Page 359 - Boiler_Operators_Handbook,_Second_Edition

344 Boiler Operator’s Handbook

matically with modulating capability, it isn’t done con- this requires an independent feedwater pump and sup-
sistently. The only pumps that can provide modulating ply line for each boiler and will not work with a pump
control are chemical feed pumps which use a reciprocat- with a relatively flat curve. When the boiler is operating
ing hydraulic pump acting on a diaphragm with an ad- at low loads, recirculating some of the feedwater back
justment of the stroke of the reciprocating section. That’s to the deaerator, especially on boilers with economizers,
what the knob is for on the pump in Figure 10-70. will eliminate problems with surging of the pump and
The centrifugal pump, which serves the majority of flashing of the feedwater in the economizer.
applications, is self aligning so the flow through it is de- A more common means of boiler feed pump speed
termined by the system. By throttling the flow in the sys- control consists of maintaining a constant header pres-
tem at some point, preferably after the pump discharge, sure. That can be enhanced, and should be, by actu-
the differential pressure required to force liquid through ally maintaining a differential between the feedwater
the system increases and the flow through the pump de- pressure and the steam pressure. It’s invaluable for
creases as it follows the differential up the pump curve. good control during plant startup and in systems with
Applications with some centrifugals and most screw varying steam pressure. The control system should also
and gear pumps normally incorporate recirculation con- incorporate a set point adjustment relative to load to ac-
trol where the flow through the pump doesn’t change count for the pressure drop in the feedwater piping and
and a portion of that flow is diverted back to the pump in the steam piping before the pressure sensing point.
suction to achieve a final control of delivery pressure. Where I’ve seen systems that simply failed to
Advances in motor speed control have made some work the major problem was with piping friction losses
pump control projects possible that were not possible and most of those were in chilled water distribution
before. They are limited; varying the speed of a cen- systems. Application to systems with three way bypass
trifugal pump with a relatively flat performance curve valves on cooling equipment have to be changed to
doesn’t produce much of a savings in horsepower above simple two way valves to produce a change in demand
what the pump automatically provides. A pump with that can be sensed. The three way valves simply re-
a nearly flat curve will supply a reasonably constant directed flow so there was no way to reduce system
differential pressure automatically so there is no need flow. Problems also appear when significant differ-
for control. If the pump has a very steep curve varying ences between actual operating conditions and design
the speed will save power costs and allow differential conditions are introduced by such things as buildings
pressure control. If the differential required for varying shutting down or varying significantly in load. A com-
flow also varies with load then some potential savings mon significant shift in load is an auditorium or gym
by controlling pump speed is possible, even with pumps where cooling is required for the high concentration
with nearly flat curves. of people and/or activity on a cool or cold night when
Application of variable speed drives on pumps has the other buildings don’t require cooling. That’s usu-
resulted in significant operating cost savings because ally solved with application of a number of differential
(1) a pump is typically oversized like every other piece pressure transmitters in the system, artfully applied
of equipment, (2) systems seldom operate at the design so the pump speed controls can operate to maintain a
capacity, and (3) it permits tuning the pump to operate differential at the lowest measured differential in the
most efficiently under different load conditions. Don’t system.
be surprised, however, if you encounter a pump with Another potential problem is a pump was selected
a VSD control that’s simply operating at it’s highest carefully for the design condition and the efficiency of
speed. It’s not unusual for the system to have problems the pump drops off dramatically as speed is reduced.
that were not anticipated by the designer or other opera- The best one being to operate equipment that matches
tors that simply don’t understand the application and, the load and stretching out operating cycles.
therefore, solve problems by eliminating the automatic In those cases I tend to look for a replacement
control and running the pump full out. pump because it isn’t hard to find one that wasn’t the
I have one customer that manages to control the lowest first cost pump but, in terms of life cycle cost-
drum level in each boiler by controlling the speed of the ing, the most economical pump for that application.
respective boiler feed pump and it’s successful despite If it doesn’t seem to be working, or creates problems
many engineers telling me it can’t be done. I would have occasionally, seek out some help with the application.
said the same but came to understand how it works by Otherwise, do what makes sense to achieve an optimum
observing that customer’s system operation. Note that economical operating state for the pump.