This course is designed to provide marine engineers and support staff with
an introduction to water treatment for steam boiler systems it begins with an
introduction to steam boilers and basic water chemistry the course goes on to
look at the problems caused by corrosion and scaling and the water treatment
solutions that are available the course examines the importance of correct
chemical dosing and control and the procedures for safe sampling and testing
there is a section on monitoring water test results and a look at the tools and
service that are available at the end of the course there is a troubleshooting
guide
a steam boiler is a piece of equipment into which water can be fed the water is
heated under pressure and is continuously evaporated into steam steam
is an excellent medium for the supply of heat and can easily be piped to the
required point of use click on the question marks for some examples of
typical uses for steam shell and tube boilers can be found on ships and are so
called because the water is described as being on the shell side of the
construction heat is provided by a burner firing inside a large diameter
furnace tube a series of smaller diameter tube banks above the main
furnace tube provides secondary heat recovery because of the large diameter
of the furnace tube the maximum pressure limit of operation of Shell boilers is
around 20 bar vertical water-tube boilers are the most common type of
boiler found on marine vessels they're almost exclusively used in the marine
industry because of their compact construction and ease of operation let
us look at the principle of operation the vertical water tube boiler has a
cylindrical shell and contains a hemispherical or cylindrical furnace
chamber at its base the burner is fired into this space and the hot gas is
produced pass around a series of tubes containing boiler water and exhaust
through an internal flue to the stack here the water is on the tube side of
the construction to compensate for the short retention time of hot combustion
gases water tubes can be fitted horizontally vertically or obliquely
cross tubes in the furnace section to maximize heat transfer inside a water
tube boiler the water circulation is controlled by having a series of
non-heated downcomers connecting the upper steam water chamber
to the lower ring shaped bottom header the water in the downcomers is cooler
and denser than in the hot Rises and this causes a natural circulation of the
boiler water the maximum operating pressure can be in the order of a tea
bar but is typically 10 to 30 bar the pipework inside water-tube boilers
is configured to extract as much heat from the furnace area as possible common
pipework configurations attempt o d and a type boilers in a typical steam boiler
system feed water is pumped into the boiler and heat is applied from a burner
system temperature and pressures increase according to the boiler design
valve arrangements are used to allow steam to flow in a controlled manner
from the headspace above the water level steam flows to the point of use and
cools and condenses back to water condensate water is typically recovered
and recycled as boiler feedwater once the ship is underway it is usual
practice to utilize the heat rejected from the main engine exhaust gas to
generate steam this means that the boilers do not need to burn valuable
fuel oil whilst at sea steam is raised by circulating water in a large non
fired heat exchanger these types of boilers are referred to as exhaust gas
boilers the fired and non fired boiler systems are usually interconnected in
general water is never pure normal tap water contains a number of constituents
that are important and beneficial to our Constitution the table shows that sea
water has a predominance of sodium chloride with significant amounts of
calcium and magnesium salts when introduced to marine steam boilers or
engine cooling systems these constituents will cause operational
problems if not removed for example dissolved gases such as oxygen and
carbon dioxide will cause corrosion and dissolved salts such as calcium
carbonate will lead to scale deposition as we will see corrosion and scaling are
serious problems if not controlled corrosion is an electrochemical process
where metal in contact with an electrolyte water is oxidized to
corrosion products local differences in chemistry metallurgy and oxygen
concentrations will induce potential differences on the metal surface and a
cathode and anode is formed at the anode will take place and metal will be lost
characteristic deposits of iron oxide rust are also formed broadly speaking
corrosion can be divided into two types uniform corrosion and pitting corrosion
click on the pictures for more information
oxygen chloride salts and pH all contribute to corrosion corrosion in
steam condensate systems can be particularly aggressive due to partially
wet and hot conditions carbon dioxide and oxygen in the boiler feedwater
flashover with steam and rhe dissolve in the condensate water the condensate
water is very pure and has no buffering capacity so relatively small amounts of
carbon dioxide will make the condensate water very acidic aggressive corrosion
will rapidly deteriorate metal surfaces and lead to pipe work failure and leaks
specialized treatments have to be applied to inhibit condensate line
corrosion scale deposits are formed when impurities such as calcium carbonate are
allowed to enter the boiler due to the negative solubility properties that is
decreasing solubility with increasing temperature of these salts deposition of
scale is favored on hot surfaces scale deposits form hard and tenacious layers
inside the boiler tubes such deposits will reduce heat transfer efficiency
cause overheating and potential tube failure to address the problems of
scaling and corrosion a range of water treatment chemical technologies have
been developed these are applied to adjust the properties of the feed boiler
and condensate waters and provide protection of the system methyl work a
typical boiler treatment program consists of a combination of chemicals
in open boiler systems it is usual practice to raise the temperature of the
feed water to minimize the level of oxygen present oxygen scavengers are
then applied to further reduce the oxygen to sub ppm levels for best
corrosion protection there are various types of oxygen scavenger available to
carry out this task and selection of the best approach is a function of the
mount of oxygen present feed system design economics and any particular
limitations required by the process using the steam some oxygen scavengers
such as D ha hydrazine and carbohydrate promote a protective iron oxide film
called magnetite that further enhances corrosion protection steel surfaces are
best protected in an alkaline environment alkalinity builders are used
to increase pH and alkalinity to the desired levels for best corrosion
protection alkalinity builders also help with scale control in alkali phosphate
treatment programs the alkalinity of the water also acts as a buffer a property
that will absorb pH changes if acid components are introduced and maintain
an alkaline pH in alkali phosphate treatment programs soluble phosphate is
incorporated into the treatment program to precipitate any trace hardness salts
passing into the boiler and disperse the salts before they can deposit on the
heat transfer surfaces scale dispersants are also used to ensure that any
precipitated scale salts are maintained free flowing in the boiler water
precipitated salts are then removed by routine blowdown this treatment approach
relies on good quality pretreatment usually see water evaporators elevated
pH and free caustic alkalinity helped the mechanism of precipitation carbon
dioxide is present in most waters and in the boiler system it will evaporate and
follow the steam into the condensate system when condensation occurs the
carbon dioxide will read azov' and lead to a low pH which consequently causes
corrosion neutralizing a means of volatile alkaline materials that flash
over with the steam they combined with the condensate water to elevate pH and
provide corrosion protection many Marines steam boiler systems operate at
low pressure and run at a steady-state this allows for the application and
control of multifunctional treatment products and for small and moderate
sized boilers this is the most common treatment approach typical
multifunctional treatments are shown here
a complete boiler treatment program can be furnished by two or three products
whether is variable steam load variable feed water temperature and with high
pressure boilers control of the selected water treatment program is best obtained
by dosing product types separately it is best practice to install chemical dosing
and control systems for steam boilers click on the question marks to see the
benefits of such a system this chart describes how good control of water
treatment applications can optimize cost and improve system protection water
sampling and testing is used to control the performance of water treatment
programs most treatment chemicals have a set of control parameters that should be
maintained for optimum system protection correct sampling frequency and
procedures are important to achieve inconsistent results system control
parameters should be in line with BS 1170 1983 treatment of water for marine
boilers consult your water treatment technical manual for further guidance it
is important that test results are recorded both for quality purposes and
to help with the interpretation of trends this enables corrective decisions
to be made quickly and effectively water testing is an essential shipboard
activity that should be performed on a regular basis water proof is a software
package developed to enable the crew onboard the vessels to electronically
record store and display results from water tests when entering the test
results the user immediately gets notified if the results are within the
set limits or if corrective actions are needed the test results can be sent to
Williamson ship service for review this is done via the internet or email a
report from our water treatment experts will be returned within days after the
defined test period is closed