How to Destroy a Boiler -- Part 3

9/22/13 How to Destroy a Boiler -- Part 3

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William L. Reeves, P.E. President, ESI, Inc. of Tennessee

Category : Incidents

Summary: The following article is a part of the National Board Technical Series. This article was originallypublished in the Fall 1999 National Board BULLETIN. (4 printed pages)

The third article of a three-part series describing some potentially catastrophic events that power and recoveryboilers are prone to if not properly maintained.

Editor's note: The following article covers the concluding topics of the most common ways to "destroy a boiler"which include improper blowdown techniques, improper storage, flame impingement, pulling a vacuum, andpreventive measures. Part one and part two of the series were printed in the Winter and Summer 1999BULLETINs.

Improper Blowdown Techniques

The concentration of undesirable solids in boiler water is reduced through proper feedwater treatment and theproper operation of a continuous purge ("blowdown") system, and by performing intermittent bottom blowdownson a regular basis.

The sodium zeolite water softening process is the predominant method of water treatment for boilers operatingat low pressures with saturated steam. In this ion exchange process, harmful scale-producing calcium andmagnesium ions are exchanged for sodium ions. The resultant water has a total dissolved solids concentrationequal to the previous combined total of sodium, magnesium, and calcium concentrations.

The main purpose of blowdowns is to maintain the solids concentration of the boiler water within certainacceptable limits. A blowdown system is shown in Figure 1. The blowdown rate can be determined by severalfactors which include total dissolved solids, suspended solids, silica, and alkalinity. Table 1 shows thesemaximum recommended concentration limits in the water of an operating boiler according to American BoilerManufacturers Association (ABMA).

TABLE 1

Drum OperatingPressure (psig)

Total DissolvedSolids (ppm)

TotalAlkalinity

(ppm)

Silica (ppmSi02)

Total SuspendedSolids (ppm)

0-300 3500 700 150 15

301-450 3000 600 90 10

451-600 2500 500 40 8

601-750 1000 200 30 3

751-900 750 150 20 2

901-1000 625 125 8 1

As the operating pressure increases, the limits become substantially more stringent, which can potentiallyrequire an extremely high blowdown rate if sodium zeolite softening is the feedwater treatment method. Tosubstantially lower the blowdown rate and control the concentration of silica, a total demineralized watertreatment system should be used. A demineralized water treatment system removes the anions and cationsinstead of substituting them for other ions. This results in very low blowdown rate requirements.

The continuous blowdown rate is set to control the boiler water within these ABMA-recommended acceptablelimits. A well-designed continuous blowdown system will constantly monitor boiler water conductivity (solidsconcentrations) and adjust the blowdown rate to maintain the control range. If the boiler water exceeds therecommended limits, potential problems can occur which include scale and sludge formation, corrosion, andmoisture carryover due to foaming and poor steam drum separation equipment performance. When this occurs,solids and silica are carried over in the steam. This results in silica and scale formation on the superheater andother process equipment, including steam turbine blading. This foaming phenomena associated with high



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other process equipment, including steam turbine blading. This foaming phenomena associated with highconductivity can also cause drum level instability leading to nuisance water level alarms and potential boilertrips.

Sometimes it is necessary to perform intermittent bottom blowdowns to dramatically reduce solidconcentrations in the boiler water. Also, intermittent bottom blowdowns of water wall headers and the muddrum are critical to remove potential sludge buildup to keep all water circuitry clear. Generally, the only bottomblowdown that can be performed while the unit is being fired is from the mud drum. The blowdown of lowerwater wall headers, particularly the furnace wall headers, should not be performed while the unit is being fired.This action could potentially result in water wall tube overheat damage due to the interruption of the boilersnatural circulation. The lower water wall headers should be routinely blown down every time the unit is broughtout of service after fuel firing has been halted and the unit is still under pressure. Care should be taken toperform a blowdown of a limited duration to maintain visibility of the boiler water level in the sight glass.Additional bottom blows can be performed once feedwater is added to raise the level back up in the sight glass.

The single biggest problem caused by poor blowdown practices is the failure to periodically blowdown the boilerwater columns. This results in sludge and debris buildup in the water columns, which renders the low watertrips inoperative. All well-designed boiler installations should include a push-button momentary low water tripoverride system located at the water column blowdown valves. This system allows the low water trip devices tobe blown down, thus cleaning the system and verifying that the low water trip alarm is activated without causingan actual boiler trip.

Improper Storage

Steam plant operations frequently require the long-term storage of boilers either used as standby units or unitsoperated only during maintenance periods. Attention to proper storage techniques can be critical to maintainingboiler longevity as a standby unit. The improper storage of a boiler can lead to corrosion on either the fire orwater sides.

Fireside corrosion damage often occurs on a boiler that is in cold standby and that has previously fired sulfur-laden fuels. There are inevitably areas of the boiler where ash is not removed from the tube surface duringnormal sootblower operation. One of the most vulnerable areas is the interface where the tubes enter the drumat tube-baffle interfaces and refractory-to-tube interfaces. When the boiler is hot, corrosion is generally not aproblem since moisture is not present; however, upon shutdown, this ash and refractory can absorb moistureand concentrated corrosive attack will occur over time in these areas. Localized pitting can be quite deep,rendering an otherwise sound tube in need of at least partial replacement.

When possible, store a standby boiler in a hot condition to prevent fireside corrosion of the tubes. Hot storagetechniques such as utilizing mud drum heaters or routing the blowdown from an operating boiler through theinactive unit is generally sufficient to keep the temperature of the boiler tubes above the acid dew point. Thesesame techniques for keeping a boiler hot are critical if a unit is required to quickly move from standby status tooperation in the event of another unit failure. Maintaining the boiler in hot standby will prevent problemsassociated with improper warm-up.

A boiler stored in hot standby with the unit full of water must be properly managed to prevent oxygen corrosionof the unit. The unit must be slowly brought down in rating while raising the water level as high in the gaugeglass as possible while still delivering export steam to the line. As the steam pressure stabilizes at the hotstandby pressure, make sure deaerated feedwater is introduced into the unit to maintain the water at the properlevel so that immediate firing can commence when necessary.

If a unit is in cold wet standby, follow the procedures above in bringing the unit out of service. The most effectiveway to control corrosion is to build up a sodium sulfite concentration of 100 ppm in the boiler water. A nitrogensource should be attached to the drum vent once the pressure is almost depleted. Pressurizing the boiler to 5psig with a nitrogen blanket system will ensure that oxygen will not be introduced into the boiler. Cold storageis recommended using the wet procedures above; however, if dry storage is necessary, make sure that amplequantities of desiccant are placed in the drums and that the waterside is closed up tight.

Flame Impingement

Flame impingement is a subtle problem that is primarily characteristic of high-capacity package boilers. Sinceshipping clearances dictate the design geometry of package boilers, the boilers naturally get long and narrowas size increases. This results in a significant challenge for burner manufacturers to shape the flame so that itis long and narrow, while simultaneously trying to stage combustion to mitigate NOx formation.

When the flame washes the furnace side walls, the result is potential corrosion on the tubes at the flameinterface, particularly if firing heavy oils with contaminants. The corrosion is accelerated due to high metaltemperatures associated with flame impingement and chemical deposits placed on the tubes resulting fromquenching the flame when it touches the tube wall. Water treatment problems can accentuate the problemsassociated with flame impingement because internal deposits at this localized high temperature zone areformed on the inside tube wall driving the tube operating temperature even higher.

Pulling A Vacuum

When boilers are designed to operate at very high pressures, they are not designed to operate under even theslightest vacuum. A potential vacuum is created when a boiler is shut down. As the unit cools, the steamcondenses and water level drops, which allows the pressure to drop.

If the steam drum vent is not open when the unit is cooling, a vacuum condition can result. A vacuum on aboiler can cause problems with leaks on rolled tube seats of generating tubes, which are designed for amechanical fit to withstand positive pressures.

Preventive Measures

In conclusion, some common practices that should be followed in order to avoid "destroying a boiler" include:



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Frequent observation of the burner flame to identify combustion problems early.Investigate the cause of any trip before numerous attempts to relight.Before lighting a boiler, always purge the furnace thoroughly. This is particularly important if oilhas spilled into the furnace. The purge will evacuate the inventory of unburned gases until theconcentration is below the explosive limits. If in doubt, purge, purge, purge!Verify that the water treatment system is operating properly, producing boiler feedwater ofsufficiently high quality for the temperatures and pressures involved. Although zero hardness isalways an absolute criteria, other water quality standards based on operating pressures andtemperatures as recommended by ABMA should be followed. Never use untreated water in aboiler.Blowdown all the dead legs of the low water trips, water column, etc., on a regular basis toprevent sludge buildup, which leads to device malfunction. Never under any circumstance disable

a low water trip.Verify that the water leaving the deaerator is free of oxygen, that the deaerator is operated at theproper pressure, and that the storage tank water is at saturation temperature. A continuous ventfrom the deaerator is necessary to allow the discharge of non-condensable gases.Continuously monitor the quality of condensate coming back from the process to enable thediversion of the condensate in the event of a catastrophic process equipment failure.Adjust continuous blowdown to maintain conductivity of the boiler water within required operatinglimits and operate the mud drum blowdown on a regular basis (consult your water treatmentspecialist). Never blowdown a furnace wall header while the boiler is operating.The boiler waterside should be inspected on a regular basis. If there are any signs of scaling orbuildup of solids on the tubes, water treatment adjustments should be made and the boilershould be mechanically or chemically cleaned.The deaerator internals should be inspected for corrosion on a regular basis. This is an importantsafety issue because a deaerator can rupture from corrosion damage. All the water in thedeaerator will immediately flash to steam in the event of a rupture, filling the boiler room withdeadly steam.The boiler's warm-up curve should be strictly followed. The standard warm-up curve for a typicalboiler is not to increase the boiler water temperature over 100F per hour. It is not unusual for acontinuous minimum fire to exceed this maximum warm-up rate. Consequently, during start-up,the burner must be intermittently fired to ensure that this rate is not exceeded.Make sure all personnel working on boilers understand that the thin tubes are quite fragile.Encourage workers to report any accidental damage so that it can be inspected and/or repairedas necessary.If production demands necessitate overfiring of the boiler, make periodic assessments of potentialeffects of overfiring and communicate these to management.When a boiler is shut down for an extended period of time, a nitrogen blanket system should beused to prevent the introduction of air and oxygen into the boiler during cooling and storage, andsodium sulfite should be injected to react with any free oxygen in the boiler water. When a boileris stored dry, desiccant should be placed in the boiler drums along with the nitrogen blanket toabsorb any free moisture.Always ensure that the steam drum vent valve is opened whenever the boiler pressure is lessthan 5 psig.

In conclusion, some common practices that should be followed in order to avoid "destroying a boiler" include:

In summary, a boiler is much like the human body. If properly cared for, it will give many years of reliableservice. It will often withstand abuse and keep on functioning. However, certain seemingly minor mistreatmentcan have catastrophic effects. You can impose a nasty cut on most parts of your body with little more thanminor discomfort. However, if the cut severs the carotid artery in your neck, it is fatal. A boiler as well has itsparticular vulnerabilities.

Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because ofadvances in material technology and/or actual experience. The reader is cautioned to refer to the latest editionand addenda of the ASME Boiler and Pressure Vessel Code for current requirements.

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How to Destroy a Boiler -- Part 3