Page 77 - Boiler_Operators_Handbook,_Second_Edition

62 Boiler Operator’s Handbook

It provides a lot of time to think about why the boiler remove the guns when the boiler is not to be fired and
tripped or how it did as you continue with the start-up; you can readily remove them. That way you have some
valuable time if you use it. degree of confidence that fuel can’t possibly get into the
I had a few service technicians working for me that boiler. It’s better for a leak to appear at the front where
thought it a nuisance and always shortened the purge you can see or smell it than to quietly create an explosive
time (remember, all they had to do was spin a knob on condition inside the boiler. The most expensive boiler ac-
a timing relay to change it). Their skill and experience cident to date, a billion dollars worth, was the result of
allowed them some leeway in breaking the rules and, leaking fuel. If you can’t break a fitting to show leakage
luckily for them, they normally got away with nothing at the front then you should check an idle boiler regularly
more than a few singed eyebrows when they stretched when there is (or could be) fuel in the burner piping.
it too far. You don’t have that skill and experience so Once a purge is complete modulating boilers
don’t play Russian Roulette with a boiler explosion. Do should be positioned for light-off. Because most boilers
a complete purge. light off at low fire we commonly refer to this as the low
A purge must be proven before you start timing fire position, not the light-off position and I will follow
it and the purge conditions must be proven during the suit. You should be aware that light-off position and low
entire purge period. Purge proving is one thing very few fire do not have to be the same. Once a burner is operat-
systems do well and you should assure yourself that the ing it can usually remain stable at firing rates lower than
system on your new burner really proves a purge air rates required to achieve a smooth light-off.
flow exists. I insist on installing a purge air flow prov- Where loads can require a boiler to operate at very
ing device that actually measures the air flow but I dis- low firing rates on occasion, and it’s more desirable to
covered that can be defeated (see why they fail) so I’m keep a boiler going, separate minimum (low) fire and
confident no automatic control system can really prove light-off positions may be established. In those instances
the purge air flow. the position switches have to prove the settings are high
The boiler operator should be the final authority enough for ignition as well as low to minimize input
on purge air flow and ensure the automatic system’s during light-off.
acceptance of the condition is correct. On small boilers Low fire position switches have always proven to
the typical proof is a fan running and the controls at be difficult to maintain and set because the low fire is at
high fire position. As the boiler size increases, a device the minimum stops described earlier. I’ve never quite
to monitor flow should be provided and one that mea- understood why they’re such a problem because the
sures air flow just like I suggested for combustion air position proving switch(es) do not have to be set right at
flow is best. A proven purge is imperative for safe boiler the minimum position. I do know that many technicians
operation. Many of the explosions and regular puffing try to do just that but it’s simply easier than doing the
I’ve experienced were the result of an inadequate or more logical thing which is to determine an acceptable
non-existent purge (see that chapter on why they fail) so upper limit for light-off and adjust the low fire switch
don’t be satisfied with anything less and don’t trust the accordingly. The acceptable upper limit is determined
sensors entirely. by increasing the firing rate until lightoff gets rough.
That’s above the upper limit and you back off it a little.
Open Fuel Supply, Prove Lightoff Conditions: There’s plenty of room for switch adjustment on a
When you’re satisfied the purge system is working multiple burner boiler because low fire has to be estab-
properly you can open manual fuel shut-off valves to lished by an independent means of control. The mini-
bring fuel up to the safety shut-off valves. Don’t open mum stop on the main fuel flow control valve should
the burner shut-off valves yet. The piping should be be set so the flow produces a pressure slightly less than
checked to ensure the fuel is up to the safety shut-off desired at low fire with one burner in operation. The ad-
valves and there are no leaks before proceeding. You ditional flow can then be provided by the minimum fire
should also perform a leak test of the fuel safety shut- controls. I always use minimum fire pressure regulators
off valves (see maintenance) to ensure they’re working which bypass the main fuel control valve to maintain a
properly before proceeding. I know, they’re new valves; certain minimum pressure in the burner header regard-
but I also know that valves leak, even new ones! less of the number of burners in operation. Setting the
After that final check you can install the oil guns or main control valve with its minimum stop to produce
gas guns that you intentionally left out so no fuel could almost enough flow for one burner helps make it pos-
get into the boiler. It’s a good habit to get into, always sible to keep the boiler from losing all burners in the