tip 0416-06 keys to successful chemical cleaning of boilers · 2020. 1. 9. · chemical cleaning of...

TIP 0416-06 ISSUED – 2000

REVISED – 2004 REVISED – 2010 REVISED – 2018

2018 TAPPI The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published.

TIP Category: Automatically Periodically Reviewed (Five-year review)

TAPPI

Keys to successful chemical cleaning of boilers Scope This Technical Information Paper is designed to answer the question, “How do we avoid having failures in chemical cleaning of boilers?” It covers key points in assuring successful boiler chemical cleaning projects, but is not intended to be an exhaustive treatment of the subject of chemical cleaning. It is an overview, with references for those interested in pursuing the subject further. This TIP does not address how the decision is made as to when chemical cleaning is necessary. That is a major topic that would require a paper of its own, larger than this one, for adequate treatment. Keys to successful chemical cleaning projects can be divided into four major topics:

• Solvent selection • Engineering and execution of the project • Post acid cleaning inspection and flushing • Start-up/passivation

Once a solvent is selected, boiler preparations must be made in order to accommodate: • Safety • Temporary piping for filling and, draining and venting the boiler. • Boiler venting, filling and draining. • Permanent instrument protection. • Solvent agitation and uniform distribution within the boiler. • Protection of the superheater and sweetwater condenser. • Solvent heating. • Solvent sampling. • Monitoring of solvent level in the steam drum. • Solvent disposal • Inspection and flushing

- Scaffolding - Hoses - Special tools, videoprobe, digital camera, etc. - Manpower needs

• Start-up/passivation Safety precautions Personnel safety and industrial hygiene should be ensured by complying with established mill safety, lockout and confined space entry procedures. This TIP may require use and disposal of chemicals which may present serious flammability and health hazards to humans. Procedures for the handling such substances shall be as set forth in the Material Safety Data Sheets which must be provided by the chemical manufacturer or vendor. Prior to using inspection procedures in this

TIP 0416-06 Keys to successful chemical cleaning of boilers / 2

TIP, the user should determine which chemicals to be used are potentially hazardous in order to strictly follow the procedures specified in the MSDS and to meet local, state and federal requirements for safe use and disposal of these materials. Personnel protection must be integrated into the project engineering. The following steps should be taken:

• Barricading of the boiler, manifold area, injection hoses, and temporary equipment in order to prevent nonparticipants from entering areas where they might be exposed to the chemicals.

• No welding or torch cutting is to be performed on the boiler during the cleaning process, nor in the immediate area because chemical cleaning usually generates flammable hydrogen gas.

• Personal protective equipment including chemical proof clothing, goggles, and respirators should be worn when entering the barricaded areas when chemical exposure is a risk.

• Hydrogen sulfide monitors should be in place. • Escape routes should be planned and communicated. • Adjacent operating areas should be informed of the operation and the associated hazards. • First aid and eye wash facilities should be immediately accessible. These should be tested prior to

starting the chemical cleaning project to ensure good operation. • A safety meeting, with both plant and cleaning contractor personnel attending, should be held before the

project starts. • Check valves should be on all water and steam supply lines. • A spill response plan should be developed in case of leaks or accidental spills of chemicals.

Definitions Beaker tests – Laboratory tests to determine the solubility of the deposit in various cleaning solutions. These tests are done at the same conditions (temperature, concentration, etc.) as may be used on the cleaning project to determine the best solvent. These beaker laboratory tests should be performed well before the actual chemical cleaning of the boiler. Chemical cleaning – the removal of deposits by dissolving or disintegrating the deposits in chemical solutions. Corrosion inhibitors – Substances added to acids and other corrosive liquids to mitigate corrosion of the equipment materials of construction that occurs during cleaning. Deposit – Unwanted solid matter, which separates out of fluids in a process stream, coating equipment surfaces and interfering with process efficiency. Deposit weight density (DWD) – The weight of deposit material per equipment surface area unit. It is generally expressed as milligrams per square centimeter (mg/cm2) or as grams per square foot (g/ft2). Neutralization – The elevation of the pH of the cleaned metal surfaces to above 7.0 in order to prevent acid corrosion during post cleaning activities including operation. Passivation – The formation of a thin, nonporous layer of magnetic iron oxide on the surface of the cleaned metal in order to prevent rusting before startup. Solvent – Any liquid, aqueous or non-aqueous, used to chemically clean equipment. Introduction Waterside deposits are usually not evenly distributed in boiler tubes. High heat flux areas of boilers tend to have heavier deposit buildups. Deposit composition varies, and is a key factor to consider when chemically cleaning a boiler. Deposits act as insulating layers, reducing heat transfer and raising tube metal temperature unevenly. A light, fluffy, insulating organic deposit may be even worse than a heavier amount of a dense adhering deposit that conducts heat better. Areas where metal temperature becomes excessive experience significant loss of yield strength, and they are frequently the sites of tube failures. Several waterside corrosion mechanisms also contribute to tube

3 / Keys to successful chemical cleaning of boilers TIP 0416-06

failures (1). Cleaning to remove the waterside deposits will improve heat transfer, and can mitigate some of the corrosion mechanisms. Mechanical methods of cleaning boiler tubes include “rattling the tubes”, sending scrapers through the tubes, or high-pressure water jetting. Chemical cleaning of boilers is generally favored over mechanical methods because of complete removal of deposits from the entire boiler and shorter cleaning times. Paper mills have sometimes experienced failure to adequately remove the deposits when chemical cleaning their boilers. Such failures can generally be avoided by detailed execution of good procedures. Chemical cleaning uses detergents or high temperature alkalis to remove organic deposits such as oil, grease, tar or carbon. Inorganic deposits (minerals) are removed by low pH cleaners such as acids, or by chelants. Chelant cleaning solutions can have low, neutral or high pH. Acids tend to work faster than chelants, but can be more corrosive to the metal equipment if not properly inhibited. Solvent selection Sampling Proper selection of the solvent system to clean a boiler requires laboratory analysis of a recent sample of the deposit. Great care should be exercised in taking the sample and in shipping it properly, since the analysis is essential to the proper selection of a solvent for the project. A guide to solvent choices typically used for various deposit compositions is shown in Table 1. The best sample is a tube cut from the water wall at an area that historically had the heaviest deposit, usually the high heat flux area of the boiler. At times, it is known that the boiler is being cleaned because of known high deposits in the generating bank section. During these occasions, a tube sample from the generating bank is the best representative sample to be taken, otherwise; generating bank tubes, economizer tubes or superheater tube sections should not be used as representative location of boiler deposits. Similarly, many powerhouse superintendents are reluctant to remove composite tube sections from the high heat release zone of the boiler, but not doing so leaves you at risk for not selecting a representative sample of high deposit weight density areas and potentially a poor chemical cleaning of the boiler. Several tube cuts should be taken if uncertainty exists as to where to sample. Deposit samples chipped off of the metal surface may suffice in place of a tube cut if no tube section is available. All deposit layers must be included in the sample. Tube sections generally are better for six reasons:

• The DWD can be determined. • A tube section allows determination of whether the deposit can be removed from the metal surface even

though it may not all be dissolved. • A cross section of the deposit on the metal can reveal layers that may be missed in a loose scale sample. • In solubility tests, loose deposits in the beaker may react differently with the solvent than the deposit on a

metal surface, such as a boiler tube. • The surface of the cleaned tube section can be examined. • The removed tube section allows waterside access for videoprobe inspection of other areas of the tube.

All tube sections and deposit samples should be labeled with clear distinguishing descriptions to prevent confusion, including the hot and cold sides of the tube sample and orientation of the tube sample (flow direction). Labeling on exteriors of tube sections should be in paint that will not rub off. Metal stamping should not be used, as the pounding can knock some of the deposit loose and it may be lost. The ends of the tube sections should be sealed to avoid sample loss in transit and to avoid ash from the outside of the tube contaminating the inside. A tube section being shipped should be padded and packaged in a hard case, such as a wooden box or a plastic pipe with hard caps. Cardboard or paper packages are often cut through in transit by the sharp edges of the heavy tube sections. If a current tube cut cannot be obtained, a sample from the most recent opening of the boiler can be submitted for analysis. Historical information from past cleanings is also helpful, but both of these approaches may fail to account for later episodes of contamination of the boiler water that can cause layers of deposit which may be more difficult to remove. Deposit samples taken from drums provide only limited information about the deposits in boiler tubes. Such limited information can be misleading, and should not be relied upon as the sole basis for solvent selection. Laboratories usually require minimum sample sizes. A typical minimum size for a loose deposit sample is 28.3 g (one ounce). When selecting a tube sample, ASTM D 3483 specifies a minimum tube length of 0.6 meter (2 feet) for removal by dry sawing or grinding. For torch cut tube sections, NACE recommends a minimum tube length of 0.9 meter (3 feet). The longer length requirement when the tube section is taken using a cutting torch is because


the ends are contaminated by slag and will be cut off and discarded by the lab. Tube sections from tubes that failed should not be used since the violence of the tube blowout will have removed some of the deposit. Some boiler owners prefer to cut a long tube section into shorter ones to send to different labs for analysis. Typical would be three sections, one to the chemical cleaning contractor, a second to the water treatment company, and the third to an in-house lab or to an outside engineering consulting company. Even though the sections may be adjacent cuts from the same tube, there will sometimes be significant differences in the reports. This is due to differences in uneven deposits along the length of the tube, and to differences in test procedures used by the laboratories. All the results should be taken into account for an accurate picture of the deposit unless one is obviously out of line with the others. The lab report When the tube samples are sent to the lab, a clear understanding should exist about what will be reported. Allow two to four weeks for normal handling and reporting. At a minimum, the lab report(s) should provide the following information for each tube section:

• DWD. • Chemical composition of the deposit. • Results of solubility tests. • Descriptions of unusual observations or appearances, including photos.

Deposit weight density. There are several methods for determining DWD (2, 3). Each tends to give different results. The most commonly used methods are:

• Shot blasting with glass beads, • Scraping with brushes and spatulas or a metal scriber, and • Chemical method for deposit removal.

Each method has its advantages and disadvantages. Extensive testing by NACE found that glass bead blasting and mechanical scraping gave comparable deposit load results. Scraping tends to contaminate the sample with metal. Glass bead blasting tends to contaminate the sample with silica. Chemical deposit removal also removes some of the base metal by corrosion. Specifying the method to be used will reduce, but not eliminate, some of the variation in numbers reported. Chemical composition of the deposit. There are several methods used to identify the composition of the deposit. In one method, the sample is digested in a boiling acid. This is usually followed by spectrometer analysis of the solution. Results of solubility tests. Beaker tests are usually sufficient to indicate which solvents will work in cleaning the boiler, and which is best. Low pH cleaning is often done around 140 to 160oF (60 to 70oC). High pH cleaners are frequently applied at 180 to 200oF (80 to 95oC).


Table 1. Choices of solvents versus boiler waterside deposits and metalsa

Deposits

Commonly used solvents

Normal temp.

oC (oF)

Construction material

incompatibilities

Magnetite, Fe3O4

Hydrochloric acid, HCl

60 - 71 [140 – 160]

Austenitic stainless steels, Incoloy 800, soft metals (i.e., aluminum, zinc, tin)

Magnetite, Fe3O4

Ammonium bifluoride (ABF additive), NH4F.HF, or formic + citric

Depends on acid choice

Titanium, zirconium, stainless steels, Incoloy 800, soft metals

Magnetite, Fe3O4

Chelants: EDTA salts, citric acid salts; Mixed organic acids: formic + hydroxyacetic

65 - 150 [150 – 300] 75 - 95 [170 – 200]

Soft metals

Copper oxide, CuO

HCl + thiourea

60 - 71 [140 – 160]

Copper alloys, austenitic stainless steels, Incoloy 800, soft metals

Copper metal + oxide

Buffered aqua ammonia + oxidizer; Ammoniated citric or EDTA salts + oxidizer

50 - 65 [120 – 150]

Copper alloys, Soft metals

Water scale, mainly Calcium carbonate, CaCO3

Hydrochloric acid, HCl

50 - 65 [120 – 150]

Austenitic stainless steels, Incoloy 800, soft metals

Water scale, mainly Calcium carbonate, CaCO3

Chelants: EDTA salts Weak acids: formic, sulfamic

65 - 150 [150 – 300] 50 - 80 [120 – 180]

Soft metals

Carbon, C

A separate stage of Sodium hydroxide + potassium permanganate solution.

95 [200] Minimum

Soft metals

Silica, SiO2, or high silica water scale

HCl + ABF

50 - 65 [120 – 150]

Titanium, zirconium, stainless steels, Incoloy 800, soft metals

Calcium sulfate, CaSO4

Proprietary gypsum solvent followed by acid stage.

Varies widely

Depends on choice of acid.

aAll acids and chelants should be used with corrosion inhibitor additives. All solvents indicated are used as dilute solutions in water. Concentrations of solvents, solution temperatures and other parameters will vary, depending on the specific solvent used, DWD, deposit composition, and other factors.


Corrosion inhibitors There are many corrosion inhibitors for acids and chelants on the market. They are generally complex proprietary mixtures. The chemical cleaning contractor or consultant will often recommend an inhibitor suitable for the project at hand. The owner may want to request supporting corrosion test data from the party making the inhibitor recommendation. Actual corrosion rates during a project run somewhat higher than the laboratory data due to salts in solution from the deposit removal and other non-ideal conditions. Therefore, the lab data should be used for inhibitor and solvent comparison purposes only, and not for writing specifications for the corrosion rates during the cleaning project. Corrosion rates during a chemical cleaning project are sometimes measured by securing metal test specimens in the boiler. Neutralization and passivation When an acid solution is used, after the cleaning solvent is drained from the boiler, it should be refilled with a neutralization solution, and then passivated. When chelants are used, neutralization and passivation are commonly done using the same solution (one volume). This reduces the amount of liquid waste created by the project. Neutralization usually involves filling the boiler with a high pH solution of soda ash, caustic or ammonia. If a chelant solvent is used for deposit removal and the pH of the chelant is below 9.0, then the neutralizing chemical may be added directly to the solvent in the boiler with proper agitation to ensure adequate mixing. Passivation is generally achieved by:

• Dissolving/injecting an oxidant such as air, oxygen, or sodium nitrite into an alkaline solution on into a neutralized chelant solvent and allowing the resulting solution to remain in the boiler for a specified minimum amount of time at proper temperature, or

• Firing the boiler to a minimum of 150 psi (10.5 Kg/Cm2) while filled with a soda ash or caustic solution, and maintaining this pressure for a specified minimum amount of time.

Common reasons for failures Even with laboratory testing, good engineering and execution, there are still a few cases where the chemical cleaning procedure fails to remove all of the deposit. Some reasons that have been identified as causing such failures are:

• The sample was not representative of the deposit in the boiler. This is often because of one or more of the following: the boiler deposit had grown thicker since the tube sample had been taken; the composition of the tube sample was not representative of the composition of the deposits in the boiler; or the sample may have been taken from a low deposit area of the boiler.

• Layers in the deposits were not reported by the laboratories. Oil leaking into the boiler water system may result in a carbon layer in the deposit. Incursions of untreated plant water or black liquor may leave layers high in organics or hardness. Laboratory solubility tests may fail to detect this as a problem when the solvent eats out under a layer from the edges of the tube piece in the beaker and the insoluble material sloughs off the metal. In the boiler, the solvent mostly removes the deposit from only the waterside, stopping when it reaches a layer it cannot dissolve.

• The solvent did not circulate into all areas of the boiler. See the engineering discussion that follows. • The treatment procedure performed on the boiler was different than indicated by the lab report. For

various reasons, some cleaning projects are done under different conditions than were used in the successful laboratory solubility tests. If different cleaning parameters are proposed than indicated by the lab report, additional laboratory solubility tests should be done to indicate whether the deviation would work in cleaning the boiler.

Laboratory analysis of tube sections taken after a failed chemical cleaning are sometimes helpful in identifying the causes, so that the boiler can be re-cleaned successfully, or so the problem may be avoided next time.


Engineering the project A well-planned cleaning job is likely to be a successful job and an inadequately planned job is likely to result in inadequate cleaning, equipment damage or other problems. Goals of the well-planned job include:

• The project must be completed safely, without injury to personnel or damage to equipment. • Control of concentration of solvents and inhibitors throughout the entire boiler must be maintained

throughout the process. • The system must be provided for rapid fill and drain of the unit to assure that solvent contact time and

concentration is as homogeneous as possible. • Boiler metal temperature and solvent temperature must be controlled and as consistent as possible

throughout the unit to assure uniform cleaning and inhibitor effectiveness. • Project design must provide for compliance with all environmental requirements. • The project design should have a spill response plan developed in case of leaks or accidental spills of

chemicals. • Adequate, pre-chosen inspection points to verify cleaning success.

Filling and draining It is critically important that all wetted parts of the unit be at a safe temperature before filling begins. Thick-walled drums and headers can raise solvent temperature locally to unacceptable levels if they are not adequately cooled, before starting the cleaning process. Filling and draining systems should be properly sized in order to allow filling and draining to be rapidly performed. Filling should usually be done in less than an hour and draining should take not more than an hour and a half. Rapid boiler filling and draining reduces overall base metal loss by reducing unnecessary amounts of time a corrosive solvent contacts the boiler metal. Rapid filling and draining also results in a shorter outage schedule thus minimizing lost production. Another benefit in providing for rapid draining of the boiler would be to enable some sludge removal from lower headers and tubes as the boiler is quickly drained. Shorter drain times also help to prevent settling of larger suspended particles in the cleaning solution and rinse fluids. Typically, boilers are blend filled with both hot water and the inhibited acid from the bottom to assure good mixing and to also ensure that all the wetted surfaces of the boiler are filled with the proper acid concentration at the time of filling. There are some methods of cleaning where the boiler is filled to slightly below the desired level, ensuring that the metal temperatures are correct and then the properly inhibited acid is then injected into the boiler. This method requires that there is some type of circulation in the entire boiler to assure that the inhibited acid reaches all wetted surfaces that are to be chemically cleaned. A typical filling and draining system is shown in Fig.1. This system, incorporating a chemical-blending manifold, is used for the simultaneous injection and heating of a solvent as well as for the proper dilution of solvent concentrates. This system also provides important control of filling temperature, pressure, and flow. For safety, welded carbon steel or stainless steel piping and components should be used. The initial cost of piping may be greater than hose, but piping will prove to be less costly over the life of the boiler if the boiler is cleaned routinely on a 5-7 year basis. Also, welded piping greatly reduces the possibility of leaks. The temporary piping used for all of the chemical cleaning process should be hydroed prior to starting the chemical cleaning of the boiler to ensure there will be no leaks once the cleaning process has started. The manifold is connected to pressurized hot condensate or feed water supply for heating of the liquid [to temperatures up to 200oF (95oC)] as the boiler is filled with the solvent. In order to prevent overheating of the solvent and also to supply proper dilution of the solvent concentrate, pressurized cool condensate or treated water is also piped to the manifold.


Fig 1. Filling and draining manifold Flow meters are used in order to monitor total flow and to quickly obtain the proper dilution of the solvent concentrate. A sample point is installed for sampling the solvent for active component concentration analysis. Distribution of the solvent during filling from the manifold is by separate lines to the lower water wall headers, lower screen header, downcomers, and economizer inlet header. A good feature is to have a totalizer incorporated into the flow meter. This helps when filling and draining the boiler to know when the total amount of water/chemicals are either in the boiler or almost completely drained out of the boiler. The manifold also allows for used solvent draining via piping leading to an approved sewer or temporary solvent storage tanks. If used solvent neutralization is required, 50% liquid caustic may be injected (metered) into the chemical concentrate injection nozzle while the used solvent is draining from the boiler. Prior to filling the boiler with a low pH solvent containing an inhibitor, a test should always be used to assure that the acid is adequately inhibited. Several test methods exist. Most of them depend on observation of hydrogen generation when ferrous metal is immersed in a sample of the acid. The ferrous metal used varies from steel wool to coupons or even iron filings. A test method is subjective unless the procedure measures the amount of hydrogen generated or of metal lost in a specified time interval under specified conditions and sets a limit for that amount. A test failure should mean that the acid is not to be injected into the boiler until more inhibitor is mixed with the acid, and a new sample passes the test. Solvent agitation After the boiler is filled with the solvent at proper concentration and temperature, agitation of the solvent is usually required in order to maintain uniform heat distribution, to increase the dissolution rate of the deposit and to expose freshly cleaned tube deposit surfaces to the cleaning solvent to be effective in complete deposit removal.


The means of solvent agitation include:

• Circulation by external pump. • Circulation by gas lift with nitrogen or air. • Draining a portion of the solvent from the boiler to an external tank and pumping the drained solvent

back into the boiler. Circulation by external pump. Circulation yields the best temperature uniformity and control of flow. Fig. 2 illustrates a typical scheme. The circulation path normally consists of pulling suction from the lower water wall headers and lower screen header(s) and discharging into the economizer drain. This method’s disadvantage is higher piping costs. For aggressive mineral acids, sometimes circulation is intermittent to minimize corrosion. For weak organic acids and chelants, continuous circulation is acceptable from a corrosion standpoint, and is often necessary to assure good deposit removal.

Fig 2. Solvent injection and circulation Circulation by gas lift with nitrogen. Gas lift is another method that also yields good temperature and solvent concentration uniformity. Using the Darcy Equation (4) as a basis, the gas flow to yield the required circulation rate can be calculated. The gas is injected into the down-comers or lower headers which creates lift of the solvent. The solvent rises and flows into the steam drum. From the steam drum the solvent flows into the upper water wall headers and down the water walls to the lower headers. When the flow reaches the point of gas injection, the circuit is completed. The disadvantage of this method is the exact flow rate of the solvent cannot be observed. If proper venting is not available for exhausting the gas from the boiler, only low flow of the solvent can be obtained.


Thermal circulation by firing and cooling the boiler. Thermal circulation is obtained by heating the solvent with the boiler igniters to a specific solvent temperature followed by the immediate cooling of the solvent to a specific temperature by forcing outside air through the boiler using the boiler forced draft and induced draft fans. This method can be very effective and usually does not require an elaborate filling manifold. However, some boilers are not conducive to this method. Also, this method should only be used for the medium to high pH chelant solvents and alkaline degreasing or neutralization solutions. It should never be used for low pH (acid) solutions. Great care should be exercised to prevent overheating of the solvent. Solvent temperature should be monitored with thermocouples installed at strategic points of the boiler. Draining a portion of the solvent from the boiler to an external tank and pumping the drained solvent back into the boiler. Draining and pumping back into the boiler is a relatively inexpensive method of solvent agitation and may be used when unusual circumstances prevail. This method has the inherent disadvantages of:

• Intermittent contact of the upper headers and connecting wall tubes of some boilers by the solvent. • Cooling of the solvent during the draining and pumping if an external heat exchanger is not utilized. • Non-uniform mixing of the solvent in the boiler resulting in concentration differences in the solvent.

Solvent level indication within the steam drum Monitoring of the solvent level in the steam drum is very important to insure that the liquid level is high enough that the upper wall headers and riser relief tubes are filled. Level monitoring is also important to ensure the solvent does not completely fill the steam drum and exit the vents, relief valves or enter the superheater or sweetwater condenser. Solvent steam drum level monitoring is done by use of a temporary sight gauge. Use of the permanent level indicator could result in damage to this instrument and this device(s) should be valved out during the entire cleaning process. An example of a temporary sight gauge for atmospheric pressure and temperatures up to 93° C (200° F) is illustrated by Fig. 3. This gauge is constructed of 1.3 cm (1/2 in.) carbon steel pipe taps, stainless steel ball valves, and 2.5 cm (1 in.) OD × 0.32 cm (1/8 in.) wall thickness flexible polymeric tubing. The horizontal carbon steel lines to/from the drum (particularly on the bottom leg) are sometimes specified to be a larger diameter (e.g., 2.5 cm or 1-inch) to reduce the risk of plugging. A vent and sample valve is included. This gauge is very effective and accurate. However, caution is generally advised against connecting the upper piping to a vent being used to vent the boiler for the cleaning. This will cause a false high level indication which will lead the observer during boiler filling to believe the level in the drum is higher than it actually is. The magnitude of the error in drum level indication can be very large. If there is a high gas velocity out the vent (e.g., due to either rapid fill rate or gas induced circulation) a plume of liquid has been rapidly drawn up the sight glass and out the drum vent due to the eductor effect. For high temperature solvents, >93o C, 200oF, the temporary sight gauge should be obtained from a supplier that specializes in high temperature, high pressure gauges. Superheater protection Superheater protection is of utmost importance since allowing solvent to spill into the superheater can result in permanent damage. As a minimum, the superheater should be filled with high quality water containing an oxygen scavenger. Although this will not prevent solvent from entering the superheater, it will provide some protection by diluting any solvent that may enter the superheater from the steam drum. If solvent enters the superheater, immediately backfilling the superheater with high quality water from the superheater outlet to the steam drum will likely flush the solvent from the superheater. To fill the superheater with water, first fill the boiler with water to an indicated level in the steam drum. Then backfill the superheater with high quality water and an oxygen scavenger just upstream of the non-return valve. The water will flow through the superheater to the steam drum. Indication that water is flowing into the steam drum from the superheater is by noting a rise in the level of the water in the steam drum as indicated by the temporary sight gauge. A well-designed and constructed temporary steam drum level gauge that is constantly monitored by a person with good communications with the cleaning crew will greatly reduce the possibility of superheater contamination. Added protection of the superheater can be achieved by installing plugs at the steam drum in the superheater inlet header supply-lines, or in individual inlet tubes.


Fig 3. Typical temporary steam drum level gauge

Solvent sampling Samples of the solvent should be collected on a periodic basis in order to determine the progress of the cleaning. Hourly sampling is adequate for most high pH chelant projects, while 30-minute sampling intervals are usually used for low pH cleaning solutions. When a deposit dissolves in the solvent, metal ions of the deposit are released into the solvent. The concentrations of these ions will continuously increase until all of the deposit is dissolved or the solvent is spent. If the solvent spends, additional acid or chelant is added, or the boiler is drained and filled with fresh solvent. The time for solvent contact termination is indicated when the concentration of the ions stabilizes and there is adequate free solvent. Therefore, analysis of the solvent should be performed for dissolved ions and solvent concentration while the solvent is in the boiler. Properly placed sampling points should be provided in order that samples of the solvent may be collected. If the solvent is being circulated, sampling the solvent from the steam drum will be adequate due to the uniformity of the solvent within the boiler. This sample is to be collected from the drain on the bottom of the steam drum temporary sight gauge. (When using high temperature solvent [greater than 93°C (200°F)], sample coolers should be installed at the sample points.) Solvent samples may also be collected from the temporary piping if external circulation is employed. For the case where the solvent is drained to a tank and pumped back into the boiler, samples should be collected from the steam drum and from the solvent drained to the tank. This is because the concentrations of


components within the solvent at the top and bottom of the boiler may be non-uniform resulting in a termination indication at one sampling point that is not indicated at the other sample point. Inspection A complete chemical cleaning of the boiler is not finished until the final inspection is done. Regardless of; the cleaning method used - either solvent agitation or circulation - there will be left over sludge in the boiler. The sludge may contain trace acid residual which can lead to very localized corrosive conditions if left in the boiler upon start-up. Acid becomes aggressive when heated and will attack the freshly cleaned watersides of the boiler. At a minimum; both the mud and steam drums need to be opened and inspected, the generating bank tubes should be video probed for sludge accumulation, the lower water wall headers and lower screen tube headers should be opened and inspected for sludge accumulation. Sludge found in the steam and mud drums should be shoveled or vacuumed out and not flushed down the downcomers to accumulate in lower areas of the boiler. If there are large amounts of sludge present in the generating bank tubes, lower water wall headers or the lower screen tube headers then additional handholes may need to be removed for inspection. NOTE: Some newer boilers have nipples and pipe caps in lieu of traditional handhole caps for header access and inspection. Only nipples and caps that are oriented in the downward direction should be removed for header access and inspection. The post cleaning inspection should include an inspection of the watersides of the boiler by use of a video probe. There should be a long (~200 ft) video probe available for inspection of the generating bank tubes, screen tubes and wall tubes. Flushing All flushing of boiler circuits should be done with demineralized water or better quality. Many mills flush all circuits of the boiler from the steam drums and mud drums (depending on design) after a chemical clean regardless of the conditions of the lower screen tube headers, lower water wall headers, mud or steam drums. They have found that the additional time spent in flushing all circuits is time well spent to assure themselves that they have done all they can to assure that there is no remaining sludge in the boiler. A good practice is to video probe the same tubes both before and after flushing to compare the results of the flushing. Start-up/passivation The last phase of the chemical clean of the boiler is to re-form a magnetite layer on the freshly cleaned internal water surfaces of the boiler. This phase is commonly called passivation. Magnetite formation is important to protect the tubes from corrosion or attack. This is done by filling the boiler with the proper chemical dosage as prescribed by the acid clean consultant or mill water treatment consultant and properly conditioned feedwater prior to firing the boiler. The dispersant dosage for start up may be different from the regular chemical dosage to remove suspended solids such as iron from the boiler. Another method to remove iron and suspended solids is to aggressively blow down the boiler during the warm up and start up process. The water treatment consultant or chemical cleaning consultant can assist in developing the chemical dosages and blowdown procedures. The two commonly used passivation processes include a fired passivation procedure and a no-fire passivation at lower temperatures. The boiler should not be placed into the main process header until boiler water iron residuals have dropped and stabilized below a previously agreed upon number by the chemical cleaning consultant, water treatment consultant and the mill operating personnel. At the appropriate time the permanent steam drum instrumentation will need to be valved back in during the start-up/passivation process. The acid cleaning consultant should provide the process and procedures to re-establish these instruments. Other prerequisites In addition to mechanical preparation, other prerequisites are important:

• A procedure covering each aspect of the project. The entire chemical cleaning procedure should be in place and be understood by personnel involved in the project. The procedure should be flexible in order to accommodate unusual occurrences. It should include practical contingency plans for upset conditions.


• A schedule allowing sufficient time for the project. The schedule should be developed well before the project begins. However, means of keeping the project schedule as brief as possible should be examined and, if technically and mechanically sound, implemented.

• Personnel. Personnel involved should have chemical cleaning experience, or at least be trained and certain of their duties. Both the cleaning contractor and the owner should provide around-the-clock coverage by experienced supervisors especially when the cleaning chemicals are in the boiler.

• Temporary equipment. All equipment should be proven and mechanically capable of performing its designed function. Also, all temporary equipment must be inspected and cleaned of all foreign material before it is made part of the cleaning project.

• Hydro-testing. All piping, hoses and the boiler should be hydro tested, and leaks repaired or faulty hoses replaced, prior to solvent injection.

• An environmental plan. The plan should cover disposal of used solvents and accidental spills or leaks. • Spill plan. The spill plan should include what temporary barriers need to be installed in case of leaks or

accidental spills of chemicals. • Header caps. These need to be taken off for inspection and for removal of sludge. They should be

identified and their removal and re-installation scheduled, with adequate time allowed in the planning. For boilers with nipples and caps oriented in the downward position, theses will have to be removed and flushed out to be certain there are no residual cleaning chemicals remaining in them.

• Personnel briefing. All personnel, including those not involved in the project but in the area or in adjacent areas, should be fully briefed on the associated hazards. A loss prevention plan should be implemented and understood by all personnel.

• Post-Cleaning Flushing. Thorough flushing of all riser release tubes, generation tubes, screen tubes, downcomers, and upper and lower headers should be done following the project in order to remove sludge from the boiler. Otherwise, sludge could adhere to cleaned tubes resulting in corrosion.

• Inspection. Flushing should be followed by a thorough inspection. There have been chemical cleaning projects that left large deposits of thick scale and sludge loose in lower headers.

Conclusions The keys to the successful chemical cleaning of a boiler are:

• Detailed boiler deposit analysis and solvent testing. • Proper procedure development. • Loss prevention of both personnel and equipment. • Environmental control. • Mechanical preparation identification and implementation. • Trained and experienced personnel with process understanding. • Solvent analysis, monitoring, and control. • Properly designed temporary equipment. • Inspection and flushing. • Start-up/passivation. • Solvent disposal plan.

With proper attention to these details, an unsuccessful chemical cleaning project will be a rare and unusual event.


Literature cited 1. Herbert H. Uhlig, The Corrosion Handbook (1948), John Wiley & Sons, Inc., New York, NY, p. 520-537. 2. ASTM Designation: D 3483 – 83, Standard Test Methods for Accumulated Deposition in A Steam Generator

Tube, Annual Book of ASTM Standards, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103.

3. George Totura and Tom Spry, Interpretation of Deposit Weight Density Analytical Results As A Measure Of Boiler Tube Cleanliness, TAPPI Fall Technical Conference – San Diego – CA, September 2002.

4. The Crane Engineering Division, Technical Paper No. 410, Flow of Fluids Through Valves, Fittings, and Pipes (1969), Crane Co., 4100 S. Kedzie Ave., Chicago, IL 60632, Chapter 1, p. 1-7, Chapter 3, p. 2-3.

References TPC Publication No. 8, Industrial Cleaning Manual (1982), National Association of Corrosion Engineers, 1440

South Creek Dr., Houston, TX 77084. McCoy, James W., Industrial Chemical Cleaning (1984), Chemical Publishing Co., Inc., New York, NY. Gutzeit, Joerg, MTI Publication No. 51, Cleaning of Process Equipment and Piping (1997), Materials Technology

Institute of the Chemical Process Industries, Inc., 1215 Fern Ridge Parkway, Suite 116, St. Louis, MO 63141-4401.

TPC Publication No. 6, A Bibliography on Chemical Cleaning of Metal, Volumes 1, 2, and 3, NACE International Membership Services Department, P.O. Box 218340, Houston, TX 77218-8340.

Standard Test Method 0193-93, Laboratory Corrosion Testing of Metals in Static Chemical Cleaning Solutions at Temperatures below 93°C (200°F) (1994), NACE International Membership Services Department, P.O. Box 218340, Houston, TX 77218-8340.

ASTM Designation: G 4 - 95, Conducting Corrosion Coupon Tests in Field Applications, Annual Book of ASTM Standards, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103.

Keywords Boilers, Cleaning, Cleansers, Corrosion inhibitors, Tubes, Deposits Additional information

Effective date of issue: August 29, 2018. Working Group Members: James Graham – Chairman, ChemTreat, Inc. Robert Bartholomew, Sheppard T Powell Associates, LLC Wayne Bucher, WB Consulting Mike Bayse, George Bodman, Inc. Susan Childress, International Paper Norris N. Johnston, Water Wizard Consulting

Acknowledgment Thanks to H. Dewey Johnson of HydroChem Industrial Services, Inc. Dewey was co-author of the original paper on which this TIP is based, and he made contributions to the writing of the TIP.

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