Considerations for High Pressure Boiler Chemical Treatment ... · spray water quality are the same as those for steam purity. In some cases boiler feedwater is suitable. In all cases

Considerations for High Pressure Boiler Chemical Treatment – Equipment and

Chemistry As the ammonia plant design envelope is continually pressed for larger and more efficient plants, the boiler and steam systems become even more critical. In the middle 1960s, the then new single train ammonia units came on-line with 1500 psig (104 bar) steam systems. At that time, experienced ammonia producers said it was like “Operating a power plant with ammonia as the by-product.” Today, such a comparison is still appropriate as heat fluxes, steam pressures, and design production rates have increased. The plant designers and operators have choices when it comes to both the boiler internal treatment and the feed equipment for proper dosing. This paper describes the various systems available and the strengths and weaknesses of each.

Daniel M. Setaro

GE Water & Process Technologies

Chemical Treatment Considerations

For purposes of this paper, the targeted operating boiler pressure range is 1500 to 2000 psig (104 to 140 bar) as this is the typical HP steam range found in modern ammonia plants. In this pressure range, the recommended feedwater quality is quite pure so as to assure the highest levels of steam purity. For further conciseness, this paper will address phosphate based internal treatment chemistries such as the coordinated phosphate and congruent phosphate programs commonly employed in high-pressure industrial systems. In special cases all volatile treatment programs are employed where there are serious concerns for phosphate hideout problems. Much more stringent demands are placed on boiler feedwater quality when AVT chemistry is utilized.

Boiler Feedwater

The feedwater quality task group of the industrial subcommittee of the ASME Research and Technology Committee on Water and Steam in

Thermal Power Systems has published Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers.1 Many refer to these practices as the ASME guidelines. The recommended feedwater quality for boiler systems in the pressure range of concern is shown in Table 1.

Table 1: ASME Feedwater Consensus Guidelines for 1501 to 2000 psig (104 to 140 bar) systems BOILER FEEDWATER PARAMETER

ASME CONSENSUS

Dissolved oxygen ppm (mg/l), as O2, measured at point prior to addition of oxygen scavenger

< 0.007

Total iron ppm (mg/l), as Fe

<0.010

Total copper ppm (mg/l), as Cu

<0.010

Total hardness ppm (mg/l) , as CaCO3

ND [not detectable]

pH @ 25oC 8.8 – 9.6

Chemical for preboiler system protection

Use only volatile alkaline materials upstream of attemperation water source*

2006 279 AMMONIA TECHNICAL MANUAL

*As a general rule, the requirements for attemperation spray water quality are the same as those for steam purity. In some cases boiler feedwater is suitable. In all cases the spray water should be obtained from a source that is free of deposit forming and corrosive chemicals such as sodium hydroxide, sodium sulfite, sodium phosphate, iron, and copper. The suggested limits for spray water quality are: <30 ppb (ųg/l) TDS, < 10 ppb (ųg/l) Na, <20 ppb (ųg/l) SiO2, and it should be essentially oxygen free.

As shown in earlier work presented at the American Power Conference, the release of iron oxide from steel feedwater heaters is minimized as the feedwater pH is increased to the 9.3 to 9.6 range.2 In a similar fashion, the feedwater in route to the steam drum(s) in ammonia plants should be at elevated pH levels to minimize release of iron oxide from the various process coolers and convection section economizers. Data from an ammonia plant study indicated similar reduction in iron oxide release from the pre-boiler circuit as pH was increased. The data shown in Table 2 comes from a paper presented at this conference at an earlier date.3

Table 2: Iron release from BFW circuit as function of pH

Day BFW pH BFW Iron, ppb (ųg/l), as

Fe* 15 8.4 40 16 8.5 35 17 9.1 11 18 9.1 9 19 9.0 <5 20 9.0 6 21 9.1 <5 22 9.0 <5 23 9.0 <5 24 9.0 6

*Data from ammonia plant study where deaerator outlet iron was in 5 to 10 ppb range, as Fe

Feedwater Oxygen Control

To assure operation under a reducing environment, volatile oxygen scavengers are added to the deaerated boiler feedwater. Commonly used chemical scavengers include carbohydrazide, hydrazine, and hydroquinone. Both carbohydrazide and hydroquinone based

scavengers were developed to be a replacement for hydrazine, as it has suspected carcinogenic properties. Since a detailed treatise of volatile oxygen scavenging is beyond the scope of this paper, let’s leave it that small excess of scavenger assures a reducing environment for the formation of protective magnetite on the surfaces of the boiler and steam systems. High purity feedwater in all steel systems without dissolved oxygen present is passive and protective at elevated pH levels of 9.0 to 9.6. Minimizing the amount of feedwater iron entering the steam drum is a key component of successful internal boiler water treatment. Even a very small amount of transported iron oxide over time can cause problems in boiler systems.

Internal Treatment Chemistry for Corrosion Control

Phosphate – a general review

Most high-pressure industrial boilers with high purity feedwater use a phosphate-based chemical treatment program for corrosion control. The evolution of phosphate-based boiler chemistries followed improvements in feedwater quality. Prior to the advent of demineralizers, sodium phosphates were used to precipitate calcium to a calcium hydroxyapatite that was a fluid sludge removable by lower drum blowdown. When demineralized water systems came about, the need for hardness precipitation was replaced by a need for phosphate buffering. The buffering action of the phosphate treatment will minimize the potential for acid and caustic based corrosion. Keeping a clean heat transfer surface is key to minimizing under-deposit corrosion cells. Deposit minimization relies on both the feedwater treatment and polymeric dispersants. Besides causing boiler tube overheat failures, deposits play a role in corrosion. Formation of concentration cells under and within deposits can lead to corrosive conditions at the tube metal surface. As such, the maintenance of clean boiler tube surfaces is the most important step that one can take to minimize corrosive attack.

2006AMMONIA TECHNICAL MANUAL 280

Coordinated and Congruent Phosphate

The first use of phosphate chemistry programs 4/5 for corrosion control dates back to the early 1940s when Whirl and Purcell developed coordinated phosphate/pH for control of caustic embrittlement. The basis of the chemistry is:

NaOH + Na2HPO4 Na3PO4 + H2O

As shown above, feedwater caustic will react with disodium phosphate in the boiler water forming trisodium phosphate.

In a similar fashion feedwater acidic species will react with trisodium phosphate to form disodium phosphate as follows:

HCl + Na3PO4 Na2HPO4 + NaCl

By controlling the boiler chemistry to maintain di-basic phosphate in the boiler (sodium to phosphate molar ratio less than 3.0 to 1), caustic (NaOH) cannot concentrate and cause corrosion. This assures that all the sodium is associated with phosphate and no free NaOH is in solution. By maintaining the alkalinity as a captive phosphate alkalinity, under-deposit concentration of caustic is eliminated, thereby minimizing caustic gouging potential.

Coordinated phosphate programs were designed to prevent caustic gouging, but such problems persisted even when coordination was maintained. The gouging was associated with boiler deposits and it suggested that the gouging was related to the extent of the deposition. 6 It was also noted that phosphate precipitation occurred at a lower ratio that the 3.0 originally prescribed for coordinated control.

These observations made researchers re-think the sodium phosphate buffer chemistry under the boiler environments where problems were observed. As a result of further studies of solution chemistry and two-phase equilibrium, a new definition of phosphate control was

developed around solubility data. Research data7

showed that pure sodium phosphate solutions at 572oF (300oC) form precipitates at a 2.85 to 1 sodium-to-phosphate ratio. This led researchers8 to propose an upper limit for maintaining sodium-to-phosphate congruency. Further definition of the chemistry9 found two invariant points, one at 2.85:1 and one at 2.15:1. These boundaries became the upper and lower limits for the congruent control utilized by most industrial boiler systems today.

Even though congruent control can maintain pH during minor contaminant ingress and to a minor extent inhibit calcium deposition during hardness intrusion, its primary purpose is to mitigate under-deposit corrosion. It performs this task by maintaining under-deposit sodium phosphate solution chemistry between the congruency limits. By doing this, the concentrated solution under a boiler deposit does not result in either caustic gouging or acidic phosphate corrosion. Figure 1 shows the relative corrosion of carbon steel boiler tubing operating at 590oF (310oC) as a function of pH and concentration of corroding species – HCl and NaOH.

For years congruent phosphate programs have been applied with the assistance of a logarithmic control chart as that shown in Figure 2. Several upper molar ratio boundaries are presented depending on the operating pressure of the boiler. These boundaries were set to assure minimal chance of phosphate precipitation at the corresponding temperatures. Staying below the upper molar ratio limit will minimize the chance of caustic concentration and gouging of the tube metal. The lower control boundary of 2.2: 1 molar ratio is set to minimize the chance of acidic phosphate attack. When the boiler pH and phosphate levels stay within the control boundaries, the boiler water should be non-corrosive. The vector diagram indicates the direction a point will go on the phosphate-pH diagram if a certain chemical is added to the system. Also shown is the affect of increased continuous blowdown. In thinking of powdered


sodium phosphates, one can imagine the need for three phosphates as well as caustic. When dealing strictly with powders, this could be the case, but normally only tri and di-sodium phosphate will be required to counter normal contaminant variation in the incoming boiler feedwater. Most water treatment companies can provide blends of sodium phosphate products at various sodium to phosphate molar ratios.

Figure 1: Relative corrosion of carbon steel by HCI and NaOH at 590°F (310°C). (Berl and vanTaack, 1930; Partridge and Hall, 1939)

Figure 1: Relative corrosion of carbon steel by HCl and NaOH at 590ºF (310ºC) (Berl and vanTaack, 1930; Partridge and Hall, 1939)

Figure 2: Coordinated Phosphate/pH Control Chart

Staying within the control boundaries of phosphate and pH is key to successful operation of high-pressure industrial boilers. But both of the corrosion pathways are connected with deposits. Figure 3 depicts caustic gouging mechanism. Figure 4 shows a photograph of acidic phosphate attack.

Figure 4: Acidic phosphate caused corrosion of this boiler tube.

Figure 3: Porous deposits provide conditions that promote high concentrations of boiler water solids, such as sodium hydroxide (NaOH).


All Volatile Treatment [AVT]

In an AVT program only volatile chemicals are added to the feedwater primarily for pre-boiler circuit corrosion control. Normally, the AVT chemicals include ammonia or amines for pH control and an oxygen scavenger (hydrazine, carbohydrazide, hydroquinone, etc.) for oxygen control. Secondarily, these volatile chemicals control boiler corrosion by maintaining a protective magnetite film through pH and oxygen control. Since ammonia is highly volatile typically, the boiler pH is 0.2 to 0.3 units lower that the feedwater pH value when ammonia is used. In addition the boiler water has essentially no capacity to buffer the pH in event of feedwater contamination. Consequently, where possible, it is advisable to use an amine with a low distribution ratio at boiler water temperature. This will provide a small increase in the ability to maintain boiler water pH in the event of acidic ingress.

It is prudent to have 100% polishing of the makeup and condensate return streams as the feedwater should be maintained below 0.2 microSiemens/cm cation conductivity. Any identified in-leakage of contaminants to the feedwater such that the cation conductivity exceeds 0.2 microSiemens/cm requires immediate implementation of a congruent phosphate program to protect against small levels of either acidic or alkaline contamination. Depending on the magnitude of the contamination, more serious measures may be required.

Table 3: Comparison of Congruent Phosphate to All Volatile Treatment Parameter AVT Phosphate

Control Comments

Steam Purity Concerns – Solids

Favored Less favored

Plants with inadequate steam separations are forced to AVT

Steam Purity Concerns – Silica

Less Favored

Favored Higher (more favorable) pH is possible with

Parameter AVT Phosphate Control

Comments

PO4 Ability to cope with contamination

Less Favored

Favored PO4 allows buffering of both acidic & caustic contamination.

Phosphate Hideout Concerns

Favored Less Favored

AVT has no solids

Hydrogen Damage (Embrittlement) Concerns

Less Favored

Favored PO4 buffers against acidic contamination.

Ability to cope with dryout conditions

Favored Less Favored

Unfortunately, these conditions cannot be predicted.

Solubility of Turbine Deposits

Less Favored

Favored

PO4 program at higher pH reduces silica vaporization.

Operator skill level required

Same Same

Minimizing Deposition

Constant vigilance over the proper operation and maintenance of the makeup water pretreatment system and the condensate polishing system is a must. Earlier mention was made regarding keeping the feedwater pH elevated in order to assure minimal release of iron oxide from the feedwater preheat train. For best results in an all steel system, feedwater pH should be within the range of 9.2 to 9.6.

Even with the best programs, safeguards, and instrumentation in place, feedwater contamination cannot be completely eliminated. As such, there is potential for troublesome deposits to develop. Consider a system that averages only 10 ppb (ųg/l) of feedwater iron and produces an average of 650,000 pounds per hour (295 tonnes per hour) of steam. It will accumulate 57 pounds (26 kg) of iron deposit (as Fe) in one year. This amount of deposit is not that significant if it is spread uniformly over the boiler system’s entire heat transfer surface. But deposits do not distribute uniformly, and in systems with varying heat fluxes and circulation patterns, the majority of the deposition can accumulate in small areas.


The use of a polymeric agent to control the deposition of feedwater contaminants, even in systems with good feedwater quality, is usually of benefit. In systems with demineralized makeup and condensate that is polished, iron will be the major contaminant to address. So in the case of the modern day ammonia plant, an effective iron dispersant that will function effectively at higher temperature and pressure would be necessary. A polymer with phosphate functional groups has proven to be exceptionally useful in this regard.

Monitoring the boiler system cleanliness is of key importance. Even with the aid of effective polymeric dispersant, no system will exhibit 100% iron transport. With that in mind, plant operators should assess the various ways to measure deposit tendency and accumulation within the boiler circuits. In fired boilers, a planned testing program for cutting tube sections for deposit weight density and deposit composition determinations could be initiated. Tube selection, removal techniques, handling, and test method can significantly affect the results obtained. Guidance in these matters can be found in references 10, 11, and 12. On-line analysis of deposit accumulations through use of strategically placed chordal thermocouples can be useful.13 Chordal thermocouples are special devices that measure the temperature gradient through the tube wall of a fired boiler. Data collected is an indirect measure of the accumulation of boiler tube internal deposits.

Lastly, in the case of high purity systems, it is good to regularly monitor the iron levels in the feedwater and boiler water for an approximation of iron transport. While not without flaws, such data will give good qualitative trends particularly when going from no dispersant to an added dispersant or from on type of polymeric dispersant to another.

Figure 5 shows a bar chart summarizing the relative effectiveness of various boiler water

dispersants under high pressure research boiler conditions.

Listed below are the highlights of our experience with two different dispersants in 1500 psig ammonia plant boiler systems. At GE Water & Process Technologies, laboratory and field trial work has indicated the two dispersants for consideration in high pressure industrial plants are polymethacrylate (PMA) and poly (isopropenyl phosphonic acid). Of these two, the latter (branded as HTP) is the stronger performer.

Figure 5: Relative Effectiveness of Various Boiler Water Dispersants

Iron transport data from 1500 psig ammonia plant boiler system locations indicate improved performance by the HTP polymer.

Plant A: Internal treatment program was congruent-phosphate/pH with PMA polymeric dispersant. Boiler feedwater iron levels were in the range of 5 to 10 ppb throughout the duration of the trial period. Below Tables 4 and 5 summarize the iron transport data in the ammonia plant field trial with PMA dispersant followed by HTP dispersant feed.

EQUIVALENT POLYMER DOSAGE

% D

EPO

SIT

INH

IBIT

ION

PAAPMA PAAM NONEHTP

Research Boiler Deposit Inhibition Comparison (1450 psig)

100

0


Table 4: Comparison of Boiler Water PMA dispersant levels with Iron levels.

Day PMA Level, ppm

Boiler Water Iron, ppb (ųg/l) as Fe

1 15 44 2 15 38 3 15 40 4 15 37 5 30 73 6 30 51 7 30 65 8 30 56 9 60 96

10 60 114 11 60 119 12 60 100 13 90 109 14 90 115 15 90 123 16 90 94

Table 5: Comparison of Boiler Water HTP Dispersant Levels with Iron Levels

Day HTP Level, ppm

Boiler Water Iron , ppb (ųg/l) as Fe

1 12 382 2 12 311 3 12 346 4 12 361 5 12 333 6 15 487 7 15 511 8 15 530 9 15 492

10 15 481 11 20 671 12 20 634 13 20 660 14 20 628 15 20 647 16 40 1031 17 40 1006 18 40 1025 19 40 1013 20 40 1046

A less rigorous HTP trial test was performed at another ammonia plant location. The results shown in Figure 6 indicate similar iron transport improvement as demonstrated in the previous ammonia plant site. In this case neither plant used a dispersant as the internal treatment

program was just a phosphate based congruent program.

Polymeric dispersants can generally be supplied as separate liquid products or they can be blended with the phosphate based liquid products at the proper concentrations. Working with a qualified water treatment specialist to obtain the chemical treatment program to meet your system’s needs is just the beginning point. One must select the chemical feed and control system that meets their needs and budget. In the next section there will be descriptions of the various feed systems that are available. It has been the author’s opinion that one should only consider the high-end feed and control equipment if they are strongly committed to the proper calibration and maintenance required.

Chemical Feed and Control Considerations for Congruent Phosphate Internal Treatment

Standard Day Tank Systems

Powdered Chemicals – For those without concerns of the added labor costs and time required for handling powdered sodium

Figure 6: HTP Trial Test – Demonstrated Iron Transport Improvement


phosphate chemicals, use of such chemicals is certainly the most economical from a chemical cost standpoint. In such a case, a day tank is equipped with a dissolving basket, mixer, and properly sized pump to deliver the made down solution to the steam drum’s chemical feed header. Operators should always log the amounts of each phosphate chemical, caustic and polymeric agent [if in the program] that is charged to the day tank along with time of day and the volume of the tank when charging and mixing began. The plant needs to decide how changes will be made based on the boiler test results – change the mixture recipe in the day tank while leaving the pump speed and stroke constant, or changing the pump settings. It is important that all shifts of operators follow the same procedure. The operators should log the day tank’s sight glass reading on a once every two-hour basis at a minimum. Manual changes and responses are made based on operator testing of boiler water for phosphate and pH. Normally this testing is done once per shift – every eight to twelve hours. While this type of system bears the lowest capital cost, it does not do provide the capability to react to system variations in a timely fashion.

Liquid Chemicals – Water treatment companies have the capability to blend sodium phosphate and caustic to provide liquid products at a given sodium to phosphate molar ratio. Within limits of solubility and compatibility, product formulations can be made to include polymeric dispersants. Due to the possible variation of sodium in the boiler feedwater, the use of one specific blend is a challenge. If there is a desire to use just one blend, then it is best to have a low sodium to phosphate blend and have the operators add the necessary amount of caustic to the day tank to yield the proper blend in the day tank for the current situation. That way, the caustic will be able to react to feedwater variation. In such a case, the equipment described above [other than the mixing basket] will be used for mixing and delivery of the chemical solution. Other than not handling

powders, the requirement and limitations of such a liquid day tank system is similar to that of powders.

To minimize the handling and possible errors involved in using one liquid product and caustic, proprietary blends of low and high sodium to phosphate ratio products are formulated. In such a system there are two internal treatment products, or a matched-pair. Each product will have the same amount of phosphate and polymeric agent [if included]. But each will be formulated to have a specific sodium to phosphate ratio -- one high and one low. That way the operator will not have to handle and measure out caustic at each makedown of chemical. The operator will add the required amounts of the two products to the day tank based on results of boiler water phosphate and pH measurements. Depending on product requirements, systems can be set up to deliver the liquid products from bulk containers into a measuring pot prior to the day tank. This minimizes operator’s exposure to chemicals. Such a liquid system should allow for less potential operator error as compared to powders and caustic or the one liquid product and caustic to the day tank. The capital cost of the day tank system would be approximately the same as that for powders. The matched pair product application should allow for less day tank batch-to- batch variation.

Automated Control of Congruent Phosphate/pH Treatment – Maintaining effective control of boiler water chemistry can be difficult. Some of the system variables that are changing are the steam rate, boiler cycles, blowdown flow, boiler feedwater sodium level, and dosing chemistry and rate. So with manual adjustment of blowdown, chemical makedown, and dosing rate, the operator will have to base changes on experience factor and best prediction. Inaccurate predictions can have a negative impact on the boiler chemistry.


With this in mind, GE Water & Process Technologies developed an automated system for control of congruent phosphate / pH systems. Most automated chemical feed systems utilize a controller with a typical proportional-integral-derivative (PID) algorithm to maintain a federate passed on a primary control variable and a secondary trim variable. But congruent control requires a more complex algorithm that accommodates the interdependent nature of the pH and phosphate as well as the dynamics of the boiler and the chemical feed network.

In the automated system described in Figure 7, two variables are used to control the chemical dosage: blowdown flow rate and boiler water pH. Knowing precisely the amount of phosphate fed and the blowdown flow rate, the phosphate concentration in the boiler is calculated by mass balance. A blowdown flowmeter is required, and the phosphate is fed based on blowdown. Typically, the blowdown valve will be fixed to optimize cycles, with changes occurring during startup, shutdown, or when in upset situations.

Figure 7: Schematic of Boiler Precision System

The continuous pH measurement of the blowdown uses feedback control that compares the on-line pH to the controller set point. Based on the deviation from the set point, the controller adjusts the feed ratio of two custom formulated products with different sodium to phosphate molar ratios.

To assure the needed accuracy of chemical feed, the feed system was re-engineered to allow for precise neat feed and verification by a patented Verifeed system. Reliable low-pressure metering pumps supply neat chemical to the automatic makedown module (AMM) that is kept flooded with high purity water. Note the controller output changes the dose rate of the chemical [high or low ratio product] to maintain the pH set point and the phosphate concentration via mass balance. From the AMM, the dilute chemical is fed via normal high-pressure pump to the steam drum. The HP pressure pump is normally set at a high delivery rate and left unchanged.

Since pH measurement is very critical to the control scheme, a sample conditioning station is mandatory as it incorporates precise control and verification of sample flow and temperature to maximize the reliability of the on-line pH measurement.

This automated system maintains the boiler water chemistry in control as it responds to changes in feedwater chemistry and steaming rates as they occur. The frequency of pH and phosphate testing can be reduced. The periodic testing of pH will serve to assist in the calibration of the on-line pH probe. Alarm systems are built in to indicate any system failures such as loss of sample flow; chemical feed pump failure, and loss of level in the make-down module. Communication with the plant’s DCS is possible. The computerized data acquisition allows for a review of historical operating data, resulting in easier system diagnostics and simplified troubleshooting.


Phosphate/pH Controller Field Trial Results

The following lists the pre-trial conditions: neat chemical feed to steam drum with a single blend of phosphate and polymer; boiler water pH and phosphate were manually measured once per eight hour shift; operators made adjustments to chemical feed and blowdown based on sample results and operating conditions. Due to the neat chemical feed, no continuous changes could be made in the sodium-to-phosphate ratio. Despite the complexity of periodic manual chemical feed and blowdown adjustments, the control was “in the box” most of the time prior to the installation of the controller. See Figure 8.

Figure 8: Boiler Chemistry Control Prior to System Installation

The controller system was installed as previously described such that two products that differed only in their ratio of sodium-to-phosphate could be fed to the automatic makedown module via low pressure metering pumps. The dilute chemical solution was pumped to the steam drum via high pressure pump. Boiler water pH was continuously measured and the blowdown flow was measured using a orifice plate flow meter. The metered blowdown results were verified by tracer studies.

To assure our trial system accuracy, a phosphate analyzer was installed to continuously measure boiler water phosphate residual. Besides

providing phosphate data more frequently, this trial tool allowed for a measure of the controller’s robustness in regard to responding to system upsets. Feedwater sodium concentration changes can have a marked affect on the boiler water chemistry. An on-line sodium analyzer was installed on the BFW sample to continuously measure this component. Data from the analyzer would define periods of upset such that the ability of the controller system’s response could be measured. These on-line analyzers are not used in standard installations of the controller system.

Figure 9 shows the chemistry results after the controller system was installed. For the trial period studied, the control was “inside the box” for the entire duration. The pH target of 9.9 was controlled with a precision of 0.04 pH units and the phosphate concentration target of 18.0 ppm was maintained within a precision of 0.5 ppm during the same time period.

Figure 9: Boiler Chemistry Control after System Installation

During the field trial, the ability of the controller to respond to changes in blowdown flow was assessed as follows: Blowdown flow was decreased via a 35% step change. The results, shown in Figures 10 and 11, show chemical feed was correctly ratioed during the flow change as reflected by the phosphate and pH results.


Figure 12 shows the ability of the controller to respond to an excursion in feedwater sodium content. During the trial there was pretreatment system upset that caused the sodium level in the BFW to rise from 50 to 170 ppb followed by a plateau of 100 ppb. During this period, the pH control of the boiler water was maintained in control (9.85 to 9.95).

Figure 10: Phosphate Control during Blowdown Flow Change

Figure 11: pH Control during Blowdown Flow Change

Figure 12: pH Control during Feedwater Sodium Excursions

Summary

The boiler precision control system provides very high levels of boiler chemistry control and reliability. Plant operating personnel uses their once per shift manual testing for confirmation of the on-line pH measurement and the output of the controller’s algorithm. The boiler chemistry control is “in the box” resulting in improved system reliability. The data acquisition system allows for improved diagnostics and system troubleshooting. Review of data regarding chemical dosage, boiler blowdown dynamics, and operating data allows for increased reliability and system improvement.

References

1. American Society of Mechanical Engineers. “Consensus on Operating Practices for the Control of Feedwater and Boiler Water Quality in Modern Industrial Boilers,” 1994

2. “Control of Iron Pick-up in Cycles Utilizing Steel Feedwater Heaters,” from Proceedings of the American Power Conference, Chicago, 1966.

3. Improved Water Treatment for High-Pressure Boilers, AIChE Ammonia Safety Symposium, November, 1991.


4. T. Purcell, S. Whirl, Trans. ASME 64, (1942) p. 397

5. Whirl, S. F.; Purcell, T.E.; “Protection Against Caustic Embrittlement by coordinated Phosphate pH Control,” International Water conference, p. 45 (Engineers’ Society of Western Pennsylvania, Pittsburgh, PA, 1945

6. H. Klein, Combustion, (1962), p. 1. 7. M. Ravich, L. Scherbakova, Izvest Sektora

Fiz, Khim, Anal., Inst Obschei Neorg. Khem, Akad Nauk SSSR 26, (1955), p. 248

8. V. Marcy, S. Halstead, Combustion, 1964, p. 45. Panson, et. Al., J. Electrochem. Soc. 122, 7 (1975), p. 915

9. Mayer, P., “Information Required for Boiler Tube Failure Investigation”, NACE Publication 7H290, 1990

10. Atwood, K.L., Hale, G.L., “A Method for Determining the Need for Chemical Cleaning of High Pressure Boilers”, American Power Conference, Chicago, IL, 19717

11. Selby, K.A., ET. AL., “Evaluation of Boiler Tube Deposit Weight Density Methodology”, NACE Corrosion 97, New Orleans, LA 1997

12. Strub, J.W., “The Use of Chordal Thermocouples for Monitoring the Thermal Resistance of Boiler Waterside Deposits”, American Power Conference, Chicago, IL, 1961


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Considerations for High Pressure Boiler Chemical Treatment ... · spray water quality are the same as those for steam purity. In some cases boiler feedwater is suitable. In all cases