More TOC (> 1 ppm) in feed water will contribute to lowering
pH due to formation weak acids (organic) due to decomposition of
TOC at 350 deg. C.
Boiler pHNatural water is usually between 6.5 and 7.5 pH. A
common recommendation is to maintain boiler water at 8.5 pH.Acidic
water is corrosive. Alkalinic water is more prone to
scaling.Alkalinity is a measure of the bicarbonate (HCO3),
carbonate (CO3) and hydroxyl (OH) ions in the water. pH and
alkalinity ratings are NOT the same and are NOT proportional. pH is
rated on the Scale and alkalinity is measured in parts per million
(ppm). A typically recommended alkalinity rating is 140 - 700 ppm
for boilers operating below 300 psi.
Low pH Background Use of deionized water as a makeup source has
increased the need for operator awareness of boiler water chemical
control, testing, and response. High purity makeup water does not
contain measurable alkalinity, therefore boiler water usually
contains a very low to zero level of hydroxide alkalinity. Boiler
water alkalinity acts as a buffer. Caustic can be added to increase
alkalinity in boiler water; however as operating pressures increase
over 1000 psig, this can lead to a problem of caustic concentration
and corresponding corrosion. Boiler water alkalinity is a pH buffer
against feedwater contamination. A buffer is defined as a substance
in solution which accepts either hydrogen ions (acids) or hydroxyl
ions (bases) entering the system, thereby minimizing a change in
pH. With very low hydroxide alkalinities, the buffering capacity of
the boiler water becomes reduced and feedwater contamination more
significantly influences the pH. Where a coordinated phosphate
program is being used, buffering capacity is minimal and pH can
change rapidly with the slightest amount of feedwater
contamination.
Contamination sources Contamination of high purity feedwater can
occur from several sources. 1. Salts entering the feedwater from
condenser leaks or demineralizer leakage. If the leakage is high in
magnesium chloride (brackish water) the chance of a low pH
condition is greater. A small amount of hard water leakage could
cause a pH of 4 or less in an unbuffered boiler water. 2.
Demineralizer regenerant leakage. Cation exchange resin requires a
strong acid for regeneration, usually sulfuric or hydrochloric. The
regenerant can enter the feedwater when valves do not close
properly during regeneration. The most common causes of this
phenomenon are a power outage and leaking valves or both. 3. In
boilers over 1000 psig pressure, black liquor contamination may
cause the boiler water pH to drop due to organic acids, high
magnesium, or high sulfidity.
Immediate action Low pH exclusions require immediate action to
minimize the effects on boiler waterside surfaces. A summary of pH
levels and appropriate actions to be taken are shown. Boiler water
pH tests should be conducted frequently until the problem is
corrected. For the following boiler water pH ranges, continuing
actions should include:
pH 8.6 to 9.2 Action Add trisodium phosphate and back out any
monosodium phosphate being fed. Begin aggressive search for
contaminant source including alignment of regeneration valves,
hardness checks on boiler feedwater, and black liquor
contamination. Maintain phosphate in normal range.
pH 8.0 to 8.6 Action Start feeding caustic to re-establish
normal operating pH. Increase continuous blowdown to maximum
allowable rate. If dispersant is being fed, increase feed to match
increased blowdown. Add or increase boiler water antifoam to
minimize carryover. Monitor steam purity: (A) Monitor condensed
steam for sodium level(B) Monitor condensed steam for conductivity
level (C) Monitor superheater outlet temperature.
3. pH 7.0 to 8.0 Action Notify downstream users of potential
impact to their system. Continue actions shown in Step 2. Reduce
boiler steaming rate by 20%.
4. pH 6.0 to 7.0 Action Reduce steaming rate by an additional
20% (total 40%). Start to use all possible blowdown, both
continuous and manual. 5. pH 5.0 to 6.0 Action Prepare to take
boiler offline. If pH continues to decrease (below 5) the fire
needs to be pulled immediately. Continue heavy blowdown of boiler
and drain at proper time. Refill with treated demineralized water,
caustic, trisodium phosphate, or regular water treatment
chemicals.
If the boiler is continuing to run at pH lower than 5 for more
than 4 h and the pH was brought back up with caustic addition,
large amounts of iron (corrosion product) will precipitate as iron
hydroxide. Iron deposits will form on the hot surfaces of the
radiant section and could cause tube failure short-term (within
weeks) or long-term (within months). Chemical cleaning as soon as
practical is recommended to remove the deposited iron.
Boiler blowdown is the removal of water from a boiler. Its
purpose is to control boiler water parameters within prescribed
limits to minimize scale, corrosion, carryover, and other specific
problems. Blowdown is also used to remove suspended solids present
in the system. These solids are caused by feedwater contamination,
by internal chemical treatment precipitates, or by exceeding the
solubility limits of otherwise soluble salts.In effect, some of the
boiler water is removed (blowndown) and replaced with feedwater.
The percentage of boiler blowdown is as follows:quantity blowdown
waterX100 = % blowdown
quantity feedwater
The blowdown can range from less than 1% when an extremely
high-quality feedwater is available to greater than 20% in a
critical system with poor-quality feedwater. In plants with sodium
zeolite softened makeup water, the percentage is commonly
determined by means of a chloride test. In higher-pressure boilers,
a soluble, inert material may be added to the boiler water as a
tracer to determine the percentage of blowdown. The formula for
calculating blowdown percentage using chloride and its derivation
are shown in Table 13-1.The primary purpose of blowdown is to
maintain the solids content of boiler water within certain limits.
This may be required for specific reasons, such as contamination of
the boiler water. In this case, a high blowdown rate is required to
eliminate the contaminants as rapidly as possible.The blowdown rate
required for a particular boiler depends on the boiler design, the
operating conditions, and the feedwater contaminant levels. In many
systems, the blowdown rate is determined according to total
dissolved solids. In other systems, alkalinity, silica, or
suspended solids levels determine the required blowdown rate.For
many years, boiler blowdown rates were established to limit boiler
water contaminants to levels set by the American Boiler
Manufacturers' Association (ABMA) in its Standard Guarantee of
Steam Purity. These standards were used even though they were of a
general nature and not applicable to each individual case. Today,
the ASME "Consensus on Operating Practices for the Control of
Feedwater and Boiler Water Quality in Modern Industrial Boilers,"
shown in Table 13-2, is frequently used for establishing blowdown
rates.This consensus applies to deposition control as well as steam
quality. Good engineering judgment must be used in all cases.
Because each specific boiler system is different, control limits
may be different as well. There are many mechanical factors that
can affect the blowdown control limits, including boiler design,
rating, water level, load characteristics, and type of fuel.In some
cases, the blowdown control limits for a particular system may be
determined by operating experience, equipment inspections, or steam
purity testing rather than ASME or ABMA water quality criteria. In
certain cases, it is possible to exceed standard total solids (or
conductivity), silica, or alkalinity limits.Antifoam agents have
been applied successfully to allow higher-than-normal solids
limits, as shown in Figure 13-1. Chelating and effective dispersant
programs also may allow certain water criteria to be exceeded.The
maximum levels possible for each specific system can be determined
only from experience. The effect of water characteristics on steam
quality can be verified with steam purity testing. However, the
effects on internal conditions must be determined from the results
observed during the turnaround for the specific unit.Certain
boilers may require lower than normal blowdown levels due to
unusual boiler design or operating criteria or an exceptionally
pure feedwater requirement. In some plants, boiler blowdown limits
are lower than necessary due to a conservative operating
philosophy.MANUAL BLOWDOWNIntermittent manual blowdown is designed
to remove suspended solids, including any sludge formed in the
boiler water. The manual blowdown take-off is usually located in
the bottom of the lowest boiler drum, where any sludge formed would
tend to settle.Properly controlled intermittent manual blowdown
removes suspended solids, allowing satisfactory boiler operation.
Most industrial boiler systems contain both a manual interm
Blowdown ControlBlowdown from industrial steam boilers is
typicallycontrolled automatically using a continuous
system.Automatic systems usually maintain the specific
conductanceof the boiler water at a specified level, especially
ifsodium zeolite softened makeup water is used. Otherboiler water
constituents such as chlorides, sodium, andsilica are also used as
a means of controlling blowdown.Continuous blowdown is supplemented
by manualblowdown to discharge suspended solids from the lowpoints
in the boiler system.Here are some guidelines for blowdown control
[9]:
Dissolved solids specific conductance gives an indirect measure
of dissolved solids and can usually be used for blowdown control.
Conductance is caused by the ionization of the various salts that
are present; in dilute solutions there is almost complete
ionization; however, in concentrated solutions ionization is
suppressed. Therefore, it is best to correlate specific conductance
with dissolved solids for each system. For a rule of thumb
estimate, very dilute solutions such as condensate may be
calculated witha factor of 0.5 to 0.6 ppm of dissolved solids per
microsiemens of specific conductance. For a more concentrated
solution such as boiler water, the factor varies between 0.55 and
0.90 ppm of dissolved solids per microsiemens of specific
conductance. It is common practice to neutralize any hydroxide
ionswith gallic acid or boric acid prior to measuring
conductivity.
Chloride if the chloride concentration in the feedwater is high
enough to measure accurately, it can be used to control blowdown
and to calculate the rate of blowdown.Chlorides do not precipitate
in the boiler water, therefore the relative chloride concentrations
provide an accurate basis for calculating the rate of blowdown. The
chloride test is unsuitable when the feedwater chloride is too low
for accurate determination. Silica on-line methods of silica
monitoring are primarily applicable to boilers operating at 600
psig or greater, utilizing a demineralized makeup and where the
steam is used in turbines. Silica in the steam must be limited to
0.01 to 0.02 ppm; to do this the silica in the boiler water must be
limited to a specified maximum that is dependent on boiler pressure
and system design.
Caustic Embrittlement
Whenever free caustic (NaOH) is present in a boiler, failure
from caustic embrittlement may occur. Embrittlement is a form of
stress corrosion cracking. It occurs in areas where there are
specific stress conditions and free NaOH in the boiler water.
Boiler tubes usually fail from caustic embrittlement at points
where tubes are rolled into sheets, drums, or headers. However, it
also occurs at weld cracks and if there is tube-end leakage.Sodium
nitrate is usually used to inhibit embrittlement in boilers
operating at low pressures. The US Bureau of Mines recommends that
the ratio of sodium nitrate to sodium hydroxide be maintained in
accordance with Table 9-5. At pressures above 7000 kPa (1000 psig),
coordinated phosphate/pH control is usually employed which
precludes the development of high concentrations of caustic.
Calculate the ratio using the following formula:
Conductivity versus Dissolved Solids
For a quick estimate of total dissolved solids (TDS) inwater one
can run a conductivity measurement. The unitfor the measurement is
mhos/cm. A mho is the reciprocalof an ohm. The mho has been renamed
the Sieman (S) byISO. Both mhos/cm and S/cm are accepted as
correctterms. In water supplies (surface, ground, etc.)
conductivitywill run about 10_6 S/cm or 1 mS/cm.Without any data
available the factor conductivity toTDS is:
However, the local water supplier will often supply TDSand
conductivity data. Table 9-4 gives conductivityfactors for common
ions found in water supplies.
