Page 103 - Boiler Operator’s Handbook

88 Boiler Operator’s Handbook

tom blowoff valves are not there to regulate the water formation in those tubes to promote subsequent failure.
level; if the water continuously runs high then get the Water tube boilers are very susceptible to that form of
level controls fixed. The bottom blowoff valves are not damage. There should be written procedures in any
there to lower the concentration of solids in the boiler plant for performing a bottom blow and they should be
water, that’s what the continuous blowdown system complied with.
is for. Continuous blowdown removes water with the Since the purpose of a bottom blow is to remove
highest concentration of solids and, when diverted to solids from that mud drum you want to have enough
a blowdown heat recovery system, waste very little en- water flowing out to flush it well so the first step in pre-
ergy. paring for a bottom blowoff is to either temporarily raise
They are definitely not for maintaining boiler oper- the drum level controller setpoint, use manual control,
ation; I had a hard time believing an operator was blow- or bypass the feedwater valve to raise the boiler water
ing his boiler down every fifteen minutes so enough level up to within a couple of inches of the top of the
cold water was added to prevent the boiler cycling off; glass. That provides the maximum reservoir of water for
he was wasting fuel, water, and his own energy to keep a good flush of the mud drum.
the boiler from doing something normal. Operating Open the first valve (more later on which valve
the bottom blowoff valves without concern for operat- gets opened first) then crack (see valve manipulation)
ing conditions can interrupt boiler water circulation to the second valve to allow some water to slowly drain
result in an eventual failure. Use them only for their out of the boiler and heat up the blowoff piping and
intended purpose. flash tank or blowoff tank. When the level in the gauge
The first and principal consideration for a bottom glass has dropped an inch, open the valve completely
blow is to make certain you are in control of it. I prefer to provide full flow to flush the mud out of the boiler.
they be done at the change of shift so two operators Then, when the level is about two inches from the bot-
are there to do it. You can do it yourself if, and only if, tom of the glass close the valves. Restore the setpoint or
you can see the gauge glass while you’re operating the automatic control to establish normal water level.
valves. There are very few boilers set up so you can do Continue to monitor the level until it returns to
that and it’s still a good thing to have another person on normal and check it frequently for about an hour af-
hand in case something goes wrong; I once had a blow- terward. I like to blow down the water column a few
off valve stick open. times at two minute intervals after the bottom blow; if
Whatever you do, don’t consider the blowoff any mud was left in the boiler it was loosened and will
an option to test the low water cutoff. I see that done show as color in the fresh water in the gauge glass. If I
regularly and ask the operator the question “What are see some color then I know I have to blow down more
you going to do the day the low water cutoff doesn’t frequently; usually when that happens I knew it was
work?” Oh, I get a lot of assuring answers but the only coming because the water supply or some other factor
right one is that operator will finally decide that some- that would increase solids accumulation in the boiler
thing’s wrong, close the blowoff valves and walk to the had changed.
front of the boiler to see the gauge glass empty and, as As to which valves to operate first; forget the argu-
in one case related to me, look into the furnace to see all ments about the valve closest to the boiler, that’s seldom
the tubes glowing red! If there’s no one there to keep an the criteria. It depends on the valves. If the two valves
eye on the gauge glass don’t blow the boiler down until are identical Y pattern globe valves then the closest is
someone is. Watch the glass every second until the entire opened first and closed last so all the erosion is concen-
process is complete. trated on the valve furthest from the boiler; however,
A bottom blow removes a considerable amount of such arrangements are unusual.
water in a very short time and can change the natural The most common mistake I see is associated with
circulation in the boiler. Unless the manufacturer’s in- the systems that have one slow opening valve and one
structions specifically state that a bottom blowoff can be quick opening valve. The operators tend to believe the
performed below a certain load never perform a bottom slow opening valve should be opened first and closed
blow without shutting down the burners. Never blow a last so you can give the boiler a real quick blow with that
boiler with loads above the limit prescribed by the boiler quick opening valve; after all, that’s why they put it on
manufacturer either. there, right? Nope, the quick opening valve is there be-
A bottom blow can temporarily stall flow in ris- cause you can open it quickly without anything flowing.
ers resulting in high concentration of solids and scale With the slow opening valve you don’t produce