A.
Shipboard Water Cycle
1.
Terminology
a.
Although the shipboard water cycle is
continuous, different terminology is used to describe the water at different
points. These distinctions are
necessary because water quality standards vary throughout the system. The following terms identify the water at
various points.
b.
Distillate is the evaporated water that is
discharged from the ship's distilling plant. Water in the shipboard water cycle
normally begins as distillate. This
distillate is stored in the reserve feedwater tanks until needed as makeup
feedwater.
c.
Reserve Feedwater and Makeup Feedwater: Distillate stored in the feedwater tanks is
called reserve feedwater. The water
flowing to the condensate system from the feed tanks is makeup feedwater. In ships equipped with demineralizers, the
water flowing out of the demineralizer to the condensate system is termed
demineralized makeup feedwater.
d.
Condensate:
After steam has done work, it is returned to the liquid state by cooling
in a condenser. The condenser is a heat
exchanger in which steam, under vacuum, flows over and is condensed on tubes
through which seawater flows. The
liquid condensed from steam is called condensate. Water from other sources (such as makeup feedwater and low pressure
drains) is mixed with and becomes part of the condensate.
e.
Feedwater:
The term feedwater defines the water contained between the DFT and the
boiler. This water is of special
quality, as it has been heated and deaerated in the DFT.
f.
Boiler Water:
The deaerated feedwater, as it enters the boiler steam drum, is defined
as boiler water. The term boiler water
describes the water in the steam drum, water drum, headers, and generating
tubes of the boiler.
g.
Freshwater drains: Steam drains from various steam systems throughout the
engineering plant are collected and returned to the system via the freshwater
drains.
(1) Steam
drains are collected from various steam systems by means of funnel drains with
swing check valves installed to prevent backflow of the drains. This drain collecting system is designed to
collect condensate from piping during light-off, securing and casualty
situations.
(2) Equipment
and steam systems using steam below 150 psig, are designed with a continuous
drainage capability. Some examples of
equipment include: lube oil heaters,
saltwater heater drains from distilling plants, and drains from air ejector
condensers.
h.
Low pressure drains: As a great deal of low pressure steam is used for auxiliary
functions outside the engineering spaces, as much of the condensed water as
possible is recovered and reused. Some
examples of contributors to the LP drain system are: galley, laundry, ship's water heaters and the scullery. This is a common source of boiler water
contamination.
B.
Goals of Chelant
1.
The goals of successful boiler water treatment
are to prevent scale formation and to minimize corrosion and carryover. These conditions can be caused by saltwater
or shore water contamination. Saltwater
or shore water contamination will introduce calcium, magnesium, and chloride
ions into the feedwater and boiler water systems.
2.
Three results of contamination
a.
Scale formation: Calcium and magnesium are the major hardness constituents and the
primary sources of scale in boilers and feed system heat exchange equipment.
All scale deposits act as insulators and reduce the heat transfer across the
particular surface. Deposits will cause
the temperature of the metal to increase until overheating, metal softening, blistering,
and failure occur.
b.
Corrosion:
There are basically eight types of corrosion which are found as a result
of contamination.
(1) High
dissolved oxygen: causes hematite (red
rust)
(2) Ammonia
corrosion: found in copper alloy piping
(such as the condensate system).
(3) Chloride
stress corrosion: transgranular
cracking will occur where there is stainless steel.
(4) Oxygen
pitting: high levels of dissolved
oxygen will result in pitting and scabs will form over the pits.
(5) Caustic
stress corrosion: where there are
deposits present, hydroxyl ions will collect and cause corrosion, cratering and
gouging of tube metal.
(6) Acid
attack: an excess of hydrogen ions will
corrode tube metal which will create a methane gas which will stress the tube
and cause it to crack.
(7) Chelant
corrosion: an over treatment of boiler
water with EDTA will thin the walls of boiler tubes.
(8) Electrolytic
corrosion: when two dissimilar metals
are joined together, and there is an electrolytic solution present (i. e..
boiler water), electrons will move from one metal to another leaving behind a
defect in the wall of the tube.
(9) Chloride
pitting: an aggressive ion which
deteriorates the walls of boiler tubes.
c.
Carryover:
Steam slugs will normally break and release dry steam in the steam
drum. A high concentration of suspended
dissolved matter in boiler water will stabilize steam slugs and prevent them
from breaking. This will result in
carryover, which enables wet steam and water to impinge on turbine
blading,valves and create water hammer which can damage piping and pipe
hangers.
C.
Chelant Treatment Chemicals
1.
Trisodium EDTA:
one of a class of organic chemicals known as chelating agents or
chelants. EDTA chelates dissolved
metals, including the scale formers magnesium and calcium, by surrounding and
bonding to the metal at several places.
Thus, magnesium and calcium are prevented from forming scale on the
boiler tube surfaces. Since the
chelates remain dissolved, they do not contribute to sludge formation in the
boiler. Chelation is the primary method
of removing magnesium and calcium ions from boiler water. EDTA also generates a magnetite layer, which
is a microscopic film formed on the walls of the boiler tubes to help protect
the metal from corrosion.
2.
Trisodium Phosphate/Disodium Phosphate
(TSP/DSP): the secondary method of removing
magnesium and calcium ions from boiler water is through the introduction of
phosphates and alkalinity. If a large
amount of contamination causes all of the EDTA to be consumed by magnesium and
calcium ions, then the excess ions will form scale. By having a residual level of phosphates in boiler water, which
is kept slightly alkaline, then the excess magnesium and calcium ions will form
sludge instead of scale.
3.
Hydrazine:
enhances the magnetite layer and acts as an oxygen scavenger. The DFT cannot remove all of the dissolved
oxygen from feedwater. Hydrazine will
consume any oxygen which is not removed in the DFT.
D.
Chelant Boiler Water Control Limits (table 1)
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Table
1
1.
Alkalinity:
There is a minimum limit for alkalinity in order to prevent acid attack
and the formation of scale if EDTA is depleted. There is a maximum for alkalinity in order to prevent caustic
corrosion and carryover of water with the steam.
2.
Phosphates:
There is a minimum limit for phosphates in order to provide a level of
residual phosphates for the formation of sludge vice scale if EDTA is
depleted. There is a maximum limit in
order to prevent carryover of water with the steam.
3.
Conductivity:
There is a maximum limit in order to prevent carryover of water with the
steam.
4.
Chloride:
There is a maximum limit for dissolved chloride ions in order to prevent
the types of corrosion mentioned above.
E. Continuous Treatment System (figure 1)
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Figure 1 |
1. The purpose of the continuous treatment
system is to inject EDTA, TSP/DSP, and hydrazine into the DFT.
2. The treatment tank holds 30 gallons of
treatment solution which is a mixture of the three chemicals condensate. Nitrogen is used to mix the chemicals in the
tank with the condensate and pressurize the tank in order to force the
chemicals into the DFT. The nitrogen pressure on the mixing tank is adjusted
between 20 and 60 psig to achieve proper treatment flow. The required pressure will vary between
systems depending on the location of the mixing tank relative to the DFT.
3. There are two alarms associated with
the treatment tank. The five gallon
alarm indicates that preparation of fresh treatment solution will be required
shortly. The continuous treatment tank
is normally filled when the two gallon alarm sounds, but may be filled when the
level is below five gallons if drills or other operations are expected to
conflict. When the treatment level
reaches two gallons, a solenoid valve closes, securing the flow of treatment
chemicals to the DFT. The continuous
treatment tank shall be refilled and
placed back on line within one hour after the two gallon level alarm sounds,
except when it is anticipated that all the boilers in the space will be secured
within twenty-four hours. This will
minimize any waste of chemicals.
F. Batch Treatment (see figure 2). There are two times when batch treatment is
done:
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Figure 2 |
1. Freshly fill and treat (FFT): Accomplished when initially filling a dry
boiler with untreated feedwater. A
standard dosage of chemicals must be added in order to raise the level of
alkalinity and phosphates to be within the boiler water control limits and
provide EDTA and hydrazine.
2. Casualty control: a dosage of treatment chemicals is injected
when there is a boiler water casualty resulting from seawater or shorewater
contamination. This casualty dose is
necessary because the continuous treatment system discharges at a constant rate
regardless of any of contamination.
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Figure 3 |
G. Continuous Blowdown System (figure 3)
1. The continuous removal of boiler water
is necessary in order to control the level of conductivity caused by the
chelated metal ions. This is
accomplished by allowing the continuous flow of boiler water through the blowdown
piping, past the boiler sample cooler, to the bilge.
2. A continuous monitor is being installed
on all new construction ships and ships already in commission are being
retrofitted. The continuous monitor
allows supervisors to monitor the conductivity of the boiler water.
H. Surface and Scum Blowdowns
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Figure 4 |
1. Surface blowdowns (figure 4): shall be performed on a steaming boiler as
necessary to reduce high levels of alkalinity, phosphates, conductivity, or
chloride and to remove light suspended solids.
The surface blowdown pipe is located on the steam drum three inches
below the normal water level. One
surface blowdown will remove the top three inches of steam drum water. The
surface blowdown procedure will result in approximately a 5% decrease in
alkalinity, phosphate, conductivity, and chlorides. This is the only
method for reducing high levels of chloride in a steaming boiler. Surface blowdowns are required less
frequently under chelant treatment due to the use of the continuous blowdown
system. When a surface blow is
required, boiler water chemistry limits must first be checked to ensure the
boiler chemistry is never reduced below the minimal chemical limits. For example, scale formation may occur if
the boiler is steaming with alkalinity less than .050 epm and acid corrosion
will occur if alkalinity is 0 epm. The surface blowdown evolution takes
approximately 45 minutes. Throughout the surface blowdown the water level will
be controlled in remote manual and the speed of the ship should be maintained
steady with little or no change in steam demand.
2. Scum blowdown (figure 4): A scum blowdown is performed in order to
prevent the accumulation of light suspended matter in the steam drum. A scum blowdown utilizes the surface blowdown
piping, but only removes the top one inch of water from the steam drum. If the boiler has been secured for more than
two hours, then a scum blowdown is required following the first boiler water
sample that is above the lower alkalinity and phosphate limits after coming on
line. At a minimum, a scum blowdown
must be conducted weekly. When a
surface blowdown is performed, that will zero the hours used to calculate the
next scum blowdown. The scum blowdown evolution takes approximately 15 minutes
and the water level will be controlled in remote manual.
I. Boiler Lay-ups
1. The method of idle boiler lay-up chosen
depends on considerations such as maintenance or repair work scheduled, planned
duration of lay up, effectiveness of the lay up method, availability of support
for the lay-up method, and boiler recovery time. With a cold boiler it takes approximately 2-3 hours
minimum to light off and bring it to operating pressure. Lighting off from a steam blanket lay up
takes approximately 1½-2 hours
minimum to be at operating pressure. If
done faster, could cause damage from thermal shock. The following is a description of boiler wet lay-up methods and
time limitations.
a. The steam blanket lay-up method uses 150 psig
steam from the 150 psig desuperheated steam system or shore source steam that
meets specifications outlined in NSTM chapter 220 V2. This provides reasonable protection for the boiler watersides,
firesides and superheater but, does not provide protection for the
economizer. This lay-up is applied as
soon as residual steam pressure has dropped below steam blanket supply
pressure. The steam keeps the boiler
warm and requires no special preparation to get the boiler underway ready. A steam blanket lay up is limited to thirty
days. At the end of thirty days, the
boiler must be placed on a hydrazine wet lay-up or a dry type lay-up if not
lighted off.
b. The nitrogen blanket lay up uses one or
more compressed nitrogen gas bottles connected to the boiler through a
regulator and small piping system. The
regulator is adjusted to maintain 5 psig constant pressure on the boiler to
keep oxygen out. This provides
reasonable protection for the boiler watersides and superheater, but does not
provide protection for the economizer.
This lay up is applied when residual steam pressure is at or below 5
psig. Since the boiler is cold, heat
should be applied to the firesides to protect them from corrosion. This lay up requires no special preparation
to get the boiler underway ready. A
nitrogen blanket lay up is limited to thirty days. At the end of thirty days, the boiler must be placed on a
hydrazine wet lay-up or a dry type lay-up if not lighted off.
c. The hydrazine/morpholine lay up can be
applied at any time by ship's force.
This lay up requires the boiler to be completely filled and all air
expelled. A constant pressure must be
maintained with the use of a head tank or pump. Preparation for light off requires a portion of the
hydrazine/morpholine to be drained and properly disposed of in accordance with
pollution control NSTM chapter 593.
This lay up also has no time limit.
2. Dry lay up methods are used when boiler
watersides are to be opened for long term inspection or repair. There are two dry lay up methods that can be
used and the preliminary procedures are the same for each method. Dry lay up methods provide reasonable
protection for boiler watersides, economizer and firesides provided the boiler
is completely dried out after dumping.
Both methods have no time limit.
The following is a description of boiler dry lay up methods.
a.
When boiler pressure has dropped to 0 psig dump
the boiler completely. After the boiler
is thoroughly drained and opened it must be completely dried out. Starting from the top of the boiler using
low pressure air to blow out all accessible surfaces including all boiler
tubes, drums and headers. Do not
contaminate the boiler with wet or oily low pressure air. Thoroughly inspect all waterside surfaces to
ensure they are dry, then apply one of the following lay up methods.
b.
The hot air lay up method uses an electric
heater and blower to circulate hot air through the boiler watersides and
firesides as long as the boiler is idle.
This requires all accesses to be closed with the exception of the hot
air entry and exit point. Special
adapters are needed to attach the blower connections to the boiler.
c.
The desiccant lay up method uses bags of
desiccant to keep the boiler dry. The
amount of desiccant used varies by boiler capacity. Follow the procedures in Boilers NSTM chapter 221. The bags are evenly distributed through all
boiler drums, headers and the economizer.
A count of bags is made as they enter the boiler and the count is logged
in the engineering log when removed. A
desiccant bag inadvertently left in a boiler will cause adverse effects in the
boiler water chemistry after light-off.
Humidity cards are placed inside the boiler to indicate when the bags
need to be changed. The boiler is then
closed up. It is recommended to have
plexiglass covers made to fit the boiler steam and water drum accesses so that
humidity cards can be observed easily.
d.
Preparation for light off can be very time
consuming depending on boiler integrity and repairs. Remove the desiccant or hot air blower connections, clean all
gasket mating surfaces as required and close the boiler. Flush the boiler by refilling the economizer
and superheater with feed‑quality water and dumping. Refill the boiler and conduct a hydrostatic
test accordance with boilers NSTM chapter 221.
J. Safety
1. There are a number of safety
precautions to be observed when testing or treating boiler water and feedwater.
Many of the chemicals employed are either acids or alkalies. All are poisons
when ingested. Sampling can be dangerous if procedures are not followed exactly
or if equipment is faulty. Immediate medical attention shall be obtained if any
chemical is swallowed.
a. Treatment chemicals
(1) A
face shield shall be worn when mixing treatment chemicals to prevent contact
with alkaline treatment solutions. A
faceshield is worn when treating the boiler because of the possibility of hot
feedwater spraying back on the operator should an error or equipment
malfunction occur. A faceshield,
goggles, apron, and plastic or rubber
gloves shall be worn when injecting caustic soda, handling hydrazine or
morpholine solutions.
2. Obtaining samples
a. Sampling of boiler water or deaerated
feedwater can be dangerous because live steam can discharge if instructions are
not followed exactly. Coolers can rupture because of the pressures involved. For boiler water sampling, the cooling
water flow must be established prior to opening any valve in the sample line.
For dissolved oxygen sampling, pressure in the secured cooling water line must
be relieved.
b. A face shield shall be worn when obtaining
a deaerated feedwater or a boiler water sample. In addition, a finger cot or surgical glove shall be worn for
dissolved oxygen testing.
3. Testing, chemical preparation and
equipment
a. The testing of feedwater and boiler water
samples requires the use of highly concentrated stock chemicals in diluted
solutions known as reagents, such as nitric acid, mercuric nitrite, and
hardness titrating solution. Some of
the reagents are supplied ready for use and need little or no preparation. For
accurate test results, it is necessary that the reagent be prepared strictly
according to NSTM 220 V2.
b. Goggles and plastic or rubber gloves shall
be worn when pouring stock reagents. Additionally, an apron is worn to prepare
dilute caustic soda.
K. Hazardous Material
1. The testing of feedwater and boiler
water samples results in the collection of hazardous material. These chemicals must be collected and
disposed of properly.
a. Mercuric
Nitrite and Silica One Regent shall be disposed of by turning into PWC.
b. Hydrazine:
is a known carcinogen and all personnel involved in its handling must be
aware of the proper procedures, including the emergency spill clean up
procedures. The use of an enclosed
transfer system ensures that personnel receive zero exposure to hydrazine
solution or vapors.
(1) There are two types of stock hydrazine
spills, flushable (a spill that can be flushed directly to the bilge or
overboard) and non-flushable. The
flushable spill requires the following:
tape off area, place ventilation on high, and flush to bilge. Personnel
must remain clear of the bilges until all of the hydrazine is removed. Non-flushable spill requires taping off the
area, spreading absorbent, placing absorbent into plastic bags and then into a
drum and turning it over to PWC. The safety gear required for stock hydrazine
spills are goggles, gloves, boots, coveralls and an apron.
(2) There
are also two types of leaks. Leakage of
stock hydrazine solution (prior to dilution) and leakage of treatment solution
(after the stock hydrazine is diluted
with feedwater). Leakage of stock
hydrazine requires the following: place ventilation on high, place bottle in
piercing apparatus, and drain the tank to the bilge. Remain clear of bilges until hydrazine is removed and bilges are
thoroughly flushed. Leakage of treatment solution requires the following: tape
off area, secure leak, drain tank to bilge, fill tank with condensate, fix
leak, hydro system to 90 psi to ensure system integrity. The safety gear for
both types of leak requires goggles, gloves, boots and an apron.
L. Inspections
1. SGPI
a. Boiler inspection done every 18 months (± 6
months)
b. Boiler Overhaul Related Inspections
2. Ship's Force
a. Fireside
(1) Engineer Officer's inspection conducted every
1800-2000 steaming hours
b. Watersides
(1) Boiler watersides shall be water-jet cleaned
within 500 steaming hours prior to or after initiation of chelant treatment.
The first waterside inspection following initiation of the chelant treatment is
required within 2,000 steaming hours of the last inspection.
(2) Following the initial inspection, watersides
shall be inspected as part of the 18 month inspection unless one of the
following conditions occurs:
(a) Steaming
without continuous treatment due to treatment system malfunction for more than
168 steaming hours (continuous), requires that the boiler watersides be
inspected during the next upkeep or repair period.
(b) Steaming
with moderate-long term contamination requires that the boiler watersides be
inspected.
(c) Steaming
with boiler water conditions that meet the criteria for significant damage,
requires that the boiler watersides be inspected during the next upkeep or
repair period. If conductivity exceeds 8,000 mmho/cm,
then waterside inspection shall be conducted prior to further operations.
(d) If
conductivity exceeds 2,000 mmho/cm, then superheater steam sides
shall be flushed and inspected prior to further operations.
M. Off Ship Reports
1. BIRMIS
a. TYCOM Boiler Inspectors report on condition
of the boiler inspected.
2. Messages to TYCOM and info ISIC, NAVSEA
and NAVSURFWARCEN
a. Continuous Treatment System will be OOC for
more than 168 steaming hours.
b. Hideout is suspected.
c. Significant damage.
d. Boiler steamed for more than 8 hours under
a condition of serious contamination.
e.
Any time the Commanding Officer does not reduce
the steaming rate under conditions of serious contamination after one treatment
action fails to correct boiler water parameters to moderate contamination or
within limits.
f.
Deviations from mandatory requirements of NSTM
220.
g.
Defective chemical standards.
3. Correspondence to NAVSURFWARCEN and
copy to TYCOM, ISIC and NAVSEA
a. Theoretical conductivity above 30% and
source of unusual contamination cannot be determined.