INFORMATION SHEET

THREE ELEMENT FEEDWATER CONTROL

Information Sheet Number 62B-225

INTRODUCTION

The Three Element Feedwater Controls operate to maintain the boiler steam drum water level at a desired setpoint. This task is accomplished by controlling the flow of feedwater to the boiler at all firing rates. It is important to know how the components work both together and independently of each other. The following assignment will help in the understanding of this system.

REFERENCES

(a) Principles of Naval Engineering NAVPERS 10788-Series

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

NAVSEA 0951-LP-036-9010

INFORMATION

A. Three Element Feed Control System.

1. The feedwater control system is a three‑element type, designed to monitor changes in steam flow, water flow and drum level. Steam flow is the rate of steam leaving the boiler - the demand. Water flow is the rate of feedwater flow into the boiler - the supply. Drum level reflects the amount of water in the boiler - the inventory. With changes in boiler load (steam flow), steam and water flow become unbalanced and water level consequently deviates from the normal position. In such an event, the system changes water flow to the extent necessary to restore the balance between steam flow and feed flow and return the water level to normal.

2. Three Elements Measured by this System:

a. Steam flow - considered demand signal

b. Feed flow - considered response signal (feedback)

c. Drum level - considered supervisory signal

3. Components of the Three‑Element Feed Control System (Refer to figure 1)

THREE ELEMENT FEEDWATER SYSTEM BLOCK DIAGRAM:

Figure 1

a. Steam and feed flow transmitter - The steam flow and feed flow transmitters are identical in construction. These transmitters incorporate a differential pressure unit (DPU) which measures the pressure drop across an orifice. The transmitter generates a pneumatic output signal that is proportional to flow rate.

b. Drum level transmitter - The drum level transmitter compares a constant and variable that gives a differential pressure. A continuous output signal is provided that is proportional to drum level.

c. Steam flow-feed flow relay - The purpose of this relay is to subtract feed flow signal from the steam signal.

d. Feedwater flow controller - The purpose of this controller is to compare demand signal to supervisory signal and develop an output to return drum level to normal.

e. Feedwater automatic-manual transfer station - Allows the operator either automatic or manual mode of control.

f. Feedwater flow control valve and positioner - The diaphragm control valve is normally operated by changes in pressure to the control air connection from the positioner. A handwheel is fitted on the top of the valve to permit manual closing during maintenance and local manual operation. The feedwater control valve positioner is used to physically position feedwater control valve. A mechanical linkage on the positioner connected to the valve stem provides feedback of the actual valve to stem movement. When the controller output calls for the valve stem to change position, the positioner acts as a pneumatic relay, through a separate air supply, to increase or decrease the pressure to the valve actuator and change the position of the stem as required by the controller output signal.

4. Operation of Three-Element Feed System:

THREE ELEMENT FEEDWATER CONTROL SYSTEM BIG PICTURE

Figure 2

B. Increase in Boiler Demand:

1. Increased steam flow is sensed by steam flow transmitter.

2. Steam flow transmitter output increases.

3. Drum level transmitter output increases due to swell.

4. Feed flow controller is unchanged due to increased inputs from steam flow transmitter and drum level transmitter balance.

5. As swell decreases, feed flow controller output increases. Thus, the controller has "held-up" the increased feedwater demand signal until swell subsides.

6. Feedwater automatic/manual transfer station passes signal through in automatic.

7. Feedwater control valve positioner decreases, to position control valve stem in correct position to increase feed flow.

8. Feed flow transmitter senses increased flow and increases output signal.

9. Steam flow and feed flow transmitters balance.

10. Steam flow/feed flow relay returns to 30 psig.

11. Drum level returns to normal.

12. Drum level transmitter at 30 psig.

13. Feed flow controller balanced at new increased valve.

C. Decrease in Boiler Demand:

1. Decreased steam flow sensed by steam flow transmitter.

2. Steam flow transmitter output decreases.

3. Drum level transmitter output decreases due to shrink.

4. Feed flow controller unchanged due to decreased inputs from steam flow transmitter and drum level transmitter balance.

5. As shrink subsides the feed flow controller output decreases.

6. Feedwater automatic/manual transfer station passes signal through in automatic.

7. Feedwater control valve positioner increases to position control valve stem in the correct position to decrease feed flow.

8. Feed flow transmitter senses decreased flow and decreases output signal.

9. Steam flow and feed flow transmitters balance.

10. Steam flow-feed flow relay output returns to 30 psig.

11. Drum level returns to normal.

12. Drum level transmitter at 30 psig.

13. Feed flow controller balanced at new decreased value.