Reference Manual

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Chapter 18
Boilers — Water/Steam Cycle
The efficient production of steam and electricity is
an important function in the overall process of pulp
and paper production. These items are basic raw
materials required in large quantities for the
manufacturing of pulp and paper.
In years past, the cost of operating the powerhouse
may not have been a priority; however, with
increasing prices of fuel and purchased electricity
affecting bottom line profits, the industry has
evolved to a more efficient and conservation
conscious powerhouse. One key item in a more
efficient powerhouse involves utilization of a
reliable control system. The final element for many
of these controls is the control valve.
Although design will vary from mill to mill, a
generic water/steam cycle is shown in figure 18-1.
This process is commonly referred to as
cogeneration due to the simultaneous use of fuel
energy for both process steam requirements and
electrical power generation via steam driven
turbines. Figure 18-2 details the upper, or
convective, section of a boiler and indicates valves
required for process control.
Condensate Return System
Water pumped to the boilers for production of
steam is composed of condensate returned from
process and demineralized make-up water. This
mixture is commonly referred to as boiler
feedwater (BFW).
The condensate returned from process is
demineralized water, which was used to produce
steam in the boiler and has condensed after giving
up vaporization energy to process. In a typical mill,
about 40-50% of the condensate is returned to the
powerhouse. Most of this comes from indirect
heating such as paper machine dryers and various
heat exchangers. A large portion will also be
returned from the condenser if a condensing turbine
is used. Losses occur in direct heating or cleaning
applications such as pulp cooking and sootblowers.
Other losses occur in the transport system
(pumps, valves, tanks, and piping) and condensate
contaminated by leakage (mixing with process).
Serious operational problems may result if
contaminated condensate (with black liquor, white
liquor, etc.) enters the boilers. To prevent this, a
condensate dump system is used. A conductivity
element is used to sense contaminants and sends
a signal to valves which dump the condensate to
sewer until the problem is corrected. Automation
of this system can save the time lost to a manual
operated system.
Demineralized or deionized (DI) water involves
treatment with ion exchange resins to remove
hardness (minerals) and silica, which would
deposit on boiler tubes. This process involves
large equipment and operational expense. Most
mills are equipped to supply only a portion of the
feedwater required with demineralized water.
Thus, the return of as much clean condensate as
possible is critical from an economic and
operational standpoint.
Condensate is usually brought to a single
collection tank from various process users and the
turbine condenser. It is then sent through a
condensate polisher to remove any scale deposits
picked up in the return system. From the collection
tank the condensate is pumped to the deaerator
(DA) heater. In some mills an indirect feedwater
heater is used before the DA to raise temperature
to near saturation. Low pressure (150 psig) steam
is used for heating and the resulting condensate is
returned to a lower pressure reservoir such as the
condenser hotwell or DA.