INFORMATION SHEET

INTRODUCTION TO AUTOMATIC BOILER CONTROL SYSTEMS

Information Sheet Number 62B-223-3-24-I

INTRODUCTION

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

REFERENCES

(a) Principles of Naval Engineering NAVEDTRAPERS 10788-B-Series

(b) Boiler Technician 3&2 NAVEDTRA 10535-H25-Series

(c) Automatic Combustion and Feedwater Control System NAVSEAavsea S9221-BJ-MMO- 010/0200951-LP-036-9010

INFORMATION

A. Definitions.

1. Automatic Control System - A system designed to control a parameter or setpoint without human intervention.

2. Setpoint - The desired pressure, temperature or flow that a ABC system will maintain (e.g. . 600 psi on boiler steam drum pressure, Normal water level in the steam drum.)

3. Controlled Variable - Is defined as the physical element of the setpoint. In other words, the the water that is being maintained at normal in the steam drum, the pressure of the steam drum being maintained at 600.

4. Measure - The measurement of a controlled variable. This is normally accomplished by the means of a transmitter.

5. Compare -: The comparison of a controlled variable against a setpoint defined in an Automatic Control System.

6. Compute - : The comparison of a controlled variable to the setpoint and analyzing the difference required to return the controlled variable to setpoint.

7. Correct -: The degree and position of the final control element must change to return the controlled variable to setpoint.

8. Variables -: Any parameter or element that is controlled or manipulated to maintain or establish a setpoint. e.g.. 600 PSI on the bBoiler, fFuel pPressure and cCombustion air flow.

9. Final Control Element - The component that will manipulate the controlled variable to maintain setpoint. The steam admission valve controls the speed on the FDB’s, the fuel oil control valve opens or closes to increase or decrease the fuel to the firebox.

10.Shrink - A false and temporary drop in steam drum water level caused by a decrease in boiler load.

11. Swell - A false and temporary rise in steam drum water level caused by a increase in boiler load.

B. Low pressure air requirements.

1. ABC’sutomatic boiler controls utilize low pressure air for a control medium. Air signals (defined as an “air signal”) are sent from one component to another. These signals will vary depending on the current boiler load. (We shall see in later lessons just how these signals vary and why).

2. Quality and (this is a test) constant pressure are important considerations when discussing control air. While the air signals may vary within the control systems the source of airsupply must remain constant and moisture-free.

3. Advantages to using low pressure air for a control medium:

a. Can withstand small air leaks without effecting system operation.

b. Heat has little effect on system operation.

C. Automatic boiler controls (Ffigure 13-24-1I)

1. Automatic combustion control system.

a. The Automatic Ccombustion Ccontrol (ACC) system is designed to provide maintain the proper amount of fuel and air necessary to maintain drum pressure at set point. To achieve this, end the speed of the forced draft blowers and the flow rate of fuel is carefully regulated.

2. Three-element feedwater control system.

a. The three-element FfeedWwater Ccontrol (FWC) system's primary function is to maintain normal boiler water level at set point - , within plus or minus one inch of normal water level, at all boiler loads. The three element FWCfeedwater control system is also designed to compensate for the effects of boiler water "shrink" and "swell".

3. Main feed pump constant pressure control system

a. The main Feed Pump constant pressure Control (FPC) system is designed to maintain a constant supply of feed water to the boiler at sufficient pressure under all load conditions.

The main Ffeed Ppump constant pressure Ccontrol (FPC) system is designed to maintain a constant supply of feed water to the boiler at sufficient pressure under all load conditions.

D. Automatic combustion control system (Ffigure 23-24-2I)

Purpose - :

To maintain steam pressure within 5 PSIpsi, plus or minus, of setpoint.

1. Steam pressure is sensed at the crossover pipe leading from the steam drum to the superheater or at the superheater outlet. For example, the following occurrs once a drop in steam pressure is sensed:

a. The air signal is sent first to begintell the forced draft blowers, causing them to increase in speed, thus increasing the volume of air to the firebox. to start speeding up.

b. With the forced draft blowers increasing in speed, the airfuel signal to the Fuel Oil System will begin to increase,causin an increase in the amount of fuel to the firebox. t

c. This increase of air and fuel to the firebox will have an overall increase in the rate of combustion or firing rate.herefore having an overall increase in combustion.

d. In the following example, Tthe air signal is being sent to an actuator which will convert the air signal to mechanical movement.

e. The steam admission valve is defined as a final control element in the ACC system. TThe mechanical movement will take place on the forced draft blower steam admission valve. By opening or closing to increase or decrease the amount of combustion air being provided to the firebox, being directly proportional to the speed of the forced draft blower. The steam admission valve is defined as a final control element.

f. The fuel oil pressure is being increased to sustain the increase in boiler demand. This fuel oil pressure component will make the conversion from an air signal to a mechanical movement.

g. The fuel oil control valve is defined as a final control element in the ACC system. The fuel oil control valve will open or close as required to maintain the new boiler demand. The fuel oil control valve is defined as a final control element.

2. Once theNow that air and fuel have changed, the steam pressure will return to setpoint.

E. Three3-Element feedwater control system (Ffigure 3-24-3II)

1. Purpose - To maintain boiler water level within plus or minus one inch of the normal mark.

1. Purpose:

a. To maintain boiler water level within plus or minus one inch of the normal mark.

2. Steam flow is detected first either due to an increase or decrease in boiler load.

3. As steam flow is changing, the feed flow must be changed to meet the new boiler load

4. . The feed flow must be equal to the proportion of steam flow.

5. The drum will be affected by “shrinkSHRINK” and “swellSWELL” during boiler demand changes.

a. ShrinkHRINK - : 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) ). When the throttle(s) close, a momentary back pressure develops in the boiler as the ACC system is still firing for the higher the demand. 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 apparent level volume of the water in the boiler to momentarily decrease. Once the until the boiler’s firing rate has lowered adjusted iitselftself to match the the new demand , the steam drum pressure will return to setpoint, thus allowingbringing the . As soon as the boilers heat balance has been corrected, water level back up will begin to return to register a correct reading (actually, the water level will be slightly above normal due to the decrease in steam leaving the boiler and the FWC systems designed lag in responsenormal.

b. SwellWELL - : This is the opposite occurrence of shrink from an up-power maneuver. A false and temporary rise in steam drum water level caused by an up-power maneuver ( i.e. opening up of the main engine throttle(s) ). Steam pressure in the boiler will drop in direct proportion to the increase ed 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 ACBC system begins to respond to the new demand (i.e., an increase in the firing rateof both combustion rate and feed flow)), pressure will build back up to setpoint and the water drum llevel in the steam drum will decrease, returning a correct reading back return to normal.

6. The steam drum level is the “supervisory signal” for the three3- element FWCfeedwater control system. It will be monitored by the system closely and will be considered the supervisor to of ensureing the water level remains within the setpoint of the system.

7. The feedwater control valve is the final control element in the three3- element FWCfeedwater system It will open and close based on the demand of the boiler. This is the final control element in this system.

F. Main Ffeed Ppump constant pressure Ccontrol (FPC) (Ffigure 43-24-4)

1. Purpose - To maintain constant feed pressure to the boiler, 150 to 200 PSIpsi above steam drum pressure, under all load conditions.

A.

1. PURPOSE:

a. To maintain constant feed pressure to the boiler, 150 to 200 psi above steam drum pressure, under all load conditions.

2. The three-element FWCfeedwater control system opens and closes the feedwater control valve. As this valve opens, less restriction to flow results in a decrease in feed pressure.

3. The FPC system senses this decrease and speeds up the Mmain Ffeed Ppump (MFP), developing more discharge pressure until setpoint is reached. The reverse is true on a decrease in demand.

4. The steam admission valve is the final control element in the FPC system. It is physically opensed or closesd to control the main feed pump’s speed. This is the final control element in this system.

A. ABC modes of operation:

1. Automatic

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

G. ABC modes of operation:

1. Automatic

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

2. Remote manual

a. In this mode, the control system is operated manually at the control console or station by manipulating the individual A/M stations.

b. This mode is used while lighting off, securing and during casualties.

3. Local manual

a. In this mode, direct manual control is taken at the final control elements (i.e., : controlling boiler water level by stationing a watchstander on the feed check valve) to manually open or shut as necessary.

b. Used when all remote means of control are lost, as in the case of a loss of control air casualty

c. Operation in this mode is very difficult if not practiced regularly.

a.

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

a. Operation in this mode is very difficult if not practiced regularly.

AUTOMATIC COMBUSTION CONTROLS

FIGURE 3-24-2I

AUTOMATIC BOILER CONTROLS

FIGURE 3-24-1I

3-ELEMENT FEEDWATER CONTROLS

FIGURE 3-24-3I

MAIN FEED PUMP CONTROLS

FIGURE 3-24-4I