_INFORMATION SHEET

INFORMATION SHEET

FEEDWATER CONTROL SYSTEMS

Information Sheet Number 62S-105

INTRODUCTION

The Automatic feedwater control systems maintain boiler water level and delivery pressure at a desired setpoint. This objective is accomplished by manipulating a feed control valve and the speed of the main feed pump. This lesson will provide the basic control functions and component interrelationships understanding.

REFERENCES

(a) Automatic Combustion and Feedwater Control Systems NAVSEA 0951‑036‑9010

(b) Steam Plant Controls NSTM Chapter 225

INFORMATION

A. Feedwater Control Systems

1. Each boiler is installed with a three element feed system (Figure 1). It is an automatic system which controls feed flow to each boiler. The three element feedwater control systems position the feedwater control valves to each boiler, maintaining normal water level in the steam drums. Steady water level indicates that a balance exists between the water flow to the boiler and the steam removed.

2. The feedwater control system is a three‑element type, designed to monitor changes in the primary sensing elements: steam flow, feed flow and drum level. With changes in boiler load (steam flow) steam and feed flow become unbalanced and water level consequently deviates from the normal position. In such an event, the system changes water flow in the proper direction and to the extent necessary to restore the balance between steam flow and feed flow and return the water level to normal.

3. Three elements are measured by the system:

a. Steam flow is considered the demand signal.

b. Feed flow is considered the feedback signal.

c. Drum level is considered the supervisory signal.

Figure 1

1. Components:

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

b. Drum level transmitters compare the weight of constant and variable legs of water that result in a differential pressure. A continuous output signal is provided that is proportional to drum level. The output of the drum level transmitter will be 30 psig when the steam drum water level is normal.

c. The steam flow/feed flow relay for a Hagan system consists of a 4 chamber totalizer with a 30 psig spring attached to chamber 3. The purpose of this relay is to subtract the feed flow signal from the steam flow signal. The spring adds 30 psig to the result. When the inputs of the steam flow and feed flow transmitters are equal the output of the relay will be 30 psig.

d. Feedwater flow controllers in a Hagan system are 4 chamber ratio totalizers which function as proportional‑plus‑reset controller. Reset action is accomplished by feeding the output of the automatic‑manual transfer station back to chamber 4 through a needle valve and volume tank. The purpose of this controller is to compare demand signal to supervisory signal and develop an output to return drum level to normal. Shrink and swell are compensated for in the operation of the feedflow controller. On an increase in boiler demand swell offsets the increased output of the steam flow/feed flow relay by causing an increase in the drum level transmitter output. These inputs received by the feedflow controller are both increasing, thus the output remains the same to the automatic/manual control station. The same is true for shrink with the exception that both inputs decrease.

e. Feedwater automatic/manual transfer stations allow the operator two modes of control: automatic or manual. Automatic mode is used for routine steaming operations and manual mode for plant startup, securing, and casualty control.

f. Feedwater control valve and positioner are diaphragm control valves which are 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 the feedwater control valve. A mechanical linkage on the positioner connected to the valve stem provides feedback of the actual valve 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.

2. System operation (Increase in boiler demand):

a. Increase steam flow sensed by steam flow transmitter

b. Steam flow transmitter output increases

c. Drum level transmitter output increases due to swell

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

e. As swell decreases, feed flow controller output increases

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

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

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

i. Steam flow and feed flow transmitters balance

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

k. Drum level returns to normal

l. Drum level transmitter at 30 psig

m. Feed flow controller balanced at new increased value

n. Decrease in boiler demand is the opposite of an increase in demand.

3. Main feed pump constant pressure control system (Figure 2)

Figure 2

a. The main feed pump constant pressure control system ensures the main feed pump maintains a constant discharge pressure.

b. The control system consists of a hydraulic system which uses lube oil as a control medium, and a pneumatic system which utilizes air as a control medium. A pneumatic bellows, which actuates the turbine control system in response to the pneumatic signal from the feedwater system, is mounted on each turbine. The pneumatic control components for the main feed pumps are located in a control panel in each fireroom, with automatic/manual stations on the combustion control console.

c. Components:

(1) Feedwater header pressure controller senses feedwater header pressure and exerts a force on an installed beam. The set point spring, which opposes the force exerted by the high pressure bellows is adjusted to balance the regulated feedwater header pressure to setpoint. The output of the feed header pressure transmitter is proportional to the pressure sensed.

(2) Signal range modifier acts as a system pressure interpreter. For example, the Hagan system pressure range is from 5-60 psi representing 100 percent range. The main feed pump governor has a working range of 3-47.5 psi representing 100 percent. The range modifier in this application establishes compatibility between the normal 5‑60 psi range and that required by the feed pump governor.

d. Main feed pump automatic/manual transfer station gives the operator two modes of control: automatic or manual. One is installed for each main feed pump. Output goes to the bellows on the main feed pump turbine control operator (governor). It is important to note that the main feed pump governor will be reverse acting to the incoming signal. On an increase of demand the increased demand signal will be reversed to a lower signal to the pump controls allowing the pump to turn faster. The opposite occurs on a decrease in demand.

4. System operation (increase demand):

a. Feed header pressure decreases

b. Feed header pressure controller output decreases

c. Signal range modifier decrease input gives decrease output

d. Automatic/manual transfer station passes signal through in automatic

e. Decrease input to main feed pump causing increase in RPM

f. Increased RPM causes rise in feed header pressure

g. Decrease in boiler demand are opposite of increase in demand.

NOTE: There are a number of ships that do not utilize the Hagan feed header pressure systems. These ships use a Leslie constant pressure regulator that simply monitors header pressure and alters the position of the steam admission valve to the main feed pump directly via a governor assembly.

B. System Deficiencies:

1. The control air must be free of moisture and particulate matter. The smallest amount of moisture or particulate matter in a control system will foul small orifices and cause a malfunction of the system. If a component in the system malfunctions, securing the plant to repair it may not be required. In many cases the control system may be placed in remote or local manual, the component isolated and then replaced with a bulkhead spare. A complete knowledge of the control system's functions is necessary to make such critical decisions. It must be understood that when a control system is in remote or local manual the plant load must be increased or decreased slowly as an operator is not as responsive as the control system would be in automatic.

2. Other problems are: improper PMS being preformed, leaking air lines or fittings, component covers not installed, loose set screws on regulators, and components not mounted correctly (missing bolts or not tight).