TM 5-810-15
CHAPTER 9
INSTRUMENTS AND CONTROLS
quirements, system security requirements, and
9-1. General.
LCCA.
This chapter addresses the criteria for the selection
(3) Distributed control systems include the
of instruments and controls to meet the
process input output sensors and actuators which
requirements of the steam plant.
are connected to the termination units which
a. Pneumatic controls. Most control system
condition and multiplex the signals for communi-
manufacturers have discontinued production of
cation to the microprocessor unit controllers where
pneumatic controls systems. Replacement parts and
the logic resides for control of the process variable.
qualified service for the equipment are difficult to
All microprocessor unit controllers communicate
procure. For these reasons pneumatic controls
with each other and to a data highway which
should not be used for new installation.
includes nodes for the operator interface station,
b. Electronic controls. Electronic systems have
engineering work station, and programmable logic
been made obsolete by the microprocessor based
controllers for balance of plant controls.
control systems. Manufacturers no longer make
(4) The distributed control system logic for the
electronic control systems and they should not be
controls and the 110 for the process are located in
specified for new installations.
the vicinity of the process. This process controller
c. Microprocessor based controls. Micropro-
will communicate with other process controllers,
cessor based control systems can provide sequential
engineers work station, data acquisition system and
logic control and modulating control in one control
the operator interface. The control system will be
device. This capability makes available boiler con-
configured to allow the process controller to
trol systems, which use both sequential logic and
continue to function upon loss of communication
modulating control, that are more flexible and
with the operator interface, data acquisition system
reliable as well as more cost effective. Processing
and other process controllers.
units can be utilized as single loop controllers or
more powerful processing units can be applied to
to ensure control system reliability will be based on
individual control subsystems, such as combustion
unit size, importance to plant operation, and the
control of ash handling control.
cost of control system failure versus the cost of
(1) Inputs from and outputs to field devices
backup hardware.
may be multiplexed. Data highways connect all
(1) Power sources. Power to the control
processing units to data storage and acquisition
system must have a backup supply. Microprocessor
components, cathode ray tube (CRT) displays and
based systems must have a backup power supply
operator consoles, loggers, and printers, providing
either from a separate ac source or an
communication among all components. Communi-
uninterruptable power supply (UPS). Loss of either
cations between modulating control devices and
the primary power source or the backup power
sequential logic flow freely and are not subject to
service must be detected and alarmed. Loss of
the restrictions inherent with analog and mechanical
either supply alone must not affect operation of the
relay electronic systems which require hard-wiring
controls. Distributed control systems will include
between components. Data acquisition and
power supplies which are redundant or backed up
operator interface for control may be accomplished
so that loss of any power supply will not cause loss
using CRT*s, keyboards and printers or through
of power to the control logic. Loss of power supply
control stations indicators and recorders mounted
should be alarmed.
on an operator console. When CRT*s, keyboards
(2) C o n t r o l
system
safeguards.
and printers are utilized redundant microprocessors
Microprocessor based controls are highly reliable
are sometimes utilized depending on unit size.
but safeguards must be provided to limit the effects
(2) Program logic can be changed or expanded
of component failure. Microprocessor based
readily with limited hardware revisions. System
control systems require grounding to a ground mat
selection can range from programmable controller
with an impedance of three ohms or less for
systems to fully distributed digital control systems.
protection of system components to reduce forced
The criteria for selection of the proper
plant outages. Spare circuit cards for critical
microprocessor based system will include unit size,
components are to be available at the boiler plant,
the amount and type of modulating control and
as well as spare microprocessing units that can be
sequential logic required, operator interface re-
9-1
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