INFORMATION SHEET

AUTOMATIC BOILER CONTROL SYSTEMS

Information Sheet Number 62P-113

INTRODUCTION

Automatic Boiler Controls (ABCs) are used extensively on modern surface ships to control the operation of the boiler and it's auxiliaries under all load conditions from minimum to 120%. The ABCs include the various sub-systems necessary in maintaining both combustion rate and sufficient feedwater supply to answer all steaming requirements within allowable tolerances. This study guide will present theory followed by sub-systems and finally system integration.

REFERENCES:

(a) Principles of Naval Engineering NAVPERS 10788 B

(b) Boiler Technician 3 and 2 NAVEDTRA 10535 G, Chapter 6

(d) Introduction to Naval Engineering (Second Edition)

INFORMATION

A. The automatic boiler control system (ABC) viewed in its entirety, can be likened to that of a computer. Unlike the electronic marvels we've grown so accustomed to, this system utilizes pneumatic signals (pressures) to process its information and effect the necessary changes to the various engineering plant components. ABC is an all inclusive term for a system which is comprised of the following four subsystems: automatic combustion control (ACC), feedwater control (FWC), feed pump control (FPC) , and air lock control (ALC). Control systems such as these are referred to as closed loop systems. A true closed loop system is self correcting in that it makes all corrective changes in direct response to its continuously monitored feedback signal. An automatic control system, regardless of its application, must meet the following criteria:

1. Measure

2. Compare

3. Compute

4. Correct

B. Terminology: As in any given trade or specialty, specific names/terms are assigned to various items, processes, or occurrences associated with it. In order to better understand the workings of these systems, a select few of these terms gleaned from Reference C are provided.

1. Actuating Signal: The pneumatic pressure (signal) sent from a relay or controller which works upon the bellows or diaphragm within an actuator.

2. Actuator: That component of a final control element that converts the actuating signal into a mechanical position change in order to change the operating point of the final control element.

3. Bias: A means of manually altering the actuating signal by either adding to or subtracting signal pressure prior to the positioner or actuator while the system is in the automatic mode of operation.

4. Boiler Load: The rate of steam generation of the boiler at set point at any instant.

5. Boiler Master: A boiler's Automatic/Manual (A/M) control station located on the boiler control console.

6. Closed Loop: A signal path that includes a forward path, a feedback path, and an error detector so arranged as to form a closed circuit.

7. Console: The boiler control panel that provides an operator with a remote manual means of controlling the system. The console also gives a visual indication of system performance during automatic operation.

8. Controlled Variable: That quantity or condition which is measured and controlled such as:

a.. Steam pressure

b. Water pressure

c. Fuel pressure

d. Air flow

9. Cycling: Control system instability which causes a continuous even variation of a systems set point.

10. Feedback: The returning of a fraction of a controlled variable output signal to the input of a systems controller input.

11. Hunting: Control system instability which causes a continuous erratic cycling around a systems set point.

12. Master Demand Signal: The input signal to both the air and fuel flow loops in the ACC system.

13. Open Loop System: A system in which the output is allowed to vary according to the characteristics of the system and of the input signal without reference to the system output (possess no feedback loop).

14. Setpoint: A reference force (generally spring tension) in a systems controller that represents a desired output (i.e. steam drum pressure, feed water header pressure).

15. Shrink: A "false" and temporary drop in steam drum water level caused by a down power maneuver (i.e. closing down on the main engine throttle/s). Because the boiler's ABC system can not anticipate a decrease in demand before it happens, a back pressure greater than boiler setpoint is created. At this point in time, this excess pressure exerts a force upon the water in the steam drum causing the steam bubbles within the entire boiler to compress. This in turn causes the volume of the water to momentarily decrease until the boiler's firing rate has adjusted itself to the new demand. As soon as the boilers heat balance has been corrected, water level will begin to return to normal.

16. Swell: This is the opposite occurrence of shrink and results from an up-power maneuver. Again, because an ABC system can not anticipate a change in boiler load (increase or decrease), steam drum pressure will drop in direct proportion to the increased amount of steam that is being admitted to the main engine/s turbines.As the pressure in the steam drum decreases the steam bubbles within the boiler are able to expand causing a "false" rise in water level. As the ABC system begins to respond to the new demand (i.e. an increase of both combustion rate and feed flow), the drum level will return to normal.

C. Subsystems:

1. Automatic Combustion Control system (ACC): The job of the ACC system is simply to maintain the proper amount of fuel and air necessary to maintain steam drum/superheater pressure at setpoint.

a. The ACC system is comprised of three loops:

(1) Steam Loop (supervisory)

(2) Air Loop (Forced Draft Blowers)

(3) Fuel Loop

b. Depending upon a particular ships engineering plant configuration, steam pressure setpoint may be measured from one of three possible locations:

(1) Steam Drum

(2) Superheater outlet

(3) A common header between boiler superheater outlets

c. Sufficient air to completely burn the fuel admitted to the firebox is referred to as the "Air to Fuel Ratio". The ACC system is designed to provide slightly more air (approximately 15% more) to the amount of fuel required for any given boiler load. This provides for more efficient combustion as well as preventing the boiler from producing black smoke.

d. Slight variations upon transient load changes are acceptable (5%) when monitoring drum pressure. However, when steaming at steady-state conditions, setpoint must be maintained.

2. Feedwater Control System (3 Element Control System)

a. The Feedwater Control (FWC) System's primary function is to maintain normal boiler water level setpoint within plus or minus one inch at all boiler loads. The three element feedwater control system is also designed to compensate for the effects of "shrink" and "swell".

b. The FWC System senses system parameters via transmitters at three different locations. They are:

(1) Steam Flow Transmitter: Senses flow at the saturated steam line leaving the steam drum. The signal from this unit is termed Demand.

(2) Feed Flow Transmitter: Senses flow at a boiler's feed pipe, prior to the economizer inlet. This is the systems Feedback signal.

(3) Drum Level Transmitter: Senses actual boiler water level directly at the steam drum. This is the Supervisory signal.

c. Steam flow and feed flow must match one for one (i.e. one pound of steam out equals one pound of feed water in) in order to maintain constant thermal / pressure equilibrium and normal water level at steady state steaming conditions.

d. A delayed response signal from the Feed Flow Controller to the Feedwater Control Valve is the means in which the system compensates for shrink and swell.

3. Main Feed Pump Constant Pressure Control System:

a. The Main Feed Pump Constant Pressure Control System is designed to maintain a constant supply of feedwater to the boiler at sufficient pressure (75 - 150 psig above steam drum pressure) under all load conditions.

b. On an increase in demand, the Feedwater Control Valve opens. When this occurs, the resistance to flow within the feedwater piping lessens resulting in a decrease in feed pressure. The Feed Pump Control (FPC) system senses this decrease and speeds up the main feed pump, causing discharge pressure to return to setpoint.

c. On a decrease in demand, just the opposite will occur. As the Feed Water Control valve closes, feed pressure will increase causing the system to slow the feed pump to its set point pressure.

D. Identification of Control Systems

1. Automatic control systems are identified by their principle manufacturer and application. Conventionally powered steam ship's currently in commission use either Hagan or General Regulator (GR) automatic boiler control systems. The individual manufacturer may use a part made by someone else much the same as Ford using Firestone tires. Barton and Moore Corporation supply several compatible components. They may be used interchangeably with either Hagan or General Regulator components.

2. Although Hagan has been the Fleet standard for years, General Regulator has recently become the predominant system installed in the majority of steam ships currently in commission.

E. Control Agent

1. Automatic boiler controls operate with clean, dry air normally supplied from the ship's low pressure air system. This however is not the only source of air as some ships are designed with their own dedicated air compressors for this system. Whatever the configuration, quality and pressure are important to the reliability of the control systems it supplies.

2. The advantages of air as a control medium are two fold in that:

a. It withstands small leaks without hampering the system's operation.

b. Heat and humidity have very little,or no effect on system components.

3. The quality of the air is dependant upon the design of "oil free" compressors and air dehydrators. Without these, the control systems will become fouled causing such problems as obstructing tiny orifices and the failure of rubber seals and diaphragms.

4. While the air signals may vary within the control systems the source of supply must remain constant. This air, normally between 115 - 125 psig, must first be reduced in pressure before it is introduced into the system/s. For example:

a. Hagan components operate at a pressure range of 0 - 65 psig

b. General Regulator systems operate on a range of 3 - 15 psig

F. Automatic Boiler Control Components

1. Transmitter (XMTTR) - These components are of varying design and are used on all the ABC systems. As the name implies, a transmitter does nothing more than MEASURE a variable and send out that information via a pneumatic signal. If a transmitter were a watchstander, it would be nothing more than a messenger, that is to say, its function is to relay information. The following are transmitters found in a typical ABC system:

a. Steam pressure (ACC)

b. Air flow (ACC)

c. Steam flow (FWC)

d. Feed flow (FWC)

e. Drum level (FWC)

2. The controller is the "brain" within any system in which it is installed. Its function is to receive the pneumatic signal or "message" sent from its transmitter, COMPARE it to its required value, and COMPUTE the difference between what it is presently receiving to what is required and send out a pneumatic signal that will bring the system back to set point. An example of this operation is as follows:

30 psig Required value (Set point)

- 23 psig Signal from Transmitter

07 psig is added to the current controller output signal of let's say 41 psig, to make a new output signal of 48 psig.

3. A Relay is similar to that of a controller in that it receives one or more pneumatic signals from a component or component and either combines, forwards or subtracts these signals to develop either an increasing or decreasing output signal. A relay is an intermediate component in that it will always be located between a controller and a final control element. Examples of these are the steam flow rate relay and fuel oil characterizing relay in the ACC system.

4. A/M Stations (Automatic and/or Manual control) are components which provide a means of selecting either automatic or remote manual control of a system or one or several of its final control elements. Despite their outward appearance and similarity in basic design, there are two different types. A/M stations are termed either 2 way or 4 way in their function.

a. 2-Way stations provide the operator with a means of selecting a mode of operation only. In other words; in the automatic mode, the automatic signal passes straight through this component totally unaltered whereas in the manual mode, the automatic signal is blocked off from passing through and the console operator generates the desired output signal. This mode of operation is used for different evolutions such as boiler light-off, specific operations (i.e. surface blowing), or troubleshooting.

b. 4-Way stations also provide the operator with the means of selecting the mode of operation and in addition to this, a means to introduce BIAS to the automatic signal. To add or subtract bias to an automatic signal is nothing more than "customizing" the output signal to a particular final control element. A four-way station is used for the parallelling of like pieces of equipment (i.e. FDBs and boilers) much like one would parallel electric generators.

5. Signal range modifiers are used whenever a dissimilar manufacturers component is used within a system's loop. The most common placement is that of a Woodward Governor which is used to position a steam driven piece of machinery's steam admission valve. Because of the disparity in signal range pressures between components, an "interpreter" or "translator" is required in order for these dissimilar components to work together.

6. A positioner is the pneumatically operated component of the final control element. It provides the positive movement for the final control element proportional to the incoming signal.

7. The Final Control Element is a valve, the actual "doing" part of the system. By manipulating the position of these various valves, the system will effect the required changes to forced draft blower and main feed pump speeds, boiler water level, and fuel oil pressure to the burner front. In short, this is the component that will CORRECT bringing its loop or system back to setpoint.

G. Air Lock System: This system is installed to provide time for the operators to shift the plant to local manual control should the air supply fail. It consists of a master air lock valve in the air supply line to the control console and an air lock valve in the air supply line to each final control element except for the main feed pump recirculating valve (if so installed).

1. In the event supply air fails, the air lock valves will shut locking in the last signal to each final control element and to the console. This keeps all final control elements "FROZEN" in their last position to prevent a loss of control of the boiler. PMS requires that the air lock system hold for a minimum of ten minutes.

2. At this point the EOOW will order the operators to shift control of the boiler to local manual control. The bridge, upon receiving this message from the EOOW, should not deviate course and speed as the control systems will not respond and the securing of the engineering plant will be inevitable.

3. In the event the air lock system does not hold, each final control element will go to a fail safe position as follows:

a. Fuel Oil Control Valve - MINIMUM

b. Feedwater Regulating Valve - OPEN

c. Forced Draft Blower Governors - MINIMUM

d. Main Feed Pump Governors - MAXIMUM

e. Main Feed Pump Automatic Recirculation Valve (if installed) - OPEN

4. Each final control element can be operated manually with the exception of the main feed pump automatic recirculation valve which immediately goes to its fail position in order to protect the main feed pump from overheating.

H. ABC MODES OF OPERATION

1. Automatic

a. In this mode, the control system will maintain the system at the set point throughout its full range without human intervention.

b. Bias can be introduced into a system having a four-way A/M station while in automatic.

2. Remote Manual

a. In this mode, the control system is operated manually at the control console or station by first shifting to the manual position. This prevents the automatic signal from passing through to the final control element down stream. The operator must now do the thinking and manually generate the necessary signal to that system's final control element/s.

b. This mode of operation is used for lighting off and securing, during casualties, and for troubleshooting the system.

3. Local Manual. In this mode, direct manual control is taken by individual watchstanders at the various final control elements.

a. Used when all remote means of control are lost such as with a loss of control air.

b. Operation in this mode is very difficult as reaction time is slowed and human error more prevalent.

I. Automatic Boiler Control System Operation

1. Increase in Demand

a. Automatic Combustion Control System - When the main engine throttle valve is opened, steam pressure decreases causing the system demand signal to increase. This causes the forced draft blowers to speed up and the fuel oil flow to increase. Even though the forced draft blowers receive an increasing signal, the blowers are slower to respond because of having to overcome inertia. The system will stabilize when setpoint reached.

NOTE: A component in the automatic combustion control system called the MINIMUM SIGNAL SELECTOR, ensures that the system functions in a series/parallel operation. That is, air leads and fuel follows on an increase in steam demand, and air and fuel oil decrease together on a decrease in steam demand.

b. Feedwater Control System - Steam Flow increases which calls for more water, but due to swell the FWC delays this increase for more water, besides the feedwater regulating valve will open returning the water level to normal. When steam flow and feed flow are equal and water level is normal the system will stabilize at the new demand.

c. Main Feed Pump Constant Pressure Control System - Feed pressure decreases below setpoint causing the main feed pump to increase in speed until feed pressure is reestablished at setpoint.

d. Main Feed Pump Recirculation Control System - Assuming that the recirc valve is open; when flow through the main feed pump increases to 90 GPM, the recirc valve will shut as there is sufficient flow through the main feed pump to prevent it from overheating.

2. Decrease in Demand

a. Automatic Combustion Control System - When the throttle valve is shut, steam pressure increases causing the system demand signal to decrease. This causes the forced draft blowers to slow down and the fuel oil flow to decrease. The system will stabilize when setpoint is reached.

b. Feedwater Control System - Steam flow decreases which calls for less water, but due to shrink the water level elements delay the decrease. When shrink subsides the feedwater regulating valve will close returning the water level to normal. When steam flow and feed flow are equal and the water level is normal the system will stabilize at the new demand.

c. Main Feed Pump Control System - Feed pressure increases above setpoint causing the main feed pump to decrease in speed until the feed pressure is reestablished at setpoint.

d. Main Feed Pump Recirculation Control System - Assuming the recirc valve is shut; when flow through the main feed pump decreases to 60 GPM, the recirc valve will open to prevent the main feed pump from overheating.

J. Effects on Plant Operations and Capabilities During Various Boiler Evolutions

1. Blowing Tubes - This normally has no effect on operations, however boiler load can possibly be limited by desuperheated steam usage. NSTM recommends a steaming rate of at least 50% to effectively remove debris clear of the stack. Windbox casing pressure is raised 2 inches of water by using the fuel/air ratio relay.

2. Surface Blowing - The bridge should not maneuver the ship or order any speed changes during this evolutions the feedwater is being controlled in remote manual at a higher level than normal.

K. Boiler Flexibility Test - This is a test conducted on the automatic boiler control system to evaluate the performance of the system. Conducted on one boiler at a time. A 70 % ramp change, as determined by fuel oil header pressure, is imposed on the boiler being tested. These ramps are conducted for 45 seconds. The controls must respond by returning steam drum pressure to setpoint plus or minus 5 PSI, water level to normal plus or minus 1", forced draft blower speeds must be within 300 RPMs of each other. There must be no smoke at any time during the test. The system must meet these requirements within 4 minutes of the beginning of the ramp load change with an additional 2 minutes settling time at the end to ensure the system is stable.

1. The ramp load change is done on the increase first and then on a decrease (up ramp and then a down ramp).

2. Prior to conducting a flexibility test the ABC technician conducts a series of On-Line Verification (OLV) Checks. These let the technician know if the ABC system will satisfactorily meet flexibility test requirements. All ships do not have the OLV package installed onboard which makes testing difficult at times.

3. The EOOW must have control of the shaft during the test. The bridge should not try to maintain course and speed as these can interfere with the test.

4. Flexibility test are conducted semi-annually per PMS. Also they are also conducted for INSURV, PEB, ETG and after each overhaul period.

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