2 ppm (mg/l)

Copper as Cu

<0.05 ppm (mg/l)

Typical Purity of Trisodium Phosphate

Na3PO4 content as Na3PO4

>44%, if water of crystallization included

>98%, if expressed as calcined product

Water insoluble substances

<0.05%

pH of 1 wt% solution, at 25oC

11-12

Sample Conditioning and Continuous Analyzers - proper sampling and sample conditioning is critical to reliable monitoring of the water and steam chemistry parameters in the unit. Proper sample conditioning includes dual sample coolers for all high-pressure/high-temperature samples so that the sample lines can operate continuously while maintaining 77°F (25°C). Sampling and sample conditioning guidelines published by ASME and ASTM should be followed.

Critical control parameters should be monitored continuously on any HRSG that produces steam for a steam turbine, regardless of the operating pressure of the HRSG. These are shown in Table 2.

Parameter

On-Line Measurement Location (1-2)

Silica

Makeup water

HP drum water

Main or reheat steam

Specific

Conductivity

HP drum water (phosphate treatment)

IP drum water (phosphate treatment)

Makeup

Feedwater downstream of ammonia feed (for alternative pH determination)

Process condensate returns

Cation

Conductivity (3)

Condensate pump discharge

HP feedwater (BFP discharge or economizer inlet)

Main steam or reheat steam

HP drum (for units on AVT)

Dissolved

Oxygen

Condensate pump discharge

Feedwater pump discharge

TOC

Process condensate returns

ORP

Feedwater pump discharge

Sodium

Condensate pump discharge

HP feedwater (feed pump discharge)

Main or reheat steam (4)

HP drum (for units on AVT)

pH

HP, IP, LP drum (blowdown)

LP, HP feedwater

Process condensate returns

  • (1) It is preferred that each of the parameters in this table have dedicated analyzers for each sample point. A list of analyzers that can be shared can be found in Table 3.
  • (2) Continuous analyzers should be verified by grab samples or by comparison with other analyzers at least daily. Increase manual testing frequency to every two hours if a continuous analyzer is not working.
  • (3) The on-line direct measurement of anions by on-line ion chromatography (IC) is an acceptable alternative to cation conductivity.
  • (4) Monitoring the HP and IP saturated steam and attemperation water (e.g., economizer inlet) can provide equivalent information as the main steam or reheat steam sample if steam and attemperation flow rates are known.

Certain sampling points should have a dedicated analyzer, but other sampling points can be connected to a shared analyzer. Shared analyzers reduce capital costs and ongoing operating and maintenance costs. The sampling frequency will vary depending on whether the plant is being operated in a cyclic or base-loaded mode.

Table 3 identifies which of these sample points can utilize a shared analyzer. The shared analyzer can be manually valved to each sample or can be cycled through all points by means of an automatic sequencer. If sample points are to be manually valved, they should be set to operate as outlined in Table 3 during normal operation.

Table 3. Sampling Points with Shared Analyzers

Measurement

Samples Sharing Analyzer

Set for Normal Operation

TOC

Makeup, process return condensate

Process return condensate

Dissolved

Oxygen

Feedwater heater inlet HP/IP economizer inlet

HP economizer inlet

Silica

LP saturated steam (standalone system)

IP saturated steam Hot reheat steam HP main steam

HP main steam

Sodium

Makeup

Hot reheat steam HP main steam HP saturated steam IP saturated steam LP saturated steam

HP main steam or hot reheat

Cation

Conductivity

LP saturated steam IP saturated steam HP saturated steam

HP saturated steam

Phosphate

LP drum water (stand-alone) IP drum water HP drum water

sequenced

Since there are many different plant configurations, additional sample points may be needed to continuously monitor a system, provide chemistry control and identify contamination ingress. For example, additional on-line analyzers would be needed for any condensate process returns. The user should examine their specific system when considering what parameter(s) to monitor and the required sampling point(s).

For additional discussion on this topic, refer to the ASME "Consensus on Operating Practices for the Sampling and Monitoring of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers" (CRTD-81) and "Steam and Water Sampling, Conditioning, and Analysis in the Power Cycle" (ASME PTC 19.11-2008).

Steam Purity - The required steam purity for the steam turbine or any process equipment must be defined clearly in the project design stage. Critical decisions regarding makeup treatment and condensate treatment will all be determined by the required steam purity.

Unit Commissioning Considerations - Timely and successful unit commissioning is highly dependent on proper planning in the predesign and design stage. Provisions for chemical cleaning need to be evaluated and factored into the design of piping and system equipment in the plant design stage. All required connections should be accessible once construction is complete and any connections that will remain as part of the system after commissioning should be completely drainable.

Early design of chemical feed systems can save money later in the project. The options for chemical treatment for the HRSG should be evaluated early in the project and a plan of treatment (i.e. selection of chemicals, chemical injection locations, and anticipated chemical feed rates) chosen in the design phase. Chemical feed equipment can then be properly specified to avoid problems of inadequate feed systems (i.e. oversized pumps, undersized pumps, wrong type of pumps) during unit commissioning.

Sampling systems (conditioners and analyzers) must be functional prior to introducing steam into the turbine, while venting steam or bypassing to the condenser, so that chemistry can be monitored. Refer to the ASME "Consensus on Operating Practices for the Sampling and Monitoring of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers" (CRTD-81) and "Steam and Water Sampling, Conditioning, and Analysis in the Power Cycle" (ASME PTC 19.1 1-2008)

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