Page 104 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 89

sudden changes in flow and you crack the valve So you should only use bottom blowoff valves to
to warm up the piping slowly. I can still remember remove sediment or drain the boiler and operate them
watching one operator whip a valve open to imme- properly so you don’t thermal shock them and the pip-
diately fill cold blowoff piping with hot boiler water. ing too much. If you do use them to drain the boiler be
Then I watched the little puffs of steam where the sure to close them off once it’s drained and you’re ready
cracks in the piping had formed from repeated shocks to open the boiler. It’s very embarrassing when you
of that nature rise up between his legs (he was strad- blow a lot of dirty water into a boiler you just drained
dling the blowoff piping). He obviously had no con- because you forgot to close the valves. It’s downright
cern for the life of the family jewels. It’s bad enough to dangerous too. The valve closest to the boiler in Figure
hit cold piping with 212°F water, let alone water well 2-6 should be removed and the remaining valve locked
over 350°F. and tagged closed before anyone enters the boiler.
Seatless blowoff valves (Figure 2-6) must be oper-
ated in a manner based on their arrangement. The piston
assembly in the valves creates a void as they’re opened CONTINUOUS (SURFACE) BLOWDOWN
and closes one as they’re closed. The valve closest to the
boiler in this picture is opened last and closed first. The Only we old-timers use the term surface blow-
piston in the second valve in line creates a void in the down. Both continuous blowdown and surface blow-
blowoff piping when it’s opened, drawing back some air down are basically the same thing. In the early years
or water, and pushes it out as it’s closed. If the second of steam power we used a lot of reciprocating steam
valve were closed first the piston in the valve closest to engines and they required lubrication. As a result lubri-
the boiler would act to compress the water between the cant ended up in the condensate and was returned to
two valves as it’s closed. the boiler. We did have systems to remove the lubricant
I’ve watched operators do that, many times using but there’s no reason for discussing them today. Today
a valve wrench because they had to apply enough force a reciprocating engine would use Teflon seals that can
to squeeze the water out the packing or gasketed joint, operate without oil lubricants. We used the surface
and they always complained because the valve was so blowdown to remove the oil and similar contaminants
difficult to close. If they’re unlucky they’ll compress the from the surface of the boiler water.
water and produce pressures so high that the gasket will Today, we should always use the term continuous
blow out of the flange and hit them in the head or, more blowdown. The purpose of continuous blowdown is
likely, those family jewels. removing concentrations of minerals dissolved in the
boiler water. It should be noted that with the advent
of chelants and polymers that solids and suspended
as well as dissolved minerals are also removed by con-
tinuous blowdown. When we were using phosphates
to remove dissolved minerals, by converting them to a
nonadherent sludge, that sludge had to be removed by
bottom blowoff. With current modern chemical water
treatment very little bottom blowoff is required.
Why is it connected to the boiler near the surface if
it isn’t surface blowdown? It’s because that’s where the
steam and water rising in the boiler separated so the wa-
ter at that point has the highest concentration of solids,
more than anywhere else in the boiler.
Unlike bottom blowoff continuous blowdown
should be continuous as its name implies. One reason
is there are limits to the concentration of TDS (total dis-
solved solids) in the boiler water, normally based on
operating pressure. If we blowdown intermittently then
we have to maintain an average concentration of TDS
that is lower than the limit. With continuous blowdown
Figure 2-6. Seatless blowoff valves we can hold the TDS concentration near the limit. That