Page 37 - Boiler Operator’s Handbook

22 Boiler Operator’s Handbook

That’s very true for condensate that’s formed in the main ery pound of vented steam represents about 1,000 btu
steam piping. The cycle diagram in Figure 1-7 is provided wasted. Some systems waste condensate that has to be
to explain how some little changes in the cycle can make replaced by cold makeup water and heated to saturated
a difference. First, I’ll provide a few explanations for the steam in the boiler. In our low pressure plant example
diagram. Steam is represented by two lines, sort of like that’s 1,180 btu per pound. Part of the wise operator’s
along a highway, where the steam flows down the middle job is to constantly monitor vents for wasted steam and
of the lines. Water, being much heavier, is shown in black try to do something about it.
between the two lines. The boxes containing a capital “T” In plants where the steam is primarily used to gen-
represent the steam traps for the systems. “SRV” repre- erate power the steam and water cycle becomes more
sents a safety relief valve (not a safety valve) it’s one that complex. Some of these are referred to as supercritical
relieves a fluid proportional to the pressure above its set boiler plants because the steam is generated at pressures
point. “PRV” represents a pressure reducing valve or a exceeding 3,190 psig where the density of water and
complete pressure reducing station. steam are the same. Density is simply a value represent-
What’s going on here is a system that doesn’t ing the weight of a substance in pounds per cubic foot.
waste heat like so many I’ve seen. It contains a flash tank Because there’s no difference in density those boilers do
that does what it’s supposed to do—unlike so many that not have a level indicator or gage glass because there’s
are installed and do nothing or simply separate the flash no water level. After the water is converted to steam in
steam from the condensate and throw it away. Some of the boiler it passes through the superheater, more tubes
the condensate that’s drained from the steam piping exposed to the boiler flue gases or radiant heat from the
will flash into steam. Condensate from the main header fire where it’s heated further increasing its temperature.
drains, kitchen and laundry is bound to be hotter than Steam heated to temperatures higher than the satura-
212°F and once it leaves the steam traps some of it will tion temperature is superheated steam. The superheated
flash into steam. By piping that condensate to the flash steam is piped to a steam turbine or steam engine where
tank that flash steam is recovered at a lower pressure the energy in the steam is converted to power. The con-
and can be used for heating the building in the winter version of energy to create power is associated with a
and service water (domestic hot water at 120°F) in the drop in steam pressure and temperature. Each horse-
winter and summer. Note that the condensate from dif- power of mechanical energy is produced by removing
ferent pressure sources is always piped independently 2,545.1 btu per hour from the steam plus an allowance
to the flash tank so flashing condensate from the higher for losses due to inefficiency.
pressure sources doesn’t block the flow of condensate Many industrial plants produce superheated steam
from lower pressure sources. I do not like the use of in higher pressure boilers to generate power with steam
check valves that, supposedly, prevent high pressure turbines that exhaust at a nominal high-pressure for use
condensate entering low pressure equipment. in the facility. The turbines are referred to as “back pres-
If the flash steam is more than the demand for heat- sure turbines.” I have worked on plants that exhaust
ing and service water the safety relief valve (in this sys- steam at 15 psig after a turbine where the steam was
tem set at 2.5 psig) will open and waste the excess steam used mostly for evaporators in the process plant and
because there’s no place else to use it. Under normal other facilities with turbine exhaust pressures as high as
operating conditions there is no waste to atmosphere 150 psig for distribution to all users in a chemical plant.
because all the flash steam from the flashing higher pres- Now compare that low pressure plant with an
sure condensate is used in the 2 psig system. Conden- electrical utility like the one depicted in Figure 1-8. The
sate from the flash tank would produce more flash steam plant consists of two boilers and two turbine sets. Each
when entering the condensate return unit and that’s set of turbines (H.P. for high pressure, I.P. for intermedi-
why it is piped to a subsurface diffuser (pipe with holes ate pressure, and L.P. for low pressure) in this plant are
drilled in the bottom) so it can heat the colder conden- all driving one shaft connected to their respective elec-
sate from the heating and hot water systems. The steam tricity generator.
condenses as it heats the low pressure condensate. The Steam generated in the boiler is piped to headers
system practically eliminates any steam venting from either external to or within the boiler that connect to
the condensate unit and boiler feed tank (unless a trap superheater tubes which raise the temperature of the
has failed). steam from saturation to a maximum that’s normally
Why all that to save a little vented steam? Never… determined by the strength of the metal the superheater
never… never… think of vented steam as “a little.” Ev- tubes are made of. Utility plant superheaters are typi-