Page 268 - Boiler plant and distribution system optimization manual

Steam System Optimization 253

The challenge is to get this heat back to the Acid Corrosion and Oxygen Attack In
boiler plant. The flash steam formed as the con- Piping Systems
densate leaves a trap is of first concern, approxi- A great part of condensate recovery system
mately half the energy in the trap exhaust is the failure can be traced to a feedwater treatment
energy in this flash steam. In an open system it problem that allows carbon dioxide to enter the
can escape to atmosphere through a vent in the distribution system piping. Oxygen also causes
receiver tank. problems but most are confined to the boiler and
If the condensate piping is not adequately economizer.
insulated, additional losses will occur. Carbon dioxide that carries over in the steam
Some pumping systems cannot handle con- forms carbonic acid which has the capacity to
densate above 180°F because of cavitation. There combine with one and a quarter pounds of steel
may be other operational problems that have per pound of CO , forming a groove in the bot-
2
lead to lower condensate return temperatures. tom of the piping. Over years this can eat up a lot
Some plants have actually devised ways to cool of metal and cause countless problems. Oxygen
condensate so they could get their pumps would pitting and scale formation can also destroy pip-
work. ing and boiler tubes as well as interfere with heat
transfer and the operation of pressure reducing
Good management of the condensate systems has valves and trap mechanisms.
additional benefits:
a. Reduction in replacement water costs.
b. Reduction in water treatment chemicals.
c. Reduction in fuel consumption used in pre-
heating the make-up water.

One solution to getting the hot or even boil-
ing condensate back to the boiler room is the
steam pressure powered pump (Figure 13.5). It
collects condensate and discharges it when the
liquid level reaches a certain point. It uses system
steam pressure for the pumping action, a small
investment for high overall energy savings.
The condensate pressure powered pump
eliminates many problems connected with han-
dling hot condensate with electric pumps. When
the condensate goes above 190°F they tend to
cavitate, forming steam in the suction end of the
pump. One common solution had been to cool
the condensate to a temperature where the old
pumps could handle it. Another common prac-
tice is to dump the condensate down a conve-
nient drain.
This type of pump is reported to use only 3
pounds of steam for every 1,000 pounds of liquid Figure 13.5—The operation of
pumped. When exhaust is vented back in a closed a pressure powered conden-
system, the steam is recovered and the cost of op- sate pump: (a) is the fill cycle,
eration is negligible. (b) is the pumping cycle and
(c) is the venting cycle.