Reference Manual

12−4
tolerated as a leak could result. A cleaning
process for the boiler tubes is needed even while
the boiler is in operation. This process is called
soot blowing.
Process:
When firing fuels such as coal, oil, biomass or
other waste products fouling of the boiler tubes
becomes a concern. Deposits from the
combustion process can collect on the heat
exchanging tubes reducing thermal efficiency and
can cause operational difficulties. In order to keep
the unit operating, an online cleaning method must
be used. This is usually accomplished by using
what are called sootblowers.
Most sootblowing systems utilize either air or
steam. Widespread use of water has been limited
due to the possibility of thermal shock on the tube
banks. Air or steam systems each have their own
advantages, but one is not considered better than
the other.
Air systems have much simpler piping
arrangements. This is due to the elimination of
condensate drain piping. The number of
compressors, compressor capacity and the
sootblower flow requirement; however, limits this
system.
Steam systems have an advantage in terms of
expansion. The supply of steam (typically
removed after the primary superheater) is virtually
unlimited, but leads to additional maintenance
concerns related to the numerous valves required.
Also, as stated above, the steam systems require
additional piping to address the possibility of
condensate in the steam lines.
As high pressure air or steam is required to
remove the deposits from the boiler tubes, the
control valve must be able to withstand high
pressures. Steam systems present a greater
challenge due to the combination of high pressure
and temperature. Because of the high inlet
pressure, downstream pressure and pipe size, the
valve must also withstand issues with noise and
vibration. As the sootblowers operate
intermittently, tight shutoff (class V) is required for
valve trim protection and when using steam,
maintaining unit efficiency. These valves modulate
over a wide range of flows and are required to
maintain downstream pressure.
Design Considerations and Service
Conditions:
D High pressure class rating due to the
pressures and temperatures.
D Tight shutoff so valves don’t leak valuable
steam.
D Large pressure drops can create noise,
vibration, and excessive wear to trim.
D Cyclic conditions as valve are operated
numerous times a day.
Typical Process Conditions:
D P
1
= 800 - 1400 psig
D P
2
= 0 psig
D T = 400 - 800°F