drum level control

THREE ELEMENT DRUM LEVEL CONTROL

1.0 Introduction:

In the boiler the steam flow changes according to the load demand from the turbine or from the process consuming the steam energy. To match the steam take off from the boiler, the feed water flow has to be increased or decreased as required. The feed water is automatically controlled by suitably positioning Feed Water Control Valve (FCV) provided in the feed water line at boiler inlet.

2.0 Three Element Feed water Control:

For automatic control of the feed water flow to the boiler, following three (elements) primary inputs are normally being considered.

2.1 Drum level.

2.2 Main Steam flow.

2.3 Feed water flow.

In addition, measurements of Drum Pressure, Main Steam Pressure and MainSteam temperature are also taken for correction/compensation purpose as detailed below.

3.0 Drum level measurement:

Normally two numbers of level transmitters (dP transmitters) one on left hand side and the right hand side of the Boiler Steam Drum are provided to measure drum level.

3.1 The two level signals from these transmitters are averaged and connected to a manual three-way selector switch. The other two inputs for the three-way selector switch are directly from the two transmitters. Normally the average signal is selected for the control. In case any problem with any one of the transmitters; the healthy transmitter is selected in the three-way selector switch for the control.

3.2 The functioning of two level transmitters is continuously monitored through a deviation monitor. In case of difference between the two level transmitters is higher than the preset value, and then the control is automatically transferred to Manual with an annunciation.

1.1 The selected drum level signal is compensated for density variation in the drum water due to pressure changes (actually due to temperature changes) in the drum, through a function generating with drum pressure signal.

2.0 Drum Level Control:

The compensated drum level signal fed to a reverse acting P I controller as process variable. This measured drum level is compared with set point and a correcting signal (with proportional plus integral action) is generated in the controller according the deviation between the measured process variable (drum level) and the set point.

3.0 Main steam flow measurement:

The dP Transmitter measures the dP across the ventury nozzle, provided in the main steam line (The dP measured is proportional to the square of the flow through the ventury nozzle and hence the square root of the dP taken as proportional to flow.) The measured dP is square rooted and compensated density variations due to change of Pressure and Temperature.

4.0 Feed water flow measurement:

The dP Transmitter measures the dP across the orifice fitting is provided in the Feed water line. The measured dP is square rooted to arrive at the feed water flow.

7.0 Feed water flow control:

The compensated steam flow is added with the corrected signal generated in the drum level controller. This sum is taken as the demand of feed water flow (cascade set point) for the boiler and compared with the actual feed water flow to the boiler. The deviation between the demand and the actual feed water flow, fed to the P & I controller. The processed out put from the controller is given to the final control element through a 'Hand selector switch' and an 'Auto – Manual station' – the Feed water control valve as positioning signal.

8.0 Single Element Control:

During low load operation, three-element control is not required. Drum level measurement with it's set point is adequate. Hence the output of the Drum Level Controller is directly given to the Feed water Control Valve through the selector switch provided after Cascade P & I Controller.

9.0 Auto Manual Operation:

An Auto Manual Control Station is provided as the last element of control system. Auto or Manual operation can be selected from the Auto Manual Selector Switch in this Control Station. If auto selected, the positioning signal from the selected controller (through Hand selector switch) will go to the Final control Element – Feed water Control Valve. If manual is selected, the controller's outputs are isolated and the Feed water Control valve can be positioned from this control station.

Raise/Lower push buttons are provided in this control station through which Feed water Control valve can be opened or closed as desired by the operator. Bump less transfer facility shall be built in as part of the Control Station to avoid sudden jump of control valve position during change over from auto to manual or vice versa.

Hope the above meets the requirement.

R.Thiyagarajan

Hi thiyagarajan,

thanks for your ans, this is very useful.

gobi

Hi R.Thiyagarajan

do you have any drawings on how the drum level transmiter is connected to the boiler drum?

shan

rajant,

In your explanation of Drum Level Measurement, section 1.1 deals with compensation for density. Do you know how the function generator correction outputs from the drum pressure signal are calculated? My company had a subcontractor install logic that added a gain and a bias function generator that corrects the drum level. The subcontractor convinced my company that he had a proprietary program and that only he was capable of doing this? Can this be true or are there programs available on the internet? Thanks.

your answer about drum level control is good and clear.if u have the control scheme send to my mail jeganrajasekaran@yahoo.co.in