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Pressure Vessel Care
exercised to make sure that the proper relationship of pressure
to temperature exists within the boiler so that all of its internal
surfaces are fully wetted at all times. It is for this reason that
the internal boiler pressure, as indicated on the water pressure
gauge, must be held to the level shown in Figure 3-2.
It is advisable to install a thermometer in the return line to
indicate return water temperature. With the return water
temperature and the supply water temperature to the system
known, the temperature differential will be established.
Knowing the flow rate, the operator easily can detect any
excessive load condition and take appropriate corrective
action.
Minimum Boiler Water Temperature
The recommended minimum boiler water temperature
(outlet or supply) should be no less than 160°F (71°C) when
the boiler is operating. This is specifically relevant during
times when there is little load or during intermittent firing.
The recommended minimum system return water
temperature shall be 140°F (60°C) when firing natural gas
or 150°F (66°C) when firing oil. And for special burner
applications firing digester gas, minimum return water should
always be above 160°F (71°C) and supply outlet temperature
greater than 170°F (77°C).
These recommendations are provided to reduce or minimize
the formation of condensation on the fireside of the boiler.
Failure to do so will lead to corrosion of the boiler tubes and
will not be covered by warranty. When supply temperature is
lower than 160°F (71°C), the combustion gases are cooled to
the point where the water vapor in the gases condenses. When
this occurs, fireside corrosion may result if the condensed
moisture stays on the steel fireside surfaces. This may occur
more frequently during seasonal or light load conditions and
specifically if the boiler is oversized for the system load.
If the system water temperature requirement is to be less than
160°F (71°C), mixing valves or blend pump should be
employed to mix the supply water with return water. NOTE:
When system 3-way valves are used, they should be set so
that the boiler is not by-passed, thus impairing circulation
within the boiler when the burner is firing. This could lead to
over-temperature and nuisance shutdowns, as the high limit
control will trip, requiring a manual reset of the control.
Repeated overheating could lead to other damage.
Multiple Boiler Installations
When multiple boilers of equal or unequal size are installed,
care must be taken to ensure proportional flow through the
boilers. Proportional flow can best be accomplished by use of
balancing valves and gauges in the supply line from each
boiler. If balancing cocks or orifice plates are used, a
significant pressure drop (for example, 3-5 psi) must be taken
across the balancing device to accomplish proportional flow.
Variations in water temperature and firing rates will result if
care is not taken to ensure proportional flow through the
boilers. In extreme cases, differences in firing rates could
750-364
result in a net header water temperature below the desired
temperature.
C. WATER TREATMENT
Properly treated boiler water will result in maximum
effectiveness and long trouble-free life of the pressure vessel.
Contact your local Cleaver-Brooks Representative or water
management consultant for complete information on how to
prevent damage resulting from inadequate water treatment.
The objectives of water treatment in general are to:
1.
Prevent hard scale and soft sludge deposits that inhibit
heat transfer and that could lead to overheated metal and
costly downtime and repairs.
2.
Eliminate corrosive gases in the supply or boiler water.
To accomplish these objectives, the boiler requires proper
water treatment before and after introduction of water into the
unit. The selection of pretreatment processes depends upon
the water source, its chemical characteristics, the amount of
makeup water needed, system operation practices, etc.
Because of the variables involved, no one boiler compound
can be considered a cure-all; nor is it advisable to experiment
with homemade treating methods. A sound treatment
program should include a periodic analysis of the water in the
system.
Table 3-2: FLX Water Chemistry
Parameter
pH
Chloride
Oxygen
Specific
Conductivity
Total Hardness
Chapter 3
Limit
8.3 - 10.5
30 mg/liter
0.1 mg/liter
3500μmho/cm
<10 ppm
3-3
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