SUBSECTION C5INSTRUMENTATION, CONTROLS, AND

INTERLOCKS

C5.100 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

C5.200 Indicators and Recorders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69C5.210 Preparation for and Placing in Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70C5.220 Checking and Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

C5.300 Controls and Interlocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70C5.310 Controls. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70C5.320 Interlocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71C5.330 Preparation for and Placing in Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71C5.340 Checking and Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

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SUBSECTION C5INSTRUMENTATION, CONTROLS, AND

INTERLOCKS

C5.100 GENERAL

Indicating and recording devices and associated in-struments are the operators basic tools for monitoringand controlling boiler operation; they may include steam,air, and feedwater flowmeters, thermometers, draft andpressure gages, and analyzers. These devices are in-stalled to monitor normal and abnormal conditions andare important for equipment operation, safety, andenergy conservation. Therefore, these devices shouldbe maintained in calibration at all times.

Modern boilers that are in compliance with acceptedpractice for safe operation include interlock systems.The interlocks establish the safe sequence for startupand provide for a safe shutdown. They should neverbe bypassed or jumpered.

Older boilers which do not include interlock safetysystems require that the operator carefully follow themanufacturers instructions when placing a boiler inand taking it out of service. During startup and shutdownall interlocks including the low water fuel cutoffs andthe flame failure devices, where installed, should bechecked for proper functioning. It is recommended thateach automatically fired boiler have two independentlow water cutoffs. Moreover, at each annual outage,the low water cutoffs should be dismantled, inspected,cleaned, and checked for proper calibration and per-formance.

On older units, the meter and control boards werelocated on the operating floor so that the operatorcould observe firsthand the critical operating functions:namely, proper combustion and the water level. Onmodern units designed with centralized control roomsincluding complex instruments, controls, and data log-gers, the firsthand observation feature is unrealisticunder normal operating conditions. In these cases redun-dancy must be provided in the instrumentation so thatthe operator is ensured of accuracy and reliability.

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C5.200 INDICATORS AND RECORDERS

Operation of a boiler requires proper regulation offeedwater flow, air flow, and fuel flow. Instrumentationshould be provided so that the operator may have anawareness of the operating conditions within the boilerthat affect these functions. In order to ensure safe,efficient, and reliable operation, a list of basic instrumen-tation is provided below.

(a) Indicating devices may include the following, ifapplicable:

(1) water level indicator;(2) steam drum pressure gage;(3) superheater outlet pressure gage;(4) feedwater pressure;(5) furnace draft or pressure;(6) boiler outlet draft or pressure;(7) forced draft fan discharge and induced draft

fan inlet pressure or draft;(8) windbox pressure or windbox furnace differen-

tial for pressurized systems;(9) economizer inlet and outlet draft or pressure;

(10) air heater inlet and outlet draft or pressure,both flue gas and air side;

(11) fuel supply and burner pressure for oil or gasfiring;

(12) atomizing supply and differential pressure andoil temperature for heavy oil systems.

(b) Recording devices may include the following, ifapplicable:

(1) steam flow;(2) air flow;(3) feedwater flow;(4) drum level;(5) steam temperatures leaving superheater and

attemperator;(6) air and flue gas temperatures as follows:

(a) air to air heater;(b) air to burners;

C5.200 1998 SECTION VII C5.310

(c) flue gas leaving boiler;(d) flue gas leaving economizer;(e) flue gas leaving air heater.

(7) smoke or opacity indicator for coal or oilfired units;

(8) flue gas analyzer, such as an O2, CO, or CO2meter.

(c) Pulverized Fuel Operation(1) Pulverized fuel fired boilers require instruments

for operation of the pulverizers. The instrument require-ments depend on pulverizer design. Instrument require-ments common to all pulverizer designs include:

(a) recorder, with high alarm, for the coal /airmixture temperature;

(b) pressure indicator for coal /air mixture tothe burners;

(c) flow device for primary air flow;(d) pulverizer drive motor ammeter.

Additional instruments may be required for specialequipment or fuels when recommended by the equip-ment manufacturer.

C5.210 Preparation for and Placing inService

All essential instruments must be installed, calibrated,and ready for operation before the boiler or any ofthe major equipment is placed in service. This procedurewill not only aid the operators of the boiler and fuelburning apparatus in getting the equipment properlyadjusted, but it will also provide for a record ofperformance during the period when equipment is beingplaced in service.

Records of instrument readings are often invaluablein preventing failures and determining the causes offailures or emergency conditions after their occurrence.A checklist of all important items to be acknowledgedby the operator during the startup provides assurancethat essentials are not overlooked in preparation forservice.

C5.220 Checking and Testing

A complete conditional and operations check of allinstruments and interlocks should be made by a compe-tent person at least once each year. Pertinent informationand dates of the operational checks should be enteredin the operating log.

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C5.300 CONTROLS AND INTERLOCKS

C5.310 Controls

Boiler controls are used to maintain steam pressure,load, drum water level, and fuel /air ratios at selectedconditions. Some elements may be omitted from smallinstallations and still provide adequate protection. Basicto the boiler control is the control of combustion andfeedwater.

(a) Combustion Control(1) A simple combustion control system should

have as a minimun the following controls:(a) a steam pressure control;(b) a subsystem for proportioning air and fuel

flows in the proper ratio to maintain safe and efficientcombustion;

(c) a furnace pressure control for balanceddraft units.

(2) More extensive systems may have additionalcontrol subloops and manual-automatic selector stations.There are many different arrangements by which ele-ments of a combustion control system may be assembledand still give the required control, but the function ofeach element should be carefully checked to ensurethat adequate protection of the installation is obtained.

(b) Feedwater Control. Automatic control of thefeedwater supply for maintaining the proper steamdrum water level must be provided on modern steamgenerating units where water volume is comparativelysmall, and it is recommended for all natural and con-trolled circulation boilers regardless of size. The typeof feedwater regulation selected should be governedby the boiler design, operating pressure, type of fuelused, method of firing, and load characteristics.

(c) Control of Superheat(1) Steam temperature control is very important,

not only for protection of the superheater but also forthe equipment served by the boiler. Factors that affectsteam temperature are as follows:

(a) excess air(b) type of fuel burned(c) slagging conditions(d) feedwater temperature(e) load

(2) Steam temperature control can be either bythe use of attemperators, steam bypasses, or gas propor-tioning dampers as well as by gas recirculation, highexcess air, or burner selection. The method utilizedwill depend on the design of the particular boiler andsuperheater.

(a) Attemperators. Spray attemperation is amethod of steam temperature control of superheatedsteam. This method regulates steam temperature by

C5.310 SUBSECTION C5 INSTRUMENTATION, CONTROLS, AND INTERLOCKS C5.340

diluting high temperature steam with makeup water orcondensate. Utilization of the spray attemperators isnot recommended below 10% steam flow. A temperaturedetector with a low temperature alarm should be in-stalled at the attemperator outlet to safeguard againstwater entering the downstream superheater sections.

(b) Steam Bypass. Spray attemperators at thesuperheater inlet are not effective during startup or atlow loads. The steam bypass system performs threefunctions during startup and low loads the following:

(1) drum pressure control by a superheaterbypass to the condenser;

(2) main steam temperature control with steamattemperation.The bypass system is used to control throttle pressureand steam temperature to meet the turbine manufactur-ers conditions during startup, shutdown, and over theload range. The unit can, however, be started andoperated without use of the bypass system.

(d) Control Device Features(1) Automatic devices for both combustion and

feedwater control should be arranged to failsafe in caseof failure of the control signal, driving medium, ormechanical failure. Locking systems that maintain feed-water control valves at the position which existed justprior to the control failure are recommended.

(2) Remote manual control of feedwater and com-bustion control should be provided to supplement auto-matic regulation so that the operator may control driveelements during startup and during periods of mainte-nance and emergencies. When combustion controls areon manual, both the fuel and air flow should also beon manual control.

C5.320 Interlocks

(a) Interlocks or safety controls, as they are some-times called, are provided to interrupt fuel flow to theboiler and shut down equipment in the event that unsafeconditions develop. These conditions could include thefollowing:

(1) loss of FD Fan(2) loss of ID Fan(3) furnace pressure

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(4) low water level(5) loss of flame(6) fuel supply out of normal range(7) atomizing fluid pressure or temperature out of

normal range(8) steam temperature out of normal range

(b) The permanent installation of cutout devices torender the interlocks inoperative and to permit theoperation of individual pieces of equipment should notbe provided. Temporary arrangements can readily beinstalled for the purpose of testing individual piecesof equipment.

(c) Aspirating-air or other types of lockout are re-quired on all observation doors, soot blower wall boxes,and oil or gas burner withdrawal openings on pressurizedfurnace boilers.

C5.330 Preparation for and Placing inService

All controls and interlocks should be carefullychecked for ability to perform as intended with equip-ment being controlled, such as dampers and valves, sothat any condition of the equipment which would makethe controller or interlock inoperable may be detectedand corrected before putting the boiler in operation.The original control setting should be made by acontrols and instrument engineer and/or Manufacturersserviceman.

Boilers should be operated on manual control duringstartup and promptly shifted over to automatic aftersteady-state conditions have been reached. Manual oper-ation under these conditions assists in training operatorsfor this mode of operation and for transferring frommanual to automatic control and vice versa.

Some package boilers are equipped only with auto-matic controls, in which case the procedure in thepreceding paragraph will not apply.

C5.340 Checking and Testing

A complete check and test of all controls and inter-locks is recommended at least once a year. An actualoperating check on interlocks can best be done duringa scheduled startup and shutdown of a boiler.

SUBSECTION C5 INSTRUMENTATION, CONTROLS, AND INTERLOCKSC5.100 GENERALC5.200 INDICATORS AND RECORDERSC5.300 CONTROLS AND INTERLOCKS