Page 405 - Boiler_Operators_Handbook,_Second_Edition

390 Boiler Operator’s Handbook

ing for those differences. The typical HTHW boiler load prevent crushing them so they get heavier and the float
control system is shown in the schematic in Figure 11-22. chamber has to be thicker (see strength of materials) so
Refer to the following discussion on two element boiler they become uneconomical. They do work fine for open
water level control, for an explanation of this particular tanks at small flow rates. One place where float controls
type of control loop. have problems that you can relate to is in brine tanks
used with water softeners. The salt tends to crystalize on
the float and surrounding materials, usually a still pipe
(a pipe placed around a float to prevent swinging opera-
tion due to wave action) so it can be trapped in the brine
crystals and fail to operate.
A float that only has to open and close an electri-
cal contact can be quite small by comparison to some-
thing that has to open and close a valve so we have
many systems controlled by float operated switches.
The switch can energize a solenoid valve to open it
and admit fluid to the tank or boiler. All the energy
required to operate the valve is provided by the elec-
tricity (and in many cases the fluid itself, see pilot
operated valves in the general discussion on controls)
so a small float can control any volume of fluid at any
difference in pressure. The float still requires a change
in level to function and only provides on-off control
of the fluid flow but that’s satisfactory in many situ-
ations. The switch can also be set to power a valve as
the level rises to provide a system that allows fluid to
flow on controller failure.
The typical heating steam boiler and small com-
Figure 11-22. HTHW boiler control
mercial and industrial boilers use float controls that start
and stop the boiler feed pumps to control feedwater flow
for maintenance of the water level instead of controlling
FLUID LEVEL MAINTENANCE
a valve. These systems solve some of the problems with
There are several locations where water level must valve control by preventing operation of the feed pump
be maintained in a boiler plant but the most important is when the control valve shuts off, a situation that would
the level in the boiler itself. The method of control varies overheat the pump. It also eliminates feedwater control
significantly depending on the size and complexity of valves as a maintenance item.
the plant. The simplest is a float controlled valve and the Each boiler has to have its own pump for this con-
most complex (and expensive) is a three element boiler trol method to work and operation of standby pumps
drum level control loop. Each has its place, its advan- is complicated because the electrical control has to be
tages, and its problems. I’ll try to give you all those in switched along with pump isolation valves. It is a sim-
the following paragraphs. ple and inexpensive method for level control and works
A float controller for level control is common in well in many applications. However, it can’t be used
boiler water feed tanks, condensate tanks, make-up with economizers and the higher electrical demand
tanks and other source tanks of water for the boiler (pump and motor are normally sized at twice the boiler
plant. They are found in boiler feed service only on capacity) can create higher electrical power costs.
residential and small commercial boilers. We’ve covered If you’re operating a boiler with very little reserve
their operation in our earlier general discussion of con- capacity like most water-tube boilers, you have an econ-
trols. They’re not found on large or high pressure boilers omizer, or you can’t tolerate the swings in load associ-
because the float would have to be very large to produce ated with feed pump on-off control a variable feed level
the force needed to operate the control valve while oper- control is required; one that modulates the feedwater
ating on a very small change in level. flow control valve to maintain the level.
As they get larger they have to get stronger to If a boiler has little reserve in it the cold feedwater