Page 99 - Boiler_Operators_Handbook,_Second_Edition

84 Boiler Operator’s Handbook

supply of one is interrupted. It also provides a basis for and a short interruption in steam supply or a dip in
negotiating price with the suppliers. Most boiler opera- steam pressure or water temperature is not considered a
tors don’t make the fuel supply or price decisions but problem. At a prescribed cold temperature the controls
they should be prepared to choose, and choose wisely, stop boiler operation then automatically restart it on the
which fuel to burn. alternate fuel. When the temperature rises to a higher
In most northern states the operator is informed by value the boiler is stopped then restarted on natural gas.
a phone call when to switch from natural gas to oil firing. Today there’s another reason for switching fu-
Their natural gas is purchased in accordance with a spe- els and it’s more important for the boiler operator to
cial contract so the supply is “interruptible.” It’s a method become involved because it relates to the fast paced
that benefits the gas supplier and the consumer. The large financial situations of today. Many gas contracts today
pipelines that transport gas from the southern states, do not set a fixed price for gas. The price varies accord-
principally Texas and Louisiana, have a maximum capac- ing to any one or more sets of rules or price indices. A
ity. The pipeline owners want to optimize the use of those typical index is “well-head price” meaning the price of
pipelines. They are limited by the pipeline capacity to the the gas where it is extracted from the ground. Currently
customers that are supplied “firm” gas. Those firm gas that price is set for each month but it could easily be set
customers don’t use much, if any, during the summer and hourly in the future.
when outdoor temperatures are mild so there’s always The boiler operator may have to watch the Inter-
room in the pipelines for more gas to flow except on very net on a computer in the control room to be prepared
cold days. By selling interruptible gas the pipelines make to switch fuels when the gas price goes high enough.
use of that extra room in the pipeline. The purchaser gets 2000-2001 produced some significant swings in natural
a discount, paying less for interruptible gas, and that’s gas pricing with prices ranging from $2.97 to $10.81 per
why both benefit. The only compromise for the purchaser Decatherm when fuel oil cost was about $7.12 per De-
is a switch to an alternate fuel when notified by the sup- catherm. There were a few plant chiefs called upon to
plier of an interruption. answer why they continued firing natural gas when it
Once you’re familiar with your plant you will was cheaper to fire oil.
know an interruption is coming most of the time. On Pricing is the principle reason for fuel switching
rare occasions the supplier may have to take a pipeline but loss of service is another. During an earthquake
out of service for maintenance or repair and will require buried gas piping is typically interrupted. I’ve also ex-
an interruption but most of them are due to load (see perienced interruptions due to contractors digging into
Know your Load, page 93). Most of the time a weather the gas mains and gas piping breaks from flooding that
forecast will forewarn you that you will have to stop fir- washed the line out. Sudden ruptures can also interrupt
ing gas and change to an alternate fuel. You’ll also know your gas service so having an alternate oil supply is a
about when you will receive a call that allows you to way of recovering from those situations.
switch back to gas. As with AIS the simple way to switch fuels is to
Here’s an appropriate word of caution when shut the boiler down then restart it on the alternate fuel.
considering a fuel transfer. There’s no such thing as a One of the reasons AIS is seldom utilized today is many
“flick of the switch” fuel transfer. I’ve had to observe people didn’t manage to get that right. There were sev-
the cleanup from a couple of boilers where someone eral failures in the 1970’s associated with systems creat-
thought it was that simple. Most boilers have to shut ed that simply switched fuel valves (the flick business).
down and go through a regular boiler start-up to change The installer or designer didn’t understand that could
from one fuel to the other. The idiots that believe in result in a loss of flame with continued admission of
“flick of the switch” end up blowing up boilers. fuel and a subsequent explosion. So, unless your system
You might even have a plant that automatically is specifically designed as one of the two “on-the-fly”
switches from gas to oil and vice versa. You’ll have switching systems I’m about to describe, shutting down
what is called an “automatic interruptible gas service” then starting on the second fuel is your only option.
controlled by an automatic interruptible system (AIS). One favored method of fuel switching is the “low
Those consist of a set of controls in a panel, normally fire changeover” method. The alternate fuel system (for
sealed by the gas supplier, that sense outdoor air tem- the one presently not firing) is placed in service to bring

perature and control the boilers to automatically switch the fuel supply up to the safety shut-off valves. The
fuel. These are typically small heating boiler plants operator also makes certain the manual burner shut-off
where only one boiler is required to carry the peak load valve for the alternate fuel is closed. The controls are