3element Boiler

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HC900 Boiler Control Application BriefIndustry: Manufacturing

Honeywell Industrial Process Control 1 http://content.honeywell.com/imc/pi/hybrid/

Ox en

Pressure

Level

Fuel

Air

BackgroundUntil recent years, only the largest boilers could justifysophisticated boiler controls. Now high fuel costs andoccasional limited fuel availability make it necessaryto improve boiler efficiency and minimize costly steamlosses and disturbances.

Government regulations force compliance with airpollution and safety standards. Combustion controlshave also become more important because boilerloads are being varied to meet needs rather thanoperating at full capacity and wasting fuel and steam.

Similar concerns are causing the metals processingand heat treating industries to improve control for theirfurnaces and other fuel fired processes.

SolutionModern controls such as the HC900 Hybrid Controllercan provide the most efficient boiler operation.

Drum Level Control. The steam drum is an integralpart of a boiler. The vessels primary function is toprovide a surface area and volume near the top of theboiler where separation of steam from water canoccur. It also provides a location for chemical watertreatment, addition of feedwater, re-circulation water,and blowdown.

Blowdown removes residue and maintains a specifiedimpurity level to reduce scale formation. Because

these functions involve the continual addition and lossof material, the water-steam interface level is critical.Low level affects the re-circulation of water to theboiler tubes and reduces the water treatmenteffectiveness. High level reduces the surface areaand can lead to water and dissolved solids enteringthe steam distribution system.

The objective of the drum level control system is tomaintain the water-steam interface at the specified

level and providea continuous mass balance byreplacing every pound of steam with a pound offeedwater.

Types of Drum Level Control. There are 3 basictypes of drum level control systems: single element,two element and three element. Their applicationdepends upon specific boiler size and load changes.

The Single Element System is the simplestapproach. It measures level and regulates feedwaterflow to maintain the level. This is only effective forsmaller boilers supplying steady processes whichhave slow and moderate load changes.

The Two Element System uses two variables, drumlevel and steam flow to manipulate the feedwater flow.Steam flow load changes are fed forward to thefeedwater valve providing an initial correction for loadchanges. The steam flow range and feedwater flowrange are matched so that a one pound change insteam flow results in a one pound change infeedwater flow. This system is adequate for loadchanges of moderate speed and magnitude and canbe applied to any size boiler. It does have twodrawbacks. It cannot adjust for pressure or loaddisturbances in the feedwater system and cannotadjust for phasing interaction in the process becauseonly the relatively slow responding drum level iscontrolled.

The Three Element System adds a third variable,feedwater flow rate to manipulate the feedwatercontrol valve. This system provides close controlduring transient conditions because the twocontrollers minimize phasing interaction present in thetwo element approach. The feedwater control assuresan immediate correction for feedwater disturbances.The drum level control provides trimming action. Thissystem can handle large and rapid load changes andfeedwater disturbances regardless of boiler capacity.This approach is required on multiple boilers having acommon feedwater supply. It is ideal for plants withboth batch and continuous processes where sudden


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HC900 Boiler Controland unpredictable steam demand changes arecommon.

Figure 1 shows a Drum Level Control Solutionimplemented with the Hybrid Control Designerconfiguration software for the HC900 controller.

Figure 1

Fuel-Air Ratio Control: A fuel-air metering controlsystem is essential for efficient combustion in boilers,furnaces, and other large fuel fired heating processes.There are 3 basic types of Fuel-Air Ratio Controlsystems: series metered system, parallel meteredsystem, and cross limiting system.

The Series Metered System is fairly common whereload changes are not large or common. Both fuel andair are metered. The steam pressure controllerregulates the fuel flow that is measured, linearizedand then used as the remote set point to the air-flowcontroller. This positions an air damper to maintainthe specified ratio between the fuel and air.

This system is adequate for near steady stateconditions. However lags in response to load changescan result in temporary smoking, incompletecombustion and fuel-rich conditions.

The Parallel Metered System operates the fuel andair control controllers in parallel from a setpointgenerated by the steam pressure controller. The

setpoint signal is ratioed before being used as thesetpoint to the air controller to establish the fuel-airproportions. This system relies on similar responsesfrom both controllers to prevent improper fuel-airmixtures. This system is best applied to processesthat experience relatively slow load changes.

The Cross Limiting System is used when large or

frequent load changes are expected. This is adynamic system that helps compensate for thedifferent speed of response of the fuel valve and airdamper. It prevents a fuel-rich condition andminimizes smoking and air pollution from the stack.

The system is also known as a lead-lagsystem. Whendemand increases, a low selector function blocks thesignal, forcing the air flow signal to become thesetpoint to the fuel flow controller. A high selectorpasses the increase to the airflow controllers setpoint.This means fuel flow cannot increase until air flow hasbegun to increase, i.e. air increase leads fuel

increase.



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HC900 Boiler Control Application Brief

When demand decreases, the low selector passes thesignal to the fuel flow controller setpoint. A highselector blocks the signal to the Air Controller andpasses the fuel flow signal to the air flow controllersetpoint. This means air flow cannot decrease untilthe fuel flow begins to drop hence air decrease lags

fuel decrease. This means a fuel rich condition is

avoided, regardless of the direction of load change.

Figure 2 shows a Fuel-Air Ratio Control solutionimplemented with Hybrid Control Designersoftware for the HC900 Controller.

Figure 2

Oxygen Trim. Automatic oxygen trim of the fuel-airratio is used to reduce excess air and thereby excessoxygen to nearly stoichiometric combustion efficiency.

Too much air results in energy lost up the stack.Insufficient air results in loss of heat generation andincreased pollution due to incomplete fuelcombustion. A certain amount of excess air isrequired to insure that complete combustion occurswithin the combustion chamber and to compensate fordelays in fuel-air ratio control action during loadchanges.

In addition to improved efficiency, lower excess airhelps reduce corrosion and air pollution by minimizingthe formation of undesired gases.

Figure 3 shows the firing rate demand signal andsignal tracking during manual operations for the aFuel Air Ratio control with Oxygen Trim solutionimplemented with Hybrid Control Designer softwarefor the HC900 Controller.



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HC900 Boiler Control

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Benefit Summary

The Honeywell HC900 provides all of the controlfunctions required for drum level control, fuel-air ratiocontrol and oxygen trim. In addition, the HC900

provides the following benefits when used in boilercontrol applications:

Integrated loop and logic control minimizesequipment cost

Integrated control and operator interfacesimplifies troubleshooting

The ability to trend and log process data forregulatory agency reporting

A common configuration tool for both control andOI minimizing engineering costs.

Autotuning and fuzzy overshoot protection forquick startup and proper control operation

ImplementationOverview: The HC900 Hybrid Controller consists ofa panel-mounted controller connected to a dedicatedOperator Interface (OI). All field signals terminate atthe controller. The controller has universal analoginputs, analog outputs and a wide variety of digitalinput and output types. This controller will provide allthe boiler control functions.

Configuration: The Hybrid Control Designerconfiguration software provides advanced

configuration techniques to allow a variety ofstrategies to be easily implemented. The run-mode

HC900 Hybrid Controller, Model 1042 OI and Hybrid

Control Designer Software

configuration monitoring and editing capability allowsthese strategies to be tested and refined as processknowledge is gained

Operation: The complete operation can be monitoredand controlled from the easy to use displays of theModel 1042 Operator Interface.

Data Storage: The data storage feature of the 1042Operator Interface can be used to record processinformation during operation to an integral floppy disk.


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