Page 145 - Boiler_Operators_Handbook,_Second_Edition

130 Boiler Operator’s Handbook

could be overheated. The pump seal or packing would ing a boiler and putting it in service. The temperature
also be exposed to those high temperatures and few can differences between atmospheric and operating condi-
handle it. Any leakage of the hot water along the shaft tions are significant.
would start flashing into steam and that could do seri- You should be careful so you don’t suddenly ex-
ous damage to shaft and seal or packing. pose metal at 80°F to high temperature water at 390°F. In
To prevent problems with the seals or packing the some circumstances that’s difficult to do but operations
circulating pumps are normally fitted with sealing fluid that mix the two fluids (hot and cold) to gradually warm
systems. Where the seal or packing is exposed to the suc- up a boiler, pump, or piping system can be managed.
tion side of the pump sealing fluid is commonly drawn Steps in bringing a boiler on line and taking one off line
off the pump discharge. Some may extract water using a can get very involved because the pumping and piping
Pitot tube inside the discharge of the pump so the velocity arrangements have to be reconfigured to ensure even
pressure is used to generate the differential to move the distribution of the load on the boilers.
sealing fluid. In others it may be necessary to have a seal I have encountered plants with piping arrange-
pump draw water off the system and produce the dif- ments that restricted single boiler operation during peri-
ferential necessary to force the water through the sealing ods of low load to a particular boiler because the system
fluid system. Newer pumps may be fitted with a special arrangement didn’t permit isolating the other boilers.
impeller on the shaft inside the seal housing that pumps In another plant where the facility load had increased
liquid through the cooler and back to the seal. significantly the design did not permit operating two
Sealing fluid systems typically consist of two ele- boilers to carry the load because there was no way to
ments, a strainer to remove any particulate that might arrange the piping to parallel the boilers. It’s possible
damage the pump seal, packing, or shaft, and a cooler to for HTHW boilers to operate in series but its uncommon
reduce the water temperature to values that the seal or and the piping arrangement has to provide for it.
packing can accommodate. After the sealing fluid passes Unlike low pressure hydronic plants HTHW boiler
through the strainer and the cooler it is returned to the systems seldom have accumulators large enough to hold
pump to flow over the seal and back into the pump and, all the expansion of the system from atmospheric to op-
in the case of packing provide the little leakage that erating conditions. A large pressure vessel designed to
separates the packing and the shaft. In the case of pack- hold several hundred gallons of water is very expensive
ing it’s supplied to a lantern ring (see pumps). Proper so they are occasionally reduced to a size that provides
control of the cooling of the sealing fluid is required to a cushion on the operation instead of allowing for com-
ensure the fluid isn’t overcooled to cause thermal shock. plete expansion and contraction.
The expansion tanks for HTHW plants are occa- Those larger plants are equipped with provisions
sionally called accumulators. They can serve the typical to fill the system as it cools from normal operating tem-
expansion tank role but can also become a storage space peratures and tanks that allow steam to flash off and
for the hot water. To limit corrosion problems at the high recover the remaining hot water as the system expands.
temperatures they are always pressurized with pure ni- In some cases the requirement for expansion tanks to ac-
trogen instead of air, although a true accumulator might commodate normal operating temperature swings is so
be pressurized with steam and can contain electric heat- great that even smaller tanks with operating and standby
ing coils to build up the steam pressure on a system provisions for fill and drain are installed instead, a lower
start-up and to maintain pressure when the system is pressure or open storage tank being used to prevent wast-
shut down. ing the treated water as the system heats and cools.
It’s common for the low water cutoffs to be mount- Any HTHW system requires makeup water pumps
ed on the accumulator because the generators don’t have to force the makeup water into the system. The pres-
any point where a low water level can be detected. To sure in a city water supply just isn’t adequate. Lack of
avoid thermal shocks in the system the makeup water is electric power in these plants can’t be tolerated because
added to the accumulator where there’s a considerable the liquid in the system will cool and shrink to require
volume of water for it to mix with before it hits any metal. makeup. A drop in pressure will result in steam flashing
Preventing thermal shock is even more of a prob- in some systems and driving water to others with much
lem in HTHW boiler plants. Most HTHW plants have noise and pipe rattling. The emergency electric genera-
more than one boiler (unlike the hydronic plant that tor is very important and some plants even have engine
typically has one) and the higher temperature operation driven makeup pumps as a backup.
requires careful management of the system when start- There is one more point I would like to make about