Page 435 - Boiler_Operators_Handbook,_Second_Edition

420 Boiler Operator’s Handbook

sor based controls using the stack temperature could running (in case one failed) in the summertime when
permit varying the setpoint to minus two tenths for one was four times larger than the actual load. Main-
start-up increasing to minus five hundredths for normal taining the header pressure by recirculating the water
operation. ensures that the pump runs at full capacity (maximum
horsepower) all the time.
That industrial plant was running one pump at 30
FEEDWATER PRESSURE CONTROLS horsepower for nothing but the mental comfort of the
operators, in case the other one failed. Total cost for that
I decided to add this control consideration because pump operation was 52.5 horsepower more than neces-
it is unique to boiler plants. In many cases I consider it to sary, equal in 1997 energy costs to about $5,000 per year.
be done improperly so I’ll cover what’s been done, why That’s money that never became a bonus for the opera-
it was needed, and what you should consider for your tors. If you have one of those systems, don’t abuse the
plant. power bill further by lowering the pressure setpoint one
Why even control the feedwater pressure? If you psi lower than it has to be.
read the chapter on pumps you know the differential If it’s necessary to control feedwater header pres-
follows the pump curve and as long as the discharge sure with electric motor driven feed pumps try to get
pressure is less than the maximum pressure rating of an evaluation of the application of one or more variable
the pump and piping there’s no way the pressure can speed drives (one for each pump preferably) because
get too high. Some pumps do have a rather steep curve they can be used to maintain pressure by slowing the
so we may choose to do something about the pressure pump down and saving on the horsepower. There’s a
getting too high but most of the time the problem is with practical limit to how slow they can go but most of the
the feedwater control valves. time they will provide all the pressure control that’s nec-
A pneumatic or electro-hydraulic actuated feed- essary. As with other things technology improvements
water control valve can be selected with an adequate and manufacturing cost reductions has made such con-
diaphragm or enough hydraulic pressure to keep the trols a wise investment.
valve closed under conditions of the maximum feed All constantly operating boiler feed pumps have
pump discharge pressure and no pressure in the boiler. a potential problem with overheating, cavitation, and
The thermo-hydraulic and thermo-mechanical valves pump damage that can occur if all the feedwater control
described earlier had limited power and in most cases valves shut off. Temporary upsets in plant operations
couldn’t operate with a pressure differential greater can result in high water levels in all the boilers so that
than thirty to fifty pounds per square inch. happens. If the water doesn’t flow through the pump
Another reason for pressure control was to im- then it just sits there and churns; all the energy put in by
prove operating efficiency; turbine driven boiler feed the motor is converted to heat that raises the tempera-
pumps could be controlled using feedwater header ture of the water. The water that left the deaerator was
pressure or the difference between feedwater and steam nearly at saturation conditions so the additional heat
header pressures to throttle the steam to the turbine. It will most certainly result in steam generation, cavita-
reduced steam flow through the turbine to save energy. tion, and pump damage (see the discussion on pumps).
Actually it saved by allowing operation of more auxil- To prevent the damage from such an incident some
iary turbines to eliminate motor operating costs. feedwater circulation is provided. You could argue that
The normal practice for maintaining a constant the recirculation provided by pump discharge pressure
feedwater header pressure, or a differential between control solves this problem, and it does, but at a very
feedwater and steam headers, consisted of installation significant operating expense. The standard practice in
of a control valve that dumped feedwater back into the my early days was to install a small recirculating line on
deaerator or boiler feedwater tank. I’ll admit that I de- each pump that returned enough water to the deaerator
signed and installed a lot of systems that did that before to prevent overheating of the water.
I became more energy conscious and started question- An orifice nipple is made for those recirculating
ing why we did what we did. lines and the recirculating lines were usually 3/4-inch
Today I know that method maintains a header pipe size so we could make the orifice out of a 1-inch
pressure or differential but it also wastes a lot of energy. diameter steel rod. One was installed on each pump to
I think the first time it was apparent to me was in an in- provide protection in the event we were so dumb as to
dustrial plant where the operators had two feed pumps start a pump with the discharge valve shut then forget