MIL-HDBK-1003/7
HDBK-1003/6 Section 6, Table 13. For small packaged boilers and boilers with constant
loads a single or two element feedwater control system will suffice. Larger boilers and
boilers with widely fluctuating loads should use a three element type. An automatic
fuel trip device is required for low drum water level.
Steam Turbine Generator Control. Steam turbine controls should be supplied by
11.2.4
the turbine manufacturer. With the rapid advancement of electronics, an electro-
hydraulic control (EHC) system is desirable over an all mechanical system. The EHC
system is more versatile and faster responding, which gives improvement in control
reliability and accuracy which is manifested in improved turbine performance. See
Figure 38 for a typical
EHC system. The Mechanical Hydraulic Control System (MHC) has not been used since the
late 1970's. Many are still in use but they are for
the most part not a currently manufactured unit. Automatic startup and shutdown is
easily done with microprocessor based control equipment. A standard EHC system is
comprised of four primary functions:
a)
Speed control
b)
Load control
c)
Flow control
d)
Trip
Safety Devices and Interlocks. Safety devices such as boiler shutdown
11.3
devices, startup and shutdown interlocks, and alarms should be installed as recommended
by the equipment manufacturer. See Table 14 in MIL-HDBK-1003/6 Section 6 for selection
factors and reasons for use. Coal, oil, and gas fired boilers can have different
shutdown safety trips. Check boiler manufacturer's recommendations and ASME Boiler and
Pressure Vessels Code Section VII for standard trip conditions.
Safety controls for boilers are needed for protection against explosions and
implosions. Criteria for protection against boiler explosions have been well
established. Protection against boiler implosions is not so well understood. For a
boiler, implosion would be the result of a negative pressure excursion of sufficient
magnitude to cause structural damage. Boiler implosions are caused by one of two basic
mechanisms: (1) The induced draft fan of a balanced draft boiler is capable of providing
more suction head than the boiler structure is capable of withstanding and (2) the so-
called flame collapse or flameout effect.
Implosion concerns have resulted in many new control-system developments.
Each steam generator manufacturer has recommendations and the National Fire Protection
Association has issued NFPA 85-G on the subject. For a detailed discussion of boiler
implosions, see also Combustion Engineering, Inc, 1981.
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