Question

With reference to Boiler feed regulation:

(a) Describe, with the aid of sketches, the operation of a boiler feed water regulator controlled by at least two other parameters besides water level in the drum. (6)

(b) Give reasons for the inclusion of the other elements besides water level in controlling feed flow. (6)

(c) Deduce the possible effects on the system when the drain valve in the constant leg in the level transmitter starts to leak. (4)

Answer 1

Part (a)

Three-Element Boiler Feed Water Control

The three elements (parameters) used are:

Steam flow rate
Feed water flow rate
Water level in the drum

Each parameter transmits a signal proportional to its measured value.

Steam flow and feed flow signals pass through individual square-root converters and are compared in a relay.
The relay sends a signal to the controller only when steam flow and feed flow are in a 1:1 ratio.
Once this condition is met, the controller compares the drum level signal (from a float level transmitter) with the setpoint.
Based on the deviation, the controller sends an air signal to the feed water control valve, which opens or closes to maintain the desired water level.

Part (b)

In feed water regulation for boilers, elements like steam flow rate and water flow rate are included along with water level to provide precise control and avoid phenomena such as swell and shrinkage, which can distort the actual water level in the boiler. A sudden increase in steam demand, for example, may lower steam pressure and saturation temperature, causing the water to temporarily exceed the saturation point. This results in bubble formation and a rise in water level, known as the "swell effect." Consequently, the control system may mistakenly close the feed water valve when more water is actually needed.

As steam demand normalises, the saturation temperature rises, and bubble formation ceases, causing the water level to fall—known as the "shrinkage effect." Including steam flow and water flow, elements help counteract these effects, ensuring an accurate reflection of the true water level and allowing the feed water control system to respond appropriately.

Part (c)

If the drain valve on the constant head of the level transmitter begins to leak, it disrupts the ability to maintain a steady head pressure, as the condensing steam and overflow cannot sustain the constant pressure needed. This leads to reduced pressure exerted on the bellow of the differential pressure (DP) transmitter. As a result, the flapper in the transmitter moves left, causing an increased air leakage from the nozzle.

The Proportional-Integral (P+I) controller misinterprets this as a higher water level and reduces the feed water flow by closing the feed control valve. This incorrect response leads to instability within the system and results in erratic water level indications.

✓ Verified Model Answer (Text Solution)

Three-Element Boiler Feed Water Control

The three elements (parameters) used are: