Page 412 - Boiler_Operators_Handbook,_Second_Edition

Controls 397

sition settings without permission. That last one is a real discussed the importance of maintaining an optimum
key because many states have adopted the ASME and air to fuel ratio. Part of the job of firing rate controls is
NFPA standards that relate to burner management and to maintain an air to fuel ratio that is adequate for safe
both standards are very exacting about the requirements and efficient operation of the burner and boiler. There
for changes in burner management systems. are different control schemes for controlling the fuel and
No discussion of a burner management system air, to maintain the air to fuel ratio, and their ability to
should be left without mentioning the important con- do the job varies with system cost and complexity.
cept of fail-safe design. Every element of the system The choice of control system for your boilers will
should be arranged so it’s failure will not compromise depend primarily on the size of the boilers. Size of the
the safety of the boiler operation. Each wire, relay, pres- boilers implies a certain annual fuel consumption and
sure switch, etc., should be evaluated for failure modes the increasing cost of more refined controls has to be
and analyzed for what will happen if the device fails. weighed against the savings that can be produced by
Only when every evaluation indicates the result will be improving the controls for better control of air to fuel
safe should the system be considered fail-safe. ratio. There’s also the question of maintaining a certain
Fail-safe concepts should be applied to all controls steam or vapor pressure or a boiler outlet temperature
and applied in a sensible manner. Too many designers that may, or may not, be critical to the facility served by
view fail-safe solutions as only resulting in a complete the boiler plant. If the pressure or temperature is critical
burner shutdown. That’s not necessarily the safest thing the controls will be more refined.
to do because, while that burner is operating, most of I have seen boiler plants where there were no pres-
the furnace and boiler is full of inert gas. There are many sure controls. In one the operators increased the firing
other examples where a shutdown is not necessarily the rate when the pressure got down to around 5 psig and
safest solution to a failure. backed it down when the pressure got up to 90 psig.
There are always arguments as to what is safe as They raised that low point in the winter to 40 psig be-
well. Is it better to have a feedwater valve fail open, so cause anything less produced complaints in the college.
the boiler will not run dry? Most of the time we have the That’s an extremely clumsy operation that could
valve fail closed because there is no safety to prevent cause a considerable number of problems both for the
water flying down the steam lines and hammering them operators and the equipment but they managed to keep
apart but we should expect the low water cutoff to safely the facility happy with that performance and that’s all
shutdown the boiler. they cared about. Not very wise was it?
If you’re replacing a component of a control sys- Swinging pressures will vary blowdown rates, in-
tem, and it’s operation isn’t exactly the same as the piece crease the opportunities for carryover, and if not caught
you’re replacing, consider what will happen if it fails. at the right time, result in boiler shutdown or lifting of
Much thought has gone into deciding if a particular safety valves which do reflect on the performance of the
component will fail in the safest manner and replacing it operators. The changes in temperature are adequate to
with one of another action could reduce the safety and/ define the operation as cycling and the standard boiler
or reliability of your plant. is constructed for a life of 7,000 cycles; swinging opera-
tion shortens boiler life. My perception of that operation
provoked words like careless, lazy, and inconsiderate
FIRING RATE CONTROL—GENERAL to name some of the printable ones. The boilers were
equipped with firing rate controls but they were ei-
Firing rate controls regulate the flow of fuel and ther inoperable due to no maintenance or not used for
combustion air to the burner to produce a flame and reasons I can’t begin to understand. If the temperature
heat input that satisfies the demand for heat at the boiler swings you are inviting problems with thermal stress.
outlet. We’ll also call them combustion controls. These A low pressure steam plant can swing from a low
are independent of the steam pressure controls on any of 8 psig to a high of 12 psig with a temperature swing of
system except a simple jackshaft system. Typically we 9 degrees; to me that’s the limit. Higher pressure plants
don’t talk of combustion controls or firing rate control have thicker boiler parts and swings of more than 4 or 5
with a jackshaft system. degrees can cause problems with thermal stress in them
The heat input is primarily a function of the amount so normal pressure swings should be held to less than 10
of fuel flowing to the fire; control of air is also required pounds.
to produce the heat input. In the chapter on fuels we Another common trick when maintenance is lack-