Page 142 - Boiler_Operators_Handbook,_Second_Edition

Special Systems 127

lot will flash off as it drops in pressure at a leak and flow can be warmed up without pumping it’s cold contents
out as pure steam. All the mud and sediment that was into the system piping. There might be situations and
in that water stays in the system. It’s one reason leaks conditions where you have slugs of cold water in the
aren’t as much of a problem, the remaining mud and piping even though the boiler is up to temperature and
sludge plug the leak. careful manipulation of the boiler’s isolating valves is
It’s safe to say you can blow down a new system required to warm up that piping.
once a month as long as makeup is minimal. Remember It’s best to crack open one of the two valves (re-
that blowing down removes water so you will have to turn or supply) connecting the boiler to the system be-
add makeup water and more treatment chemicals with fore starting the boiler to maintain consistent pressures
it to replace what you blew down. Watching the first throughout the system. Leaving one valve open when a
gush out the drain valve will be the clue to frequency. boiler is out of service but not isolated for repair or other
Normally a hydronic system should be tested for TDS purposes is not a bad idea. The selected valve should be
(see chemical water treatment) just like a steam system in a position where thermosyphoning will not generate
and the blowdown should be managed to keep TDS any thermal shock, sometimes warming the boiler up
below a prescribed value (usually 2500 ppm). However, with a valve open allows thermosyphoning to warm up
if you see a slug of mud (the water will be discolored) piping to avoid thermal shock. Since every plant is differ-
for more than ten seconds you’re not blowing down ent you should develop an SOP that allows starting and
frequently enough, increase the frequency. No sludge, engaging a hydronic boiler with minimal thermal shock.
decrease it. Arrangements of hydronic boilers in multi-boiler
TDS is dissolved solids, not settled solids so there’s plants come in two forms. Parallel installations (Figure
a distinct difference and unlike a steam system (where ev- 4-4) are most common and can be used with any number
erything solid stays in the boiler because it can’t become of boilers. Serial installations (Figure 4-5) are less com-
a gas and leave with the steam) the settled solids tend to mon and the number of boilers is limited to two or three.
pick many points in the hydronic system to accumulate. In parallel installations each boiler handles a portion of
Don’t believe that old lie that you don’t have to do the system water and care is recommended to ensure the
any water chemistry testing and maintenance in a hy- water flows to each boiler uniformly.
dronic system. Even systems with zero leaks have prob- In some parallel installations the system water is
lems with the water chemistry changing as it reacts with left flowing through each boiler so a boiler that is shut
the metals in the systems and any air it comes in contact down acts as a radiator, wasting heat to the air that is
with. It’s essential to maintain the proper pH of the sys- drawn through it by stack effect to actually cool the sys-
tem and a supply of Nitrite or Sulfite to prevent corrosion tem water. If you can’t do anything else about this type
due to oxygen getting in. (See water treatment.)
If you have system leaks that must be replaced by
makeup water then that water has to be treated. As sys-
tems grow older the number of leaks tend to increase,
despite good maintenance practices; and the water
treatment program has to improve to handle the large
volumes of makeup water. Many hydronic systems are
equipped with nothing to pretreat the water (see water
treatment) so more chemicals are required and in many
cases adding pretreating equipment is justified.
In my experience the major concern with hydronic
boilers is preventing thermal shock. Be sure to read the
chapter on thermal shock in the section on why boilers
fail. It’s particularly important when the plant has more
than one boiler because you have to avoid sending a
slug of cold water from an idle boiler into a system op-
erating on another boiler and avoid dumping hot water
into a boiler that’s cold.
Most hydronic heating plants permit firing the
boiler without any water flow through it so the boiler Figure 4-4. Hydronic boilers in parallel