Page 271 - Boiler plant and distribution system optimization manual

256 Boiler Plant and Distribution System Optimization Manual

Severe damage can occur when the incom- generator operating procedures may also have to
pressible slug of water is forced to change direc- be investigated and changed.
tion by a pipe bend, fitting or valve. It produces All of the steam generated by the boiler must
a shock wave of tremendous momentary force eventually leave the system as condensate, except
which can cause great physical damage. It is a for leaks and steam directly used for some pur-
function of the mass of water and the velocity pose. It is obvious that if the steam can somehow
squared, so the greater part of its energy comes escape from the system without going through a
from the high velocities (6,000-12,000 feet per trap as condensate, the system will be inefficient
minute) achieved in the steam piping. to that degree. Properly working traps insure that
Some steam boilers have a tendency to pro- steam gives up its energy efficiently.
duce low quality steam, which contains an ap-
preciable amount of moisture and possibly boiler Air and Noncondensibles in the
chemicals and contaminated boiler water. This in- Steam System
troduces water into the boiler header and distri- Accumulations of air and noncondensible
bution piping. Depending on the severity, this can gases in the steam system can limit steam flow,
overload the distribution and trapping systems. steam temperature and heat energy release. Air
To detect this problem a Throttling Calorim- is present in the system on startup and is intro-
eter can be used to test for steam quality (percent duced through vacuum breakers.
moisture in the steam). This instrument measures Noncondensible gases are liberated in the
the temperature of a very small amount of steam boiler. Carbon dioxide and oxygen are dissolved
discharging from the steam main. Dry steam will in boiler feedwater as carbonates and bicarbon-
become superheated due to the pressure drop. If
there is any moisture present, the temperature of
the discharge will be reduced. Measuring tem-
perature and comparing it to the maximum pos-
sible temperature will indicate steam quality.
Other methods that can be used are:

a. Ion exchange

b. Conductivity

c. Sodium tracer flame photometry

d. Specific ion electrodes.

These methods determine the solids content
of the steam, including the solids carried over by
water droplets.
The throttling steam calorimeter is the only
direct way to determine steam quality and is most
accurate below 600 psi and where moisture con-
tent is above 0.5 percent.
When low water quality is detected, it indi-
cates a problem with the water level control, feed-
water treatment, blowdown cycle, boiler sizing or Figure 14.4—Shows several forms of steam and con-
steam drum internal problems. Boiler and steam densate mixing in piping.