Process Control

# Three Element Control

## What is 3 Element Control?

Boilers in power plants that encounter quick and wide-ranging load variations need three-element control.

Control with three elements is quite similar to control with two elements, with the exception that the water flow loop is closed rather than open.

In order to determine the feedwater flow’s index or set point, the level and steam flow signals are added.

The flow of feedwater and steam both have an impact on the drum level. They give precise water level control in the drum when added to the drum level element, and this control method is known as “three-element control.”

## What are the three elements in a three element boiler feed water control system?

It is necessary to have the following three inputs in order to create 3-element boiler feed water flow control:

1. Steam drum level.
2. Boiler feed water flowrate.
3. Steam flowrate.

## What is 3 element control of boiler drum level?

The most effective approach is to “feed forward” (FF) the steam flow into the level PID loop and then “cascade” the output of that loop into the setpoint of a boiler feedwater (BFW) flow loop.

Due to the fact that the algorithm takes into account level, BFW flow, and steam flow, this arrangement is referred to as “Three Element Drum Level Control.”

Three-element control is the term used to describe the method of controlling boiler drum level that combines measurements of the feed flow, steam flow, and drum level.

## Why is 3 element drum level control used?

• With varying steam load, the three-element system offers tighter control over drum level. Ideal for systems that experience varying feedwater pressure or flow.
• Higher degree of control is necessary.
• Required additional input for feedwater flow

The next few paragraphs explain briefly how the flow of feed water is controlled to match the steam demand from the boiler for drum level measurement:

• There are two sets of level transmitters (dP transmitters), one on each side of the boiler steam drum, to measure the level of the drum.
• A manual three-way selection switch is attached to the averaged two level signals from these transmitters.
• The three-way selector switch’s other two inputs come straight from the two transmitters. The average signal is often used as the control.
• The healthy transmitter is chosen in the three-way selection switch for the control if there is a problem with any of the transmitters.
• A deviation monitor continually monitors the performance of two level transmitters.
• When the difference between the two level transmitters exceeds the predetermined value, the control switches automatically to manual and an alarm is produced.
• A function that is generated using the drum pressure signal is used to correct the chosen drum level signal for variations in the density of the drum water that are brought on by pressure changes (actually brought on by temperature changes) in the drum.

## Drum Level Control:

• The signal from the compensated drum level is sent as a process variable to a P I controller that works in the opposite direction.
•  This measured drum level is compared to the set point, and the controller sends out a correcting signal (with proportional plus integral action) based on how far away the measured process variable (drum level) is from the set point.
• The equation that is used to determine the drum level is as follows:

h = DP + H(γr −γs) ÷ (γw −γs)

Where:

• h = True drum level
• DP = Measured DP head
• H = Distance between taps
• γ s = Steam Specific Gravity (S.G.)
• γ r = Reference leg (S.G.)
• γ s = Drum Water (S.G.)

### Drum Pressure:

• Since the density of steam and water at saturation temperature changes with pressure, the drum level calibration will only be accurate at a single boiler drum pressure.
• Using a drum pressure transmitter, the drum level signal can be made to work with any pressure.
• Pressure compensation is usually used with utility boilers, which have much higher pressures than industrial boilers. However, this method can be used with either type of boiler.

### Main steam flow measurement:

• The DP Transmitter measures the DP across the venturi nozzle, provided in the main steamline (The flow through the venturi nozzle is proportional to the square of the differential pressure, so the flow is proportional to the square root of the differential pressure.)
• The measured differential pressure is square-rooted and adjusted for changes in density due to changes in pressure and temperature.

### Feed water flow measurement:

• The DP Transmitter measures the DP across the orifice fitting is provided in the Feed waterline.
• The feed water flow is found by taking the square root of the measured differential pressure.

### Feed water flow control:

• The signal that is adjusted in the drum level controller and the compensated steam flow are combined.
• This total is used as the demand of feed water flow (cascade set point) for the boiler and compared to the actual feed water flow to the boiler.
• The difference between the actual water flow and the required water flow is sent to the Proportional Integral controller.
• Through a “Hand selector switch” and a “Auto–Manual station,” the controller’s output is sent as a positioning signal to the final control element, which is the Feed water control valve.

## What is single element control in boiler?

• When the load is low, you don’t need three-element control. The set point is a good way to measure the level of the drum.
• So, the output of the Drum Level Controller is linked directly to the Feed Water flow Control Valve through using selection switch that is located after the Cascade P&I Controller.
• Single-element control is the basic minimum for feedwater control, and it should be used for the following:

1) When the equipment is starting up or running with a low load, when flow measurements are usually not accurate.

2) When the rate of change of steam flow is nominal and the pressure of the feedwater supply is basically the same.

### Pressure compensation:

• If the sensors that are used to monitor the level of the drum are sensitive to fluctuations in density, then density compensation technologies need to be applied.

By adding an integral/reset control effect, the level will bring it back to the setting it was set. The swelling or shrinking makes the level on the steam rise. Integral control needs to be set up to be slow so that the effect of the initial rise is lessened.

## Three-Element Drum Level Control

• A three-element drum level control checks the level of the drum, the flow of steam, and the flow of feedwater.
• The reset value in the drum level PID function block makes up for any difference between the two flows.
• But this compensation is slow because of the tuning parameters that are needed to control the level of the drums.
• Changes in drum pressure affect the flow rate of feedwater, which makes the drum level go up and down.
• This is corrected by adding a feedwater flow control loop to the three-element drum level control.
• This separates the control of the feedwater flow from the control of the drum level.
• This makes it easy to respond quickly to changes in the feedwater flow without changing the drum level.
• The sum of the steam load and the drum level trim is the set point for the feedwater control loop.
• Most of the time, steam load is measured in tonnes per hour (T/Hr).
• If the density of the steam is taken into account, a differential pressure transmitter placed across an orifice plate or venturi meter can be used to measure mass flow.
• For a saturated steam boiler, the steam pressure can be used to figure out the density.
• The analogue input block is sized for full mass flow at a reference pressure. The static pressure upstream is used to measure how dense the steam is.
• Using the steam tables as a guide, a characterizer function block is set up to change absolute pressure to density.
• When you multiply the measured flow by the measured density and divide by the reference density, you get the actual mass flow.
• Most of the time, steam load is measured in tonnes per hour (T/Hr).
• Both the process variable and the set point have to be expressed in the same technical units for the PID function block to work properly.
• In 3-element control, the feedwater control loop’s set point is the sum of the steam flow trim and the drum level trim. Both steam mass flow and feedwater flow are measured in T/Hr, so there is no need to change the units of flow.
• The PID feedback signal is important because it makes it possible for the whole system to work.
• In 3-element control, the feedwater flow rate minus the steam mass flow rate is the feedback signal.
• In the feed water controller, a proportionate and integral action will be taken in response to any deviation that may occur.
• This controller output will serve as the position demand signal for the full load feed control valve.
• When the controller is in manual mode, the feedwater and drum level trim PID function blocks are forced into tracking mode.
• Three-element control turns the falling level control on its head so that, at high steaming rates, the water level goes up. This tries to make sure that the amount of water in the boiler stays the same at all loads and that the feed water control valve opens when steam demand goes up quickly. The system works by using the signal from a steam flow meter installed in the steam discharge pipe to raise the level controller set point when there is a lot of steam.

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